Processes Maintaining Calcium Homeostasis in Acinar Cells of the Rat Submandibular Salivary Gland

Using a Ca2+-sensitive fluorescent indicator, fura-2/AM, we recorded calcium transients in secretory cells of isolated acini of the rat submandibular salivary gland; these transients were induced by hyperpotassium-induced depolarization (after an increase in [K+] e up to 50 mM) of the plasma membrane of the above cells. Calcium transients were significantly suppressed by 50 μM nifedipine. Addition of 10 μM carbonyl cyanide m-chlorophenylhydrazone to the normal extracellular solution was accompanied by a rise in [Ca2+]i, whereas when hyperpotassium solution is used the effect was less expressed. Blockers of CA2+-ATPase in the cellular membrane and in the endoplasmic reticulum, eosin Y (5 μM) and cyclopiazonic acid (CPA, 5 μM), respectively, evoked a significant increase in [Ca2+]i and a decrease in the K+-depolarization-induced calcium transient. Extracellular application of caffeine (2, 10, or 30 mM) was accompanied by a concentration-dependent rise in [Ca2+]i. Therefore, potassium depolarization of the plasma membrane of acinar cells of the rat submandibular salivary gland activates both the voltage-dependent Ca2+ influx and Ca2+-induced Ca2+ release from the endoplasmic reticulum; the initial level of [Ca2+]i was restored at the joint involvement of Ca2+-ATPases in the plasma membrane and the membranes of the endoplasmic reticulum and mitochondria.