Biomedical subjects

S L Alexander

Publications and source records attributed to S L Alexander.

At least 37 records · Page 2Linked to original sources

PubMed1995

The effect of naloxone administration on the secretion of corticotropin-releasing hormone, arginine vasopressin, and adrenocorticotropin in unperturbed horses.

We used our nonsurgical method for collecting equine pituitary venous blood to study the role of endogenous opioids in the basal regulation of the hypothalamo-pituitary-adrenal axis. We gave mares the opioid antagonist, naloxone (NAL), at either a high (0.5 mg/kg i.v. bolus, followed by infusion of 0.25 mg/kg.h; n = 4) or low (0.2 mg/kg i.v. bolus; n = 6) dose rate. Pituitary venous blood was collected continuously, divided into 0.5- or 1-min segments for 15-30 min before and 1 h after the NAL bolus, and assayed for CRH, arginine vasopressin (AVP), and ACTH. The mares tolerated NAL administration well, with little difference between dose rates in the mild transient side-effects. Both NAL doses increased jugular cortisol concentrations (high, P = 0.0022; low, P = 0.0001) and the ACTH secretion rate (high, P = 0.0056; low, P = 0.0103). High dose NAL raised the secretion rates of AVP (P = 0.0252) and CRH (P = 0.0106); however, the magnitude of ACTH responses exceeded those in AVP and CRH, as shown by increased ratios between ACTH and AVP (P = 0.0246) or CRH (P = 0.0122) secretion rates. After low dose NAL, neither CRH nor AVP secretion was altered. Indeed, CRH declined as ACTH rose in 4 mares and was unchanged in a fifth mare. When data from the 10 mares were pooled, mean secretion rates of ACTH and CRH were correlated after (P < 0.05), but not before, NAL treatment. Overall, mean ACTH and AVP secretion rates were not correlated during any 30-min period, but in individual mares, minute to minute AVP and ACTH secretion patterns were always correlated. We conclude that endogenous opioids inhibit the equine hypothalamo-pituitary-adrenal axis under basal conditions; however, their sites of action do not appear to lie solely on CRH and/or AVP neurons. It seems likely that endogenous opioids also inhibit the release of a third ACTH secretagogue or promote the secretion of an ACTH release inhibitory factor.

Adrenocorticotropic Hormone↗

PubMed1994

Factors affecting the circadian rhythm in plasma cortisol concentrations in the horse.

In horses, a circadian rhythm in plasma cortisol concentrations has been reported in some but not all studies. When a rhythm occurred, horses were accustomed to a management routine, comprising stabling, feeding and sometimes exercise, which may entrain a circadian pattern. In this work, we monitored plasma cortisol by collecting jugular blood through indwelling cannulae from four groups: 1): 10 untrained, unperturbed mares grazing excess pasture, bled hourly for 26 hr; 2) 4 mares housed in a barn for 48 hr before sampling every 15 min for 20-24 hr; 3) 5 mares placed in an outdoor yard for sampling every 30 min from 0930-2100 hr; and 4) 4 stabled racehorses in training, bled every 30 min from 0730-2000 hr and once the following morning at 0830 hr. Plasma cortisol showed a similar-timed circadian rhythm (P < 0.0001) in all Group 1 horses, with a peak at 0600-0900 hr, and a nadir at 1800-2100 hr. By contrast, cortisol concentrations did not vary with time in either Group 2 or 3. Neither daily mean nor peak cortisol values differed in Group 1 and 2 (i.e. bled for > or = 20 hr); however nadir values were higher (P < 0.05) in Group 2. In Group 4, cortisol declined (P = 0.004) during the sampling period but had returned to initial concentrations the next morning. Values did not differ from those for Group 1, except between 1000 and 1300 hr when cortisol in Group 4 was lower (P < 0.05). We conclude that a circadian cortisol rhythm exists in horses in the absence of any known cues imposed by humans. However, this rhythm can be obliterated by the minor perturbation of removing the horse from its accustomed environment. By contrast, the rhythm occurs in trained racehorses, suggesting either that they have adapted to their environment thereby allowing an endogenous rhythm to emerge, or that the rhythm is entrained by their daily routine. These observations highlight the difficulties in determining the cortisol status of a horse, since measurements will be affected by time of day, the occurrence of short-term fluctuations, and how accustomed the horse is to its environment.

Adaptation, Physiological↗

PubMed1994

Short-term secretion patterns of corticotropin-releasing hormone, arginine vasopressin and ACTH as shown by intensive sampling of pituitary venous blood from horses.

To characterize the short-term ACTH secretion pattern and to investigate factors regulating it, pituitary venous (PV) blood was collected using our nonsurgical method from 8 unperturbed horses every 20 or 30 s for approximately 1 h. In all but 1 horse, sampling occurred during the broad circadian maximum in plasma cortisol concentrations. Concentrations of corticotropin-releasing hormone (CRH; n = 7 horses), arginine vasopressin (AVP), ACTH and cortisol were measured by radioimmunoassay. In all horses, CRH, AVP and ACTH secretion patterns appeared irregular in time and amplitude. The mean (+/- SEM) numbers of peaks per hour detected by the cluster program were 2.8 +/- 1.2, 10.1 +/- 1.9 and 10.2 +/- 1.4 for CRH, AVP and ACTH, respectively. However, when 2- and 5-min sampling frequencies were simulated by meaning consecutive values, significantly fewer peaks were detected in each hormone. There was no correlation between the prevailing cortisol concentration and peak frequencies of CRH, AVP or ACTH. Secretion patterns of ACTH and AVP were closely related in all horses as assessed by cross correlation analysis and coincidence of peaks, although the ratio between PV ACTH and AVP concentrations fluctuated markedly within each horse. In contrast, the relationship between CRH and ACTH secretion was variable. Bivariate spectral analysis showed only a modest degree of underlying periodicity in CRH, AVP and ACTH secretion during the very short term studied. Nevertheless, distinct peaks exceeding the 95% confidence limits of white noise were observed at periods between 2 and 30 min in 5 of 7 CRH, 6 of 8 AVP and 5 of 8 ACTH spectra. Furthermore, the slope of the regression line through each spectrum did not become indistinguishable from zero, i.e. the flat white noise continuum, until mean (+/- SEM) periods of 2.6 +/- 0.8, 1.6 +/- 0.2, and 2.0 +/- 0.2 min, for CRH, AVP and ACTH spectra, respectively. At the ACTH spectral maximum, the coherence coefficient, which is analogous to the squared correlation coefficient, exceeded 0.5 in comparisons of all ACTH and AVP spectra and of 5 of 7 ACTH and CRH spectra.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenocorticotropic Hormone↗

PubMed1994

The dynamics of gonadotrophin-releasing hormone, LH and FSH secretion during the spontaneous ovulatory surge of the mare as revealed by intensive sampling of pituitary venous blood.

Conflicting views exist on the mode of gonadotrophin-releasing hormone (GnRH) secretion during the ovulatory LH surge and the relative importance of changes in pituitary responsiveness to GnRH in generating the LH surge. This disagreement may stem from species differences and/or methodological problems. To provide data on the exact relationship between GnRH and gonadotrophin secretion during the spontaneous LH surge, we collected pituitary venous (PV) blood every 30 s for 3-4 h from eight mares and then assayed GnRH (in six of the mares), FSH and LH. Jugular blood was also collected from twelve mares without PV cannulae either thrice daily during the surge (n = 8) or hourly for 24 h when close to ovulation (n = 4) and assayed for LH. Hormone peaks in PV blood were detected by the Cluster program and PV hormone patterns were scanned for underlying periodicity using spectral analysis. Jugular LH concentrations rose slowly and steadily without abrupt increase during the prolonged ovulatory surge, suggesting that hormone secretory patterns seen during the periods of rapid sampling were typical of the surge. Jugular LH concentrations were similar in mares with and without PV cannulae. Intensive sampling of PV blood showed that GnRH, FSH and LH were secreted in frequent (two to five per h) brief (5-7 min) peaks. Secretion was not detectable in 24%, 28% and 57% of the total sampling time for GnRH, LH and FSH respectively. GnRH and LH peaks appeared to be irregular in time and amplitude in most mares. However, spectral analysis of the data revealed an underlying periodicity in the secretion of all three hormones, with the dominant period ranging from 20 to 65 min in individual mares. The spectra of GnRH, FSH and LH were highly coherent at this dominant frequency, and 90% of GnRH peaks were concurrent with LH peaks, which is consistent with the dogma that GnRH is the primary secretagogue for both FSH and LH. Although PV FSH and LH concentrations were closely correlated, PV GnRH and gonadotrophin concentrations were only weakly correlated, implying that there was no consistent relationship between the magnitudes of changes in GnRH and gonadotrophin secretion. When compared with our published mid-luteal phase values, the daily GnRH secretion rate during the LH surge was trebled, while the LH responsiveness to endogenous GnRH, as assessed by the ratio between newly secreted LH and PV GnRH concentrations, was four times greater.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Secretory patterns and rates of gonadotropin-releasing hormone, follicle-stimulating hormone, and luteinizing hormone revealed by intensive sampling of pituitary venous blood in the luteal phase mare.

We used our unique nonsurgical technique for collecting pituitary venous (pit) blood to study GnRH, FSH, and LH secretion patterns in midluteal phase mares. This method does not perturb endocrine function and allows continuous monitoring of GnRH and gonadotropin (Gn) secretion, determination of the amount of GnRH perfusing gonadotropes, and direct measurements of the amounts of Gn secreted. In a total of 80 h of 5-min sampling in four mares, eight Gn peaks occurred; however, more frequent sampling was needed to define secretory events precisely. Therefore, pit blood was collected continuously and split into 30-sec segments in six mares. To ensure a peak during sampling, the opioid antagonist naloxone was given after 4-6 h of sampling to try to replicate a physiological signal for GnRH release. Naloxone induced Gn peaks in jugular blood that were indistinguishable in amplitude from spontaneous peaks. Intensive sampling of pit blood showed that jugular peaks reflected major episodes of GnRH and Gn secretion lasting 30-55 min, which were similar in profile whether naloxone induced or spontaneous and consisted of a train of three to six peaks of diminishing amplitude. Peaks of GnRH and, less often, Gn also occurred outside major episodes. Despite markedly variable size, GnRH peak maxima were correlated with the amount of LH and FSH secreted in concurrent peaks. Likewise, cross-correlation analyses (n = 960 samples/mare) showed close correspondence between patterns of GnRH and secreted FSH and LH. The delay (+/- SEM) between GnRH and Gn maxima was 0.62 +/- 0.18 min for LH and 0.18 +/- 0.22 min for FSH. The majority of GnRH and Gn peaks were concurrent; however, 34.7% of GnRH peaks occurred without Gn peaks. These peaks had a lower amplitude than those with Gn peaks (P < 0.001). For Gn, secretion (i.e. ratio between pit and jugular concentrations, > 1.5) continued at a low level for 40 +/- 9% (LH) or 64 +/- 14% (FSH) of the time between Cluster-defined peaks during the basal period. We conclude that in the luteal phase 1) the predominant mode of GnRH and Gn secretion is as concurrent, large amplitude, prolonged episodes that appeared to be the summation of a train of peaks; and 2) a GnRH dose-Gn response relationship operates endogenously. This along with the synchronicity of secretion patterns of the three hormones suggest that GnRH is the major secretagogue for both LH and FSH.

Animals↗

PubMed1993

The acute effect of lowering plasma cortisol on the secretion of corticotropin-releasing hormone, arginine vasopressin, and adrenocorticotropin as revealed by intensive sampling of pituitary venous blood in the normal horse.

The effect of an acute fall in plasma cortisol on the secretion of CRH, arginine vasopressin (AVP), and ACTH was studied using our nonsurgical technique for collecting pituitary venous (PV) blood from horses. PV blood from six mares was collected continuously and divided into 30-sec segments for 0.5 h before and during a 3-h infusion of metyrapone, an 11-beta-hydroxylase inhibitor. During treatment, plasma cortisol fell (P < 0.01) to a mean nadir of 15% of pretreatment levels, and 11-deoxy-cortisol rose (P < 0.02). Three mares became mildly agitated during treatment. Mean PV concentrations of CRH (P < 0.025), AVP (P < 0.05), and ACTH (P < 0.005) were higher during the second hour of treatment than before. For AVP (P < 0.05) and ACTH (P < 0.01), the amount secreted in peaks detected by CLUSTER analysis increased during treatment, whereas peak frequency did not. Responses, particularly in CRH and AVP, tended to be amplified during agitation. Increases in CRH, AVP, and ACTH secretion commenced when cortisol had fallen to 50-59% of the initial value (P < 0.005 for each). By contrast, the cortisol concentration at this point varied 3-fold among mares. The ratio between PV concentrations of ACTH and CRH, which was used as an index of pituitary responsiveness to endogenous CRH, also rose (P < 0.005) as cortisol fell. The increase in this ratio preceded any significant change in CRH secretion and was maintained to the end of the experiment. We suggest that the initial response to falling cortisol in the horse is at the pituitary, via increased responsiveness to CRH. If cortisol continues to fall, AVP and then CRH secretion are stimulated. However, the magnitude of the hypothalamic response to hypocortisolemia may be augmented by concurrent stress. Last, the hypothalamo-pituitary-adrenal axis of the horse appears to monitor changes in plasma cortisol and not concentrations, at least in the short term.

Adrenocorticotropic Hormone↗

PubMed1991

The effect of acute exercise on the secretion of corticotropin-releasing factor, arginine vasopressin, and adrenocorticotropin as measured in pituitary venous blood from the horse.

We have used the technique which we have developed for collecting pituitary venous blood from conscious, undisturbed horses to study the effect of acute vigorous exercise on the secretion of CRF, arginine vasopressin (AVP) and ACTH. Pituitary venous (pit) blood was collected every 1-5 min from nine trained racehorses at rest in the stable. The horses then trotted quietly for 10 min, after which they galloped as fast as possible for 4-6 min, before returning to the stable where sampling continued. In Exp 1 (n = 5) no blood samples were taken during exercise, whereas in Exp 2 (n = 4), pit blood was collected every 30 sec during exercise. Immediately after exercise, significant elevations in heart rate (P less than 0.001), body temperature (P less than 0.01) and hematocrit (P less than 0.001) were observed as compared with preexercise values. Jugular cortisol levels were higher after exercise (301.9 +/- 35.2 nmol/liter; mean +/- SEM) than before (187.3 +/- 34.8; P less than 0.01; n = 9). Likewise, jugular AVP levels increased with exercise (before, 0.65 +/- 0.11 pmol/liter; after 3.2 +/- 0.6; P less than 0.01; n = 6), whereas jugular CRF was not altered by exercise (before, 0.38 +/- 0.08 pmol/liter; after, 0.93 +/- 0.31; n = 6; NS). In Exp 1, no significant changes in pit ACTH, AVP, or CRF were observed after exercise. However in Exp 2 when pit blood was sampled during exercise all horses showed an immediate and dramatic rise in ACTH (P less than 0.01) and AVP (P less than 0.005) secretion which peaked during galloping with mean fractional changes above resting levels of 23.6 +/- 9.9 for ACTH and 51.7 +/- 24.0 for AVP. After exercise pit AVP levels were not different from resting, whereas ACTH remained elevated (11.4 +/- 6.9-fold above resting levels). By contrast, pit CRF levels were not altered by exercise. In both experiments together, pit AVP and ACTH concentrations were correlated in eight of the nine horses, whereas pit CRF and ACTH concentrations were positively correlated in only one of seven horses. We conclude that acute exercise causes a transient increase in ACTH secretion which occurs synchronously with an increase in AVP secretion. CRF does not appear to play a major role in mediating the initial ACTH response to exercise.

Adrenocorticotropic Hormone↗

PubMed1991

Comparison of the microheterogeneity of horse LH and FSH in the pituitary with that secreted into pituitary venous blood at oestrus.

For aqueous extracts of pituitary glands of oestrous mares, luteinizing hormone (LH) profiles were found to be similar to each other and to earlier work after chromatofocussing (CF) and isoelectricfocussing (IEF). After CF, both LH and follicle-stimulating hormone (FSH) in pituitary extracts focussed in multiple peaks in the acidic range, with 86% of LH and 80% of FSH found between pH 4 and 6. By contrast, in pituitary venous plasma, only 18% of the LH focussed in this range, whereas a significantly greater proportion (P less than 0.01) eluted above pH 7 than occurred in pituitary extracts (37% vs 2%, respectively). For pituitary venous FSH, there was only a slight shift in the distribution of isoforms compared with the pituitary extract, with a rise in the percentage of strongly acidic molecules in pituitary venous plasma (pH less than 3.65; 34% vs 16%). These results show that at oestrus, horse LH (which differs from that of other species because it has a heavily sialylated C-terminal extension to the beta-subunit, as does eCG), is much more alkaline when secreted as opposed to when it is stored in the pituitary. The authors of this report suggest that this modification is made after entry into a preferentially released pool of LH. Modulation of the forms of LH and FSH that are secreted may play a role in regulating target tissue responses.

Animals↗

PubMed1991

Effect of sexual arousal on gonadotrophin-releasing hormone, luteinizing hormone and follicle-stimulating hormone secretion in the stallion.

In an experiment conducted late in the physiological breeding season, 5 stallions were fitted with indwelling pituitary venous cannulae that permitted unobtrusive collection of blood coming from the pituitary and the hypothalamo-hypophyseal portal vessels. The next day, blood samples were collected at 5 min intervals for several hours while the stallions were resting. Pulses of gonadotrophin-releasing hormone (GnRH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) occurred approximately once per hour. After this, an oestrous mare was brought into contact with each stallion for 8-30 min. This exposure rapidly induced pulses of GnRH, FSH and LH secretion in all stallions, showing that sexual arousal stimulates the hormones of the reproductive axis.

Animals↗

PubMed1991

In vitro and in vivo studies of equine prolactin secretion throughout the year.

In vitro, the prolactin response of perifused anterior pituitary cells of horses to thyrotrophin-releasing hormone (TRH) (0.1, 1.0, 10 and 100 nM), correlated significantly with hours of daylight (P less than 0.01). Baseline concentrations of prolactin also were significantly correlated with daylength (P less than 0.01). When response and baseline data were fitted by nonlinear least squares to a cosine function, the circannual phase was -0.06072 (+/- 0.02170) and -0.05560 (+/- 0.0255), respectively, which are not significantly different from that of daylength. In vivo, prolactin secretion was measured in jugular plasma and pituitary venous effluent. Prolactin secretion by the pituitary (20- to 30-sec samples) showed pulses of 2-7 min duration above a varying baseline. Jugular prolactin concentrations correlated significantly with daylength (P less than 0.01, n = 20). The fractional plasma clearance rate was 2.50 x 10(-2) and 3.31 x 10(-2)/min in 2 horses. Seasonal changes in prolactin secretion in vivo and in vitro may reflect the amount of prolactin available for release.

Animals↗

PubMed1991

Control of onset of breeding season in the mare and its artificial regulation by progesterone treatment.

Mean plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels and their pulse frequencies and the size of the largest ovarian follicle increased during spring in 12 mares studied twice monthly from deep anoestrus to the occurrence of the first ovulation of the breeding season. Mean FSH levels were reduced significantly in deep anoestrus and when a pre-ovulatory follicle was present, whereas mean LH levels were highest close to ovulation. Five of these 12 research mares and 20 of 40 maiden or barren Standardbred mares at a commercial studfarm were given daily intramuscular (i.m.) injections of 150 mg progesterone in oil for 12 days when their ovaries were considered to contain developing follicles on rectal palpation. In the research mares, the size of the largest follicle in the ovaries during progesterone treatment did not differ from pre-treatment or control values; nevertheless, the 5 mares ovulated synchronously 11.2 +/- 0.4 (s.d.) days after progesterone withdrawal. Ovulation was less synchronized after progesterone withdrawal in the commercial mares (23 +/- 9.5 [s.d.] days) and in neither group did progesterone treatment advance the mean date of first ovulation compared with the untreated control mares. Mean plasma LH levels were not affected and mean FSH levels were slightly, but not significantly, elevated during progesterone administration in the research mares. In contrast, mean FSH levels fell steadily with time in both the progesterone treated and the control commercial mares. LH and FSH pulse frequencies in treated and control research mares differed only after progesterone withdrawal, when pulse frequency was higher in controls. We conclude that exogenous progesterone has no consistent effect on follicular development and gonadotrophin secretion patterns in transition phase mares, and in the present study it did not advance the mean date of the first ovulation of the breeding season compared with untreated control mares.

Animals↗

PubMed1989

Effect of an osmotic stimulus on the secretion of arginine vasopressin and adrenocorticotropin in the horse.

Arginine vasopressin (AVP) is released in response to changes in blood osmolality and is also a putative secretagogue for ACTH. However, it is unclear whether osmotically generated increases in AVP in the physiological range influence ACTH secretion. We have studied this question using our unique noninvasive technique for collecting pituitary venous blood in six normal conscious horses that received an iv infusion of hypertonic saline (HS; 5%, 0.07 ml/kg.min) for 45-60 min. Pituitary and jugular venous samples were collected every 5 min for 40 min before, during, and for 20 min after HS. During HS, mean blood osmolality rose (P less than 0.01), with a mean peak increase of 14.8 mosmol/kg (range, +6-+37 mosmol/kg). Jugular AVP rose (P less than 0.01) from 0.56 +/- 0.18 pmol/liter (mean +/- SEM) before HS to 2.16 +/- 0.86 pmol/liter during HS. Mean jugular AVP and osmolality were correlated (r = 0.82; P less than 0.05) during HS. Mean jugular ACTH concentrations increased (P less than 0.01) from 49 +/- 9 ng/liter before HS to 148 +/- 54 ng/liter during HS, while mean cortisol levels during and after HS exceeded basal levels (P less than 0.05). Pituitary AVP and ACTH concentrations exceeded jugular concentrations by up to 100-fold, and mean (P less than 0.01 for both) and peak (P less than 0.001 for both) levels increased during HS. AVP and ACTH secretion during HS were pulsatile. The mean and peak changes in pituitary AVP were significantly correlated with those in ACTH. For the six horses together, pituitary ACTH and AVP concentration changes occurred synchronously during the experiment (P less than 0.001), and the paired AVP and ACTH concentrations were highly correlated (r = 0.73; n = 129 pairs; P less than 0.001). We conclude that 1) physiological changes in AVP secretion are closely associated with comparable changes in ACTH secretion, and 2) osmotic signals that presumably activate the magnocellular neurons of the supraoptic and paraventricular nuclei may be physiologically relevant regulators of corticotrope function.

Adrenocorticotropic Hormone↗

PubMed1988

The effects of cortisol, vasopressin (AVP), and corticotropin-releasing factor administration on pulsatile adrenocorticotropin, alpha-melanocyte-stimulating hormone, and AVP secretion in the pituitary venous effluent of the horse.

Plasma ACTH, arginine vasopressin (AVP), and alpha MSH were measured in pituitary venous effluent at 5-min intervals from five unanesthetized horses during cortisol infusion and after an iv bolus of AVP or ovine (o) CRF. In control experiments (no hormone) there was a significant overall correlation between the timing of concentration changes in ACTH and alpha MSH. Cortisol infusion increased jugular cortisol levels by 70% and was associated with a reduction in mean ACTH, AVP, and alpha MSH secretion rates and ACTH peak secretion rate, but did not alter the observed pulse frequencies of these hormones. Administration of AVP raised plasma concentrations to a level comparable to the spontaneous peaks in pituitary venous blood and resulted in an increase in the secretion of ACTH and alpha MSH in all horses. Furthermore, spontaneous AVP peaks occurred in pituitary venous blood between 90 and 180 min after AVP injection, indicating that the exogenous hormone did not suppress AVP secretion. oCRF administration led to a prolonged elevation in plasma CRF and an increase in secretion of ACTH and alpha MSH, but not AVP, in all horses. The pulsatile secretion of ACTH and alpha MSH was maintained despite plasma CRF levels in excess of 400 pmol/liter, and the timing of concentration changes in AVP and ACTH continued to be highly correlated. It is concluded that pulsatile ACTH secretion continues during cortisol, oCRF, or AVP administration. Like that of ACTH, alpha MSH secretion is stimulated by oCRF and AVP administration and suppressed by cortisol. Although the timing of concentration changes in ACTH and alpha MSH is highly correlated, the correlation of the actual concentrations of these two hormones varies considerably in different animals.

Adrenocorticotropic Hormone↗

PubMed1988

Effect of isolation stress on concentrations of arginine vasopressin, alpha-melanocyte-stimulating hormone and ACTH in the pituitary venous effluent of the normal horse.

A non-surgical, non-stressful technique was used for collection of pituitary venous blood from five conscious horses every minute for two 10-min periods before and during isolation from the herd, which caused a predictable, yet humane and physiological, emotional stress. Pituitary blood was also sampled every 5 min for two approximately 90-min periods before and after isolation, while jugular blood was sampled every 15 min throughout the experiment. During isolation, all horses became agitated, hyperventilating and sweating. Packed red cell volume increased, as did pituitary venous concentrations of adrenaline (mean +/- S.E.M. concentration before isolation, 621.5 +/- 112.3 pmol/l; peak during isolation, 2665.4 +/- 869.8 pmol/l; P less than 0.05) and noradrenaline (before, 871.8 +/- 111.8 pmol/l; peak, 2726.1 +/- 547.4 pmol/l; P less than 0.02). Concentrations of arginine vasopressin (AVP) were higher in pituitary venous but not in jugular blood during isolation than during the preceding 10-min period (P less than 0.05). Although AVP secretion increased in all horses, in three of the five it rose dramatically in the first minute of isolation to 25.7 (horse 1), 13.6 (horse 4) and 145.1 (horse 5) times the level in the last sample collected before isolation. Mean pituitary venous concentrations of ACTH and alpha-MSH increased during isolation in the three horses which had large increases in AVP secretion, but, overall, stress did not significantly affect ACTH or alpha-MSH secretion. Similarly, mean jugular cortisol levels were not significantly altered by isolation. However, the magnitudes of ACTH, AVP and alpha-MSH responses to isolation were negatively correlated with the jugular cortisol level before isolation. The changes in pituitary venous concentrations of ACTH and AVP were synchronous under resting conditions, whether samples were collected at intervals of 1 (P less than 0.01) or 5 (P less than 0.005) min; however, this synchrony was lost during isolation. The changes in pituitary venous concentrations of ACTH and alpha-MSH were synchronous both at rest (P less than 0.025 for 1-min sampling, P less than 0.01 for 5-min sampling) and during isolation (P less than 0.01). We conclude that isolation stress increases AVP secretion and may alter the temporal relationship between pituitary venous concentrations of AVP and ACTH. Furthermore, the magnitude of the responses of AVP, ACTH and alpha-MSH to isolation is significantly affected by the prevailing cortisol level.

Adrenocorticotropic Hormone↗

PubMed1988

Secretion rates and short-term patterns of gonadotrophin-releasing hormone, FSH and LH in the normal stallion in the breeding season.

Pituitary venous blood was collected by a painless nonsurgical cannulation method from five ambulatory stallions at 5-min intervals for 5-6 h during the breeding season. In four adult stallions, statistical analysis showed that pulses of gonadotrophin-releasing hormone (GnRH) and LH were coincident (P less than 0.01), as were pulses of FSH and LH (P less than 0.05). Furthermore, the patterns of changes in concentration of FSH and LH were highly correlated in each of the four stallions. However, seemingly ineffective pulses of GnRH were also observed, with 28% of GnRH pulses failing to induce a significant gonadotrophin pulse. In the four adult stallions the amplitude of pituitary venous gonadotrophin pulses varied markedly but no correlation with GnRH pulse amplitude was observed. Peak secretion of FSH, but not LH, during pulses was correlated with the length of the interpulse interval. Consequently, the ratio of FSH to LH during peaks was least (P less than 0.02) when the interpulse interval was 30 min or less. Thus, differential FSH and LH secretion was achieved within a constant steroid milieu. Two stallions had regular contact with oestrous mares, and in these horses the secretion of GnRH and gonadotrophins occurred almost continuously with rapid, rhythmic pulses superimposed upon a tonic background. Mean (+/- S.D.) interval between GnRH pulses was 31.4 +/- 9.8 min and 27.7 +/- 10.1 min. This secretory pattern was not observed in the two stallions which had infrequent contact with oestrous mares, although the small numbers precluded statistical testing of this apparent difference. No GnRH pulses were observed in one of these stallions, while in the other mean (+/- S.D.) GnRH pulse interval was 45.0 +/- 48.7 min, the large variance being partly due to rapid pulses during a period in which the stallion teased mares. The fifth stallion was pubertal, and GnRH and LH secretion occurred in 15 and 0% of samples respectively, while low levels of FSH secretion were observed in 37% of samples and jugular testosterone levels were immeasurably low. We conclude that there is a statistically significant synchrony between pulses of GnRH, LH and FSH in the pituitary venous blood of stallions. Furthermore, decreasing intervals between gonadotrophin pulses result in a significant reduction in secretion of FSH but not LH.

Animals↗

PubMed1987

A novel technique for measuring hypothalamic and pituitary hormone secretion rates from collection of pituitary venous effluent in the normal horse.

We have described a novel technique for collecting pituitary venous effluent in the horse by placing a cannula in the intercavernous sinus close to the outlet of the pituitary veins using a venous pathway unique to equids. Cannula placement and blood collection are carried out painlessly in fully conscious, ambulatory, unstressed animals. There is no interference to hypothalamic, pituitary or target organ function. The blood collected contains readily measurable concentrations of gonadotrophin-releasing hormone, and LH concentrations which can be up to 40 times those in concurrent peripheral blood samples. Four millilitre blood samples, a quantity which permits simultaneous measurement of many hypothalamic and pituitary hormones, can be collected at 2-min intervals for several days. Intercavernous sinus blood flow can be calculated allowing secretion rates of hypothalamic and pituitary hormones to be determined for any time-period. This model is uniquely useful for investigating the normal functional characteristics of several neuroendocrine and endocrine systems.

Animals↗

PubMed1987

Secretion rates and short-term patterns of gonadotrophin-releasing hormone, FSH and LH throughout the periovulatory period in the mare.

We have developed a non-surgical technique for long-term collection of pituitary venous blood which consists of slightly diluted hypophysial portal blood into which pituitary hormones have been secreted. In these experiments jugular and pituitary venous blood samples were collected from five unmedicated, ambulatory mares at 5-min intervals for 2-6 h on 11 occasions during the 6 days surrounding the ovulatory LH peak. Jugular blood only was collected from another five periovulatory mares without pituitary cannulae. The duration of oestrus was similar in mares with and without pituitary cannulae and all mares ovulated, showing that the procedure did not affect the reproductive axis. In all pituitary-cannulated mares the secretion of gonadotrophin-releasing hormone (GnRH), FSH and LH occurred almost continuously with broad, concurrent pulses of the three hormones superimposed upon this tonic background. Only 9% of the GnRH pulses appeared to be ineffective in inducing a rise in gonadotrophin levels. When measured in pituitary blood, gonadotrophin pulse frequency varied from 0.45 pulses/h early in the LH surge to 1.87 pulses/h at the time of ovulation. In contrast, mean pulse frequency measured in jugular blood did not exceed 1 pulse/h throughout the periovulatory period in cannulated or non-cannulated mares. The low amplitude of jugular pulses (less than 50% fractional increase) may have caused problems in identifying the pulses. In the two mares in which pituitary venous blood was sampled during more than one period before ovulation, GnRH secretion tended to be lower on the day of ovulation (day 0) than earlier in oestrus (ratio day 0:day -1; mare WV = 0.58, mare LS = 0.66), whereas LH secretion rate was higher on the day of ovulation (ratio day 0:day -1; mare WV = 1.54, mare LS = 6.68). These studies show that the painless and non-invasive collection of pituitary venous blood, which is possible only in horses, can provide a useful tool for studying hypothalamic-pituitary interactions under completely physiological conditions.

Animals↗

PubMed1987

Release of LH, FSH and GnRH into pituitary venous blood in mares treated with a PGF analogue, luprostiol, during the transition period.

Nine mares received cannulae to collect blood from the pituitary venous outflow in the intercavernous sinus (ICS) and the jugular vein; in 4 mares, only jugular cannulae were used. Those 4 mares and 3 of the mares with cannulae in both positions received 7.5 mg luprostiol i.m. and 1 mare with both cannulae was treated with 3.75 mg uprostiol i.v. Blood samples were kept before and after treatment at 2-, 5- or 10-min intervals and concentrations of LH, FSH and GnRH were determined by RIA. Treatments resulted in an immediate sharp rise of LH and FSH in ICS and jugular blood samples within 2-10 min, with ICS concentrations rising earlier, and with peak levels of LH 8 to 100 times higher, respectively. In ICS samples, GnRH was elevated consistently only after LH and FSH had reached peak levels. At both locations, LH and FSH concentrations remained elevated 60-120 min after treatment, but had returned to baseline by 240 min. In 5 untreated mares with cannulae at both locations, sampling at 5-min intervals for 12 or 24h revealed no pulses of LH or FSH in 3 mares, and only one pulse a day, preceded by several small rises of GnRH during the hour before the pulses, in 2 mares.

Animals↗