Effects of GABA on spontaneous [Ca2+]c dynamics and electrical properties of rat adrenal chromaffin cells.

A large fraction of rat adrenal chromaffin cells (about 60%) shows spontaneous [Ca2+]c oscillations and spontaneous action potentials. In the present study the effects of gamma-aminobutyric acid (GABA) on the spontaneous [Ca2+]c oscillations and electrical properties of rat adrenal chromaffin cells were investigated using Fura-2 [Ca2+]c imaging and patch clamp techniques. GABA inhibited the spontaneous [Ca2+]c oscillations in a reversible manner. The effect of GABA was mimicked by the GABAA and GABAC receptor agonist, muscimol, but not by the GABAB receptor agonist, baclofen. Moreover, the effect was antagonized by the selective GABAA receptor antagonist, bicuculline. The mode of the inhibition was all-or-none, and the threshold concentration at which the inhibition occurred varied widely (50 microM to over 1 microM) from cell to cell. GABA (100 microM) elicited a transient burst of action potentials of diminished amplitude, which was followed by arrest of action potentials. Further analysis showed that GABA (100 microM) induced inward whole-cell currents in voltage-clamp experiments and produced depolarization and membrane conductance increase in current-clamp experiments. The effects appear to be due to an increase in chloride ion conductance since the degree of GABA-induced depolarization depended on the pipette [Cl-]. These results suggest that GABA, acting through GABAA receptor, may play a role in the physiological regulation of rat adrenal chromaffin cells by directly modifying the discharge of spontaneous action potentials and spontaneous [Ca2+]c oscillations.