Theoretical Bases for Vascular Selectivity of Ca2+ Antagonists

Summary: The purpose of these studies was to characterize mechanisms of activation of vascular tissues in order to clarify possible theoretical bases for Ca2+-antagonist (CAt) selectivity. Activation of rabbit aorta, mesenteric artery, and mesenteric resistance vessels by agonists and depolarizing potassium (K+) solution, and inhibition by CAts were studied by measurement of contractions, 45Ca fluxes, and membrane potentials (via intracellular electrodes). We found that the CAts studied (D-600, diltiazem, and nisoldipine) inhibited K+-induced Ca2+ influx and contractions in a closely correlated manner, suggesting inhibition of Ca2+ entry as their primary, if not sole, mechanism of action. Diltiazem and nisoldipine had no effect on intracellular Ca2+ release or on the contractile protein system. The following evidence was obtained to support the tenet that norepinephrine (NE) and 80 mM K+ open two distinct Ca2+ channels, one receptor-operated (ROC) and the other potential-operated (POC): (a) NE activated the rabbit aorta without eliciting a change in membrane potential; (b) NE activation of the rabbit mesenteric resistance vessels was accompanied by membrane depolarization, but this depolarization was blocked by 10-5M diltiazem, whereas that induced by 80 mM K+ was not; (c) 45Ca influx stimulated by 80 mM K+ and that stimulated by a maximal [NE] were additive when the two agents were administered together in the aorta and the resistance vessels; (d) the Ca2+-channel agonist Bay K8644 opens the POC but not the ROC in rabbit aorta, as it is capable of stimulating Ca2+ influx in addition to 10−5M NE, but not to 80 mM K+; (e) the CAts show selective inhibition of the POC over the ROC in the aorta, whereas diltiazem preferentially inhibits the ROC in the resistance vessels; and (f) alpha adrenoreceptor occupation, phosphodiesterase inhibition, or dibutyryl cyclic AMP selectively inhibits the POC over the ROC in the aorta. We have also found that ROCs vary widely in their sensitivity to CAts, depending upon the level of activation of the tissue and upon the vessel studied; the POC, in contrast, retains a rather constant CAt sensitivity, regardless of the level of activation and of the vessel activated. The degree of CAt sensitivity of the ROC is inversely correlated with the degree to which NE releases intracellular Ca2+. The release of intracellular Ca2+ by NE appears to gate the ROC, as it cannot be further opened when the NE-releasable intra-cellular Ca2+ store has been previously released, whereas the POC can be opened. We conclude that differing mechanisms of activation by NE in various tissues can be largely responsible for differences in their sensitivities to CAts.