Evidence that a pertussis-toxin-sensitive substrate is involved in the stimulation by epidermal growth factor and vasopressin of plasma-membrane Ca2+ inflow in hepatocytes.
1. In hepatocytes, epidermal growth factor (EFG) (a) increased the rate of 45Ca2+ exchange in cells incubated at 1.3 mM extracellular Ca2+, (b) increased the activity of glycogen phosphorylase a and the intracellular free Ca2+ concentration (measured with quin2) in a process dependent on the concentration of extracellular Ca2+, and (c) enhanced the increase in glycogen phosphorylase activity which follows the addition of Ca2+ to cells previously incubated in the absence of Ca2+. It is concluded that EGF stimulates plasma-membrane Ca2+ inflow. 2. The effects of the combination of EGF and vasopressin on the rate of 45Ca2+ exchange and on the rate of increase in glycogen phosphorylase activity were the same as those of vasopressin alone. 3. The amount of 45Ca2+ released by EGF from internal stores was about 30% of that released by vasopressin. No detectable increase in [3H]inositol mono-, bis- or tris-phosphate was observed after the addition of EGF to cells labelled with myo-[3H]inositol. 4. In hepatocytes isolated from rats treated with pertussis toxin, the effects of EGF and vasopressin on phosphorylase activity (measured at 1.3 mM-Ca2+) and on the rate of Ca2+ inflow (measured with quin2) were markedly decreased compared with those in normal cells. 5. Treatment with pertussis toxin did not impair the ability of vasopressin to release Ca2+ from internal stores, but decreased vasopressin-stimulated [3H]inositol polyphosphate formation by 50%. 6. It is concluded that the mechanism(s) by which vasopressin and EGF stimulate plasma-membrane Ca2+-inflow transporters in hepatocytes involves a GTP-binding regulatory protein sensitive to pertussis toxin, and does not require an increase in the concentration of inositol trisphosphate comparable with that which induces the release of Ca2+ from the endoplasmic reticulum.
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D004789:Enzyme Activation / Q000187:drug effects; D004815:Epidermal Growth Factor / Q000493:pharmacokinetics*; D007295:Inositol Phosphates / Q000378:metabolism; D008099:Liver / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D037342:Pertussis Toxin; D006005:Phosphorylases / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D014667:Vasopressins / Q000493:pharmacokinetics*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology*
D007295:Inositol Phosphates; D010566:Virulence Factors, Bordetella; D014667:Vasopressins; D004815:Epidermal Growth Factor; D006005:Phosphorylases; D037342:Pertussis Toxin; D002118:Calcium