EFFECT OF SK-AND-F-96365 ON NICOTINIC OR MUSCARINIC AGONIST-INDUCED AND SPONTANEOUS CA2+ DYNAMICS IN RAT ADRENAL CHROMAFFIN CELLS

The effect of a receptor-mediated and voltage-dependent Ca2+ entry blo cker, SK&F 96365, on secretagogue-induced and spontaneous Ca2+ dynamic s was examined in isolated Fura-2-loaded clusters of rat adrenal chrom affin cells. SK&F 96365 at 50 mu M completely abolished 56 mM K+- or 1 0 mu M nicotine-evoked [Ca2+](c) elevation. Stimulation with a muscari nic receptor agonist, methacholine (200 mu M), induced [Ca2+](c) dynam ics consisted of the initial transient followed by a sustained plateau phase. Delayed applications of methacholine after block of Ca2+ entry by 50 mu M SK&F 96365 led to a progressive decline of the [Ca2+](c) r esponse with substantial difference in the susceptibility to SK&F 9636 5 of the initial and the sustained phases. These results indicate that methacholine-induced [Ca2+](c) response may consist of two components : initial Ca2+ release from intracellular store followed by Ca2+ entry . Most rat adrenal chromaffin cells (74%) exhibited spontaneous Ca2+ o scillations in the absence of any stimuli. The spontaneous oscillation s were completely abolished by 50 mu M SK&F 96365. The spontaneous osc illations coincided with step-wise quenching of Fura-2 fluorescence ex cited at the isosbestic wavelength in the presence of 1 mM Mn2+. The s tepwise quenching was arrested by 50 mu M SK&F 96365. These results in dicate that Ca2+ entry is an important factor for the spontaneous Ca2 oscillations, whereas the possibility that intracellular Ca2+ release could also be involved in the mechanism of the oscillations still rem ains.