INVOLVEMENT OF PERTUSSIS-TOXIN-SENSITIVE G-PROTEIN IN MUSCARINIC-RECEPTOR-MEDIATED INHIBITION OF K+-ACTIVATED 4-NITROPHENYLPHOSPHATASE ACTIVITY OF CARDIAC SARCOLEMMA

The effects of the cholinergic agonist carbachol on ouabain‐sensitive K+‐activated 4‐nitrophenylphosphatase (K+‐O2NPhPase) activity of rabbit and pig ventricular sarcolemma were examined. Carbachol (0.01–1000 μM) alone had no effect on K+‐O2NPhPase. However, in the presence of GTP (100 μM) or its analog guanosine 5′‐[γ‐thio]triphosphate (GTP[S], 1 μM) the agonist reduced this enzymatic activity (IC50= 0.3 μM) by about 45% in a concentration‐dependent manner. The GTP[S]‐dependent effect of carbachol was blocked by 10 μM atropine, an antagonist of muscarinic acetylcholine receptor (mAcChoR). In the presence of micromolar concentrations of ATP or the GDP analog guanosine 5′‐[β‐thio]diphosphate, carbachol did not change sarcolemmal K+‐O2NPhPase activity. GTP[S] alone reduced this activity (IC50= 2 μM) by about 40% in a concentration‐dependent manner with a lag period of about 3 min. This lag disappeared in the presence of carbachol. Treatment of sarcolemmal membranes with 20 μg/ml pertussis toxin, which catalyzed ADP‐ribosylation of the 40–41‐kDa α‐subunits of inhibitory GTP‐binding protein (Gi), abolished the GTP[S]‐promoted inhibitory effect of carbachol. Immunochemically, these α‐subunits were identified as αi2‐ and αi3‐subunits. It is suggested that the carbachol‐induced inhibition of ouabain‐sensitive K+‐O2NPhPase activity of mammalian myocardial sarcolemma is a result of a negative coupling between mAcChoR and Na+/K+‐ATPase via Gi protein. Copyright © 1990, Wiley Blackwell. All rights reserved