Synchronized oscillation of Ca2+ entry and Ca2+ release in agonist-stimulated AR42J cells.

Oscillation in [Ca2+]i induced by agonists has been described in many cell types and is thought to reflect Ca2+ release from and uptake into internal stores. We measured [Ca2+]i and Mn2+ entry in single cells of the pancreatic acinar cell line AR42J loaded with Fura 2 to examine the behavior of Ca2+ influx across the plasma membrane (Ca2+ entry) during agonist-evoked [Ca2+]i oscillation. Addition of extracellular Ca2+ (Ca2+out) to agonist-stimulated cells bathed in Ca2(+)-free medium resulted in a marked [Ca2+]i increase blocked by La3+. The use of Mn2+ as a congener of Ca2+ to follow unidirectional Ca2+ movement reveals an oscillatory activation of Ca2+ entry by Ca2(+)-mobilizing agonists. The frequency at which Ca2+ entry oscillated matched the frequency of Ca2+ release from intracellular stores. Ca2+ entry is activated after completion of Ca2+ release and is inactivated within the time span of each [Ca2+]i spike. These studies reveal a new aspect of [Ca2+]i oscillation in agonist-stimulated cells, that is the oscillatory activation of [Ca2+]i entry during [Ca2+]i oscillation.