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.

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.

The hormonal actions of corticotropin-releasing factor in sheep: effect of intravenous and intracerebroventricular injection.

The 41-residue ovine corticotropin releasing factor (CRF) was administered iv and intracerebroventricularly (icv) to merino sheep. A significant rise in plasma ACTH, beta-lipotropin (beta LPH) and cortisol was demonstrated after the administration of 200 micrograms, iv. A highly significant correlation between the increments in plasma ACTH and beta LPH was observed. The plasma ACTH rise was evident within 5 min and was abolished by the prior administration of 0.4-4.0 mg dexamethasone. No significant rise in plasma GH, LH, PRL, insulin, glucagon, pancreatic polypeptide, met-enkephalin, angiotensin II, aldosterone, or vasopressin could be demonstrated. Although smaller doses of CRF (50 ng to 5 micrograms) were effective when given icv, the ACTH response was more delayed. It is concluded that CRF stimulates a rapid increase in the secretion of ACTH and beta LPH in sheep. Suppression of this response by dexamethasone indicates that glucocorticoids are capable of acting on the pituitary to inhibit the ACTH response to CRF. The delayed response when CRF is given icv may be due to diffusion. The action of CRF appears to be relatively specific, in that the plasma concentrations of the other pancreatic, pituitary, and adrenal hormones measured were not affected.

The physiological significance of arginine vasopressin in potentiating the response to corticotrophin-releasing factor in sheep.

In order to assess the physiological importance of endogenous arginine vasopressin (AVP) in augmenting the ACTH response to corticotrophin-releasing factor (CRF), the response to CRF during hypertonic saline infusion in six Coopworth sheep was examined. A 4-h infusion of 5% (w/v) NaCl (3.8 ml/min) resulted in significantly (P less than 0.01) greater rises in ACTH and cortisol, but not aldosterone, than were observed after CRF alone. Infusion of hypertonic saline without CRF resulted in a highly significant (P less than 0.001) rise in plasma osmolality and AVP but no significant change in plasma ACTH, cortisol or aldosterone. It is concluded that a marked but physiological increase in peripheral (and presumably central) levels of AVP does not result in any demonstrable change in plasma ACTH concentration. However, under these conditions, the ACTH and cortisol responses to CRF are considerably augmented.

The influence of glucagon on sodium retention after fasting.

Two groups of five obese female subjects, having undergone a ten day therapeutic fast, were fed with either glucose 50 g/day or with L-alanine 50 g/day for three days. Plasma glucagon concentrations and urinary electrolyte excretion were compared in the two groups. Although 4.00pm plasma glucagon concentrations during refeeding were significantly greater in the alanine refeed group (P less than 0.05) the reduction in urinary sodium excretion in each of the two groups was identical. These observations do not support the hypothesis that glucose induced suppression of plasma glucagon concentrations is a mechanism whereby carbohydrate refeeding produces post-fast urinary sodium retention.

The role of condensed tannins in the nutritional value of Lotus pedunculatus for sheep. 5. Effects on the endocrine system and on adipose tissue metabolism.

1. Three experiments were conducted using Lotus pedunculatus containing high concentrations of condensed tannins (CT), and utilizing the principle that polyethylene glycol (PEG) application (molecular weight 3350) will irreversibly bind a portion of the CT and thus reduce the dietary reactive (i.e. non-PEG bound) CT concentration. Lotus diets containing 95, 45 and 14 g total reactive CT/kg dry matter (DM), induced by spraying with three PEG rates, were given to sheep at hourly intervals (600 g DM/d) for 21 d (Expt 1). In Expts 2 and 3, lambs grazed areas oversown with either lotus (89 g CT/kg DM) or clovers (Trifolium repens and Trifolium pratense; less than 1 g CT/kg DM) for 42 and 92 d respectively. In Expt 2 half the animals grazing each forage received oral PEG (75 g/d), whilst in Expt 3 half the lambs were sired by rams selected respectively for low or high levels of subcutaneous fat deposition. 2. Hormone concentrations in plasma (Expt 1 only) were determined by radioimmunoassay. Rates of [U-14C]-acetate and D-[U-14C]glucose incorporation and oxidation by subcutaneous and abdominal adipose tissue removed at slaughter, together with rate of glycerol release, were determined during in vitro incubation in all three experiments. 3. Plasma concentration of growth hormone was positively and linearly related to dietary reactive CT concentration, whilst 3,5,3'-triiodothyronine (T3) concentration tended to be negatively and linearly related to dietary reactive CT concentration. Diet CT concentration had no effect on plasma concentrations of the other hormones measured.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression and stimulation of growth hormone release in sheep by somatostatin analogs.

The inhibitory and stimulatory effects of somatostatin analogues on growth hormone secretion have been studied in the sheep. Plasma immunoreactive growth hormone (GH) was stimulated by the iv administration of the antilipolytic compound 3,5-dimethylpyrazole and was followed by infusion of somatostatin (SRIF) and analogues into the carotid artery. The analogues D-trp8-D-cys14-SRIF, des-AA1,2,4,5,12D-trp8-D-cys-14SRIF, and SRIF-Wy 40793 were shown to cause a dose-dependent suppression of plasma GH, followed by a significant rise ("rebound") on terminating the infusion. In general, the greater the suppression, the greater the rebound. A significant correlation between the rebound increase in plasma GH and dose of both SRIF and des-AA1,2,4,5,12D-trp8-cys14-SRIF was demonstrated (p less than 0.01 and p less than 0.001, respectively). This analogue and Wy-40793 showed a significantly greater rebound than did SRIF following 50% suppression of the GH response (p less than 0.01 and p less than 0.05, respectively). Although two analogues were shown to be significantly more potent than SRIF (p less than 0.01), in this sheep bioassay the duration of action did not appear to be longer than that of the native hormone for any of the three analogues.

Gastro-intestinal tract function in sheep infused with somatostatin.

The effect of a 5-day continuous intravenous infusion of somatostatin (4.6 ng min-1 kg-1) was studied, using anoestrous ewes given 791 g dry matter per day of a 60:40 lucerne hay:oat grain pelleted diet from a continuously moving belt. 51Cr-EDTA, 103Ru-phenanthroline and lignin were used as markers to determine digesta mean retention times (MRT) by a continuous infusion-total sampling procedure. The somatostatin infusion increased the concentration of somatostatin in venous plasma within the physiological range from 10 to 76 ng/l, decreased plasma concentrations of prolactin and thyroxine, but had no effect upon plasma concentrations of insulin and glucagon. It had no effect upon digesta-free weight of the rumen and omasum but consistently decreased the weight of all post-ruminal segments of the gastrointestinal (GI) tract. The infusion increased the accumulation of digesta in the abomasum and caecum. Total MRT of all three markers in the entire GI tract was unaffected by somatostatin infusion, but the proportion of total MRT spent in the abomasum + small intestine + caecum increased and the proportion spent in the large intestine and rumen decreased. Somatostatin infusion decreased apparent endogenous abomasal secretion, increased water flow from the rumen and into the abomasum and decreased voluntary water consumption. It is proposed that the prime site of somatostatin action was in the abomasal to caecal region, where somatostatin-secreting D cells are found in greatest concentration, that effects observed in the large intestine and rumen may represent secondary compensatory mechanisms and that the effects observed were due to direct action of somatostatin and were not mediated by other GI hormones.

Inhibition of spontaneous ACTH release and improved response to CRF in sheep premedicated with the enkephalin analogue DAMME.

The effect of IV injections of an enkephalin analogue (DAMME) in sheep prior to the administration of CRF has been compared with chlorpromazine-morphine (CPZ-M) injections as well as a chlorpromazine-morphine-nembutal (CPZ-M-N) regime. The results showed that the administration of DAMME 60 minutes prior to CRF injections, gave more consistent and more sustained inhibition of spontaneous plasma ACTH and plasma cortisol secretion than either CPZ-M or CPZ-M-N pre-treatment. Changes in plasma ACTH and plasma cortisol levels were more readily detectable in DAMME-treated sheep as compared with saline controls when 50 micrograms injections of CRF were given into the carotid artery. This suggests that DAMME may inhibit endogenous CRF release by an action at hypothalamic level or above.

An in vivo model for testing somatostatin suppression of growth hormone release in sheep.

To investigate the suppressive effect of somatostatin on growth hormone secretion, a consistent, potent stimulus to growth hormone release is required. The antilipolytic compound 3,5-dimethylpyrazole (DMP) gave a rapid rise in plasma immunoreactive growth hormone following iv administration to fasting sheep. The dose response relationship for iv DMP was defined in 12 sheep, and a rise in growth hormone levels from a baseline of 2.1 +/- 0.4 ng/ml (mean +/- SE) to 38.9 +/- 3.9 ng/ml was achieved in 40 min with 0.1 mg/kg bw of DMP given as a bolus. Infusion of graded doses of somatostatin (50, 100 and 200 micrograms) in 4 sheep over a 90 min period resulted in a dose-dependent suppression of the DMP-provoked growth hormone secretion below the responses observed during saline infusion. This experimental model should facilitate further studies of the inhibitory effects of somatostatin, its analogues and other drugs which suppress growth hormone secretion.

Regulation of plasma vasopressin by plasma osmolality and carotid sinus pressure in anesthetized rabbits.

The purpose of the experiments was to investigate the effect of changes in carotid sinus baroreceptor stimulation on plasma vasopressin (AVP) at different plasma osmolalities in the anesthetized artificially ventilated rabbit. Both carotid sinuses were isolated and perfused with blood at servo-controlled pressures. The vagus and aortic depressor nerves were sectioned bilaterally to eliminate input from atrial and aortic arch baroreceptors. Saline (0.3%, wt/vol) was infused to lower plasma osmolality, and 5% saline was infused to raise plasma osmolality. At three plasma osmolalities, the carotid sinus pressure (CSP) was changed from 100 mmHg to 40 and 140 mmHg and returned to 100 mmHg. There were no changes in plasma AVP in response to changes in CSP at low plasma osmolality (289 mosmol/kgH2O), but at medium (309 mosmol/kgH2O) and high (323 mosmol/kgH2O) osmolality, plasma AVP was higher at 40 than at 140 mmHg CSP. The relationship between plasma AVP and plasma osmolality was expressed as a linear regression at each CSP. Changes in CSP changed the sensitivity but not the threshold of the osmotic control of AVP release.

Endocrine regulation of metabolism in sheep given kale (Brassica oleracea) and ryegrass (Lolium perenne)-clover (Trifolium repens) fresh-forage diets.

Diets of fresh kale (Brassica oleracea) and ryegrass (Lolium perenne)-clover (Trifolium repens) herbage were fed to growing sheep in three experiments. In Expts 1 and 3 the sheep were confined indoors and fed at hourly intervals, and all were given supplementary iodine to counteract kale goitrogens. Lambs grazed the two forages for 24 weeks in Expt 2, with and without intramuscular injections of iodized oil. The kale and herbage contained respectively 11 and less than 0.1 g S-methyl-L-cysteine sulphoxide (SMCO)/kg dry matter (DM) and values for readily fermentable: structural carbohydrate (CHO) were 3.1 and 0.8, respectively. Blood samples were withdrawn from indwelling catheters (Expts 1 and 3) or venipuncture (Expt 2) and the plasma analysed for a range of hormones using radioimmunoassay procedures. Glucose irreversible loss (GIL) was measured in Expt 1 using primed continuous infusions of D-[U-14C]glucose. Samples of adipose tissue were removed from the shoulder area in Expt 3, and rates of D-[U-14C]glucose and [U-14C]acetate incorporation and oxidation were measured in vitro, together with the rate of glycerol release. In the presence of supplementary I2, kale feeding was associated with an elevation in plasma concentration of free thyroxine (T4). Regardless of I2 supplementation, sheep fed on kale had much higher plasma growth hormone concentrations than sheep fed on ryegrass-clover herbage, and this was accompanied by reduced plasma somatostatin concentrations. Plasma insulin and glucagon concentrations were similar for sheep fed on the two diets; GIL tended to be slightly but not significantly greater (9.4%) for sheep fed on kale than for those fed on ryegrass-clover herbage.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of an enkephalin analogue (DAMME) on pituitary and pancreatic function in sheep.

The hormonal secretory effects of intravenously and intracerebroventricularly administered DAMME have been studied in the sheep. Serum prolactin and growth hormone levels were significantly elevated after i.v. injection of DAMME (14 micrograms kg-1) while a significant and considerably more prolonged increase occurred following i.c.v. administration of 20 micrograms of the analogue. The responses to i.c.v. but not to i.v. injected DAMME were attenuated by prior i.v. injection of naloxone. Administration of DAMME i.c.v. resulted in a detectable concentration in the peripheral circulation within two minutes and maximal levels at 120-150 minutes. No significant changes occurred in circulating concentrations of the pancreatic hormones investigated although there was a trend to somatostatin and pancreatic polypeptide levels being elevated after systemic injection of the analogue.

Protein metabolism in growing lambs given fresh ryegrass (Lolium perenne)--clover (Trifolium repens) pasture ad lib. 2. Endocrine changes, glucose production, and their relationship to protein deposition and the partition of absorbed nutrients.

1. Glucose metabolism and changes in the concentrations of several hormones in jugular plasma were measured in growing lambs fed on fresh pasture ad lib. One group of lambs acted as control while the second received a continuous abomasal infusion supplying 44 g sodium caseinate+0.5 g L-methionine/d. 2. Hormone concentrations were determined by radioimmunoassay procedures and glucose irreversible loss measured from continuous infusion of D-[U-14C]glucose. 3. Protein infusion increased plasma concentrations of insulin, glucagon and thyroxine (T4), depressed those of growth hormone, prolactin and somatostatin and had no effect on triiodothyronine (T3) concentrations. Cortisol concentrations also tended to be slightly higher in the plasma of protein-infused lambs. 4. Increases in herbage intake within the ad lib, range were associated with increases in plasma insulin and glucagon concentrations and decreases in growth hormone concentration, and it is suggested that these effects could be mediated in part by the accompanying increases in protein absorption from the intestines. The T4:T3 value also decreased with increasing herbage intake, and it is suggested this was due to conversion of T4 to T3. 5. After correction by covariance to equal herbage intake, rates of irreversible glucose loss for control and protein-infused lambs were 9.2 and 10.0 mg/min per kg body-weight 0.75. It was calculated that respectively 0.12 and 0.19 of the total glucose production in control and protein-infused lambs could be accounted for by net synthesis from protein. 6. It was concluded that changes in the circulating concentration of several hormones in protein-infused compared with control lambs were likely to have been implicated in protein deposition forming a greater proportion of energy retention in the infused lambs (0.41 v. 0.27).

Studies with the autotransplanted ovine pancreas: glucagon and insulin secretion.

Basic studies on the secretion of glucagon and insulin by the ovine pancreatic autotransplant in the neck are described. Of the 17 transplants in the series none failed to secrete glucagon and only three failed to secrete insulin in detectable amounts. The longest surviving transplant actively secreted both hormones 3 years after transplantation and five other transplants were functional and the animals healthy after 16 months. Exocrine secretion disappears shortly after transplantation. Sodium butyrate and alanine each promoted the secretion of both hormones by the transplant. Glucagon failed to promote insulin secretion by the transplant, although it apparently stimulated the ovine in situ pancreas. The immediate (presumably direct) effect of insulin was to inhibit transplant glucagon secretion. Hypoglycaemia induced by peripheral insulin administration failed to stimulate glucagon secretion by the transplant, although it did promote glucagon secretion by the ovine in situ pancreas. Heparin did not markedly suppress basal transplant secretion of either glucagon or insulin. Phasic response patterns occurred with both hormones during long butyrate perfusions, although first-phase responsiveness was not a constant feature. In one trial, first-phase responses fell off with repeated short butyrate infusions. Glucagon and insulin secretory patterns in response to butyrate were remarkably alike, suggesting a common mechanism. Loss of specific functions by the ovine pancreas after transplantation is discussed.