EVIDENCE FOR A NON-CAPACITATIVE CA2+ ENTRY DURING [CA2+] OSCILLATIONS

Current models for the agonist-induced activation of Ca2+ entry from t he extracellular medium in non-excitable cells generally emphasize a c apacitative mechanism whereby Ca2+ entry is activated simply as a resu lt of the emptying of intracellular Ca2+ stores, without any direct in volvement of inositol phosphates. To date, the activation and control of Ca2+ entry have generally been studied under conditions where the a gonist-sensitive stores undergo a profound and sustained depletion. Ho wever, responses under more normal physiological conditions typically involve the cyclical release and refilling of the stores associated wi th oscillations in [Ca2+], and the nature and control of entry under t hese conditions has received relatively little attention. In this stud y, using isolated cells from the exocrine avian nasal gland as a model system, we show that: (a) the agonist-enhanced rate of Mn2+ quench is independent of the cyclical emptying and refilling of the agonist-sen sitive Ca2+ pool during oscillations; (b) the Ca2+ entry pathway is ma intained in an activated state for extended periods following inhibiti on of oscillations under conditions in which agonist-sensitive stores can be shown to be full; (c) no Ca2+ entry could be detected in oscill ating cells in experiments that followed a definitive protocol for the demonstration of capacitative entry; and (d) on initial exposure to l ow agonist concentrations, activation of Ca2+ entry preceded any detec table release of Ca2+ from the stores. We conclude that the essential characteristics of the control of Ca2+ entry during oscillations are i ncompatible with current capacitative models.