CROMAKALIM INHIBITS MULTIPLE MECHANISMS OF SMOOTH-MUSCLE ACTIVATION WITH SIMILAR STEREOSELECTIVITY

Purified cromakalim trans enantiomers were tested for their ability to antagonize three specific mechanisms of smooth muscle activation, i.e ., depolarization-induced Ca2+ entry through voltage-gated channels, a gonist-induced Ca2+ entry, and agonist-induced Ca2+ release. Cromakali m effects were studied in rabbit aortic rings contracted by stimuli co rresponding to the above mechanisms. First, aortic rings were contract ed by increase in extracellular [K+] (to 27 mM), which causes partial membrane depolarization. Under these conditions, (-)-cromakalim exhibi ted an EC50 of 0. 18 muM and a 150-fold higher relaxing potency than t he (+)-enantiomer. Second, in aortic rings tonically contracted by 1 m uM norepinephrine (NE) in the presence of 1 muM nifedipine, i.e., in r ings contracted mainly owing to NE-stimulated Ca+ entry through recept or-operated channels, (-)-cromakalim induced relaxation with an EC50 O f 0.68 muM and exhibited a 191-fold higher potency than the (+)-enanti omer. Third, phasic, NE-induced contractions of rabbit aortic rings in the absence of extracellular Ca2+, i.e., contractions that reflect re lease of Ca2+ from intracellular stores, were antagonized with an EC50 of 0.29 muM and a 144-fold higher potency than the (+)-enantiomer. Al l effects of (-)-cromakalim were blocked by either completely depolari zing the vessels with high extracellular [K+] (40 mM) or by addition o f the K+ channel blocker glibenclamide (10 muM). Cromakalim relaxed ra bbit aorta independent of the mechanism underlying smooth muscle tone. Cromakalim effects were equivalent with respect to dose dependence, s tereoselectivity, and sensitivity to extracellular [K+] and glibenclam ide. Our results suggest that cromakalim inhibits depolarization-induc ed as well as receptor-activated Ca2+ entry and intracellular Ca2+ rel ease by interaction with the same cellular target site, i.e., a specif ic potassium channel in the plasma membrane.