The agonist-sensitive calcium pool in the pancreatic acinar cell. Activation of plasma membrane Ca2+ influx mechanism.

The purposes of the present study were to investigate the characteristics and regulation of Ca2+ influx across the plasma membrane in pancreatic acini and to demonstrate the role of this Ca2+ influx in the mechanism of reloading of the agonist-sensitive Ca2+ pool. In pancreatic acini, depleted of intracellular Ca2+ by stimulation with carbachol in the absence of extracellular Ca2+, 25 microM LaCl3 inhibited the increase in free cytosolic Ca2+ ([Ca2+]i) and reloading of the agonist-sensitive pool that occurred with the addition of extracellular CaCl2 to the medium. LaCl3 also inhibited the increase in cellular 45Ca2+ uptake that occurred during agonist stimulation and its termination but not cellular 45Ca2+ uptake into unstimulated acini. In acini depleted of intracellular Ca2+, increased cellular Ca2+ influx and reloading of the agonist-sensitive pool occurred even if extracellular CaCl2 was added 10 min after the termination of agonist action. Maximal reloading was independent of the extracellular Ca2+ concentration between 0.5 and 2.0 mM CaCl2. However, the time to maximal reloading was longer at lower extracellular Ca2+ concentrations. These results demonstrate a plasma membrane Ca2+ influx mechanism in the pancreatic acinar cell that is activated during cell stimulation. This transport remains activated as long as the agonist-sensitive pool is not completely loaded with Ca2+ suggesting that the Ca2+ influx mechanism is regulated by the quantity of Ca2+ in the agonist-sensitive pool. The activation of this Ca2+ transport mechanism functions to allow Ca2+ influx across the plasma membrane and Ca2+ reloading of the agonist-sensitive pool. Furthermore, these results suggest that during reloading Ca2+ crosses the plasma membrane into the cytosol before entering the agonist-sensitive pool.