The role of extracellular Ca2+ in the response of the hepatocyte to Ca2+-dependent hormones.
The influence of extracellular Ca2+ on hormone-mediated increases of cytosolic free Ca2+ [( Ca2+]i) and phosphorylase activity was studied in isolated hepatocytes. In the presence of 1.3 mM extracellular Ca2+, the stimulation of phosphorylase activity produced by vasopressin or phenylephrine was maintained for 20-30 min. In contrast, the change in [Ca2+]i under these conditions was more transient and declined within 3-4 min to steady state values only 70 +/- 8 nM above the resting [Ca2+]i. Removal of the hormone from its receptor with specific antagonists caused a decline in [Ca2+]i back to the original resting values. Subsequent addition of a second hormone elicited a further Ca2+ transient. If the antagonist was omitted, the second hormone addition did not increase [Ca2+]i indicating that the labile intracellular Ca2+ pool remains depleted during receptor occupation. When extracellular Ca2+ was omitted, both the changes of [Ca2+]i and phosphorylase a caused by vasopressin were transient and returned exactly to resting values within 3-4 min. The subsequent readdition of Ca2+ to these cells produced a further increase of [Ca2+]i and phosphorylase activity which was larger than the changes observed upon Ca2+ addition to untreated cells. This reactivation of phosphorylase showed saturation kinetics with respect to extracellular [Ca2+], was maximally stimulated within 1 min of vasopressin addition and was inhibited by high concentration of diltiazem. We conclude that entry of extracellular Ca2+ into the cell is required in order to obtain a sustained hormonal stimulation of phosphorylase activity and is responsible for the maintenance of a small steady state elevation of [Ca2+]i.
D000634:Aminoquinolines; D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D004110:Diltiazem / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D005470:Fluorometry; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology*; D010762:Phosphorylase a / Q000378:metabolism*; D006005:Phosphorylases / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D014667:Vasopressins / Q000494:pharmacology*
D000634:Aminoquinolines; D002121:Calcium Channel Blockers; D014667:Vasopressins; D010656:Phenylephrine; D010762:Phosphorylase a; D006005:Phosphorylases; D004110:Diltiazem; C035739:Quin2; D002118:Calcium