PACAP-38 activates parasympathetic nerves in isolated, blood-perfused dog atria

PAC₁ receptors mediate positive chronotropic responses to PACAP-27 and VIP in isolated mouse atria

PACAP and VIP have prominent effects on cardiac function in several species, but little is known about their influence on the murine heart. Accordingly, we evaluated the expression of PACAP/VIP receptors in mouse heart and the response of isolated atria to peptide agonists. Quantitative PCR demonstrated that PAC₁, VPAC₁, and VPAC₂ receptor mRNAs are present throughout the mouse heart. Expression of all three receptor transcripts was low, PAC₁ being the lowest. No regional differences in expression were detected for individual receptor mRNAs after normalization to L32. Pharmacological effects of PACAP-27, VIP, and the selective PAC₁ agonist maxadilan were evaluated in isolated, spontaneously beating atria from C57BL/6 mice of either sex. Incremental additions of PACAP-27 at 1 min intervals caused a concentration-dependent tachycardia with a logEC₅₀=-9.08 ± 0.15 M (n=7) and a maximum of 96.3 ± 5.9% above baseline heart rate. VIP and maxadilan also caused tachycardia but their potencies were about two orders of magnitude less. Increasing the dosing interval to 5 min caused a leftward shift of the concentration-response curve to maxadilan but no changes in the curves for PACAP-27 or VIP. Under this condition, neither the potency nor the efficacy of maxadilan differed from those of PACAP-27. Neither PACAP-27 nor maxadilan caused tachyphylaxis, and maximal responses to maxadilan were maintained for at least 2 h. We conclude that all three VIP/PACAP family receptors are expressed by mouse cardiac tissue, but only PAC₁ receptors mediate positive chronotropic responses to PACAP-27 and VIP.

Alternative splicing of the pituitary adenylate cyclase-activating polypetide (PACAP) receptor contributes to function of PACAP-27

Pituitary adenylate cyclase-activating polypeptide (PACAP)-27 and PACAP-38 are neuropeptides performing a variety of physiological functions. The PACAP-specific receptor PAC1 has several variants that result mainly from alternative splicing in the mRNA region encoding the first extracellular (EC1) domain and the third intracellular cytoplasmic (IC3) loop. To characterize the molecular forms of alternative splicing variants of PAC1, we examined the binding affinity and activation of two major second messenger pathways (cAMP production and changes in [Ca(2+)]( i )) by PACAP-27. Activation of cAMP was influenced by the variant in both of the EC1 domain and IC3 loops. In the N form in the EC1 domain, the suppressive effect of the HOP1 form in the IC3 loop was enhanced. Regarding the intracellular calcium mobilization assay, the rank order of the potency of PACAP-27 for the different PAC1 isoforms was S/HOP1>>N/R~S/R>>N/HOP1. In particular, PACAP-27 exhibited remarkable potency of calcium mobilization in the S/HOP1-expressing cells at sub-picomolar concentrations even though the affinities of PACAP-27 to the four PAC1 isoforms were not significantly different. This suggests the specific functions of PACAP-27 due to PACAP-27 preferring PAC1 activation, and leads in clarification of the pleiotoropic function of PACAP.

cAMP produced by pituitary adenylate cyclase-activating polypeptide 27 inhibits atrial natriuretic peptide secretion in rabbit beating atria

The aim of the present study was to determine the effects of increased cAMP levels in response to pituitary adenylate cyclase-activating polypeptide 27 (PACAP27) on atrial atrial natriuretic peptide (ANP) secretion in rabbit atria. A perfused beating atrial model was used in the present study and cAMP efflux and ANP levels in atrial perfusates were measured by radioimmnoassay. At 100 nmol/L, PACAP27 increased cAMP production, which resulted in subsequent inhibition of ANP secretion. Nicardipine (1.0 micromol/L), an L-type Ca2+ channel blocker, attenuated inhibition of ANP secretion by PACAP27. Staurosporine (1.0 micromol/L), a non-specific protein kinase inhibitor, and H-89 (1.0 micromol/L), a cAMP-dependent protein kinase A (PKA) inhibitor, completely blocked the inhibition of ANP secretion in response to PACAP27 but had no effect on PACAP27-induced increases in cAMP. In conclusion, the results suggest that increased cAMP levels in response to PACAP27 negatively regulate ANP secretion via the adenylate cyclase-cAMP-PKA signalling pathway in rabbit atria and that L-type Ca2+ channels may be involved, in part, in the regulation of ANP secretion by cAMP.

Pituitary adenylate cyclase-activating polypeptide: localization and differential influence on isolated hearts from rats and guinea pigs

This study was done to determine if pituitary adenylate cyclase-activating peptide (PACAP)-immunoreactive nerve fibers occur in cardiac muscle as well as intracardiac ganglia of rats and guinea pigs and to clarify the chronotropic actions of PACAP27 in the same species using isolated heart preparations. PACAP nerve fibers were not detected in atrial or ventricular muscle of rat or guinea pig but a few stained nerve fibers occurred in the atrioventricular bundle of the guinea pig. Stained nerve fibers were prominent in intracardiac ganglia of both species. PACAP27 caused a dose-dependent tachycardia in isolated rat hearts (+39 +/- 3 beats/min with 1 nmol, n = 6). Positive and/or negative chronotropic responses were evoked by PACAP27 in guinea pig heart, depending on dose and prior exposure to the peptide. PACAP27 also caused arrhythmias in several guinea pig hearts. Treatment with atropine eliminated or prevented PACAP-evoked bradycardia and arrhythmias, implicating cholinergic neurons in these responses. Positive chronotropic responses to PACAP were unaffected by beta-adrenergic receptor blockade in either species, suggesting that tachycardia resulted from a direct action on the heart. These observations support the conclusion that endogenous PACAP could have a role in regulating parasympathetic input to the heart but through different mechanisms in rats versus guinea pigs. A direct positive chronotropic influence of endogenous PACAP is unlikely since atrial muscle lacks PACAP-immunoreactive nerve fibers.

VPAC Receptor Modulation of Neuroexcitability in Intracardiac Neurons

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) have been found within mammalian intracardiac ganglia, but the cellular effects of these neuropeptides remain poorly understood. Fluorometric calcium imaging and whole cell patch clamp recordings were used to examine the effects of PACAP and VIP on [Ca2+]i and neuroexcitability, respectively, in intracardiac neurons of neonatal rats. PACAP and VIP evoked rapid increases in [Ca2+]i that exhibited both transient and sustained components. Pharmacological experiments using PAC1 and VPAC receptor-selective antagonists demonstrated that the elevations in [Ca2+]i result from the activation of VPAC receptors. The transient increases in [Ca2+]i were shown to be the product of Ca2+ mobilization from caffeine/ryanodine-sensitive intracellular stores and were not due to inositol 1,4,5-trisphosphate-mediated calcium release. In contrast, the sustained [Ca2+]i elevations were dependent on extracellular Ca2+ and were blocked by the transient receptor channel antagonist, 2-aminoethoxydiphenyl borate, which suggests that they are due to Ca2+ entry via store-operated channels. In addition to elevating [Ca2+]i, both PACAP and VIP depolarized intracardiac neurons, and PACAP was further shown to augment action potential firing in these cells. Depolarization of intracardiac neurons by the neuropeptides was dependent on activation of VPAC receptors and the concomitant increases in [Ca2+]i. Although activation of PAC1 receptors alone had no direct effects on neuroexcitability, PAC1 receptor stimulation potentiated the VPAC receptor-induced depolarizations. Furthermore, enhanced action potential firing was only observed upon concurrent stimulation of PAC1 and VPAC receptors, which indicates that these receptors act synergistically to enhance neuroexcitability in intracardiac neurons.

Effects of PACAP(1-27) on the canine endocrine pancreas in vivo: interaction with cholinergic mechanism

The aim of the present study was to characterize the effects of pituitary adenylate cyclase activating polypeptide (PACAP) on the endocrine pancreas in anesthetized dogs. PACAP(1-27) and a PACAP receptor (PAC(1)) blocker, PACAP(6-27), were locally administered to the pancreas. PACAP(1-27) (0.005-5 microg) increased basal insulin and glucagon secretion in a dose-dependent manner. PACAP(6-27) (200 microg) blocked the glucagon response to PACAP(1-27) (0.5 microg) by about 80%, while the insulin response remained unchanged. With a higher dose of PACAP(6-27) (500 microg), both responses to PACAP(1-27) were inhibited by more than 80%. In the presence of atropine with an equivalent dose (128.2 microg) of PACAP(6-27) (500 microg) on a molar basis, the insulin response to PACAP(1-27) was diminished by about 20%, while the glucagon response was enhanced by about 80%. The PACAP(1-27)-induced increase in pancreatic venous blood flow was blocked by PACAP(6-27) but not by atropine. The study suggests that the endocrine secretagogue effect of PACAP(1-27) is primarily mediated by the PAC(1) receptor, and that PACAP(1-27) may interact with muscarinic receptor function in PACAP-induced insulin and glucagon secretion in the canine pancreas in vivo.

The enhancement of endogenous cAMP with pituitary adenylate cyclase-activating polypeptide protects rat kidney against ischemia through the modulation of inflammatory response

BACKGROUND

Cyclic nucleotide analogue administration improves ischemia-reperfusion damage in several organs. The neuropeptide pituitary adenylate cyclase-activating polypeptide, PACAP-38, is a potent stimulus to enhance cellular cAMP levels. This study tested the protective effect of enhancing endogenous cAMP levels by PACAP-38 in a model of warm renal ischemia.

METHODS

Sprague-Dawley rats underwent 40 min of bilateral warm renal ischemia. PACAP-38 continuous infusion began either before ischemia or at 6 hr or 18 hr after ischemia. A mini-osmotic pump infused PACAP-38 throughout 7 days of follow-up. Groups were constructed with sham, ischemic control, and dibutyryl cAMP treated animals, and four PACAP-38 treatment groups, using 16 pmol/hr or 160 pmol/hr of the compound, or delaying its administration by 6 hr or 18 hr after ischemia. Renal function was assessed by means of serum creatinine levels on days 1, 2, 3, and 7 after ischemia. Conventional histology was performed on day 7. Renal myeloperoxidase (MPO) activity, infiltrating CD45+ cells, plasma and tissue cAMP, and serum IL-6 were measured.

RESULTS

Continuous administration of the high concentration of PACAP-38 ameliorated renal function and morphologic abnormalities induced by warm ischemia. Treatment with dibutyryl cAMP produced morphologic protection but only partial functional effect on the ischemic kidney. A 6-hour delay in the administration of the compound after ischemia offered similar protective effect, whereas an 18-hr delay did not. The neuropeptide clearly increased circulating cAMP after ischemia but not cAMP in renal tissue. PACAP-38 increased circulating IL-6, and minimized renal inflammatory cell infiltration induced by ischemia-reperfusion injury, as evidenced by a reduction of MPO activity and the number of CD45+ cells in ischemic renal tissue.

CONCLUSIONS

Enhancement of endogenous circulating cAMP with PACAP-38 modulates postischemic inflammatory response and strongly protects from ischemic acute renal failure, even when administration is delayed for 6 hr after injury.

The origin and function of the pituitary adenylate cyclase-activating polypeptide (PACAP)/glucagon superfamily

The pituitary adenylate cyclase-activating polypeptide (PACAP)/ glucagon superfamily includes nine hormones in humans that are related by structure, distribution (especially the brain and gut), function (often by activation of cAMP), and receptors (a subset of seven-transmembrane receptors). The nine hormones include glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, glucose-dependent insulinotropic polypeptide (GIP), GH-releasing hormone (GRF), peptide histidine-methionine (PHM), PACAP, secretin, and vasoactive intestinal polypeptide (VIP). The origin of the ancestral superfamily members is at least as old as the invertebrates; the most ancient and tightly conserved members are PACAP and glucagon. Evidence to date suggests the superfamily began with a gene or exon duplication and then continued to diverge with some gene duplications in vertebrates. The function of PACAP is considered in detail because it is newly (1989) discovered; it is tightly conserved (96% over 700 million years); and it is probably the ancestral molecule. The diverse functions of PACAP include regulation of proliferation, differentiation, and apoptosis in some cell populations. In addition, PACAP regulates metabolism and the cardiovascular, endocrine, and immune systems, although the physiological event(s) that coordinates PACAP responses remains to be identified.

Effects of pituitary adenylate cyclase activating polypeptide27 on cyclic AMP efflux and atrial dynamics in perfused beating atria

Although pituitary adenylate cyclase activating polypeptide (PACAP) has been shown to increase cardiac force of contraction and to change the heart rate, the effect of PACAP on cyclic (c) AMP production in the atrium still has to be defined. In the present experiments, a simple protocol was developed for the evaluation of cAMP production in real-time base in the perfused beating left atria. The PACAP27-induced cAMP efflux in the atrial perfusate reflected changes in the production of cAMP in the atrial tissue. cAMP efflux was measured as an indicator of cAMP production in beating perfused rabbit atria. PACAP27 increased cAMP production in a dose- and time-dependent manner with a minor effect on atrial dynamics. These results suggest that PACAP27 has other roles besides control of force of contraction through cAMP production in the atrium.

VIP and PACAP potentiation of nicotinic ACh-evoked currents in rat parasympathetic neurons is mediated by G-protein activation

The effects of vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP27 and PACAP38) on isolated parasympathetic neurons of rat intracardiac and submandibular ganglia were examined under voltage clamp using whole-cell patch-clamp recording techniques. VIP and PACAP (</= 10 nM) selectively and reversibly increased the affinity of nicotinic acetylcholine receptor channels (nAChRs) for their agonists resulting in a potentiation of acetylcholine (ACh)-evoked whole-cell currents at low agonist concentrations. VIP-induced potentiation was observed with either ACh or nicotine as the cholinergic agonist. The VIP- but not the PACAP-induced potentiation of ACh-evoked currents was inhibited by [Ac-Tyr1, D-Phe2]-GRF 1-29, amide (100 nM), a selective antagonist of VPAC1 and VPAC2 receptors; whereas the PACAP38- but not the VIP-induced potentiation was inhibited by 100 nM PACAP6-38, a PAC1 and VPAC2 receptor antagonist. The signal transduction pathway mediating VIP- and PACAP-induced potentiation of nicotinic ACh-evoked currents involves a pertussis toxin (PTX)-sensitive G-protein. Intracellular application of 200 microM GTPgammaS or GDPbetaS inhibited VIP-induced potentiation of ACh-evoked whole-cell currents. GTPgammaS alone potentiated ACh- and nicotine-evoked currents and the magnitude of these currents was not further increased by VIP or PACAP. The G-protein subtype modulating the neuronal nAChRs was examined by intracellular dialysis with antibodies directed against alphao, alphai-1,2, alphai-3 or beta G-protein subunits. Only the anti-Galphao and anti-Gbeta antibodies significantly inhibited the effect of VIP and PACAP on ACh-evoked currents. The potentiation of ACh-evoked currents by VIP and PACAP may be mediated by a membrane-delimited signal transduction cascade involving the PTX-sensitive Go protein.

Enhanced reactivity of the adrenal medulla in response to pituitary adenylate cyclase activating polypeptide1-27 (PACAP) during insulin-induced hypoglycemia in anesthetized dogs

The present study was to test the hypothesis that the reactivity of the adrenal medulla to pituitary adenylate cyclase activating polypeptide1-27 (PACAP27) is enhanced during insulin-induced hypoglycemia (IIH) in anesthetized dogs. Plasma catecholamine (CA) concentrations in adrenal venous and aortic blood were determined by an HPLC method coupled with electrochemical detection, and the plasma glucose concentration in aortic blood was measured using a glucometer. PACAP27 (25 ng) was administered locally via the adrenolumbar artery to the left adrenal gland. The resulting CA responses were compared before and during IIH following an intravenous bolus injection of insulin (0.15 IU/kg, iv). In the first group with normal adrenal innervation, the basal adrenal CA secretion gradually increased, reaching a maximum level 45 min after the insulin injection. The net increase in PACAP27-induced CA secretion was significantly greater 30, 45, and 60 min after the induction of hypoglycemia, compared with the initial net response to PACAP27 observed before insulin injection. In the second group receiving local adrenal denervation, neither the basal CA secretion nor the net CA response to PACAP27 significantly increased despite the presence of IIH, which developed to an extent similar to that found in the first group. In the third group, which was the normoglycemic control group, both the basal CA secretion and the net CA response to PACAP27 remained unchanged during the experimental period. The results indicate that the adrenomedullary reactivity to PACAP27 was significantly enhanced during IIH only when the sympathoadrenal system was activated. The present study suggests that PACAP27 may play a beneficial role in glucose counterregulatory mechanisms in the adrenal medulla during hypoglycemia.Key words: catecholamine, counterregulation, denervation, secretion, splanchnic, sympathoadrenal.

Modulation of adrenal catecholamine release by PACAP in vivo

The aim of the present study was to investigate whether pituitary adenylate cyclase-activating polypeptide-(1-27) (PACAP27) can modulate the adrenal catecholamine (CA) secretion induced by splanchnic nerve stimulation (SNS) and by exogenous acetylcholine (ACh) in anesthetized dogs. Plasma CA concentrations in adrenal venous and aortic blood were quantified by a high-performance liquid chromatography coupled with electrochemical detection. Adrenal venous blood flow was measured by gravimetry. Local infusion of PACAP27 (0.5, 5, and 50 ng) to the left adrenal gland via the adrenolumbar artery resulted in an increase in CA output, reaching a significant level at the highest dose tested. Either direct SNS (2 Hz) or local infusion of ACh (0.5 microgram) to the left adrenal gland produced significant increases in CA output to an extent similar to that obtained with 50 ng of PACAP27 alone. In the presence of PACAP27 (50 ng), CA responses to either SNS or exogenous ACh were significantly potentiated by approximately four- and sixfold, respectively, compared with those obtained in response to each stimulus alone. However, the enhanced CA responses to ACh were not significantly different from those to SNS. The results indicate that the increase in adrenal CA secretion, induced by either direct SNS or exogenous ACh, is synergistically enhanced by PACAP27. The study suggests that the enhanced CA secretion may result from the activation of a PACAP-mediated facilitatory mechanism(s) localized presumably at the postsynaptic level in the canine adrenal medulla in vivo, although the possible involvement of presynaptic mechanisms cannot completely be ruled out in the present study.

Pituitary adenylate cyclase-activating polypeptide expression and modulation of neuronal excitability in guinea pig cardiac ganglia

Cardiac output is regulated by the coordinate interactions of stimulatory sympathetic and inhibitory parasympathetic signals. Intracardiac parasympathetic ganglia are integrative centers of cardiac regulation, and modulation of the parasympathetic drive on the heart is accomplished by altering intrinsic cardiac ganglion neuron excitability. The pituitary adenylate cyclase-activating polypeptide (PACAP)/vasoactive intestinal peptide (VIP) family of peptides modulates cardiac function, and in guinea pig heart, PACAP appears to act directly on intrinsic parasympathetic cardiac ganglia neurons through PACAP-selective receptors. A multidisciplinary project tested whether cardiac PACAP peptides act through PACAP-selective receptors as excitatory neuromodulators amplifying the parasympathetic inhibition from guinea pig cardiac ganglia. The in vivo sources of regulatory PACAP peptides were localized immunocytochemically to neuronal fibers and a subpopulation of intrinsic postganglionic cardiac neurons. RT-PCR confirmed that cardiac ganglia expressed proPACAP transcripts and have PACAP peptide biosynthetic capabilities. Messenger RNA encoding PACAP-selective PAC1 receptor isoforms were also present in cardiac ganglia. Alternative splicing of PAC1 receptor transcripts produced predominant expression of the very short variant with neither HIP nor HOP cassettes; lower levels of the PAC1HOP2 receptor mRNA were present. Almost all of the parasympathetic neurons expressed membrane-associated PAC1 receptor proteins, localized immunocytochemically, which correlated with the population of cells that responded physiologically to PACAP peptides. PACAP depolarized cardiac ganglia neurons and increased neuronal membrane excitability. The rank order of peptide potency on membrane excitability in response to depolarizing currents was PACAP27>PACAP38>VIP. The PACAP-induced increase in excitability was not a function of membrane depolarization nor was it caused by alterations in action potential configuration. These results support roles for PACAP peptides as integrative modulators amplifying, through PACAP-selective receptors, the parasympathetic cardiac ganglia inhibition of cardiac output.

Regional differences in cardiac effects of pituitary adenylate cyclase-activating polypeptide-27 in the isolated dog heart

Recent observations indicate that several neuropeptides may be involved in the regulation of cardiac function, but the effects of these peptides on the atrium are not always the same as those on the ventricle. To compare the effect of pituitary adenylate cyclase-activating polypeptide (PACAP)-27 on the atrium with that on the ventricle, we investigated the effects of PACAP-27 on the sinus rate and atrial and ventricular contractility in isolated, blood-perfused dog heart preparations. PACAP-27 (0.01-0.3 nmol) caused transient positive followed by negative chronotropic and inotropic responses in a dose-dependent manner in the isolated right atrium, whereas it caused only a dose-dependent positive inotropic response in the left ventricle. After atropine treatment, PACAP-27 caused only positive cardiac responses in isolated atria. The order of the increase in response to PACAP-27 was atrial contractile force > sinus rate > or = ventricular contractile force. Tetrodotoxin blocked the negative chronotropic and inotropic responses to PACAP-27 in isolated atria. Propranolol did not affect the positive response. PACAP-(6-27), a type I PACAP receptor antagonist, attenuated the positive responses similarly in the atropine-treated right atrium and the left ventricle. Thus, we demonstrated that (1) PACAP-27 caused negative cardiac effects in the atrium and sinoatrial node by activation of intracardiac parasympathetic nerves, but had no negative effect on the ventricle; (2) PACAP-27 had positive effects in the atrium, sinoatrial node and ventricle mediated by type I PACAP receptors, but PACAP-27 was more effective in the atrium and sinoatrial node than in the ventricle of the dog heart.

PACAP-27 causes negative and positive dromotropic effects in anesthetized dogs

While pituitary adenylate cyclase-activating polypeptide (PACAP) has been identified radioimmunologically in the rat heart, the physiological role of PACAP has not been elucidated in the regulation of the atrioventricular conduction in the heart. We, therefore, determined the dromotropic effects of PACAP-27 injected into the cannulated atrioventricular node artery in the autonomically decentralized heart of the open-chest, anesthetized dog. PACAP-27 caused transient positive followed by negative dromotropic responses in a dose-dependent manner, whereas vasoactive intestinal peptide (VIP) caused only a positive dromotropic response. Atropine and tetrodotoxin blocked the negative dromotropic response to PACAP-27 and after blockade PACAP-27 caused only a positive dromotropic response. Tetrodotoxin and propranolol did not affect the positive dromotropic response to PACAP-27 in atropine-treated dogs. PACAP-27 altered the atrio-His bundle interval but did not alter the His-ventricle interval. These results demonstrate that PACAP-27 prolongs the atrio-His bundle interval due to the liberation of acetylcholine from parasympathetic nerves and decreases it by a non-adrenergic mechanism in the dog heart in situ.

Role of L-type Ca2+ channel in PACAP-induced adrenal catecholamine release in vivo

The aim of the present study was to investigate whether the dihydropyridine-sensitive L-type Ca2+ channel is operative in adrenal catecholamine (CA) secretion induced by a novel neuropeptide, pituitary adenylate cyclase-activating polypeptide (PACAP), in anesthetized dogs. Plasma CA concentrations in adrenal venous and aortic blood were determined by a high-performance liquid chromatography method. All drugs tested were locally infused into the left adrenal gland via the left adrenolumbar artery. PACAP, with the isoform consisting of 27 (PACAP-27) and 38 (PACAP-38) amino acid residues, significantly increased CA output in a dose-dependent manner, with doses ranging from 5 to 500 ng and 7 to 700 ng, respectively. However, the amplitude of epinephrine response to PACAP-27 was three times greater than that obtained with PACAP-38 at the highest dose tested. In a separate group, a single dose of PACAP-27 (50 ng) induced highly reproducible CA responses when the same dose was repeated with an interval of 35 min. In dogs treated with nifedipine (50 microg), 5 min before the second administration of PACAP-27, the net CA response was significantly inhibited by approximately 50% compared with that obtained in the presence of vehicle. A similar CA response to BAY K 8644 (5 microg) was completely abolished by the same dose of nifedipine. The present results indicate that both PACAP-27 and PACAP-38 have the direct local secretagogue effect on the adrenal medulla in vivo and that CA responses to PACAP-27 were greater than those observed with PACAP-38 at equivalent mole doses. The study suggests that the dihydropyridine-sensitive L-type Ca2+ channel is functionally involved in PACAP-induced adrenal CA secretion in the canine adrenal medulla in vivo.