VOLTAGE-DEPENDENT CURRENTS AND MODULATION OF CALCIUM-CHANNEL EXPRESSION IN ZONA-FASCICULATA, CELLS FROM RAT ADRENAL-GLAND

1. Whole-cell voltage-activated currents from single zona fasciculata (ZF) cells from rat adrenal glands were studied. T- and L-type Ca2+ cu rrents and a slowly inactivating A-type K+ current were the three majo r currents observed. 2. In freshly isolated cells, the A-type K+ curre nt and the T-type Ca2+ current were predominant. The A-type current wa s activated at -50 mV and inhibited by 4-aminopyridine with a half-max imal block (IC50) at 130 mu M while the T-type current was activated a t -70 mV and blocked by Cd2+, Ni2+ and amiloride with IC50 values of 2 4.1, 132.4 and 518.9 mu M, respectively. 3. Under current clamp, depol arizing 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 membra ne potential oscillations were seen after the spike. 4. In freshly iso lated cells and during the first 24 h after plating, the T-type curren t was observed in all cells, with L-type current being observed in < 2 % of cells, even in the presence of (+)SDZ 202791, a dihydropyridine C a2+ channel agonist. With time in culture, the T-type current disappea red, and a high-voltage-activated L-type current became increasingly a pparent. In cells tested after > 2 days in culture, (+)SDZ 202 791 pot entiated L-type current by 407 +/- 12% and the antagonist (-)SDZ 202 7 91 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 hor mone (ACTH) or vasoactive intestinal peptide (VIP) for 3 days resulted in a high, sustained level of expression of T-type current, with a me an 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 strongl y inhibited this effect. Neither treatment affected L-type current exp ression. 8. The expression of both Ca2+ current types was unaffected b y pre-incubation with 8-bromo-cAMP or forskolin. The protein kinase A antagonist, H89, did not inhibit the ACTH induced upregulation of T-ty pe Ca2+ currents. 7. It is concluded that the main voltage-dependent c urrents involved in cell excitability and steroidogenesis in rat adren al ZF cells are an A-type K+ current and a T-type Ca current. The phys iological role and control of expression of L-type Ca2+ channels in ra t ZF cells remain less clear.