MODULATORS OF CALCIUM AND POTASSIUM CHANNELS - THEIR EFFECTS ON ENDOTHELIN-1 BINDING TO CARDIAC MEMBRANES

Endothelin-I (ET-1) causes long-lasting vasoconstriction associated wi th a prolonged elevation of intracellular free Ca2+. Because this may be mediated through an effect on membrane ion channels, we investigate d the effects of the dihydropyridine calcium channel antagonist nifedi pine; two structurally distinct K+ channel openers, pinacidil and levc romakalim; and the inactive stereoisomer of levcromakalim (D-cromakali m), as well as ET-1 and ET-3, on binding of I-125-labeled endothelin-I to rat cardiac membranes. Specific binding of I-125-ET-1 was inhibite d in a concentration-dependent manner by unlabeled ET-I (IC50 = 1.56 /- 0.78 nM; slope = -0.49 +/- 0.10) and ET-3 (IC50 = 314 +/- 54 nM; sl ope = -0.34 +/- 0.11). Nifedipine, in concentrations less-than-or-equa l-to 10(-5) M, did not affect I-125-ET-1 binding. However, levcromakal im significantly inhibited I-125-ET-1 binding (maximum binding 49 +/- 9%; p = 0.04), whereas the inactive isomer, D-cromakalim, had no effec t. Pinacidil also inhibited I-125-ET-1 binding, although to a lesser e xtent than levcromakalim (maximum binding 63 +/- 7%). These findings p rovide evidence for a stereospecific interaction between K+-channel op eners and ET-1 binding in rat cardiac membranes. Because the slope of the logistic fit was substantially less than unity, and the effects of pinacidil and levcromakalim were incomplete, there may be two or more receptors for ET-1 in rat heart, only one of which is sensitive to K-channel openers.