EVIDENCE THAT THE CA2+ INFLOW PATHWAY IN HEPATOCYTES STIMULATED BY THAPSIGARGIN IS SIMILAR TO THAT ACTIVATED BY VASOPRESSIN

Experiments were conducted to characterize the thapsigargin-stimulated plasma membrane Ca2+ inflow pathway in hepatocytes. Ca2+ inflow was e stimated by measurement of the initial rate of activation of glycogen phosphorylase a following the addition of Ca2+ to cells previously inc ubated in the absence of added Ca2+ . Pretreatment of hepatocytes with thapsigargin caused a substantial stimulation of the rate of Ca2+ act ivation of glycogen phosphorylase a. This was interpreted to reflect a stimulation of plasma membrane Ca2+ inflow. The effect of thapsigargi n on plasma membrane Ca2+ inflow was approximately 65% of the magnitud e of the effect caused by vasopressin. When thapsigargin and vasopress in were combined as a stimulus, the degree of stimulation was similar to that caused by vasopressin alone. The thapsigargin-induced stimulat ion of the rate of Ca2+ activation of glycogen phosphorylase a was inh ibited in a concentration-dependent manner by both Zn2+ and xyphenyl)p ropoxy]-4-methoxyphenethyl}-1H-imidazole hydrochloride (SK&F 96365). T he concentration of each agent required for half-maximal inhibition wa s approximately 20 muM. It is concluded from: (i) the apparent lack of additivity in the responses of thapsigargin and vasopressin, and (ii) the sensitivity to inhibitors, that the Ca2+ inflow pathway in hepato cytes stimulated by thapsigargin is likely to be similar to that which is activated by vasopressin.