Isolation and enzymatic characterization of guinea pig cardiac sarcolemma

Sarcolemma was isolated by fractionation of salt-extracted particles on two consecutive sucrose density gradients. Salt extraction of homogenates, rather than of washed particles, was found to preserve the activities of adenylate cyclase and ouabain-sensitive (Na+,-K+)-ATPase in the isolated sarcolemmal membranes. Purified sarcolemma contained substantial adenylate cyclase and guanylate cyclase activities that were stimulable by beta-adrenergic and muscarinic agonists, respectively. Significant ouabain-sensitive (Na+, K+)-ATPase activity as well as putative digitalis receptor activity was also present in sarcolemma. Cyclic nucleotide phosphodiesterases of sarcolemma, both cAMP- and cGMP-dependent, displayed positive cooperativity of substrate interactions; Ca2+ ions were found to increase the activity of the GMP-dependent enzyme.