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 orginial 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).
|Original language||English (US)|
|Number of pages||8|
|Journal||Journal of Biological Chemistry|
|State||Published - 1985|
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology