Calcium binding properties of rat heart plasma membrane and inhibition by structural analogues of dl-propranoloL

Abstract

The isolation and characterization of a plasma membrane preparation from rat heart is described. Enzymatic, chemical, and electron microscopic analysis revealed a relative lack of contamination with nuclear, mitochondrial, ribosomal, and sarcoplasmic reticulum membrane. One calcium binding site (Kd) = 265 [mu]M, Bmax = 65 nmoles/mg protein) was detected by equilibrium dialysis. Monovalent metal ions exhibited 10-100-fold less inhibition potency than divalent metal ions when analyzed by competitive inhibition of calcium binding. The range of Ki values found for divalent metal ions was similar to the Kd value for calcium. La+3 produced a potent non-competitive inhibition. A large variety of structural analogues of d,l-propranolol, many of which have been shown to lack [beta]-adrenergic blocking activity, were competitive inhibitors of the calcium binding activity, with Ki values ranging from 40-900 [mu]M. Electrophilic, hydrophobic, and diamino substituents greatly increased the inhibitory activity. There was no significant difference between related tertiary and quaternary amines. The experimental antiarrhythmic agent UM 272 had the least ability to inhibit calcium binding to the cardiac plasma membrane preparation (Ki = 795 [mu]M). However, UM 424, another experimental antiarrhythmic agent, had an inhibitory activity similar to dl-propranolol (ki = 115 [mu]M and 108 [mu]M, respectively).