T-TYPE CA(2+) CHANNELS ARE REQUIRED FOR ADRENOCORTICOTROPIN-STIMULATED CORTISOL PRODUCTION BY BOVINE ADRENAL ZONA-FASCICULATA CELLS

The function of low voltage-activated T-type Ca2+ channels in ACTH-stimulated cortisol production by bovine adrenal zona fasciculata cells (AZF) was explored in patch clamp and secretion studies. Nearly all AZF cells expressed only a low voltage-activated T-type Ca2+ current (I(T)) that was blocked by the diphenylbutylpiperidine (DPBP) Ca2+ antagonists penfluridol and pimozide with IC50s of 0.3 and 0.5 mum, respectively. Dihydropyridine (DHP) Ca2+ antagonists, including nimodipine, nisoldipine, and felodipine, also blocked T-type Ca2+ current with IC50s ranging from 3.5-8.8 mum. Inhibition of IT by DPBP and DHP antagonists was voltage and use dependent. ACTH (1 nm) stimulated large (>50-fold) increases in cortisol production by AZF cells, which were inhibited by Ca2+ antagonists at concentrations similar to those which blocked I(T). Inhibition of cortisol production by Ca2+ antagonists was specific; ACTH-induced insulin-like growth factor-I production by AZF cells was not affected by DPBP antagonists. The L channel-specific DHP Ca2+ agonist (-)Bay K 8644 did not enhance basal or ACTH-stimulated cortisol synthesis. These results demonstrate that functional T-rather than L-type Ca2+ channels are required for ACTH-stimulated cortisol synthesis. They also suggest that these low voltage-activated channels, acting as the primary pathway for Ca2+ entry into AZF cells, couple ACTH-stimulated membrane depolarization to steroid hormone production.