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 estimated by measurement of the initial rate of activation of glycogen phosphorylase a following the addition of Ca2+ to cells previously incubated in the absence of added Ca2+. Pretreatment of hepatocytes with thapsigargin caused a substantial stimulation of the rate of Ca2+ activation of glycogen phosphorylase a. This was interpreted to reflect a stimulation of plasma membrane Ca2+ inflow. The effect of thapsigargin on plasma membrane Ca2+ inflow was approximately 65% of the magnitude of the effect caused by vasopressin. When thapsigargin and vasopressin were combined as a stimulus, the degree of stimulation was similar to that caused by vasopressin alone. The thapsigargin-induced stimulation of the rate of Ca2+ activation of glycogen phosphorylase a was inhibited in a concentration-dependent manner by both Zn2+ and 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SK&F 96365). The concentration of each agent required for half-maximal inhibition was approximately 20 microM. 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 hepatocytes stimulated by thapsigargin is likely to be similar to that which is activated by vasopressin.
School of Pharmacy and Medical Sciences, University of South Australia, North Terrace, Adelaide.
M J
MJ
Strazzari
B P
BP
Hughes
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000493:pharmacokinetics; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D004121:Dimethyl Sulfoxide / Q000494:pharmacology; D004789:Enzyme Activation; D007093:Imidazoles / Q000494:pharmacology; D007700:Kinetics; D008099:Liver / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D008297:Male; D006005:Phosphorylases / Q000187:drug effects / Q000378:metabolism; D010975:Platelet Aggregation Inhibitors / Q000494:pharmacology; D051381:Rats; D012680:Sensitivity and Specificity; D013268:Stimulation, Chemical; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin; D014667:Vasopressins / Q000494:pharmacology*; D015032:Zinc / Q000494:pharmacology
D007093:Imidazoles; D010975:Platelet Aggregation Inhibitors; D013729:Terpenes; D014667:Vasopressins; D019284:Thapsigargin; D006005:Phosphorylases; C063159:1-(2-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenylethyl)-1H-imidazole; D015032:Zinc; D002118:Calcium; D004121:Dimethyl Sulfoxide