Biomedical subjects

C H Irvine

Publications and source records attributed to C H Irvine.

At least 37 records · Page 2Linked to original sources

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↗

PubMed1991

The effect of various gonadotrophin-releasing hormone regimens on gonadotrophins, follicular growth and ovulation in deeply anoestrous mares.

Gonadotrophin-releasing hormone (GnRH) was used in several regimens in anoestrous mares in an attempt to investigate the effects of frequency and dose on follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release, and consequently on ovulation and corpus luteum establishment. Thrice daily injections of GnRH induced development of pre-ovulatory follicles, but hourly injections or constant infusions were required to induce ovulation. Hourly injections induced a much higher LH:FSH ratio in the presence or absence of ovarian hormones. When anoestrous mares were given an implant that released a potent GnRH analogue at a rate of 120 micrograms/day, 55% of mares ovulated and established corpora lutea compared with 77% at 180 micrograms/day. Even at the higher dose, all mares ovulated during or within 3 weeks of the end of treatment. Conception rate and mean day of conception were the same as in controls indicating absence of adverse effects on the reproductive axis. Although pulsatile administration of GnRH is the most effective regimen for induction of an LH surge and ovulation, GnRH analogue implants are much more easily given and are sufficiently effective to be the method of choice in many practical situations.

Anestrus↗

PubMed1990

Stairway risers and treads: acceptable and preferred dimensions.

Stairway preference and acceptability were investigated with psychophysical techniques. A series of six experiments was conducted where subjects ascended and descended 19 sets of stairways with different riser and tread (run) dimensions. Subjects were instructed to identify stairways that they considered acceptable, and the one stairway they most preferred. The optimum riser was 7.2 in (183 mm), and the optimum tread (run) was 11 or 12 in (279 or 300 mm). These dimensions were acceptable to both males and females, young and old, and subjects of greater or lesser stature. Larger dimensions were not as acceptable to shorter subjects, and smaller dimensions were not as acceptable to taller subjects. The 4-in (102 mm) riser was almost totally unacceptable, and never preferred. The 5.14 and 9-in (131 and 229 mm) risers were acceptable to less than one-third of the subjects, and rarely or never preferred. These results are compared with existing practices and recommendations.

Journal Article↗

PubMed1990

Changes in plasma progesterone concentrations from days 17 to 42 of gestation in mares maintaining or losing pregnancy.

Plasma progesterone concentrations were measured in 179 mares bled on alternate days commencing with a positive pregnancy diagnosis on Days 17 to 18 after ovulation and concluding on Days 42 to 45. During this period 17 mares (10 per cent) lost their pregnancies, 11 before Day 25. In 15 mares the timing of the pregnancy loss could be determined with adequate accuracy; in only one did a decline in progesterone precede the loss. Thus pregnancy loss between Days 17 and 42 was rarely caused by a fall in plasma progesterone.

Abortion, Veterinary↗

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↗

PubMed1987

Comparison by three different radioimmunoassay systems of the polymorphism of plasma FSH in mares in various reproductive states.

FSH was measured in the pituitary, and in pituitary venous and jugular blood collected at frequent intervals from mares in various reproductive states, using 3 validated and highly specific radioimmunoassay systems based on different antibodies, 'o', 'h' and 'e'. In the pituitary, 4 forms of FSH were found which differed in isoelectric point and relative potency in the 3 assays. In jugular blood, mean FSH concentrations and short-term patterns depended on the assay used and the reproductive state of the mare. In pituitary venous blood, although FSH concentrations were greatly elevated above jugular values, the relationship amongst the 3 assays was similar to that in jugular blood. However, the pulsatility of FSH secretion at oestrus (1-2 pulses per h in 4 of 5 mares) could be observed only in pituitary blood. When the rate of folliculogenesis was slow (acyclicity, oestrus), FSH concentrations measured in assay 'o' were relatively high and showed smaller fluctuations than in assays 'h' and 'e'. By contrast, when folliculogenesis was rapid (transitional phase, dioestrus), FSH tended to be lowest in assay 'o' and pulses had a similar magnitude in all assays. These results suggest that plasma FSH is polymorphic and that changes in the circulating form may have functional importance. Furthermore, since FSH polymorphism affects immunoactivity, the choice of a radioimmunoassay for clinical or research use is difficult without further FSH structure-function studies to determine which form of the hormone should be measured in a given situation.

Animals↗

PubMed1987

Factors affecting uterine clearance of inoculated materials in mares.

Twelve acyclic mares of various ages (2-29 years) and parity (maiden-multiparous) were given oestradiol-17 beta i.m. (winter 1982) or progesterone i.m. (winter 1983) to induce changes in the endometrium consistent with oestrus and dioestrus, respectively. After hormone treatment, mares were inoculated intrauterine with 50 ml saline containing 5 X 10(5) Streptococcus zooepidemicus bacteria, 51Cr-labelled 15-micron microspheres, and 500 mg charcoal (Groups E + B and P + B) or microspheres and charcoal only (Groups EC and PC). At 5 h after inoculation uteri were flushed with 50 ml saline containing tracer amounts of 125I-labelled HSA. In Group E + B inoculated materials were cleared more rapidly in younger than in older mares, and there was a significant positive correlation between age and bacterial concentration, total numbers of bacteria, and amounts of microspheres and charcoal in the uterus and also between age and the WBC concentration and total numbers of WBC. In Group EC there was a significant positive correlation between age and the amounts of microspheres and charcoal remaining in the uterus, and between age and WBC concentration and total numbers of WBC. We suggest that in the oestrogen-dominated uterus physical drainage may be a factor in determining whether a mare is 'resistant' or 'susceptible' to bacterial challenge of the uterus. Physical clearance is increased in younger (resistant) mares in the presence and absence of an antigenic stimulus. In mares in Group P + B there was no correlation between age and the values measured.(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗

PubMed1987

Measurement of free cortisol and the capacity and association constant of cortisol-binding proteins in plasma of foals and adult horses.

A direct method for measuring the capacity of the high-affinity binding protein, CBG, based on charcoal adsorption, was validated for use in the horse. Several unique aspects of cortisol binding in the horse were observed: (1) CBG content at birth was the lowest of any species studied, (2) CBG concentration increased with age whereas in other species it decreases, (3) the plasma of the new born foal has a binding protein, not reported for other species, which binds as much cortisol as does CBG. Its capacity and affinity are intermediate between albumin and CBG. It may be involved prenatally in increasing dam to fetus transfer of cortisol, and post-natally in buffering the effects of the huge stress-related peripartum release of cortisol.

Aging↗

PubMed1986

Clearance of bacteria and non-antigenic markers following intra-uterine inoculation into maiden mares: Effect of steroid hormone environment.

Uterine response to inoculation with Streptococcus zooepidemicus organisms (antigenic markers) and 15-mum microspheres and charcoal (non - antigenic markers) was determined in seasonally acyclic maiden mares treated with either progesterone (P) n = 4, estradiol (E) n = 4 or oil vehicle (C) n = 4. At 3, 7 and 15 d after inoculation with bacteria and the 2 non - antigenic markers, uteri were flushed and the clearance of these materials, as well as the number of white blood cells and immunoglobulin concentration, determined. P-treated mares had higher numbers of bacteria and IgA and a greater volume of purulent fluid in the uterus than E- or C-treated mares at 7 d after inoculation. Clearance of inoculated materials began within 2 h in E-treated mares, and the non-antigenic markers were completely cleared in E- and C-, but not in P-treated mares, in 3 d. This suggests that in the P-dominated uterus, reduced physical clearance may contribute to an increased susceptibility to uterine infection.

Journal Article↗

PubMed1986

Spontaneous and stimulated adrenocorticotropin and vasopressin pulsatile secretion in the pituitary venous effluent of the horse.

Plasma ACTH, arginine vasopressin (AVP), and catecholamines were measured at 5-min intervals in the pituitary venous effluent of the unanesthetized horse. Pulses of ACTH and AVP were found to be surprisingly brief (usually of less than 10-min duration) and frequent (averaging between 15-25 min). A highly significant relationship in the changes in concentration of these two hormones was demonstrated (P less than 0.0002) both at rest and after a mild hypoglycemic stimulus. Although there was also a significant correlation (P less than 0.005) between simultaneous plasma ACTH and AVP values the pulse amplitude ratio of AVP to ACTH showed a considerable variation. A rise in cortisol appeared to have a greater suppressive effect on the amplitude of ACTH than AVP pulses. The gradient in hormonal concentration between pituitary effluent and jugular plasma was at times over 50-fold for ACTH, and 500-fold for AVP. A gradient was also found for epinephrine, norepinephrine, and dopamine. A highly significant correlation (P less than 0.005) was demonstrated between changes in norepinephrine, ACTH, and AVP concentrations, but no such relationship could be shown for epinephrine and dopamine. It is concluded that there is a close temporal relationship between changes in ACTH, AVP, and norepinephrine concentrations. Pulses of these hormones are greater in amplitude and more frequent than would have been suspected from sampling peripheral plasma. The variability in the pulse amplitude ratio of ACTH and AVP may suggest that other factors are affecting ACTH secretion. The ability to sample frequently for several hormones and to obtain a marked gradient in hormonal secretion between the pituitary venous effluent and jugular plasma suggest that the horse should provide an excellent animal model in which to study the regulation of hypothalamic and pituitary hormone secretion.

Adrenocorticotropic Hormone↗

PubMed1986

Seasonal variation in the feedback of sex steroid hormones on serum LH concentrations in the male horse.

The possibility of seasonal variation in the feedback effect of testosterone or oestradiol was investigated by giving replacement treatment to geldings for 2-3 weeks during breeding and non-breeding seasons. In the non-breeding season, testosterone suppressed LH values (mean +/- s.e.m., ng/ml) in all geldings (before treatment, 7.5 +/- 2.3; final treatment week, 1.8 +/- 0.2; P less than 0.05), whereas early in the breeding season, testosterone caused a prolonged rise in LH (before, 6.8 +/- 2.3; final week, 18.9 +/- 6.4; P less than 0.05). In all testosterone experiments, LH returned to pretreatment levels within 2 weeks after treatment. Oestradiol treatment caused a prolonged increase (P less than 0.05) in LH concentrations (mean +/- s.e.m., ng/ml) in both seasons (breeding: before 5.2 +/- 1.1; final week, 16.2 +/- 4.8; non-breeding before, 10.9, 20.1 +/- 5.2). We conclude that in geldings the feedback effect of testosterone varies with season and, further, that testosterone replacement may be able to restore to geldings the stallion's seasonal pattern of LH secretion. The results suggest that, in male horses, testosterone and possibly oestradiol, are important components in the neuroendocrine pathway controlling seasonal breeding and, moreover, are essential for the generation of a positive signal for LH secretion in the breeding season.

Animals↗