Hormone-induced rise in cytosolic Ca2+ in axolotl hepatocytes: properties of the Ca2+ influx channel.
Calcium entry in nonexcitable cells occurs through Ca(2+)-selective channels activated secondarily to store depletion and/or through receptor- or second messenger-operated channels. In amphibian liver, hormones that stimulate the production of adenosine 3',5'-cyclic monophosphate (cAMP) also regulate the opening of an ion gate in the plasma membrane, which allows a noncapacitative inflow of Ca2+. To characterize this Ca2+ channel, we studied the effects of inhibitors of voltage-dependent Ca2+ channels and of nonselective cation channels on 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP)-dependent Ca2+ entry in single axolotl hepatocytes. Ca2+ entry provoked by 8-BrcAMP in the presence of physiological Ca2+ followed first-order kinetics (apparent Michaelis constant = 43 microM at the cell surface). Maximal values of cytosolic Ca2+ (increment approximately 300%) were reached within 15 s, and the effect was transient (half time of 56 s). We report a strong inhibition of cAMP-dependent Ca2+ entry by nifedipine [half-maximal inhibitory concentration (IC50) = 0.8 microM], by verapamil (IC50 = 22 microM), and by SK&F-96365 (IC50 = 1.8 microM). Depolarizing concentrations of K+ were without effect. Gadolinium and the anti-inflammatory compound niflumate, both inhibitors of nonselective cation channels, suppressed Ca2+ influx. This "profile" indicates a novel mechanism of Ca2+ entry in nonexcitable cells.
Lenz
T
T
Abteilung Klinische Biochemie, University of Göttingen, Germany.
Kleineke
J W
JW
D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology*; D000557:Ambystoma; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000502:physiology*; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D005682:Gadolinium / Q000494:pharmacology; D007093:Imidazoles / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D015290:Second Messenger Systems / Q000187:drug effects / Q000502:physiology; D013997:Time Factors; D014700:Verapamil / Q000494:pharmacology
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D007093:Imidazoles; D015124:8-Bromo Cyclic Adenosine Monophosphate; D005682:Gadolinium; D014700:Verapamil; C063159:1-(2-(3-(4-methoxyphenyl)propoxy)-4-methoxyphenylethyl)-1H-imidazole; D009543:Nifedipine; D002118:Calcium