Interaction of SKCa channels and L-type Ca2+ channels in catecholamine secretion in the rat adrenal gland

We elucidated the interaction of small-conductance Ca2+-activated K+ (SKCa) channels and L-type Ca2+ channels in muscarinic receptor-mediated control of catecholamine secretion in the isolated perfused rat adrenal gland. The muscarinic agonist methacholine (10-300 muM) produced concentration-dependent increases in adrenal output of epinephrine and norepinephrine. The SKCa channel blocker apamin (1 muM) enhanced the methacholine-induced catecholamine responses. The facilitatory effect of apamin on the methacholine-induced catecholamine responses was not observed during treatment with the L-type Ca2+ channel blocker nifedipine (3 muM) or Ca2+-free solution. Nifedipine did not affect the methacholine-induced catecholamine responses, but it inhibited the responses during treatment with apamin. The L-type Ca2+ channel activator Bay k 8644 (1 muM) enhanced the methacholine-induced catecholamine responses, whereas the enhancement of the methacholine-induced epinephrine and norepinephrine responses were prevented and attenuated by apamin, respectively. These results suggest that SKCa channels are activated by muscarinic receptor stimulation, which inhibits the opening of L-type Ca2+ channels and thereby attenuates adrenal catecholamine secretion.