The adenosine analogue N6-L-phenylisopropyladenosine inhibits catecholamine secretion from bovine adrenal medulla cells by inhibiting calcium influx

Functional distribution of Ca2+-coupled P2 purinergic receptors among adrenergic and noradrenergic bovine adrenal chromaffin cells

BACKGROUND

Adrenal chromaffin cells mediate acute responses to stress through the release of epinephrine. Chromaffin cell function is regulated by several receptors, present both in adrenergic (AD) and noradrenergic (NA) cells. Extracellular ATP exerts excitatory and inhibitory actions on chromaffin cells via ionotropic (P2X) and metabotropic (P2Y) receptors. We have taken advantage of the actions of the purinergic agonists ATP and UTP on cytosolic free Ca2+ concentration ([Ca2+]i) to determine whether P2X and P2Y receptors might be asymmetrically distributed among AD and NA chromaffin cells.

RESULTS

The [Ca2+]i and the [Na+]i were recorded from immunolabeled bovine chromaffin cells by single-cell fluorescence imaging. Among the ATP-sensitive cells ~40% did not yield [Ca2+]i responses to ATP in the absence of extracellular Ca2+ (Ca2+o), indicating that they expressed P2X receptors and did not express Ca2+- mobilizing P2Y receptors; the remainder expressed Ca2+-mobilizing P2Y receptors. Relative to AD-cells approximately twice as many NA-cells expressed P2X receptors while not expressing Ca2+- mobilizing P2Y receptors, as indicated by the proportion of cells lacking [Ca2+]i responses and exhibiting [Na+]i responses to ATP in the absence and presence of Ca2+o, respectively. The density of P2X receptors in NA-cells appeared to be 30-50% larger, as suggested by comparing the average size of the [Na+]i and [Ca2+]i responses to ATP. Conversely, approximately twice as many AD-cells expressed Ca2+-mobilizing P2Y receptors, and they appeared to exhibit a higher (~20%) receptor density. UTP raised the [Ca2+]i in a fraction of the cells and did not raise the [Na+]i in any of the cells tested, confirming its specificity as a P2Y agonist. The cell density of UTP-sensitive P2Y receptors did not appear to vary among AD- and NA-cells.

CONCLUSION

Although neither of the major purinoceptor types can be ascribed to a particular cell phenotype, P2X and Ca2+-mobilizing P2Y receptors are preferentially located to noradrenergic and adrenergic chromaffin cells, respectively. ATP might, in addition to an UTP-sensitive P2Y receptor, activate an UTP-insensitive P2Y receptor subtype. A model for a short-loop feedback interaction is presented whereby locally released ATP acts upon P2Y receptors in adrenergic cells, inhibiting Ca2+ influx and contributing to terminate evoked epinephrine secretion.

The adrenergic stress response in fish: control of catecholamine storage and release

In fish, the catecholamine hormones adrenaline and noradrenaline are released into the circulation, from chromaffin cells, during numerous 'stressful' situations. The physiological and biochemical actions of these hormones (the efferent adrenergic response) have been the focus of numerous investigations over the past several decades. However, until recently, few studies have examined aspects involved in controlling/modulating catecholamine storage and release in fish. This review provides a detailed account of the afferent limb of the adrenergic response in fish, from the biosynthesis of catecholamines to the exocytotic release of these hormones from the chromaffin cells. The emphasis is on three particular topics: (1) catecholamine biosynthesis and storage within the chromaffin cells including the different types of chromaffin cells and their varying arrangement amongst species; (2) situations eliciting the secretion of catecholamines (e.g. hypoxia, hypercapnia, chasing); (3) cholinergic and non-cholinergic (i.e. serotonin, adrenocorticotropic hormone, angiotensin, adenosine) control of catecholamine secretion. As such, this review will demonstrate that the control of catecholamine storage and release in fish chromaffin cells is a complex processes involving regulation via numerous hormones, neurotransmitters and second messenger systems.

Effect of aging on A1-adenosine receptor-mediated inhibition of norepinephrine release in the rat heart

Adenosine inhibits norepinephrine (NE) release from cardiac adrenergic nerves and reduces the postsynaptic beta-adrenergic mediated actions of NE, leading to decreased myocardial force of contraction. The actions of adenosine are mediated by pre- and postsynaptic adenosine A1 receptors (A1-AdoR). We reported that adenosine inhibition of postsynaptic beta-adrenergic receptor-mediated cyclic adenosine monophosphate (cAMP) production declines with age in male F344 rat hearts. In this study, cardiac synaptosomes, isolated intact adrenergic nerve terminals, were used to examine the effect of age on adenosine inhibition of NE release. Cardiac synaptosomes were prepared from the hearts of 6- and 24-month-old male F344 rats, loaded with [3H]NE, and placed in a superfusion system. [3H]NE release was induced by high [K+] exposure in the presence of varying concentrations of adenosine or the specific A1-AdoR agonist, N6-p-sulfophenyladenosine (SPA). [3H]NE release was significantly reduced in old rats compared with young rats. Inhibition of [3H]NE release by adenosine and SPA was significantly greater in young rats compared with old rats. The A1-AdoR antagonist, 8-(p-sulfophenyl)-theophylline, blocked the actions of adenosine on [3H]NE release, and the specific adenosine A2-receptor agonist, cyclopropylcarboxamidoadenosine, had no effect on [3H]NE release. Our data suggest that presynaptic A1-AdoR-mediated inhibition of NE release in the rat heart declines with age.

Modulation of calcium current by ATP in guinea-pig adrenal chromaffin cells

The effects of extracellular adenosine 5'-triphosphate (ATP) on voltage-dependent Ca2+ currents were examined using the whole-cell voltage-clamp technique in guinea-pig isolated adrenal chromaffin cells. ATP (500 microM) reversibly suppressed Ca2+ currents in the presence of 5mM Ca2+ in the extracellular solution. The inhibitory effect of ATP on Ca2+ currents tended to increase with increases in the peak amplitude of ATP-evoked current when the intracellular solution contained 0.1 or 1 mM ethylenebis(oxonitrilo)tetraacetate(EGTA). Using the intracellular solution containing 10mM EGTA, on the other hand, the inhibitory effect did not change regardless of the amplitude of current responses to ATP. In the presence of 10 mM Ba2+, ATP (100 microM) reduced Ba2+ currents in a manner similar to Ca2+ currents. This reduction was decreased by dialysis of cells with the internal solution containing guanosine 5'-O-(2-thiodiphosphate) (GDP [beta-S]; 1 mM) or guanosine 5'-O-(3-thiotriphosphate) (GTP [gamma-S]; 100 microM). A depolarizing prepulse to + 100 mV partly relieved ATP-induced reduction of Ba2+ currents. ADP, AMP and adenosine also reduced Ba2+ currents and the effect of adenosine was the most potent. Adenosine (0.5 and 1 mM) significantly inhibited adrenaline secretion induced by nicotine (50 microM). These results suggest that Ca2+ entry through ATP-activated non-selective cation channels results in the inactivation of voltage-dependent Ca2+ channels. In addition, ATP seems to modulate Ca2+ channels via the pathway related to G-protein. Adenine nucleotides and adenosine may play a role in controlling secretory activity in guinea-pig adrenal chromaffin cells.

Effects of caffeine on Ca2+ fluxes and secretion in bovine chromaffin cells

The effects of caffeine on Ca2+ fluxes and catecholamine secretion in bovine adrenal chromaffin cells were examined. Caffeine inhibited secretion. 45Ca2+ uptake and cytosolic Ca2+ concentration ([Ca2+]i) rise induced by the nicotinic receptor agonist 1.1-dimethyl- 4-phenylpiperazinium (DMPP) and the Na+ channel activator veratridine. The inhibitory effect of caffeine on high K(+)-induced secretion was smaller than that on DMPP- and veratridine-induced responses. Caffeine only slightly inhibited high K(+)-induced 45Ca2+ uptake and did not affect [Ca2+]i rise. Caffeine also inhibited muscarinic receptor-mediated inositol phosphate generation. Our results suggest that the inhibitory effects of caffeine on bovine chromaffin cells mainly occur at both muscarinic and nicotinic receptors as well as at the voltage-dependent Na+ channels and to a smaller extent at site(s) distal to Ca2+ entry. The effects of caffeine on nicotinic receptors but not on muscarinic receptors can be explained by its ability to raise intracellular cAMP.