Mechanism through which GABA<inf>A</inf> receptor modulates catecholamine secretion from bovine chromaffin cells

Abstract

The actions and mechanism of GABAergic modulation of catecholamine secretion from isolated bovine chromaffin cells were investigated. The GABAA receptor agonist muscimol induced a fast rise in cytosolic [Ca2+ ]. The mean peak increase was 290 ± 30nM over basal levels. The increase in cytosolic [Ca2+ ] was abolished in the absence of extracellular [Ca2+ ] and was blocked by the GABAA antagonist bicuculline and the dihydropiridine nifedipine. Muscimol also elicited the release of catecholamines and increased the bisoxonol fluorescence indicating a cell depolarization. The [Ca2+ ] entry was well correlated with muscimol-evoked catecholamine secretion. When cells were treated with muscimol and a second secretagogue, a biphasic behavior was revealed. Muscimol enhanced the catecholamine release evoked by low concentrations of nicotine or K+, whereas release obtained at high concentrations of nicotine or K+ was actually inhibited. When the muscimol effect on membrane potential was studied in the presence of low K+ or nicotine concentrations, an enhancement of the bisoxonol fluorescence was observed. This effect was reversed at high concentrations of both K+ and nicotine. Measurement of 36Cl− fluxes showed an increase in membrane permeability to Cl− during muscimol stimulation. The influx or efflux in Cl− was dependent on membrane potential. In normal conditions, with a K+ concentration of 5.4 mM, a Cl− efflux was observed by both radiometric techniques, with 36Cl− and by the use of the chloride-sensitive fluorescent probe 6-methoxy-N-(3-sulphopropil)quinolinium, as indicator of intracellular Cl−. At high nicotine (20 mM) or K+ concentrations (105 mM) a Cl− influx was observed using 6-methoxy-N-(3-sulphopropil)quinolinium. Equilibrium potential for Cl− was placed at −51.4 ± 2.1mV by independent estimation of 36Cl−distribution across membrane. These results suggest a dual, modulatory role for GABA in the adrenal medulla. Its probable mechanism and implications are discussed.