UNMASKING EFFECT OF ALAMETHICIN ON THE (NA+,K+)-ATPASE, BETA-ADRENERGIC RECEPTOR-COUPLED ADENYLATE-CYCLASE, AND CAMP-DEPENDENT PROTEIN-KINASE ACTIVITIES OF CARDIAC SARCOLEMMAL VESICLES

Abstract

A mechanism for the activating effect of alamethicin on membrane enzymes was investigated, using a purified preparation of [canine] cardiac sarcolemmal vesicles. (Na+,K+)-ATPase, .beta.-adrenergic receptor-coupled adenylate cyclase and cAMP-dependent protein kinase activities were measured. Alamethicin increased ouabain-sensitive (Na+-K+)-ATPase activity of sarcolemmal vesicle 5- to 7-fold and adenylate cyclase activity 2.5- to 4-fold. Adenylate cyclase retained its sensitivity to the .beta.-adrenergic agonist isoproterenol after membranes were treated with alamethicin. Alamethicin caused a 4- to 6-fold increase in the number of detectable (Na+-K+)-ATPase enzymic sites, but no increase was noted for the number of muscarinic-cholinergic receptor-binding sites. Phosphorylation of endogenous proteins of sarcolemmal vesicles by an intrinsic cAMP-dependent protein kinase activity was stimulated 5- to 7-fold by alamethicin. The regulatory subunit of the membrane-bound cAMP-dependent protein kinase was labeled with the photoaffinity probe 8-azido-adenosine 3'':5''-[32P]monophosphate (8-N3-[32P]cAMP) and it migrated with an apparent MW of 55,000 in sodium dodecyl sulfate polyacrylamide gels. Alamethicin stimulated autophosphorylation of the regulatory subunit by [.gamma.-32P]ATP 6-fold and incorporation of 8-N3-[32P]cAMP into the subunit 2.6-fold. Alamethicin apparently disrupts membrane barriers of sarcolemmal vesicles which are mostly right side out, giving substrates and activators access to enzymic sites in the interior of the vesicles while preserving functional coupling of enzymes to their effectors.