Interaction of the Calcium Channel Activating3H-Bay K 8644 and Inhibiting3H-Verapamil with Specific Receptor Sites on Cultured Beating Myocardial Cells

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Abstract
Binding properties of the calcium channel activating dihydropyridine (DHP), 3H-BAY K 8644, and the inhibiting 3H-verapamil were demonstrated in monolayer cultures of beating cardiac cells. 3H-BAY K 8644 specific binding was dependent on the presence of extracellular calcium, the affinity was modulated by Ca2+, but Hill coefficients remained unaffected. BAY K 8644 stimulated myocardial contractility in resting and beating myocytes. In contrast to ß-adrenoceptor agonists, however, cellular levels of cyclic AMP and cyclic GMP in cultured myocytes remained unchanged by the compound. Dihydropyridine derivatives of both the calcium channel activating BAY K 8644 as well as the Ca2+ entry blocking DHPs of the nifedipine or nimodipine type yielded very low affinity to other receptors measured in brain and heart membranes. 3H-BAY K 8644 binding sites proved to be highly specific for various potently displacing DHP derivatives and discriminated between optical isomers (stereoselectivity) with inhibition constants (Ki) in the nanomolar range. The heterogeneous shapes of the competition curves also imply interactions of these compounds with different (sub-)sites of the DHP receptor that represents one locus of interaction in regulating transmembranal Ca2+ currents. The other specific site of action for the potent diphenylalkylamines, clearly different to the DHP receptor was characterized with 3H-verapamil. The equilibrium dissociation constant, Kd in cultured myocytes ranged between 16-25 nM, and binding capacity, Bmax amounted to about 1.85 pmol/mg of protein. The different mode of competitions indicates the involvement of more than one 3H-verapamil binding site. The interrelation of the structurally heterogeneous channel modulators with the differently radiolabelled receptor (sub-)sites located in or near by the calcium channel may represent new approaches in investigating the nature of action of these potent compounds.