Vasoactive intestinal peptide stimulates proenkephalin A mRNA expression in bovine adrenal chromaffin cells

Adrenal neuropeptides: regulation and interaction with ACTH and other adrenal regulators

It is now well accepted that both the cortex and medulla of the mammalian adrenal gland receive a rich innervation. Many different transmitter substances have been identified in nerves supplying both cortex and medulla and, as well as catecholamines, a wide range of neuropeptides has been found in the adrenal gland. There have been several studies on the affects of age, sodium intake, stress, ACTH, and splanchnic nerve activity on the regulation of adrenal neuropeptide content. There is evidence that the abundance of each of these peptides is actively regulated. Although there have been many studies addressing the individual actions of various neurotransmitters on steroid secretion, adrenal blood flow, and adrenal growth, few have attempted to determine the nature of any interaction between neurotransmitters and the classical adrenal stimulants. There are, however, some significant interactions, particularly in the regulation of zona glomerulosa function. This review necessarily focuses on vasoactive intestinal peptide (VIP) and neuropeptide Y (NPY), as these are the most abundant transmitter peptides in the adrenal gland and the majority of studies have investigated their regulation and actions. However, substance P, calcitonin gene-related peptide (CGRP), neurotensin, and the enkephalins are included where appropriate. Finally, it has been suggested that certain neurotransmitters, particularly VIP, may interact with classical hormone receptors in the adrenal, notably the ACTH receptor. This review attempts to evaluate our current state of knowledge in each of these areas.

Pituitary adenylate-cyclase activating polypeptide (PACAP) evokes long-lasting secretion and de novo biosynthesis of bovine adrenal medullary neuropeptides

Recently, the pituitary adenylate-cyclase activating polypeptide (PACAP) has emerged as a potential noncholinergic neuromodulator of adrenal medullary function. In support of this hypothesis, we documented PACAP's effects on the secretion and biosynthesis of neuropeptides by cultured bovine chromaffin cells. Data presented in this study indicate that PACAP is a potent and efficacious secretagogue of leucine-enkephalin which was coreleased with catecholamines with identical profiles. In comparison to nicotinic activation, however, rates of PACAP-induced secretion were substantially slower but persisted for several hours causing a prolonged increase in the tonic release of both transmitters and peptides. Interestingly, renewal of intracellular pools of neuropeptides was also stimulated by PACAP but not the vasoactive intestinal peptide (VIP). Indeed, the higher incorporation of [35S]-labeled amino acids into atrial and brain natriuretic peptides (ANP, BNP) provided strong evidence that PACAP directly activated de novo biosynthesis. Of particular importance was PACAP's net preferential stimulation of the biosynthesis of BNP, similar to the differential regulation by protein kinase A (PK-A) and protein kinase C (PK-C) activators we have previously the differential regulation by protein kinase A (PK-A) and protein kinase C (PK-C) activators we have previously reported. PACAP-induced secretion and biosynthesis appeared to be mediated by the PACAP-specific type I receptors known to activate adenylate cyclase and phospholipase C. We verified that PACAP did indeed stimulate the production of cyclic AMP and inositol phosphates in our cell system. These findings suggest that the dual signaling properties of type I receptors may be important for PACAP's differential effect on the biosynthesis of natriuretic peptides. We conclude that PACAP might assume important noncholinergic trans-synaptic regulation of the adrenal medulla by releasing and modifying intragranular catecholamine and neuropeptide contents.

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.

Effects of [Sar1]angiotensin II on proenkephalin gene expression and secretion of [Met5]enkephalin in bovine adrenal medullary chromaffin cells

We have studied the effect of [Sar1]angiotensin II [S1-AII; a degradation-resistant analogue of angiotensin II (AII) on the release of [Met5]enkephalin (ME) and proenkephalin A (proENK) gene expression. Short-term (15-min to 1-h) stimulation of bovine adrenal medullary chromaffin (BAMC) cells with S1-AII at concentrations from 0.1 to 100 nM had no significant effect on secretion of ME, whereas high concentrations of S1-AII (3 to 100 microM) produced a concentration-dependent increase in the concentration of ME in the incubation media. In contrast, long-term (3- to 24-h) stimulation with low concentrations (0.1 nM-1 microM) of S1-AII increased the secretion of ME in a concentration-dependent manner (EC50 = 1 nM). The intracellular level of ME was not changed by long-term treatment with S1-AII (100 nM). In addition to increased ME secretion, long-term (24-h) stimulation with S1-AII increased the expression of proENK mRNA in a concentration-dependent manner (EC50 = 4 nM). Losartan (2-n-butyl-4 chloro-5-hydroxymethyl-1-[(2'-(1 H-tetrazol-5-yl)biphenyl-4-yl)- methyl]imidazole potassium salt, a type 1 AII receptor antagonist) inhibited these effects, whereas PD123319 (50 microM, a type 2 AII receptor antagonist) was inactive. Our results suggest that AII in BAMC cells exerts a major effect on the long-term regulation of expression of proENK mRNA and secretion of ME. These effects appear to be mediated by type 1-like AII receptors.

The peptide VIP is a neurotransmitter in rat adrenal medulla: physiological role in controlling catecholamine secretion

1. The perfused adrenal gland of the rat was used to establish the identity of a non-cholinergic substance involved in splanchnic nerve-mediated secretion of catecholamines. 2. The perfused adrenal medulla was rich in vasoactive intestinal polypeptide (VIP) content (28 pmol g-1 of wet tissue). VIP-immunoreactive nerve fibres were present in the adrenal medulla and the adrenal cortex. 3. Field stimulation (10 Hz for 15 min plus 1 Hz for 15 min) caused a large increase in the output of VIP in the perfusate over the spontaneous release of VIP. Secretion of catecholamines was also greatly elevated by field stimulation. Field stimulation-evoked output of VIP and catecholamines was abolished after chronic denervation of the adrenal glands. 4. Infusion of acetylcholine (ACh) did not increase the output of VIP but caused a robust secretion of catecholamines. 5. The VIP output declined when the stimulation frequency was increased (8.6 x 10(-3) fmol pulse-1 at 1 Hz and 4.0 x 10(-3) fmol pulse-1 at 10 Hz). 6. In contrast, the output of 3H-acetylcholine (3H-ACh, expressed as a fraction of tissue 3H-ACh content) increased from 7.0 x 10(-2) pulse-1 at 1 Hz to 16.3 x 10(-2) pulse-1 at 10 Hz. 7. Secretion of catecholamines evoked by low-frequency stimulation (1 Hz) was reduced by 40% in the presence of cholinergic receptor antagonists (atropine plus hexamethonium). Inclusion of a VIP receptor antagonist ([Ac-Tyr1, D-Phe2]-GRF 1-29 amide) caused about 75% inhibition. 8. The VIP receptor antagonist inhibited VIP-evoked secretion of catecholamines without affecting ACh-evoked secretion. 9. In conclusion, VIP satisfies all the essential criteria to assume the role of a neurotransmitter in the rat adrenal medulla. The contribution of VIP to the secretion of adrenal medullary hormones is more prominent at low rates of neuronal activity whereas ACh is the major contributor at higher activity.

Regulation of proenkephalin synthesis in adrenal medullary chromaffin cells

The synthesis of proenkephalin was assessed in primary cultures of bovine adrenal medullary chromaffin cells by incubation of the cells with [35S]methionine, digestion of proenkephalin-derived peptides with trypsin and carboxy-peptidase B, and quantitation of radioactivity incorporated into Met-enkephalin following reversed-phase HPLC. Nicotine, histamine, and vasoactive intestinal peptide each enhanced the rate of proenkephalin synthesis approximately 10-fold when examined between 16 and 32 h after the drug or hormone addition. Inclusion of nifedipine (1 microM) partially blocked the stimulatory effect of nicotine, but not that of vasoactive intestinal peptide or histamine, or proenkephalin synthesis. Theophylline, tetrabenazine, and angiotensin II also increased the rate of proenkephalin synthesis (three- to eight-fold). These increases in the apparent rate of proenkephalin synthesis were not attributable to altered [35S]methionine specific radioactivity or rates of turnover and did not reflect similar increases in total protein synthesis. The half-life for turnover of Met-enkephalin sequences was 3-4 days in the cultured chromaffin cell. These studies directly show that proenkephalin synthesis is the primary regulatory step in control of chromaffin cell opioid peptide content.