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The aim of the present investigation was to assess the relative contributions of cholinergic (acetylcholine) and non-cholinergic vasoactive intestinal polypeptide (VIP), and pituitary adenylate cyclase activating polypeptide (PACAP) neurotransmitters in the neuronal control of catecholamine secretion from the chromaffin tissue lining the posterior cardinal vein of the rainbow trout (Oncorhynchus mykiss). Using an in situ saline-perfused posterior cardinal vein preparation, it was demonstrated that exogenous administration of chicken VIP or human PACAP-27 caused a dose-dependent increase in adrenaline secretion; noradrenaline secretion was unaffected. Analysis of dose-response curves indicated that VIP and PACAP stimulated the secretion of adrenaline with a similar degree of potency (ED(50) for VIP=1.90x10(-11) mol/kg; ED(50) for PACAP=1.03x10(-11) mol/kg). The VIP/PACAP-elicited secretion was diminished in the presence of the VIP receptor antagonist, VIP 6-28, but was unaffected by the PACAP receptor antagonist, PACAP 6-27, or the cholinergic antagonists, hexamethonium and atropine. Thus, this is the first study to demonstrate a direct stimulatory role for VIP or PACAP in catecholamine secretion from piscine chromaffin cells. The relative contribution of cholinergic and non-cholinergic neurotransmitters in the neuronal control of catecholamine secretion from the chromaffin tissue was evaluated using an in situ nerve-stimulating technique previously validated by us in the rainbow trout. This was accomplished by comparing catecholamine secretion in the presence or absence of cholinergic and the VIP and PACAP receptor antagonists during different levels of electrical stimulation. The results demonstrated that cholinergic stimulation predominated during high frequency of electrical stimulation (20 Hz) while the non-cholinergic component prevailed at low frequency (1 Hz). Overall, the results of the present investigation demonstrate that VIP and/or PACAP may directly stimulate adrenaline secretion from trout chromaffin cells at low levels of neuronal activity. Therefore, the neuronal control of catecholamine secretion in teleosts may not be confined to cholinergic-evoked events.

The interaction between extracellular catecholamines and catecholamine secretion from chromaffin cells was assessed in rainbow trout (Oncorhynchus mykiss) using an in situ saline-perfused posterior cardinal vein preparation. This was accomplished by comparing the effects of adrenergic receptor agonists and antagonists on stimulus-evoked secretion. An acute bolus injection or extended perfusion with saline containing high levels of either noradrenaline or adrenaline did not affect the baseline secretion of catecholamines. However, catecholamine secretion in response to a bolus injection of the general cholinergic receptor agonist carbachol or electrical stimulation of the nerves innervating the chromaffin cells was abolished or reduced respectively, in preparations perfused with saline containing either catecholamine. To characterize the catecholaminergic inhibition of catecholamine release, secretion in response to carbachol and electrical stimulation was compared in preparations perfused with the adrenergic receptor agonists dobutamine (beta(1)), salbutamol (beta(2)), phenylephrine (alpha(1)) or clonidine (alpha(2)). Prior treatment with dobutamine or phenylephrine was without effect on baseline catecholamine secretion or stimulus-evoked secretion. In contrast, pre-treatment with salbutamol significantly inhibited catecholamine secretion in response to carbachol or electrical stimulation. Pre-treatment with clonidine did not affect carbachol-evoked secretion but did reduce catecholamine secretion during electrical stimulation. The significance of this adrenergic mechanism of regulating stimulus-evoked catecholamine secretion was further established using the adrenergic receptor antagonists nadolol (beta) or phentolamine (alpha). Catecholamine secretion in response to cholinergic stimulation was significantly enhanced in preparations perfused with saline containing nadolol. Furthermore, pre-treatment with phentolamine significantly enhanced adrenaline secretion in response to neuronal stimulation. These results suggest that the mechanisms of adrenergic inhibition of catecholamine secretion from trout chromaffin cells include activation of chromaffin cell membrane beta(2)-receptors and presynaptic alpha(2)-adrenergic receptors.

The individual contributions of, and potential interactions between, the renin-angiotensin system (RAS) and the humoral adrenergic stress response to blood pressure regulation were examined in rainbow trout. Intravenous injection of the smooth muscle relaxant, papaverine (10 mg/kg), elicited a transient decrease in dorsal aortic blood pressure (PDA) and systemic vascular resistance (RS), and significant increases in plasma angiotensin II (Ang II) and catecholamine concentrations. Blockade of alpha-adrenoceptors before papaverine treatment prevented PDA and RS recovery, had no effect on the increase in plasma catecholamines, and resulted in greater plasma Ang II concentrations. Administration of the angiotensin-converting enzyme inhibitor, lisinopril (10(-4) mol/kg), before papaverine treatment attenuated the increases in the plasma concentrations of Ang II, adrenaline, and noradrenaline by 90, 79, and 40%, respectively and also prevented PDA and RS recovery. By itself, lisinopril treatment caused a gradual and sustained decrease in PDA and RS, and reductions in basal plasma Ang II and adrenaline concentrations. Bolus injection of a catecholamine cocktail (4 nmol/kg noradrenaline plus 40 nmol/kg adrenaline) in the lisinopril+papaverine-treated trout, to supplement their circulating catecholamine concentrations and mimic those observed in fish treated only with papaverine, resulted in a temporary recovery in PDA and RS. These results indicate that the RAS and the acute humoral adrenergic response are both recruited during an acute hypotensive stress, and have important roles in the compensatory response to hypotension in rainbow trout. However, whereas the contribution of the RAS to PDA recovery is largely indirect and relies on an Ang II-mediated secretion of catecholamines, the contribution from the adrenergic system is direct and relies at least in part on plasma catecholamines.