title: Biochemical properties of cardiac sarcolemma: adenylate cyclase and (Na++K+)-activated ATPase.
abstract: Morphologically intact plasma membranes from guinea pig ventricles were obtained by exposing isolated cell segments to osmotic shock, followed by extraction of actomyosin in 1 M KC1. These preparations contained approximately 1/6 of the protein and 5-10 percent of the mitochondrial markers present in the original cell preparation. Both adenylate cyclase and (Na++K+)-activated ATPase activities were enriched 3-4 fold. The receptor for epinephrine stimulation of adenylate cyclase was retained. The "basal" ATPase activity of 5-6 mumoles of Pi/mg/hr, measured in 120 mM NaC1 or KC1, was approximately doubled in 100 mM NaC1+20 mM KC1. This increment, the (Na++K+)-activated ATPase, was abolished by 10(-5) M ouabain, the Ki for ouabain being approximately 3x10(-7) M. Adenylate cyclase, which had a basal activity of approximately 0.33 nmole of cyclic AMP produced/min/mg of protein, was significantly stimulated by both l-epinephrine and NaF. Half-maximal stimulation was seen at approximately 5x10(-6) M l-epinephrine. Increasing Ca2+ in the range between 10(-7) and 10(-3) M inhibited basal, l-epinephrine-, and NaF-stimulated adenylate cyclase activities. Basal rates of cyclic AMP production were more sensitive to Ca2+ than was l-epinephrine-stimulated adenylate cyclase activity, so that l-epinephrine stimulation was increased from approximately 60 percent in 0.5 mM ethylene glycol bis (beta-aminoethyl ether) N,N'-tetraacetic acid to approximately 150 percent in 10(-7)M Ca2+ and 400 percent in 10(-5) M Ca2+. The inhibitory effect of Ca2+ on adenylate cyclase activity may represent a negative feedback mechanism by which eelevation of intracellular Ca2+ concentration lowers cellular levels of cyclic AMP and thus reduces Ca2+ influx into the myocardium.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000262:Adenylyl Cyclases / Q000378:metabolism*; D000818:Animals; D001386:Azides / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D004533:Egtazic Acid / Q000494:pharmacology; D004789:Enzyme Activation / Q000187:drug effects; D004837:Epinephrine / Q000494:pharmacology; D006168:Guinea Pigs; D006352:Heart Ventricles; D066298:In Vitro Techniques; D007700:Kinetics; D008858:Microscopy, Phase-Contrast; D009206:Myocardium / Q000201:enzymology*; D011188:Potassium / Q000494:pharmacology; D012508:Sarcolemma / Q000187:drug effects / Q000201:enzymology* / Q000648:ultrastructure; D012964:Sodium / Q000494:pharmacology; D013384:Succinate Cytochrome c Oxidoreductase / Q000378:metabolism
chemical_list: D001386:Azides; D004533:Egtazic Acid; D000255:Adenosine Triphosphate; D012964:Sodium; D000242:Cyclic AMP; D013384:Succinate Cytochrome c Oxidoreductase; D000251:Adenosine Triphosphatases; D000262:Adenylyl Cyclases; D011188:Potassium; D002118:Calcium; D004837:Epinephrine