MODULATION BY L-TYPE CA2-SENSITIVE K+ CHANNELS OF MUSCARINIC RESPONSES IN CAT CHROMAFFIN CELLS( CHANNELS AND APAMIN)

In the perfused cat adrenal gland stimulated with the muscarinic agoni st methacholine chloride (100 mu M for 3 min), two components were det ected in the catecholamine secretory response: 1) an early phasic comp onent that peaked at 300 ng/5 s catecholamine release and 2) a tonic c omponent whose peak was transient and declined to a plateau of about 1 40 ng/5 s. Apamin (0.1 mu M) increased the phasic component to 1,200 n g/5 s and the tonic component to similar to 350 ng/5 s. In single fura 2-loaded cat adrenal chromaffin cells, the cytosolic Ca2+ concentrati on ([Ca2+](i)) also followed a biphasic pattern after stimulation with methacholine. Depletion of extracellular Ca2+ reduced the phasic [Ca2 +](i) peak by >50% and the phasic secretory peak by similar to 90%; bo th the tonic components of [Ca2+](i) and secretion were abolished. Dep letion of intracellular Ca2+ pools decreased the phasic and tonic comp onents of [Ca2+](i) and secretion with respect to control values; howe ver, the phasic components diminished more than the tonic components o f [Ca2+](i) and secretion. Although 3 mu M furnidipine (a dihydropyrid ine L-type Ca2+ channel blocker) inhibited the phasic component of [Ca 2+](i) and secretion, its effects were more pronounced on the tonic co mponent. omega-Conotoxin GVIA (1 mu M, an N-type Ca2+ channel blocker) did not affect the [Ca2+](i) or the methacholine secretory responses. The secretion peak seems to depend on both extracellular free Ca2+ (C a-0(2+)) entry through L-type Ca2+ channels as well as on the mobiliza tion of Ca2+ from intracellular stores; the plateau depends only on Ca -0(2+) entry through L-type Ca2+ channels. Although present, N-type Ca 2+ channels do not contribute to the muscarinic Ca-i(2+) acid secretio n signals. Both phases of the Ca-i(2+) and secretion signals seem to b e regulated by an apamin-sensitive Ca2+-dependent K+ channel.