Characteristics of cytosolic Ca 2+ elevation induced by muscarinic receptor activation in single adrenal chromaffin cells of the guinea pig

Abstract

In Fura-2 loaded-single guinea pig adrenal chromaffin cells, muscarine, nicotine and KCl all caused an early peak rise in intracellular Ca concentration ([Ca 2+] i) followed by a sustained rise. In Ca 2+-free solution, muscarine, but neither nicotine nor KCl, caused a transient increase in [Ca 2+] i, which was partially reduced by preceding application of caffeine or by treatment with ryanodine plus caffeine. In voltage-clamped cells at a holding potential of −60 mV, the muscarine-induced [Ca 2+] i, rise, especially its sustained phase, decreased in magnitude. intracellular application of inositol 1,4,5-trisphosphate caused a transient increase in [Ca 2+] i and inhibited the following [Ca 2+] i response to muscarine without affecting responses to nicotine and a depolarizing pulse. Muscarine evoked membrane depolarization following brief hyperpolarization in most cells tested. There was a significant positive correlation between the amplitude of the depolarization and the magnitude of the sustained rise in [Ca 2+] i. Muscarine-induced sustained [Ca 2+] i rise was much greater in the current-clamp mode than that in the voltage-clamp mode. The sustained phase of [Ca 2+] i rise and Mn 2+ influx in response to muscarine were suppressed by a voltage-dependent Ca 2+ channel blocker, methoxyverapamil. These results suggest that stimulation of muscarinic receptors causes not only extracellular Ca 2+ entry, but also Ca 2+ mobilization from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Voltage-dependent Ca 2+ channels may function as one of the Ca 2+ entry pathways activated by muscarinic receptor in guinea pig adrenal chromaffin cells.