Voltage‐dependent currents and modulation of calcium channel expression in zona fasciculata cells from rat adrenal gland.

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Abstract
1. Whole‐cell voltage‐activated currents from single zona fasciculata (ZF) cells from rat adrenal glands were studied. T‐ and L‐type Ca2+ currents and a slowly inactivating A‐type K+ current were the three major currents observed. 2. In freshly isolated cells, the A‐type K+ current and the T‐type Ca2+ current were predominant. The A‐type current was activated at ‐50 mV and inhibited by 4‐amino‐pyridine with a half‐maximal block (IC50) at 130 microM while the T‐type current was activated at ‐70 mV and blocked by Cd2+, Ni2+ and amiloride with IC50 values of 24.1, 132.4 and 518.9 microM, respectively. 3. Under current clamp, depolarizing current pulses produced a single Ca2+ action potential with Cs+ in the pipette internal solution. Upon replacement of Cs+ by K+, the half‐amplitude width of the action potential was shortened and membrane potential oscillations were seen after the spike. 4. In freshly isolated cells and during the first 24 h after plating, the T‐type current was observed in all cells, with L‐type current being observed in < 2% of cells, even in the presence of (+)SDZ 202,791, a dihydropyridine Ca2+ channel agonist. With time in culture, the T‐type current disappeared, and a high‐voltage‐activated L‐type current became increasingly apparent. In cells tested after > 2 days in culture, (+)SDZ 202,791 potentiated L‐type current by 407 +/‐ 12% and the antagonist (‐)SDZ 202,791 blocked this increase. The L‐type current was activated between ‐30 and ‐20 mV and was sensitive to nitrendipine and omega‐conotoxin GVIA. 5. Pre‐incubation of cultured ZF cells with adrenocorticotrophic hormone (ACTH) or vasoactive intestinal peptide (VIP) for 3 days resulted in a high, sustained level of expression of T‐type current, with a mean amplitude of 34.2 +/‐ 5.5 pA pF‐1 for ACTH‐treated cells compared with 3.4 +/‐ 1.8 pA pF‐1 for untreated cells. Cycloheximide strongly inhibited this effect. Neither treatment affected L‐type current expression. 6. The expression of both Ca2+ current types was unaffected by pre‐incubation with 8‐bromo‐cAMP or forskolin. The protein kinase A antagonist, H89, did not inhibit the ACTH‐induced upregulation of T‐type Ca2+ currents. 7. It is concluded that the main voltage‐dependent currents involved in cell excitability and steroidogenesis in rat adrenal ZF cells are an A‐type K+ current and a T‐type Ca2+ current. The physiological role and control of expression of L‐type Ca2+ channels in rat ZF cells remain less clear.