Inward currents underlying action potentials in rat adrenal chromaffin cells.

1. Current- and voltage-clamp studies were conducted on isolated rat adrenal chromaffin cells to identify the voltage-dependent ion channels mediating inward currents. 2. Mean resting membrane potential of the isolated cells was -62 +/- 3 (SE) mV. Evoked action potentials were both Na+ and Ca2+ based, and whole cell voltage-clamp studies in normal saline revealed an inward-rectifier-type current. 3. Na+ channels were studied in isolation and showed a half-inactivation of -60 +/- 2 mV with a slope factor of -6 mV and a half-activation of -26.8 +/- 2 mV with a slope factor of 6.5 +/- 0.7 mV. 4. Isolated Ca2+ currents, elicited in 10 mM external Ca2+, revealed a T-type current in a subset of cells. Ca2+ currents were sensitive to both N- and L-type channel antagonists, and blockade of the current by the L-type channel antagonist nimodipine and the N-type channel antagonist omega-conotoxin GVIA revealed a third Ca2+-current component that was unaffected by the P-type channel antagonist omega-agatoxin IVA. 5. Ca2+ currents were facilitated 5-20% by a depolarizing prepulse, and facilitation was completely blocked by nimodipine. The effects of the dihydropyridine L-type channel agonist, (+)202-791 and depolarizing prepulses on the currents were additive. 6. The results of this study show that the properties of voltage-dependent ion channels in rat chromaffin cells differ from those reported in their counterparts in bovine chromaffin cells. Na+ channels differ in activation and inactivation properties and Ca2+ channels differ in activation, sensitivity to antagonists, and the magnitude of voltage-dependent facilitation.