Failure of beta-adrenergic receptors to modulate adrenal medullary secretion

Effects of alpha- and beta-adrenoceptor agonists and antagonists on ATP and catecholamine release and desensitization of the nicotinic response in bovine adrenal chromaffin cells

The effects of a number of alpha- and beta-adrenoceptor agonists and antagonists on the modulation of secretion from bovine adrenal chromaffin cells were investigated. Secretion was induced by nicotine, 56 mM K+, histamine or Ba2+ and was detected by the ATP luciferin-luciferase bioluminescence technique or by the measurement of endogenous catecholamines (CA) by HPLC coupled with electrochemical detection. ATP release from freshly isolated cells by 5 microM nicotine was only weakly inhibited by adrenaline and noradrenaline and even then required high concentrations (greater than 500 microM), while dopamine (1 microM-1 mM) and isoproterenol (100 microM) had no effect. Clonidine (100 microM), oxymetazoline (100 microM), yohimbine (100 microM), and propranolol (5 microM) all produced inhibition of nicotine-induced ATP release with the order of potency:propranolol greater than oxymetazoline greater than clonidine = yohimbine. The inhibitory effect by propranolol could not be reversed by high concentrations of adrenaline or isoproterenol. In chromaffin cell monolayer cultures, all alpha 2-adrenoceptor agents tested (clonidine, oxymetazoline and yohimbine), produced a dose-dependent, Na+-sensitive, non-competitive inhibition of nicotine-induced catecholamine release with little effect on the catecholamine release induced by K+ (56 mM), histamine (10 microM) or Ba2+ (2.2 mM). (+/-)Propranolol caused a similar pattern of inhibition, however, this inhibition was also observed by (+)propranolol, an isomer with little beta-adrenoceptor antagonist activity. The effects of clonidine and propranolol on desensitization of nicotine-induced CA secretion were also investigated. The degree of desensitization of the nicotinic response was dependent on the concentration of nicotine to which the cells were pre-exposed. Desensitization was detected as the decrease in response to a near EC50 concentration of nicotine (5 microM) following pre-incubation of cells to nicotine in the range of 0.3-300 microM. The desensitization had a threshold of 1 microM nicotine and was maximal at 3 microM nicotine in the pre-incubation. Both clonidine (50 microM) and (+/-)propranolol (5 microM) inhibited CA secretion induced by nicotine (0.3 microM-300 microM) during the pre-incubation period. However, regardless of this inhibition of secretion, neither clonidine nor propranolol had an effect on either the onset, or the rate of nicotine-evoked desensitization subsequently observed. These data suggest that inhibition of the nicotinic response and desensitization of the nicotinic response are regulated independently.(ABSTRACT TRUNCATED AT 400 WORDS)

Local modulation of adrenal catecholamines release by beta-2 adrenoceptors in the anaesthetized dog

The release of adrenal catecholamines into the adrenal vein elicited by splanchnic nerve stimulation, was evaluated in the presence of a beta-adrenoceptor agonist and both beta-1 and beta-2 adrenoceptor antagonists in anaesthetized and vagotomized dogs. Stimulations (0.5 V pulses of 2 ms duration for 3 min at 1 Hz) were applied before and after the i.v. infusion of the beta-adrenoceptor agonist, isoproterenol (0.1 microgram/kg/min). While maintaining the infusion of isoproterenol, either ICI 118551 (0.3 mg/kg), a selective beta-2 adrenoceptor antagonist, or 204-155 (0.2 mg/kg), a selective beta-1 adrenoceptor antagonist (Sandoz Co., Dorval, PQ, Canada), were injected intravenously and the stimulation was repeated. The results show that isoproterenol increased significantly both pre-stimulation basal levels and the stimulated release of catecholamines. These potentiated responses were significantly reversed by ICI 118551, but not by 204-155. These results suggest that the release of adrenal catecholamines is locally modulated by a positive feedback mechanism through activation of beta-2 adrenoceptors.

Histamine affects release and biosynthesis of opioid peptides primarily via H1-receptors in bovine chromaffin cells

Histamine is a potent secretagogue for opioid pentapeptides (Met- and Leu-enkephalin) in adrenal chromaffin cells in vitro. This effect is dependent on extracellular Ca2+ and is reduced by Ca2+ channel blockers such as Co2+, D 600, and nifedipine. Moreover, histamine also produced a profound compensatory increase in cellular peptide content after 48 h of exposure, most likely caused by a four- to fivefold increase in the mRNA levels coding for the proenkephalin A precursor. All the histamine-induced effects (acute release, changes in peptide cell content, proenkephalin A mRNA levels) are antagonized by the H1-receptor antagonist, clemastine, whereas the H2-receptor antagonists, ranitidine and cimetidine, were less effective (approximately 20% inhibition).

Presynaptic beta-adrenoceptors

The existence of facilitatory presynaptic beta-adrenoceptors has been shown in approximately 30 tissues of 6 different species including human. A positive feed back loop for further release of the transmitter appears to be activated by an endogenous agonist, epinephrine, taken up and released as a cotransmitter with norepinephrine rather than norepinephrine itself released from peripheral noradrenergic nerve terminals. Presynaptic beta-adrenoceptors are mainly of a beta 2-subtype. Some beta 1-subtype receptors are also suggested. There coexist presynaptic beta 1- and beta 2-adrenoceptors in cat and rat hypothalamus. Higher sensitivity of peripheral presynaptic beta-adrenoceptors to isoproterenol may be implicated in the early development of hypertension in SHR. Epinephrine taken up and released initiates the development of hypertension in rats via activation of these receptors. Increased activation of these receptors by epinephrine may play a role in the development of essential hypertension. The antihypertensive action of beta-antagonists may be in part due to blockade of these facilitatory presynaptic beta-adrenoceptors.

Forskolin enhancement of acetylcholine-evoked cyclic AMP formation and catecholamine release in perfused dog adrenals

Unstimulated efflux of cyclic AMP from perfused dog adrenal glands was not altered by 0.1 microM of forskolin and was slightly increased by 0.3 and 1.0 microM of forskolin. ACh stimulated efflux of cyclic AMP which preceded CA release and the efflux was dose-dependently enhanced by forskolin. Forskolin did not affect the spontaneous CA release but enhanced ACh-evoked catecholamine (CA) release. There was a close correlation between the dose relationship of forskolin enhancement of stimulated-cyclic AMP efflux and that of evoked-CA release. ACh-evoked CA release in the presence of forskolin was further potentiated by R020-1724, a phosphodiesterase inhibitor. CA release evoked by excess K+, or by caffeine in the presence or absence of external Ca2+ was also potentiated by forskolin. These results suggests that cyclic AMP generation may increase in response to stimulation of adrenal chromaffin cells and that the resulting increase of the nucleotide may function as a facilitating modulator of CA release.

Stimulation-evoked cyclic nucleotides efflux from isolated perfused dog adrenals and possible involvement of calcium

Nicotine and muscarine caused the transient increase in cAMP and cGMP efflux from dog adrenal glands followed by a small but lasting increase in the nucleotide efflux. The initial increase preceded catecholamine (CA) release and the latter slowly developed. Nicotine and muscarine caused the maximal increase of cAMP and cGMP levels in adrenal medulla 15 sec after the treatment, and these effects were antagonized by hexamethonium and atropine, respectively. Hexamethonium in combination with atropine, verapamil and an omission of Ca2+ in the medium prevented ACh from producing the increase in cyclic nucleotide efflux and CA release. Reintroduction of Ca2+ (1.3 mM) in fluid after perfusion with Ca2+ and Mg2+-free fluid caused the transient increase in cyclic nucleotide efflux and CA release. Adenylate cyclase activity in adrenal medulla was activated by Ca2+. These results may suggest that cAMP formation was increased by activation of adenylate cyclase as a result of increased influx of Ca2+ when adrenal medulla were stimulated.

Cyclic AMP inhibits secretion from bovine adrenal chromaffin cells evoked by carbamylcholine but not by high K+

The role of cAMP in the control of secretion from bovine adrenal chromaffin cells was examined using the adenylate cyclase activator, forskolin. Treatment of chromaffin cells with forskolin resulted in a rise in cAMP levels. Forskolin inhibited catecholamine release elicited by carbamylcholine or nicotine but had no effect on secretion evoked by 55 mM K+. Inhibition of carbamylcholine-stimulated release by forskolin was half-maximal at 10 microM forskolin. The inhibition by forskolin of secretion evoked by carbamylcholine was at a step distal to the rise in intracellular free calcium concentration ([Ca2+]i), since this rise was not inhibited by forskolin, which itself produced a small rise in [Ca2+]i. The results suggest that secretion evoked by carbamylcholine is due to the activation of an additional second messenger pathway acting with the rise in [Ca2+]i. This additional pathway may be the target for cAMP action.