title: Does calmodulin play a role in the regulation of cardiac sarcolemmal adenylate cyclase activity?
abstract: The recent suggestion that calmodulin (CaM) could mediate calcium inhibition of cardiac adenylate cyclase (AC) has been reassessed. Using a purified sarcolemmal preparation (SL), the influence of different concentrations of free Ca2+ (obtained using Ca2+-EGTA solutions) was studied on dog heart AC. From 10(-9) M to 10(-3) M Ca2+ reduced basal activity, as well as epinephrine (10(-4) M)- and trypsin (1.0 microgram/mL)-stimulated activities with, in the three cases, an identical IC50 of 10(-8) M. The amount of endogenous CaM in the SL, measured using a radioimmunoassay technique, was found to be 7.5 ng/mg protein. The resulting concentration of CaM in the final AC incubation medium was lower than 50 pM, indicating the lack of a significant role for endogenous CaM in the inhibition observed. The addition of exogenous CaM to the AC assay at a concentration sufficient to stimulate other CaM-dependent systems did not modify the Ca2+ inhibitory curves for basal, epinephrine (10(-4) M)-stimulated, or trypsin (1 microgram/mL)-stimulated activities. These results indicate that CaM does not play a significant role in the Ca2+ inhibition of cardiac AC and that trypsin stimulation of cardiac AC is not mediated through a CaM-dependent process.
mesh_terms: D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002147:Calmodulin / Q000378:metabolism*; D004837:Epinephrine / Q000494:pharmacology; D009206:Myocardium / Q000201:enzymology*; D012508:Sarcolemma / Q000201:enzymology; D014357:Trypsin / Q000378:metabolism
chemical_list: D002147:Calmodulin; D014357:Trypsin; D000262:Adenylyl Cyclases; D002118:Calcium; D004837:Epinephrine