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.
Strazzari
M J
MJ
School of Pharmacy and Medical Sciences, University of South Australia, North Terrace, Adelaide.
Hughes
B P
BP
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