Effects of synthetic adrenocorticotrophin on adrenal medullary responses to splanchnic nerve stimulation in conscious calves

Aspects of autonomic and neuroendocrine function

This article provides a brief review of aspects of autonomic and neuroendocrine function studied initially in collaboration with the late Marian Silver. The importance of the sympathetic innervation to the liver in the control of glycogenolysis was established in anaesthetised animals of various species. Otherwise the work has been carried out mainly in conscious animals under strictly physiological conditions and below behavioural threshold. Investigations of the role of the autonomic innervation to the endocrine pancreas in controlling the release of pancreatic hormones, led to the realisation that the parasympathetic innervation mediates responses to glycaemic stimuli while the sympathetic innervation mediates responses to any form of stress. Studies of adrenal medullary function have confirmed that its threshold for many forms of stress is much higher than that of other components of the sympathetic system and revealed the importance of the pattern of electrical stimulation in determining the rates of release of catecholamines, enkephalins, corticotrophin-releasing factor (CRF) and adrendocorticotrophin (ACTH). The splanchnic sympathetic innervation to the adrenal cortex also plays an important role in determining glucocorticoid output by sensitising the cells to ACTH, probably mainly by the release of vasoactive intestinal peptide (VIP) from cortical nerve terminals. Finally studies of feeding in milk-fed calves have shown that suckling is associated with a remarkable hypertension and tachycardia. These cardiovascular effects are due to a selective sympathetic discharge, which does not involve the adrenal medullae, or the release of neuropeptide Y (NPY) and, at least in the calf, can be attributed to activation of adrenoceptors.

Vascularization of the adrenal cortex: its possible involvement in the regulation of steroid hormone release

This review examines the morphology of the adrenal gland with particular reference to the adrenal vasculature. It examines the possibility that variability in adrenal gland responsiveness may be attributable to neural or hormonal modulation of adrenal blood flow. Changes in the rate of blood flow through the adrenal cortex would be expected to play an important role in the regulation of steroid hormone release. It would affect both the delivery of the major stimulant (ACTH) and the removal of the end product from the steroidogenic cells (the glucocorticoids). In the past, interest in this area has concentrated on the regulation of arterial blood flow, rather than the regulation of venous drainage. The current review examines the concept of vascular damming, and attempts to link the morphological features of the gland with experimental data associated with glucocorticoid release. It is postulated that regulation of venous drainage, via the vascular dam, plays an important role in the storage of the secretory product during the animals' inactive phase, and in the initial rapid rise in plasma levels of the glucocorticoids seen in response to stress or injection of ACTH.

Muscarinic involvement in vascular and adrenal medullary responses to splanchnic nerve stimulation in conscious calves

Stimulation of the peripheral end of the right splanchnic nerve (4 Hz for 10 min) in the presence of hexamethonium caused a small but significant rise in mean aortic blood pressure which was subsequently abolished by atropine. There were also small but significant increases in the outputs of catecholamines, [Met5]-enkephalins and corticotrophin releasing factor (CRF) from the right adrenal gland. The catecholamine response was roughly halved after atropine while the outputs of enkephalins and CRF were unaffected. It is concluded that splanchnic sympathetic postganglionic neurones supplying the vasculature are completely blocked by cholinergic blockade whereas adrenal medullary responses persist in an attenuated form.

Autonomic control of adrenal function

Recent studies of adrenal function in conscious calves are reviewed. These have involved collecting the whole of the adrenal effluent blood from the right adrenal gland at intervals and, where necessary, prior functional hypophysectomy by destruction of the pituitary stalk under general halothane anaesthesia 3 d previously. The adrenal medulla was found to release numerous neuropeptides, in addition to catecholamines, in response to stimulation of the peripheral end of the right splanchnic nerve, which was carried out below behavioural threshold. Many of these responses were enhanced by stimulating intermittently at a relatively high frequency. Intra-aortic infusions of a relatively low dose of acetylcholine (4.5 nmol min-1 kg-1) elicited similar responses. In the adrenal cortex, agonists which either potentiated the steroidogenic response to ACTH or exerted a direct steroidogenic action included VIP, CGRP, CRF and ACh acting via muscarinic receptors. Stimulation of the peripheral end of the right splanchnic nerve strongly potentiated the steroidogenic response to ACTH and there is compelling evidence that the innervation normally plays an important part in cortisol secretion.

Adrenal cortical and medullary responses to acetylcholine and vasoactive intestinal peptide in conscious calves

1. Adrenal responses to intra-aortic infusions of acetylcholine and vasoactive intestinal peptide (VIP) have been investigated in functionally hypophysectomized calves given exogenous adrenocorticotrophic hormone (ACTH, 2 ng min-1 kg-1 I.V.). 2. Infusions of VIP at a dose of 0.13 micrograms min-1 kg-1 caused a small, but significant increase in adrenaline and noradrenaline output which was, however, far below the level recorded previously in response to acetylcholine (0.7 micrograms min-1 kg-1). In contrast, these doses of the two agonists produced closely similar rises in adrenal cortisol output. 3. The steroidogenic effects of acetylcholine and VIP were found to be strictly additive and no evidence of potentiation was obtained in relation to either cortical or medullary responses or in the case of any of the cardiovascular responses which were monitored. 4. Intra-aortic infusions of VIP, at a dose which produced a substantial increase in adrenal steroidogenesis (0.065 micrograms min-1 kg-1), had no effect on the output of catecholamines, enkephalin-like immunoreactivity or corticotrophin-releasing factor, either in the presence or absence of acetylcholine. 5. It is concluded that VIP is unlikely to modulate adrenal medullary responses to muscarinic stimulation in this species as it has been claimed to do in the rat and does not potentiate adrenal steroidogenesis in response to acetylcholine as it does to ACTH.

Differences between adrenomedullary adrenaline and noradrenaline cells: quantitative electron-microscopic evaluation of their differential cellular association with supporting cells

Quantitative differences in cellular association of adrenomedullary chromaffin cells with other types of cells, mainly supporting cells, were studied. Adrenaline (A) and noradrenaline (NA) cells were compared. Electron micrographs (12000 x) of profiles of A and NA cells, bordering against other types of cells, were used for quantitative evaluation. Supporting cells constituted the majority of the non-chromaffin cell types. Occurrence frequencies of chromaffin cells contiguous with other types of cells were: (1) higher for A cells (68.9%, 199/289) than for NA cells (11.0%, 34/309) in case of small contact regions (chi 2-test: P < 0.001), and (2) higher for NA cells (68.3%, 211/309) than for A cells (9.7%, 28/289) in case of extended contact regions (P < 0.001). In conclusion, the extent of cellular association with supporting cells was remarkably lower in A cells than in NA cells. Such an arrangement is likely to be appropriate for the extensive, homogeneous control and amplified response characteristic of A cells, and for the close range, complex control and more diverse responses characteristic of NA cells.

The role of corticotrophin releasing factor in relation to the neural control of adrenal function in conscious calves

1. Adrenal responses to intra-aortic infusions of pure synthetic ovine corticotrophin releasing factor (CRF), and to electrical stimulation of the preganglionic sympathetic innervation, have been investigated in functionally hypophysectomized conscious calves, in the presence and absence of a specific CRF antagonist. 2. CRF exerted a substantial steroidogenic effect on the adrenal gland of functionally hypophysectomized calves when infused intra-aortically at a dose (1.3 ng min-1 kg-1) below that which caused any fall in the arterial blood pressure. This response was significantly reduced, but not abolished by a concomitant infusion of CRF-antagonist into the aorta. 3. The steroidogenic effect of CRF was significantly reduced in the presence of exogenous adrenocorticotrophic hormone (ACTH) (2 ng min-1 kg-1, I.V.) and the surviving response was completely abolished by CRF-antagonist. 4. Stimulation of the peripheral end of the splanchnic nerve at 4 Hz in functionally hypophysectomized calves given exogenous ACTH produced a rise in mean adrenal output of the same order of magnitude as did exogenous CRF under the same conditions. The response to splanchnic nerve stimulation was apparently unaffected by CRF-antagonist although release of endogenous CRF from the gland was significantly increased thereby. 5. These results indicate that release of CRF from the adrenal gland during splanchnic nerve stimulation in the calf does not contribute significantly to the steroidogenic response thereto.

Muscarinic adrenal responses to acetylcholine in conscious calves

1. Adrenal responses to intra-aortic infusions of acetylcholine (4.5 nmol min-1 kg-1 for 10 min) have been investigated in hypophysectomized conscious calves given exogenous adrenocorticotrophic hormone (ACTH) (2 ng min-1 kg-1 I.V.) in the presence and absence of hexamethonium. 2. Acetylcholine produced a significant increase in adrenal cortisol output and plasma cortisol concentration. In the absence of nicotinic blockade with hexamethonium this was apparently accounted for by an increase in adrenal ACTH presentation secondary to increased adrenal blood flow. However, administration of hexamethonium revealed a direct steroidogenic action of acetylcholine in the presence of exogenous ACTH. 3. Adrenal medullary responses to acetylcholine, including the release of catecholamines, enkephalins and corticotrophin-releasing factor (CRF), were not significantly reduced by nicotinic blockade. 4. It is concluded that both adrenal medullary and cortical responses to intraaortic infusions of acetylcholine at a low dose are mediated mainly by muscarinic receptors, as it has previously been shown that they are substantially reduced in the presence of atropine.

Endocrine responses to intra-aortic infusions of acetylcholine in conscious calves

1. Adrenal responses to intra-aortic infusions of acetylcholine (4.5 nmol min-1 kg-1 for 10 min) have been investigated in conscious, functionally hypophysectomized, 3- to 6-week-old calves, in the presence and absence of exogenous ACTH (2 ng min-1 kg-1, I.V.). 2. Acetylcholine produced a substantial fall in adrenal vascular resistance, which was significantly reduced in the presence of exogenous ACTH, while producing minimal changes in aortic blood pressure and heart rate. 3. There was also a significant rise in right adrenal cortisol output which was sufficient to produce a measurable rise in plasma cortisol concentration. The effect could be accounted for by the increase in adrenal ACTH presentation. It was abolished by pre-treatment with atropine (0.2 mg kg-1). A small but significant rise in aldosterone output during acetylcholine infusions was also abolished in the presence of ACTH. 4. Both adrenaline and noradrenaline were released during intra-aortic acetylcholine infusions and these responses were substantially reduced, but not abolished, by pre-treatment with atropine. 5. Acetylcholine also stimulated the release of corticotrophin-releasing factor (CRF) and [Met5]enkephalins from the gland. The output of CRF was enhanced and that of free [Met5]enkephalin was significantly reduced in the presence of exogenous ACTH. All these responses were largely, but not completely, suppressed by atropine. 6. Acetylcholine also promoted the release of the pancreatic hormones glucagon, insulin and pancreatic polypeptide (PP). The amounts of pancreatic glucagon and insulin that were released were highly dependent on the concentration of glucose in the circulating plasma and all these responses were abolished by atropine. 7. It is concluded that acetylcholine is capable of stimulating the release of a wide variety of agonists from the adrenal gland when infused intra-aortically at a dose of 4.5 nmol min-1 kg-1. The increase in cortisol output appears to be secondary to an increase in blood flow whereas the adrenal medullary responses are not, and appear to be due largely, but not entirely, to activation of muscarinic receptors.

Maturational changes in expression of enkephalin peptides in adrenal and extra-adrenal tissue of fetal and adult rabbits

Met-enkephalin immunoreactivity (MET-ENKi), total enkephalin immunoreactivity (TOTAL MET-ENKi) and catecholamines were measured in adrenal and extra-adrenal tissue of fetal, newborn and adult rabbits. Met-enkephalin peptides were detected in adrenal and extra-adrenal tissue by 29 days of gestation. There were progressive increases in TOTAL MET-ENKi in both the adrenal and extra-adrenal tissue during development. In 29-day-old fetuses, MET-ENKi represented 43 and 50% of the peptide content in adrenal and extra-adrenal tissues respectively. By 3 days after birth, MET-ENKi represented only 15 and 7% of the peptide content in the same tissues. In the adult adrenals, 10% of enkephalin peptides were found as MET-ENKi. There were progressive increases in adrenal and extra-adrenal catecholamine content in the fetal and newborn rabbits throughout development. The changes in the ratio of MET-ENKi to TOTAL MET-ENKi peptides suggest differences in posttranslational processing of proenkephalin peptide during maturation. We speculate that enkephalin peptides derived from proenkephalin A are important during fetal and early newborn life and that extra-adrenal tissue may be an important source of these peptides during development.

Adrenal responses to corticotrophin-releasing factor in conscious hypophysectomized calves

1. Adrenal responses to intra-aortic infusions of pure synthetic ovine corticotrophin-releasing factor (CRF) have been investigated in functionally hypophysectomized calves previously fitted with an adrenal clamp. 2. CRF caused an increase in the output of cortisol from the adrenal gland, which was dose related over the range 4-8 pmol min-1 and maximal at the higher of these doses; this response was observed at a dose below that which produced any change in adrenal vascular resistance. Cortisol output was also found to be related linearly to the rate at which CRF was estimated to be presented to the gland during these infusions. 3. The infusions of CRF also provoked the release of small, but readily detectable, amounts of adrenocorticotrophin-like peptides (ACTH) from the gland. This was mainly in the form of ACTH1-39 with some pro-opiomelanocortin (POMC) also being released. 4. Comparison of the adrenal steroidogenic response to exogenous CRF with that to synthetic ACTH1-24 showed that CRF was the more potent; in each case cortisol output was related linearly to the presentation rate of the peptide. 5. It is concluded that the adrenal cortex in the calf is capable of releasing cortisol in response to exogenous CRF at low concentrations and is even more sensitive to CRF than it is to exogenous ACTH over the dose range that was employed.

The effect of changes in adrenal blood flow on adrenal cortical responses to adrenocorticotrophin in conscious calves

1. The effect of varying adrenal blood flow on the rate at which it was estimated that adrenocorticotrophin (ACTH) was presented to the adrenal gland was related to right adrenal cortisol output in conscious calves fitted with 'adrenal clamps'. 2. Intra-aortic infusions of endothelin at either 15.0 or 7.5 pmol min-1 kg-1 produced a substantial fall in right adrenal blood flow which was dose-related over this range. There was an associated fall in right adrenal cortisol output and cortisol output was linearly related to estimated ACTH presentation to the gland over the whole range investigated. The changes in adrenal cortisol output were reflected by changes in the concentration of cortisol in the peripheral plasma, which could be attributed entirely to the fluctuations in adrenal cortisol output. 3. It is concluded that delivery of ACTH to the adrenal gland is flow dependent over the physiological range in these animals and that changes in adrenal cortical blood flow can therefore be expected to result in changes in adrenal output due to variations in the presentation rate of ACTH.

Release of adrenocorticotrophin from the adrenal gland in the conscious calf

1. The effect of stimulation of the splanchnic nerve on the output of ACTH-related peptides from the adrenal gland has been investigated in conscious, functionally hypophysectomized calves, previously fitted with an 'adrenal clamp'. 2. Stimulation of the splanchnic nerve elicited a small, but statistically significant, increase in the output of ACTH-like immunoreactivity at each frequency tested. This response was frequency-dependent over the range 40-70 Hz when stimulating intermittently for 1 s at 10 s intervals and was potentiated by stimulating intermittently. Thus, the average mean output during stimulation in burst at 70 Hz (25 +/- 5 fmol min-1 kg-1) was significantly higher than the corresponding value during continuous stimulation at 7 Hz (6 +/- 1 fmol min-1 kg-1; P less than 0.01) even though the total number of impulses delivered was identical in each case. 3. There was also a small but significant rise in the output of cortisol from the gland with intermittent stimulation, which was linearly related to the output of ACTH-like immunoreactivity at the lower frequencies (4 and 7 Hz). 4. Separation of the ACTH-related peptides which were extracted from the adrenal effluent plasma of these animals during splanchnic nerve stimulation revealed the existence of two clear forms: ACTH (1-39) accounted for about 60% of total ACTH immunoreactivity and pro-opiomelanocortin (POMC) for about 30%. 5. It is concluded that small amounts of ACTH are released within the adrenal gland during splanchnic nerve stimulation in the functionally hypophysectomized calf and that this may possibly contribute towards the steroidogenic effect of stimulating the splanchnic nerve.

Adrenal responses to splanchnic nerve stimulation in conscious calves given naloxone

1. The effects of stimulating the peripheral end of the right splanchnic nerve in the presence of naloxone (2 mg kg-1) have been investigated in conscious 3 to 6-week-old calves. 2. Mean aortic blood pressure rose to significantly higher levels during splanchnic stimulation in bursts at 40 Hz for 1 s at 10 s intervals than it did during stimulation at the corresponding continuous frequency (4 Hz). Furthermore, naloxone significantly reduced the fall in mean vascular resistance in response to both patterns of stimulation. 3. The output of catecholamines from the adrenal gland, together with the proportion of noradrenaline released, was significantly enhanced by stimulating the splanchnic nerves in bursts in animals pre-treated with naloxone and the proportion of noradrenaline released also increased. In both cases the output of adrenaline and noradrenaline was within the same range as that reported previously in normal control animals. 4. Naloxone significantly increased the amounts of enkephalin-like immunoreactivity and corticotrophin-releasing factor (CRF)-like immunoreactivity released from the adrenal gland in response to splanchnic nerve stimulation and raised the proportion of total to free met5-enkephalin that was secreted. 5. Naloxone also inhibited the rise in plasma adrenocorticotrophic hormone (ACTH) concentration during continuous stimulation at 4 Hz, but not during stimulation at 40 Hz in bursts. Under these latter conditions the output of cortisol apparently directly from the adrenal gland was inhibited. The finding that splanchnic nerve stimulation can potentiate the output of cortisol in response to ACTH was confirmed. 6. These results provide evidence that release of enkephalins and of CRF from the adrenal is inhibited by activating opioid receptors within the gland itself.

Adrenal responses to calcitonin gene-related peptide in conscious hypophysectomized calves

1. Right adrenal and various cardiovascular responses to an intra-aortic infusion of calcitonin gene-related peptide (CGRP; 4 micrograms min-1) have been investigated in the presence and absence of exogenous adrenocorticotrophin ACTH1-24 (2 or 5 ng min-1 kg-1, I.V.). The adrenal clamp technique was employed in conscious calves in which the pituitary stalk had been cauterized 3-7 days previously. 2. At the higher dose (5 ng min-1 kg-1) the I.V. infusion of ACTH raised mean plasma ACTH concentration by about 1000 pg ml-1 and mean right adrenal cortisol output by about 750 ng min-1 kg-1. Under these conditions the intra-aortic infusion of CGRP had no apparent effect on adrenal cortisol output by about 750 ng min-1 kg-1. Under these conditions the intra-aortic infusion of CGRP had no apparent effect on adrenal function, other than to produce moderate adrenal vasodilatation. In contrast, in the absence of exogenous ACTH, the same dose of CGRP produced a substantial rise in cortisol output, which rose steadily to a peak mean value of 409 +/- 31 pg min-1 kg-1 at 10 min. It also significantly inhibited the release of free, but not of total, met5-enkephalin-like immunoreactivity from the gland (P less than 0.001) together with a significantly greater fall in adrenal vascular resistance (P less than 0.001). 3. At the lower dose of ACTH (2 ng min-1 kg-1, I.V.) CGRP raised mean plasma cortisol output from 314 +/- 31 to 486 +/- 44 ng min-1 kg-1 (P less than 0.01) and this effect was not attributable to an increase in the adrenal presentation rate of ACTH. 4. It is concluded that this peptide exerts a steroidogenic action on the adrenal cortex which is manifest in the absence of exogenous ACTH in the functionally hypophysectomized calf.

Secretion of corticotrophin releasing factor from the adrenal during splanchnic nerve stimulation in conscious calves

1. The output of corticotrophin releasing factor-like immunoreactivity (CRF) from the adrenal gland has been investigated using the 'adrenal clamp' technique in conscious calves. 2. Stimulation of the peripheral end of the splanchnic nerve for 10 min increased the mean output of CRF progressively, so that it had risen by about twentyfold, to a peak incremental value of 24 +/- 4 pmol min-1 kg-1 at 10 min. This response was significantly increased by stimulating in bursts at 40 Hz for 1 s at 10 s intervals, which raised the mean CRF output by 44 +/- 7 pmol min-1 kg-1 at 10 min (P less than 0.05). 3. The mean output of adrenaline and noradrenaline rose more abruptly in response to splanchnic nerve stimulation with peak incremental values realized within 2.5 min. However, the ratios of adrenal CRF to catecholamine output were closely similar during the later stages of stimulation (7.5-10 min). There was a similarly abrupt rise in adrenal cortisol output in response to splanchnic nerve stimulation which was, nevertheless, linearly related to arterial plasma ACTH concentration throughout. 4. In hypophysectomized calves, administration of adrenocorticotrophic hormone (ACTH1-24) at a dose of 5 ng min-1 kg-1 reduced the output of adrenal CRF in response to splanchnic nerve stimulation by about 50% (P less than 0.05). 5. CRF isolated from adrenal venous effluent plasma, collected both at rest and during splanchnic nerve stimulation, was separated by reverse-phase high-pressure liquid chromatography and found to elute in a position identical to that of human 41CRF. This suggests that adrenal CRF is structurally closely similar to its pituitary counterpart.

Roles of the pituitary-adrenocortical axis in control of the native and cryptic enkephalin levels and proenkephalin mRNA in the sympathoadrenal system of the rat

The effects of hypophysectomy (HPX) and dexamethasone (DEX) on the levels of Met5-enkephalin (ME), ME precursors, and the abundance of proenkephalin (pEK) mRNA, were examined in the adrenal medulla (AM) and superior cervical ganglia (SCG). To assess possible changes in enkephalin processing, both cryptic (after trypsin and carboxypeptidase B digestions) and native (without enzyme digestions) ME-like immunoreactivity (ME-LI) was measured. Three weeks after HPX the proportion of pEK mRNA to the total RNA content in the AM was not significantly changed when compared to sham-operated (SO) animals. Total (native + cryptic) ME-LI was decreased by 45% in the AM of HPX rats. This decrease was paralleled by a 58% depletion of AM proteins. Cryptic ME-LI was also reduced by 43%. In contrast, native ME-LI was not altered after HPX, indicating enhanced processing of ME precursors. Treatment with DEX (5 daily injections--1 mg/kg, i.p.) increased the relative abundance of pEK mRNA (+27%) and total ME-LI in the AM of HPX group, but not in SO group. Native ME-LI, cryptic ME-LI, and their ratio were not significantly affected by DEX in the AM of HPX or SO rats. In SCG, the relative abundance of pEK mRNA decreased by 25% after hypophysectomy. Total and cryptic ME-LI in the SCG of HPX rats were not changed when compared to SO rats. In contrast, HPX reduced native ME-LI suggesting decreased processing of ME precursors. Similarly, as in AM, DEX produced increase in the SCG pEK mRNA only in HPX (+68%) and not in the SO rats. In SCG, DEX produced decreases in total ME-LI which could be attributed to an increased enkephalin release. An overall reduction of cryptic ME-LI was also observed after DEX, whereas native ME-LI remained unchanged suggesting increased processing of enkephalins. Our findings indicate that the pituitary adrenocortical axis controls the relative proportions of ME to its precursors, and that this control involves both glucocorticoid-dependent (SCG) and glucocorticoid-independent (AM) mechanisms. In contrast, our studies do not suggest specific control of pEK synthesis by the pituitary adrenocortical axis. The pituitary adrenocortical axis may also influence the relative contents of ME and catecholamines in the AM and SCG. The ratio of ME/catecholamines increased after HPX (AM and SCG) and after DEX (SCG). Such regulation may contribute to the control of co-transmitter output in the sympathoadrenal system.

The adrenal contribution to the neuroendocrine responses to splanchnic nerve stimulation in conscious calves

1. The extent to which the adrenal gland contributes to neuroendocrine responses to electrical stimulation of the peripheral end of the splanchnic nerve has been investigated in conscious calves in which the right nerve was stimulated either at 4 Hz continuously for 10 min or at 40 Hz in 1 s bursts at 10 s intervals for the same period. 2. It was confirmed that the release of neuropeptide Y (NPY) and of gastrin-releasing peptide (GRP) is potentiated by stimulation in bursts at a relatively high frequency and shown that the adrenal gland made a negligible contribution to these responses. 3. There was no detectable change in the concentration of vasoactive intestinal peptide (VIP) in the arterial plasma but the existence of a very small but highly significant rise in the output of VIP from the adrenal provided evidence that it was released within the gland in response to splanchnic nerve stimulation. 4. The concentration of calcitonin gene-related peptide (CGRP) in the arterial and adrenal venous effluent plasma was consistently below the level of detection of the assay. 5. Splanchnic nerve stimulation resulted in an abrupt rise in the output of both free and total met5-enkephalin-like immunoreactivity from the adrenal gland which was substantially potentiated by stimulating in bursts. This pattern of stimulation also increased the proportion released in a high-molecular-weight form. 6. Stimulation in bursts significantly enhanced the output of both adrenaline and noradrenaline from the adrenal and resulted in the release of proportionately more noradrenaline. Small amounts of dopamine and DOPAC were also released during splanchnic nerve stimulation and the output of dopamine was significantly increased by stimulating in bursts. 7. Both patterns of stimulation elicited an abrupt rise in mean plasma adrenocorticotrophic hormone (ACTH) concentration, which was associated with an increase in mean adrenal cortisol output and the former effect was significantly enhanced by stimulating in bursts. 8. It is concluded that certain responses to splanchnic nerve stimulation are significantly potentiated by an intermittent high-frequency pattern of stimulation, including all those that are attributable to adrenal medullary activity, whereas others are apparently unaffected by changes in stimulus pattern.

Adrenal cortical responses to vasoactive intestinal peptide in conscious hypophysectomized calves

1. Right adrenal and various cardiovascular responses to an intra-aortic infusion of vasoactive intestinal polypeptide (VIP; 4 micrograms min-1 kg-1) have been investigated in the presence and absence of exogenous adrenocorticotrophin, (ACTH1-24; 5 ng min-1 kg-1, i.v.). The adrenal clamp technique was employed in conscious calves in which the pituitary stalk had been cauterized 3-4 days previously. 2. The i.v. infusion of ACTH1-24 increased mean plasma ACTH concentration by between 1000 and 1100 pg ml-1 and mean right cortisol output by about 700 ng min-1 kg-1. Under these conditions the intra-aortic infusion of VIP produced a further rise in mean adrenal cortisol output, together with a consequential rise in mean arterial plasma cortisol concentration, without affecting the concentration of ACTH in the arterial plasma significantly. In the absence of ACTH the same infusion of VIP had no detectable effect on adrenal cortisol output. 3. In each of the above respects this intra-aortic infusion of VIP closely mimicked the effect of stimulation of the peripheral end of the right splanchnic nerve in these animals, as it also did by causing a substantial fall in adrenal vascular resistance in the absence, but not in the presence, of ACTH. 4. It is concluded that release of this peptide from splanchnic nerve terminals in the adrenal gland most probably accounts, at least in part, for the powerful adrenocortical steroidogenic response to splanchnic nerve stimulation, that occurs in the presence of submaximal doses of ACTH.

The effect of splanchnic nerve stimulation on adrenocortical activity in conscious calves

1. Right adrenal and various cardiovascular responses to stimulation of the peripheral end of the right splanchnic nerve have been investigated in the presence and absence of exogenous adrenocorticotrophin, ACTH1-24, (5 ng min-1 kg-1). The adrenal-clamp technique was employed in conscious calves in which the pituitary stalk had been cauterized 3-4 days previously. 2. The I.V. infusion of ACTH1-24 increased mean plasma ACTH concentration by about 1200 pg/ml and mean right adrenal cortisol output by about 500 ng min-1 kg-1. Stimulation of the peripheral end of the right splanchnic nerve at 4 Hz for 10 min produced a further rise in cortisol output, amounting to about 400 ng min-1 kg-1 (P less than 0.01). These changes in output were reflected accurately by changes in peripheral plasma cortisol concentration. 3. Closely similar amounts of adrenaline were released in response to splanchnic nerve stimulation in the presence and absence of exogenous ACTH. In the presence of ACTH the average mean output of noradrenaline (58 +/- 2 ng min-1 kg-1) was significantly less than that of adrenaline (102 +/- 4 ng min-1 kg-1; P less than 0.001), whereas the corresponding values were not significantly different in the absence of ACTH. 4. These results also confirm the fact that the fall in adrenal vascular resistance which occurs during splanchnic nerve stimulation is substantially reduced by ACTH, as is the rise in met5-enkephalin output. 5. It is concluded that the splanchnic innervation is capable of enhancing the secretion of adrenal glucocorticoids in response to ACTH under physiological conditions in the conscious calf.