Secretion In Sheep Research Articles

Hypoglycemia-induced inhibition of LH and stimulation of ACTH secretion in the rhesus monkey is blocked by alprazolam.

Insulin-induced hypoglycemia inhibits luteinizing hormone (LH) secretion and has been used as a model to study stress-induced inhibition of reproductive function. Endogenous opioid peptides have been implicated in mediating the inhibitory effect of hypoglycemia on LH secretion in sheep and rat. The objective of the present study was to determine if corticotropin-releasing hormone (CRH) and endogenous opiates are involved in the LH response to hypoglycemia in the nonhuman primate. Blood samples were collected at 15-min intervals for 6 h from ovariectomized rhesus monkeys (n = 6). Hypoglycemia was induced by injecting insulin 1 h after initiating blood collection. Animals were pretreated 15 min prior to insulin with either saline (n = 6), naloxone, a nonselective opiate receptor antagonist (n = 4), or alprazolam (n = 6), a potent benzodiazepine which has been shown to inhibit CRH. The LH, glucose, adrenocorticotropin (ACTH), and cortisol responses to insulin were determined. Insulin-induced hypoglycemia significantly inhibited LH secretion and increased ACTH and cortisol concentrations. Alprazolam prevented hypoglycemia-induced inhibition of LH independent of an effect on glucose concentrations. The mean (+/- SEM) LH pulse interval in response to hypoglycemia was decreased in the alprazolam pretreated group compared to the saline pretreated group (77.4 +/- 6.0 vs. 130.0 +/- 18.4 min), while LH pulse amplitude and mean LH levels were significantly increased (56.2 +/- 7.1 vs. 28.3 +/- 5.5 ng/ml, and 105.6 +/- 14.4 vs. 60.9 +/- 12.1 ng/ml respectively). In contrast, naloxone did not prevent hypoglycemia-induced LH inhibition. The mean LH pulse interval, LH pulse amplitude, and LH concentration in the naloxone pretreated monkeys were 152.1 +/- 33.4 min, 37.1 +/- 8.9 ng/ml, and 63.7 +/- 9.1 ng/ml respectively. Alprazolam pretreatment also markedly attenuated the ACTH response to hypoglycemia whereas the cortisol response was only moderately affected. We conclude that insulin-induced hypoglycemia in the monkey inhibits LH secretion through a mechanism involving CRH but not endogenous opiates.

Inhibition of GH releasing factor (GRF)-induced GH secretion by intraruminal infusion of volatile fatty acids (VFA) in sheep.

A VFA mixture solution containing acetate, propionate and butyrate (the molar ratio of acetate, propionate and n-butyrate = 61.7:24.3:14.0) was infused into the rumen at various rates (53.5, 107 and 214 mumol kg-1 min-1) over 6 h to examine the effects on basal and growth hormone-releasing factor (GRF, 0.25 micrograms kg-1)-induced increase in secretion of GH, insulin, glucagon and somatostatin (SRIF) in five castrated male sheep. Intraruminal infusion of the VFA mixture into the 18-h-fasted animals at the rates of 53.5, 107 and 214 mumol kg-1 min-1 finally raised the total intraruminal VFA concentration from 91.4 to 100.2 (P > 0.05), 175.9 (P < 0.05) and 234.5 (P < 0.05) mmol l-1, respectively. A preliminary experiment showed that an infusion rate of 107 mumol kg-1 min-1 mimics the postprandial increase in ruminal VFA. The basal plasma GH concentrations (2 to 4 h after the start of VFA infusion) and the area under the profiles for GH release in response to the intravenous GRF injection, which was done 4 h after the start of VFA infusion, were significantly decreased by the VFA infusion rates of 107 and 214 mumol kg-1 min-1. Furthermore, the VFA infusion noticeably increased basal plasma concentrations of insulin, but it scarcely changed the basal levels of glucagon, SRIF and glucose. From these results we conclude that an increase in the ruminal VFA concentration, even within the physiological range, would suppress GH secretion from the ovine anterior pituitary, and that the postprandial rise in the ruminal VFA concentration may be one of the factors normally suppressing GH secretion in sheep.

Adrenergic and cholinergic modulation of insulin secretion in non-lactating cows exposed to heat

The inhibition of insulin secretion in sheep exposed to cold has been reported to be mediated through adrenergic α receptors stimulated by sympathoadrenomedullary activity (Sasaki and Takahashi, 1980). Insulin secretion in Holstein non-lactating cows in response to various secretogogues was also suppressed during heat exposure (Itoh et al., unpublished data), however it is not obvious whether these responses were related to neural controls or not. However acetylcholine (parasympathetic nervous system neurotransmitter) has been shown to promote the release of glucagon and insulin (Jones et al., 1991). Therefore, the objective of this study was to examine the participation of adrenergic and cholinergic neural control on insulin secretion in thermoneutral and hot environments in non-lactating cows by administering, adrenergic and cholinergic blockers with an intravenous injection of glucose.

Follicular dynamics and ovarian steroid secretion in sheep during anoestrus.

The dynamics of ovarian follicular development and its relationship to ovarian and pituitary hormones during seasonal anoestrus were investigated for 10 days in nine ewes with autotransplanted ovaries in a longitudinal study. The size and position in the ovary of individual follicles over 2.5 mm in diameter were recorded by daily ultrasonography. Samples of ovarian and jugular venous blood were collected at intervals of 12 h, before and after a GnRH challenge (2.50 ng GnRH, i.v.) so that basal and LH-stimulated ovarian steroid secretion could be determined. Throughout the experimental period, all animals developed at least one large antral follicle > 5 mm, which secreted increased (P < 0.05) amounts of oestradiol and androstenedione in response to an LH challenge as the diameter of the follicle increased. However, a decrease (P < 0.05) in ovarian steroid secretion preceded any significant change in follicular diameter, indicating a dissociation between morphological and functional stages of dominance in sheep. We conclude that follicular growth and ovarian steroid secretion in sheep occur in wave-like forms, with the ascending and static part of both waves being synchronous but with a decline in steroid secretion preceding any changes in follicular diameter. Therefore, in sheep, follicular size alone is not an adequate parameter to assign dominance, and the secretory status of the follicle at any given time must be taken into account when studying the dynamics of follicular growth.

Effects of central administration of highly selective opioid mu-, delta- and kappa-receptor agonists on plasma luteinizing hormone (LH), prolactin, and the estrogen-induced LH surge in ovariectomized ewes.

A reduction in endogenous opioid inhibition (disinhibition) of GnRH secretion is thought to be permissive for the preovulatory GnRH/LH surge. There are no published studies of the effects of highly specific opioid receptor agonists on the LH surge in any species, and the relative importance of the opioid receptor subtypes mu, delta and kappa in the mechanism of disinhibition is unknown. In sheep, attempts to block the LH surge with opiates have been largely unsuccessful, and there is little evidence for reduced opioid inhibition during the GnRH/LH surge. The opioid receptor subtypes regulating PRL secretion in sheep are also unknown. Conscious, ovariectomized ewes with permanent third ventricular cannulae were injected with estradiol benzoate (EB) 50 micrograms or oil im (t = 0 h). In this model, EB elicits a time-delayed surge in LH secretion after 13-18 h. Jugular venous blood was sampled at half hourly intervals between-2 and 0 h and 10 and 26 h. From 12-20 h, infusions were made into the third ventricle of either the highly specific mu-agonist DAGO (10, 20 or 40 nmol/h), the delta-agonist DPDPE (40 nmol/h), the kappa-agonist U50488 (40 nmol/h) or saline (vehicle). In oil-treated animals (n = 4-6), DAGO infusion at 20 and 40 nmol/h reduced plasma LH whereas DPDPE or U50488 had no effect. In EB-treated animals (n = 6), DAGO (40 nmol/h) delayed the LH surge (mean +/- SEM time to surge onset 21.4 +/- 0.3 h vs. 14.0 +/- 0.4 h in controls, P < 0.0001). DAGO at 10 nmol/h did not alter surge onset and at 20 nmol/h had variable effects. DPDPE or U50488 did not affect LH surge timing or amplitude. All doses of DAGO increased plasma PRL, whereas DPDPE and U50488 had no effect. We conclude that, in ovariectomized ewes, activation of opioid mu-receptors, but not delta- or kappa-receptors, inhibits GnRH secretion, can block the estrogen-induced GnRH/LH surge and increases PRL secretion. The results are consistent with the disinhibition hypothesis.

Acidaemia enhances the inhibitory effect of hypoxia on fetal lung liquid secretion in sheep.

Previous studies have shown that moderate fetal asphyxia reduces the secretion rate of fetal lung liquid. The present aim was to determine the relative effects of the individual components of asphyxia (hypoxia, hypercapnia and acidaemia) on lung liquid secretion in fetal sheep. Fetal hyperoxia was also studied to determine the extent to which lung liquid secretion is restricted by the relatively low fetal blood PO2. As each manipulation of fetal blood gas tensions and pH treatment produced alterations in more than one aspect of blood composition, data from all treatment groups were combined and a multiple analysis of variance was performed to determine the separate effects of PaO2, PaCO2, SaO2 and pHa. Lung liquid secretion rate was significantly reduced when mean PaO2 values were below 24.5 mmHg (range 12.9-24.3 mmHg). When PaO2 values below 24.5 mmHg occurred in combination with pHa values below 7.275 (range 6.934-7.268) the secretion rates were further reduced. Alterations in pHa alone or in PaCO2 had no significant effect. These results indicate that hypoxia is the principal factor responsible for the inhibition of lung liquid secretion during asphyxia and that acidaemia enhances this inhibition.

Role of endogenous CCK in regulation of interdigestive pancreatic exocrine secretion in sheep (Ovis aries)

The role of endogenous cholecystokinin (CCK) in the regulation of pancreatic exocrine secretion during the interdigestive period was studied in conscious sheep. Intravenous infusion of proglumide (15, 30 and 60 mumol/kg/min) for 70 min significantly decreased flow rate, protein and amylase output in pancreatic juice by 80% at the maximum dose, as well as atropine (10 micrograms/kg/min), without a significant decrease in plasma concentration of immunoreactive CCK. Intravenous injection of CCK (114 pmol/kg) increased the pancreatic secretion during atropine infusion, but not during proglumide infusion. These results suggest that the regulation of interdigestive pancreatic exocrine secretion in sheep depends greatly on endogenous CCK as well as on the cholinergic mechanism.

Effects of benzoic acid and its analogues on insulin and glucagon secretion in sheep

The effects of benzoic acid and its analogues on insulin and glucagon secretion were investigated in conscious sheep. Intravenous injections of benzoic acid increased plasma insulin and glucagon concentrations in a dose-dependent manner between 39–1250 μmol/kg, with ED 50s for increasing both hormones of about 625 μmol/kg. Various derivatives of benzoic acid (625 μmol/kg) were administered and structure-activity relationships were examined. A single carboxylic group was essential for stimulating insulin and glucagon secretion, since both hormone responses were abolished with compounds in which the carboxylic group was replaced by sulfonic or phosphoric groups, or in which another carboxylic element was introduced (phthalic acids). Most of the compounds which introduced other elements (amino and hydroxy groups, and halogens) onto the benzene ring had an altered stimulating activity. Thus the pancreatic endocrine system can recognize the chemical structure of benzoic acid and its derivatives in detail and induce insulin and glucagon secretion in sheep.

Effects of intravenous infusions of cholecystokinin (CCK)-8 on exocrine and endocrine pancreatic secretion in conscious sheep.

The effects of cholecystokinin (CCK)-8 on both exocrine and endocrine pancreatic functions were examined simultaneously in five conscious sheep. Intravenous infusions of CCK-8 (0, 5, 10, 20, 30, 60, 120 and 240 pmol/kg/min for 40 min) induced dose-dependent increases in flow rate, and in protein and amylase outputs in pancreatic juice. The same CCK-8 infusions induced dose-dependent increases in plasma insulin, but no change in plasma glucagon concentrations. The threshold dose (10-30 pmol/kg/min) of CCK-8 infusion for stimulating insulin secretion was similar to that for stimulating amylase output. In conclusion, using amylase output as an indicator of physiological action, CCK is one of the potential candidates as a physiological regulator of insulin, but not glucagon secretion in sheep.

The use of intragastric nutrition to study saliva secretion and the relationship between rumen osmotic pressure and water transport.

Four sheep sustained by intragastric nutrition were used to study saliva secretion and the relationship between osmotic pressure in the rumen and net water transport across the rumen wall. Different concentrations of buffer were infused into the rumen to change the rumen osmotic pressure. Salivary secretion was estimated from entrance of P into the rumen. Net water transport across the rumen wall was calculated as the difference between water inflow and water outflow from the rumen. A negative linear relationship between the rumen osmotic pressure (X, mOsm/kg) and the water absorption across the rumen wall (Y, ml/h) was found: Y = (394 SE 8.3)-(1.22 SE 0.03) X, r2 0.83, (P < 0.001), and a positive linear relationship was found between the rumen osmotic pressure (X, mOsm/kg) and the outflow rate of rumen fluid (Y, ml/h): Y = (34.0 SE 8.0) + (0.97 SE 0.03) X, r2 0.56, (P < 0.001). The implication is that rumen osmotic pressure can be a key factor in the control of the net water transport across the rumen wall, the outflow of rumen fluid to omasum and the rumen liquid dilution rate. A method is suggested by which salivary secretion in sheep may be calculated from the water balance in the rumen.

Bilateral lesions of the suprachiasmatic nuclei alter the nocturnal melatonin secretion in sheep.

The hypothalamic suprachiasmatic nuclei (SCN) constitute both the biological clock of many circadian rhythms, and the first relay in the transmission of light cues from the retina to the pineal gland, which releases, via nocturnal melatonin secretion, an endocrine expression of the daylength. The aim of the present work was to investigate the precise role of the SCN in the entrainment of the nocturnal rhythm of melatonin (MEL) in sheep. Bilateral lesions of the SCN were performed via a transsinusal surgical approach in 10 adult rams submitted to a constant photoperiod (16:8D). Lesioned rams were compared to 4 sham and 2 control animals. Blood samples were collected 8 days before, 8 days after, and one month after surgery. Plasma MEL levels were estimated using direct radioimmunoassay. At the end of the experiment, histology and immunohistochemistry of the suprachiasmatic area were performed, and the extent of lesions was evaluated using a computerized image analysis system. Six rams exhibited a complete lesion of the SCN, and in the four remaining animals, the lesions were restricted to the anterior part of the SCN. For all animals, the nocturnal melatonin secretion was altered, but depending on the extent of the lesion, two types of results were observed: Eight days after surgery, in 3 of the 4 rams bearing anterior lesions of the SCN (SCNx-), a nocturnal increase in melatonin secretion still occurred at dusk, but the duration of this secretion extended beyond the end of the night. One month after surgery, melatonin profiles were once again normal, as compared to sham animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of abomasal infusion of alanine and aspartic acid on growth hormone secretion in sheep

Effect of abomasal infusion of alanine and aspartic acid on growth hormone secretion in sheep

Photoperiodic modulation of the dopaminergic control of pulsatile LH secretion in sheep.

This study was conducted to investigate whether the photoperiodic regulation of the seasonal changes in pulsatile LH secretion in the ram involves changes in the activity of inhibitory hypothalamic dopaminergic (DA) pathways. To test this hypothesis, a series of experiments was carried out in Soay rams in which the effects of a DA-D2 receptor antagonist (sulpiride) or a DA-D2 receptor agonist (bromocriptine) on the pulsatile secretion of LH were determined under both long and short days. In each experiment blood samples were collected every 10 min for 8 h starting at the time of vehicle, sulpiride or bromocriptine injections to assess concentrations of LH. Sulpiride (0.59 mg/kg, s.c.) administered to rams under long days induced an immediate and sustained increase in the secretion of LH that lasted for approximately 4 h (P < 0.05; ANOVA); this LH response reflected both a rise in mean concentrations (0.247 +/- 0.03 vs. 0.452 +/- 0.1 microgram/l) and an increase in the frequency of LH pulses (0.5 +/- 0.5 vs. 2.33 +/- 0.42 pulses/8 h; P < 0.01). In contrast, under short days sulpiride had no effect. Bromocriptine (0.06 mg/kg, s.c.) administered to rams under long days, when LH concentrations were low, was without effect, but when given to rams under short days significantly (P < 0.05) suppressed mean LH concentrations (0.627 +/- 0.08 vs. 0.320 +/- 0.02 microgram/l) and LH pulse frequency (4.86 +/- 0.46 vs. 2.43 +/- 0.37 pulses/8 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Indirect measurement of saliva secretion in sheep fed diets of different structures and the effect of such diets on ruminal fluid kinetics and fermentation pattern

Four Suffolk x Dorset sheep were allocated in a 4 x 4 Latin square design and received a hay-barley-molasses diet in one of four different physical structures: (1) pelleted; (2) pelleted:chopped (60:40); (3) chopped:pelleted (60:40); or (4) chopped. The animals were penned individually and the diet was restricted to provide 20 g of dry matter (DM) per kilogram live weight daily. The effects of the diets on rumen fluid kinetics, fermentation pattern and microbial nitrogen (MN) supply were examined. Saliva secretion was estimated using an indirect method based on water balance in the rumen. When the animals were fed the chopped diet, the salivation rate, rumen fluid volume and rumen liquid outflow were all increased significantly (P < 0.05 to P < 0.01). No consistent effect of dietary structure on rumen fluid dilution rate, purine derivative (PD) excretion or MN supply was observed. Dietary structure had no effect on the rumen fermentation pattern, digestibility of DM or nitrogen.

Chemical specificity of short-chain fatty acids in stimulating insulin and glucagon secretion in sheep

The chemical specificity and structural requirements of short-chain fatty acids (SCFAs) in stimulating pancreatic endocrine responses was investigated in conscious sheep. Normal SCFAs with one to eight carbons were injected intravenously at seven doses of 39-2,500 mumol/kg body wt. The isomers or derivatives of SCFAs were administered at 625 mumol/kg body wt. Analysis of dose-response curves showed that n-butyric acid (4 carbons in the molecule) was most effective for both insulin and glucagon secretion among the normal SCFAs tested. In addition, one carboxylic group was absolutely required, since hormone secretion was significantly reduced or abolished with compounds in which the carboxylic element was replaced by other groups and with dicarboxylic acids. The form of the hydrocarbon chain (branched, cyclic, or benzoic ring) also affected hormone secretory activity. Most of the compounds that replaced hydrogen in the hydrocarbon chain by other groups at various positions reduced or abolished the hormone secretory effect obtained by n-butyric acid. In conclusion, a monocarboxylic acid with several numbers of hydrocarbons was required for insulin or glucagon secretion. These results suggest that the pancreatic endocrine system can recognize the chemical structure of SCFAs in detail and induce hormone secretion in sheep.

Neuroendocrine regulation of growth hormone secretion in sheep. VII. Effects of GABA

The effects of intravenous (i.v.) or intracerebroventricular (i.c.v.) administration of γ aminobutyric acid (GABA) on plasma growth hormone (GH) concentrations have been examined in sheep. Intravenous administration of GABA resulted in a rapid, significant ( P<0.001) increase in plasma GH. Administration of 10 mg of GABA i.c.v. produced a significant ( P<0.001) increase in GH release. By contrast, 100 mg given i.c.v. was inhibitory and resulted in a decrease ( P<0.05) in plasma GH levels. Concurrent administration of somatostatin (0.5 μg/min i.v. over 1 h) did not alter the plasma GH response to 10 mg GABA given i.c.v. These data are consistent with the concept of dual sites of action for GABA in regulating GH release in sheep, but the exact mechanism through which this effect is mediated remains unclear.

Effects of intravenous infusions of cholecystokinin-8 and pentagastrin on plasma concentrations of insulin and glucagon in sheep

The effects of cholecystokinin-8 ( cck-8) and pentagastrin on insulin and glucagon secretion were studied in conscious sheep. Intravenous infusions of ccx-8 (3 to 1000 pmol kg −1 min −1 for 30 minutes) induced a dose-dependent increase in plasma insulin, but did not alter plasma glucagon concentration. The threshold dose of ccx-8 for stimulation of insulin secretion was 10 to 30 pmol kg −1 min −1. Pentagastrin was infused intravenously at doses of 10 to 3000 pmol kg −1 min −1. The maximal dose of pentagastrin slightly stimulated insulin, but not glucagon, secretion. The insulin secretory activity of pentagastrin was only 1/300 that of ccx-8 on a molar basis. The threshold dose of ccx-8 for stimulation of insulin secretion was similar to that for exocrine pancreatic secretion obtained in earlier studies. In conclusion, ccx is a potential candidate as a physiological factor regulating insulin secretion in sheep.

Adrenergic regulation of growt hormone secretion in the ewe

This study examined the role of the adrenergic system in the regulation of growth hormone (GH) secretion in sheep. Intravenous infusion of noradrenaline (0.5μg/kg per min for 2 hr) totally suppressed plasma GH concentrations. Concomitant treatment of animals with the β-adrenergic antagonist propranolol completely blocked the noradrenaline-induced suppression of GH. In contrast, intravenous injection of the centrally acting α 2-agonist clonidine (2μg/kg) elicited a release of GH. To further investigate the central adrenergic regulation of GH secretion 10 μg of noradrenaline or adrenaline was microinjected (1μl) directly into the preoptic area of the hypothalamus of ovariectomized ewes. When the time of injection coincided with a GH trough period, both noradrenaline and adrenaline caused an increase in plasma GH concentrations, whereas if the injection coincided with an endogenous pulse of GH no additional GH response was obtained. In conclusion, these results provide evidence for the involvement of the adrenergic system in the regulation of GH secretion in sheep. Centrally, adrenergic pathways exert a stimulatory effect on GH release via an α 2-adrenergic system, whereas peripherally adrenergic pathways exert an inhibitory effect via β-adrenergic mediated mechanisms. Furthermore, adrenergic stimulation of the preoptic area may inhibit somatostatin activity and directly facilitate a GH pulse. Alternatively, adrenergic innervation of the preoptic area may influence neurons (somatostatin or other) that project to the arcuate nucleus and stimulate the release of GH-releasing factor.

Centrally administered vasopressin modifies stress hormone (cortisol, prolactin) secretion in sheep under basal conditions, during restraint and following intravenous corticotrophin-releasing hormone.

The effects of central (lateral ventricle) injections of vasopressin (2.5, 25 or 250 ng) on cortisol and prolactin release were investigated in castrated male sheep (N = 6) under basal (non-stress) conditions, during 120min of physical restraint and following peripheral injection of ovine corticotrophin-releasing hormone (oCRH). Cortisol and prolactin concentrations in non-stressed sheep were raised significantly (p < 0.05) in the 120 min following administration of 250 ng of vasopressin. The prolactin response to restraint stress was decreased (p < 0.05) in the 30-min period following central injection of 250 ng of vasopressin. The stimulatory effect of oCRH on cortisol release was enhanced (p < 0.05) by vasopressin (25 and 250 ng), whereas prolactin levels decreased (p < 0.02) in the 30-min period following injection of the highest dose of vasopressin. None of the central doses of vasopressin significantly altered plasma levels of this hormone, although vasopressin secretion was increased during restraint.

Influences of renal nerves and pregnancy on renin secretion in sheep

To investigate the role of the renal nerves in the control of renin secretion during pregnancy, we prepared sheep with one denervated kidney, bilateral renal vein catheters, and an inflatable aortic occluder. The renin secretory responses to step reductions of renal perfusion pressure (RPP) of the innervated and denervated kidneys were compared in eight nonpregnant and nine pregnant (115-130 days gestation) sheep in a normal conscious state. Nonpregnant sheep had average basal levels of mean arterial pressure (MAP) of 91 +/- 3 mmHg, plasma renin activities (PRA) of 2.3 +/- 0.4 ng ANG I.ml-1.3 h-1, and plasma angiotensin (ANG) concentrations of 8.4 +/- 1.1 pg/ml. Pregnant sheep had reduced levels (P < 0.01) of MAP (76 +/- 3 mmHg) but elevated levels of PRA (3.8 +/- 0.4 ng ANG I.ml-1.h 3-1; P < 0.5) and ANG II (12.0 +/- 1.6 pg/ml; P = 0.08). Four successively reduced levels of RPP were observed for 15 min each in both groups of sheep. In both groups, RPP was reduced to a final level of 51 +/- 2 mmHg. The renal venous-arterial difference of PRA (v-aPRA) from the innervated kidneys was increased as RPP was reduced. However, there was little response of v-aPRA to RPP in denervated kidneys. In nonpregnant sheep, the relationship between RPP and renin secretion rate was characterized by an initially slow phase, over which small reductions of RPP resulted in small increases in renin secretion. As RPP fell, this relationship became progressively steeper in nonpregnant ewes.(ABSTRACT TRUNCATED AT 250 WORDS)