Calcium influx mechanisms underlying calcium oscillations in rat hepatocytes

Abstract

The process of capacitative or store-operated Ca²⁺ entry has been extensively investigated, and recently two major molecular players in this process have been described. Stromal interacting molecule (STIM) 1 acts as a sensor for the level of Ca²⁺ stored in the endoplasmic reticulum, and Orai proteins constitute pore-forming subunits of the store-operated channels. Store-operated Ca²⁺ entry is readily demonstrated with protocols that provide extensive Ca²⁺ store depletion; however, the role of store-operated entry with modest and more physiological cell stimuli is less certain. Recent studies have addressed this question in cell lines; however, the role of store-operated entry during physiological activation of primary cells has not been extensively investigated, and there is little or no information on the roles of STIM and Orai proteins in primary cells. Also, the nature of the Ca²⁺ influx mechanism with hormone activation of hepatocytes is controversial. Hepatocytes respond to physiological levels of glycogenolytic hormones with well-characterized intracellular Ca²⁺ oscillations. In the current study, we have used both pharmacological tools and RNA interference (RNAi)-based techniques to investigate the role of store-operated channels in the maintenance of hormone-induced Ca²⁺ oscillations in rat hepatocytes. Pharmacological inhibitors of store-operated channels blocked thapsigargin-induced Ca²⁺ entry but only partially reduced the frequency of Ca²⁺ oscillations. Similarly, RNAi knockdown of STIM1 or Orai1 substantially reduced thapsigargin-induced calcium entry, and more modestly diminished the frequency of vasopressin-induced oscillations. Conclusion: Our findings establish that store-operated Ca²⁺ entry plays a role in the maintenance of agonist-induced oscillations in primary rat hepatocytes but indicate that other agonist-induced entry mechanisms must be involved to a significant extent. (HEPATOLOGY 2008.)