Evidence that store-operated Ca 2+ channels are more effective than intracellular messenger-activated non-selective cation channels in refilling rat hepatocyte intracellular Ca 2+ stores

Abstract

Liver cells possess store-operated Ca 2+ channels (SOCs) with a high selectivity for Ca 2+ compared with Na +, and several types of intracellular messenger-activated non-selective cation channels with a lower selectivity for Ca 2+ (NSCCs). The main role of SOCs is thought to be in refilling depleted endoplasmic reticulum Ca 2+ stores [Cell Calcium 7 (1986) 1]. NSCCs may be involved in refilling intracellular stores but are also thought to have other roles in regulating the cytoplasmic-free Ca 2+ and Na + concentrations. The ability of SOCs to refill the endoplasmic reticulum Ca 2+ stores in hepatocytes has not previously been compared with that of NSCCs. The aim of the present studies was to compare the ability of SOCs and maitotoxin-activated NSCCs to refill the endoplasmic reticulum in rat hepatocytes. The experiments were performed using fura-2FF and fura-2 to monitor the free Ca 2+ concentrations in the endoplasmic reticulum and cytoplasmic space, respectively, a Ca 2+ add-back protocol, and 2-aminoethyl diphenylborate (2-APB) to inhibit Ca 2+ inflow through SOCs. In cells treated with 2,5-di- t-butylhydroquinone (DBHQ) or vasopressin to deplete the endoplasmic reticulum Ca 2+ stores, then washed to remove DBHQ or vasopressin, the addition of Ca 2+ caused a substantial increase in the concentration of Ca 2+ in the endoplasmic reticulum and cytoplasmic space due to the activation of SOCs. These increases were inhibited 80% by 2-APB, indicating that Ca 2+ inflow is predominantly through SOCs. In the presence of 2-APB (to block SOCs), maitotoxin induced a substantial increase in [Ca 2+] cyt, but only a modest and slower increase in [Ca 2+] er. Under these conditions, Ca 2+ inflow is predominantly through maitotoxin-activated NSCCs. It is concluded that SOCs are more effective than maitotoxin-activated NSCCs in refilling the endoplasmic reticulum Ca 2+ stores. The previously developed concept of a specific role for SOCs in refilling the endoplasmic reticulum is consistent with the results reported here.