Temporal relationships between Ca store mobilization and Ca entry in an exocrine cell

Consideration of the principal current models for agonist-induced activation of Ca²⁺ entry in electrically non-excitable cells suggests that it may be possible to distinguish between them on the basis of predicted differences in the temporal relationship(s) between intracellular Ca²⁺ release and the activation of Ca²⁺ entry. Measurements of changes in Ca²⁺]_i and Mn²⁺ quench in individual exocrine cells from the avian nasal gland indicate that, whereas Ins(1,4,5)P₃-induced release of intracellular Ca²⁺ occurs within 3–5 s, the increase in Mn²⁺ quench is delayed by some 20–30 s. Mn²⁺ quench rate is similarly increased by thapsigargin, and is blocked by SK&F 96365, indicating that the increased Mn²⁺ quench observed genuinely reflects agonist-enhanced activity of the divalent cation entry pathway normally traversed by Ca²⁺. Additional experiments indicate that the observed delay is not due to inhibition of this pathway by elevated Ca²⁺]_i. Furthermore, the delay cannot be explained by the time required for Ins(1,3,4,5)P₄ generation, which is essentially maximal within 10 s of agonist addition. It is concluded that the observed delay in the activation of the Ca²⁺ entry pathway is best explained by ‘capacitative’ models where increased entry requires the generation, and transmission to the plasma membrane, of an unknown messenger as a direct result of the depletion of intracellular Ca²⁺ stores.