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Biomedical subjects

I C Robinson

Publications and source records attributed to I C Robinson.

At least 109 records · Page 6Linked to original sources
PubMed1986

Central release of oxytocin, vasopressin and neurophysin by magnocellular neurone depolarization: evidence in slices of guinea pig and rat hypothalamus.

Using slices of rat hypothalamus maintained in vitro, we have examined release of oxytocin and vasopressin under conditions of increased neuronal activity. We report here that when the supraoptic or paraventricular nucleus is depolarized with high K+ solutions, hormone is released into areas close to the nucleus. Similar experiments with guinea pig hypothalamus suggest that neurophysin may also be co-released with oxytocin and vasopressin. Use of acetylcholine to selectively stimulate vasopressin neurones appears to evoke a rise in local release of vasopressin but not oxytocin. These results suggest that under conditions of increased neuronal activity, hormones normally secreted from the neurohypophysis are secreted locally into the hypothalamus.

Acetylcholine↗
PubMed1986

Aspects of the neuronal and endocrine components of reflex milk ejection in conscious rabbits.

Nursing behaviour and reflex milk ejection were studied over 257 suckling periods in 26 Californian rabbits. Detailed plasma profiles of oxytocin were obtained for 161 suckling periods in 16 animals. Plasma oxytocin was detected by radioimmunoassay in serial samples of 0.2 ml blood collected during nursing. Oxytocin titres were below the lower limit of the assay (5 pmol/l) before suckling, and started to rise at a rate of 2.8 +/- 0.8 (S.D.) pmol/l per s 10-30 s after the onset of suckling. Peak levels of hormone were 345 +/- 113 pmol/l and were attained towards the end of nursing. In 33 of these experiments simultaneous records of activity from oxytocin neurones were taken whilst chronically sampling the blood. Each neurone gave an average of seven bursts of neurosecretory activity in suckling. Onset of this bursting pattern of discharge began 5-24 s before the rise in plasma oxytocin was detected. Oxytocin neurone activity alone was monitored in a further 55 suckling periods in eight rabbits. A marked relationship between the duration of suckling period, milk yield, peak oxytocin levels and the length of neurosecretory bursts was demonstrated over the course of lactation. All four parameters increased in parallel from day 1 to reach maximal values on days 15-20 of lactation then started to wane until the doe ceased nursing her litter on days 25-27 of lactation.

Animals↗
PubMed1986

Automated repetitive microsampling of blood: growth hormone profiles in conscious male rats.

A system is described for the automatic collection of small samples of blood from conscious rats. Rats bearing chronic indwelling i.v. catheters were connected via swivels to a multichannel peristaltic pump, solenoid valves and a fraction collector. A microcomputer controlled the operations involved in the removal of blood and its deposition into a fraction collector for subsequent direct radioimmunoassay for GH. Blood samples of 10-20 microliter could be collected, into a total volume of 100 microliter heparinized saline, from up to eight rats simultaneously every few minutes for many hours. This collection method avoided major blood loss and did not require transfusions of donor blood to maintain blood volume. Using a double-lumen cannula it was possible to inject or infuse into the animals while sampling blood. The system was used to investigate in detail the secretion of GH in conscious male rats. The 3-hourly endogenous secretory rhythm of GH was maintained for up to 44 h with episodes of GH secretion being multicomponent. Endogenous secretion was suppressed by constant i.v. infusions of somatostatin, with repetitive sampling showing in detail a rapid rebound secretion of GH after terminating the somatostatin infusions. Four injections of a fragment of GH-releasing factor, given at 3-hourly intervals, produced entrained GH responses, but the subsequent recovery of endogenous GH pulsing was delayed for up to 12 h. This method for the automatic microsampling of blood in small animals gives a very detailed description of the blood levels of hormones secreted in a highly episodic fashion, and could be widely applicable to other endocrine studies.

Animals↗
PubMed1985

Membrane retrieval in the guinea pig neurohypophysis: biochemical characterization of a retrieval structure.

[3H]Choline and [35S]methionine injected into the guinea pig hypothalamus in vivo were incorporated into the lipids and proteins, respectively, of secretory vesicles transported to the neural lobe. Prolonged in vivo stimulation of hormone secretion by dehydration decreased the [3H]choline content of secretory vesicles, with a concomitant increase in the [3H]choline content of a membrane fraction isolated on sucrose gradients. After stimulation of neural lobes in vitro in the presence of horseradish peroxidase, this extracellular fluid marker was found in the same membrane fraction. SDS electrophoresis of membrane proteins radiolabelled by [35S]methionine in vivo demonstrated that this fraction contained at least one major protein also present in the secretory vesicle membrane. These results suggest that we have isolated a membrane fraction containing the structure(s) involve in membrane retrieval in the neurohypophysis.

Animals↗
PubMed1985

Influence of endogenous and exogenous oestrogens on posterior pituitary secretion in women.

Four normally menstruating subjects were studied throughout the menstrual cycle to investigate changes in plasma LH, arginine vasopressin (AVP), oxytocin (OT) and oxytocin associated neurophysin (NPOT). A clear mid-cycle LH peak was observed in each subject. Mean levels of AVP, OT and NPOT were 2.2 pmol/l, 1.1 pmol/l and 39 pmol/l, respectively. There were no significant differences between plasma levels during follicular, mid-cycle and luteal phases for each of these. Two further subjects with anovulatory cycles were studied in a similar way. In the first subject, with polycystic ovarian disease, posterior pituitary peptide levels were in the normal range, whereas the other subject, recovering from anorexia nervosa, had raised plasma levels of all posterior pituitary peptides (AVP 8.1 pmol/l, OT 1.8 pmol/l, NPOT 131 pmol/l, mean values) despite a normal osmolality. Administration of ethinyl oestradiol, 100 micrograms or 500 micrograms, had no effect of either AVP or OT, but 100 micrograms caused a marked rise in NPOT levels in all cases within 12 h (from mean 64 pmol/l to mean 196 pmol/l) and the levels remained elevated for 3 d.

Adult↗
PubMed1985

Intravenous growth hormone: growth responses to patterned infusions in hypophysectomized rats.

Young hypophysectomized rats were maintained with chronic indwelling i.v. cannulae attached via swivels to a multichannel pumping system programmed to deliver GH in a continuous or pulsatile pattern for several days. Continuous i.v. infusions of human GH for 5 days produced dose-dependent increases in body weight and tail length, without increasing food intake. A comparison of GH infusions by the s.c. or i.v. route showed that the direct i.v. route was threefold more effective. Pulsatile i.v. infusions of human or bovine GH at two doses (12 or 36 mu./day, eight pulses/day, 5-min duration, every 3 h) produced greater increases in body weight than continuous i.v. infusions of GH at the same daily dose. Continuous infusions of bovine GH produced a lower growth rate in the second of two consecutive 5-day treatment periods, whereas the responses to pulsatile GH did not diminish with time. Both body weight gain and long-bone growth were affected by the frequency of GH pulses; nine pulses per day were more effective than three pulses per day which in turn produced larger growth responses than one pulse per day. Keeping GH pulse frequency constant and varying pulse duration (4, 16 or 64 min) did not affect growth rates. In conclusion, long-term pulsatile i.v. infusions of GH mimic the endogenous secretory pattern, and are most effective when given at the physiologically appropriate pulse frequency.

Animals↗
PubMed1985

Oxytocin and vasopressin in rat hypophysial portal blood: experimental studies in normal and Brattleboro rats.

Oxytocin (OT) and vasopressin (VP) were measured by radioimmunoassay in hypophysial portal and peripheral blood from male Wistar rats and heterozygous and homozygous Brattleboro rats anaesthetized with urethane. In Wistar rats the concentrations of OT and VP were about 50 times greater than the concentrations in peripheral blood, whether or not the pituitary gland was left in situ during collection, and also considerably greater than the reported concentrations of the peptides in the cerebrospinal fluid. The release of both peptides was increased significantly by a lesion of the supraoptico-hypophysial tract that led to diabetes insipidus, but which left intact the external layer of the median eminence (ME). Concentrations of VP were undetectable in plasma from homozygous Brattleboro rats, but the portal plasma concentrations of VP in heterozygous Brattleboro rats were not significantly lower than in Wistar rats. The concentrations of OT in portal plasma from both types of Brattleboro rat were significantly higher than in Wistar rats. The output of VP and OT into hypophysial portal blood of Wistar rats was not significantly affected by electrical stimulation of the suprachiasmatic, supraoptic or paraventricular nuclei or the ME using two types of stimuli, one of which produced an increase in peripheral plasma concentrations of VP and OT in intact rats and a significant increase in the release of LH-releasing hormone into hypophysial portal blood. The output of VP and OT into portal blood was also not significantly affected by either adrenalectomy with or without injection of dexamethasone or the injection of either the 5-hydroxytryptamine (5-HT) synthesis blocker, parachlorophenylalanine, or the 5-HT uptake blockers, alaproclate or zimelidine. These results show that large amounts of OT as well as VP are released into hypophysial portal blood from fibres of the hypothalamo-neurohypophysial system that terminate in the external layer of the ME. Although distinct from the fibres that terminate in the pars nervosa (PN), the findings in Brattleboro rats show that the VP fibres of the ME system originate in neurones with a genomic mechanism for VP synthesis similar to that of the VP neurones that project to the PN. The lack of effect of adrenalectomy and the administration of 5-HT synthesis and uptake blockers must be interpreted with caution since the results obtained with electrical stimulation suggest that when the pituitary stalk is cut the release of OT and VP into portal blood approaches a maximum and may therefore be difficult to alter by experimental manipulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenalectomy↗
PubMed1985

Effects of a fragment of human growth hormone-releasing factor in normal and 'Little' mice.

The 'Little' mouse is characterized by a body growth rate 60% of normal due to a defect in the synthesis and storage of GH in the anterior pituitary gland. We have now investigated the effects of GH releasing factor (GRF) in these mice and in normal animals. The pituitary GH content in Little mice was only 4% of that in normal C57: +/+ mice, and was not affected by twice daily i.p. injections of human (h) GRF1-29NH2 (0.2-2 micrograms) for 14 days. This treatment also had no effect on body growth. In anaesthetized normal mice, single i.v. injections of 0.1 or 2 micrograms hGRF1-29NH2 released large amounts of GH into the plasma, whereas this peptide was ineffective in Little mice, whether or not they had been pretreated with GRF. Therefore, although pituitaries of Little mice contain significant amounts of GH, this pool is not releasable by GRF. This suggests that the dwarfism in Little mice may be partly due to a pituitary defect in GRF receptors or their stimulus-secretion coupling, rather than a deficiency in hypothalamic GRF.

Animals↗
PubMed1985

Growth hormone responses to multiple injections of a fragment of human growth hormone-releasing factor in conscious male and female rats.

The GH responses to single i.v. injections of GH-releasing factor (GRF) in conscious male rats are highly variable. Although normal male rats show a pulsatile secretory pattern of GH with pulses occurring at intervals of 3-3.5 h, the peaks occur at different times in individual animals. We have compared the GH responses of young conscious male and female rats to multiple i.v. injections of 1 microgram human (h) GRF1-29NH2. The peak GH responses occurred 3-5 min after hGRF1-29NH2 injection and were lower in female than in male rats. Both males and females responded uniformly to hGRF1-29NH2 injections given 180 min apart and the GH responses became entrained with no endogenous GH pulsing. Female rats produced consistent GH peaks in response to hGRF1-29NH2 injections at 90-min intervals, whereas male rats responded only to alternate injections, so that GH peaks occurred only every 180 min despite giving GRF every 90 min. When the frequency of hGRF1-29NH2 administration was increased to once every 40 min female rats again responded consistently to each injection. Male rats responded intermittently, being able to respond to two injections 40 min apart, after which they became refractory to hGRF1-29NH2. This cycle of varying sensitivity to GRF in male rats probably underlies their 3-hourly endogenous GH secretory rhythm. Female rats can respond uniformly to repeated GRF injections, consistent with their more continuous pattern of endogenous GH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1984

Membrane retrieval in the guinea-pig neurohypophysis. Isolation and characterization of secretory vesicles and coated microvesicles after radiolabel incorporation in vivo.

We have developed small-scale methods for the isolation and biochemical characterization of subcellular fractions from single guinea-pig posterior-pituitary glands. Secretory vesicles and coated microvesicles produced in this way were of similar purity to those isolated from large amounts of tissue by conventional ultracentrifugation. [35S]Cysteine injected into the hypothalamus was found in the soluble contents of secretory vesicles isolated from the neural lobes 24 h later. High-pressure liquid-chromatographic analysis revealed that the radiolabel was incorporated into the expected neurosecretory products (oxytocin, vasopressin and neurophysin) and also into a biosynthetic intermediate in the vasopressin system. The membranes of secretory vesicles were labelled with [3H]choline 24 h after its hypothalamic injection. Little or no [3H]choline could be demonstrated in coated microvesicles at this time, although these structures were labelled 5 days after injection. Stimulating hormone secretion by chronic dehydration produced a significant fall in [3H]choline content of the secretory-vesicle membranes without any transfer of label into coated microvesicles, suggesting that coated microvesicles are not involved in membrane retrieval in the neurohypophysis.

Animals↗
PubMed1984

Conversion and release of an intermediate in vasopressin-neurophysin biosynthesis in the guinea-pig.

Guinea-pig neural lobes contain appreciable amounts of neurophysin with a glycopeptide extension (NPGP) which may represent a partially processed form of the arginine vasopressin (AVP) precursor. We have now studied the turnover and release of the NPGP component using a combination of in-vivo radiolabel incorporation and high pressure liquid chromatography. Measurement of the neural lobe content of 35S-labelled peptides at various times after hypothalamic injection of [35S]cysteine demonstrated that the oxytocin-related products accumulated more rapidly than the AVP-related products. The relative amounts of [35S]cysteine incorporated into NPGP and the AVP-related neurophysin (NPavp) changed markedly with time after in-vivo labelling. In-vitro incubation of neurosecretory granules prepared from neural lobes 4h after radiolabel injection produced a time- and temperature-dependent conversion of NPGP to NPavp. Incubation at 37 degrees C for 4h produced a 30% decrease in [35S]NPGP with a concomitant increase in [35S]NPavp, whilst there were no changes in the other 35S-labelled components. In-vitro stimulation of radiolabelled neural lobes by 56mM-K+ evoked a Ca++-dependent release of NPGP as well as the other expected neurosecretory components, and the amount of NPGP released reflected its neural lobe content. We conclude that the NPGP component found in guinea-pig neural lobes is a biosynthetic intermediate, most of which is further processed to NPavp. However, some NPGP may also be secreted from the neural lobe in an intact form.

Animals↗
PubMed1983

Characterisation of an intermediate in neurophysin biosynthesis in the guinea pig.

In addition to oxytocin (OT), vasopressin (AVP) and their respective neurophysins (NPs), another [35S]cysteine incorporating component is present in the guinea pig neurohypophysis. Gel filtration and Con A affinity chromatography revealed that this component was larger than NP and was glycosylated. NP-immunoreactivity was assessed using antisera which distinguish the OT- and AVP-related NPs. Whilst the anti-NP antiserum detected only one component (guinea pig NP), the anti-NP antiserum detected both NP and the glycosylated 35S-labelled component. These results suggest that a significant amount of NP in guinea pig neural lobes bears a glycopeptide extension and represents a partially processed form of the AVP precursor in this species.

Animals↗
PubMed1983

An intermediate in the biosynthesis of vasopressin and neurophysin in the guinea pig posterior pituitary.

Biosynthesis of oxytocin (OT), vasopressin (AVP) and neurophysins (NP) has been studied in the guinea pig using incorporation of radiolabelled cysteine and fucose, and analysis by high pressure liquid chromatography and radioimmunoassay. Using these methods it was possible to identify and quantitate OT, AVP and their respective NPs as well as the carboxy-terminal glycopeptide derived from the AVP precursor. In addition, we have also identified an intermediate component comprising the AVP-related NP with its carboxy-terminal glycopeptide still attached. The presence of this intermediate suggests that the removal of AVP is the first step in the proteolytic cleavage of the AVP precursor.

Animals↗
PubMed1983

Release of oxytocin and prolactin in response to suckling.

The oxytocin and prolactin responses to suckling were measured in 10 women in early (n = 5) and established lactation (n = 5). Oxytocin was released in a pulsatile manner during suckling in all women, but the response was not related to milk volume, prolactin response, or parity of the mother. In all 10 women plasma oxytocin concentrations increased three to 10 minutes before suckling began. In five women this occurred in response to the baby crying, in three it coincided with the baby becoming restless in expectation of the feed, while in two it corresponded with the mother preparing for the feed. There was no prolactin response to stimuli other than stimulation of the nipple associated with suckling. These results clearly indicate that the milk ejection reflex, with release of oxytocin, occurs in most women before the tactile stimulus of suckling. A second release of oxytocin follows in response to the suckling stimulus itself. Thus it is important that care is taken to protect breast feeding mothers from stress not only during suckling but also immediately before nursing, when conditioned releases of oxytocin will occur.

Breast Feeding↗
PubMed1983

Vasopressin and oxytocin content are decreased in the brain stems of spontaneously hypertensive rats.

The brain stem and neurohypophyseal content of arginine vasopressin and the brain stem content of oxytocin were measured by radioimmunoassay in 3-, 7-, and 22- to 28-week-old spontaneously hypertensive rats of the stroke-prone strain (SHRSP) and the values were compared to those measured in age-matched normotensive Wistar-Kyoto rats (WKY). When compared to WKY, the content of vasopressin in the brain stems of SHRSP was reduced in all three age-groups; in contrast, the neurohypophyseal contents of vasopressin in the two species were not significantly different at 3 and 7 weeks of age and the content was increased slightly in SHRSP at 22-28 weeks. Similar to the findings for vasopressin in the brain stem, the content of oxytocin in this tissue was reduced in SHRSP at 7 and 22-28 weeks of age. The results demonstrate that brain stem arginine vasopressin levels and neurohypophyseal arginine vasopressin levels may change differentially and that the age-related differences in the brain stem levels of arginine vasopressin and oxytocin in SHRSP and WKY are consistent with the possibility that these peptides may play a role in altering cardiovascular reflex activity.

Age Factors↗
PubMed1983

Clearance of vasopressin from cerebrospinal fluid to blood in chronically cannulated Brattleboro rats.

The antidiuretic effects of intravenous (i.v.) and intracerebroventricular (i.c.v.) infusions of vasopressin (AVP) were compared in chronically cannulated conscious Brattleboro rats. AVP infused i.c.v. for 6 h reduced urine volume and increased urinary Na+ and K+ concentrations. These antidiuretic responses to i.c.v. AVP were dose related in the range 3-12 ng/h and were delayed and prolonged when compared with i.v. infusions (0.33-3 ng/h) of this peptide. Comparison of the dose-response curves for both routes of administration demonstrated that the antidiuretic response to i.c.v. infusion of AVP was approximately 10-fold lower than to AVP given intravenously. Since measurable amounts of AVP-like material appeared in the urine during the antidiuretic response, we conclude that a significant amount of AVP infused into the cerebroventricular system reaches the bloodstream in a biologically active form.

Animals↗
PubMed1983

Release of oxytocin into blood and cerebrospinal fluid by electrical stimulation of the hypothalamus or neural lobe in the rat.

Oxytocin (OT) was measured in blood and cerebrospinal fluid (CSF) samples obtained from anaesthetized rats. Release of OT was evoked by electrical stimulation (15 s) of the hypothalamus or the neural lobe. Stimulation of the neural lobe increased plasma OT sevenfold without altering CSF levels. Hypothalamic stimulation released OT into both blood and CSF, although the rise in CSF was both delayed and prolonged compared to that in plasma. Stimulation sites which released OT into CSF extended over a large medial area of the hypothalamus. More lateral stimulation sites were much less effective. Our results show that the release of OT into blood and CSF can be dissociated and that CSF levels reflect the activity of those hypothalamic OT-neurones which do not project to the neural lobe.

Animals↗