Effects of Bay K 8644 on 45Ca uptake and efflux and on contraction in the rabbit aorta

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Abstract
1 Contractions induced by K+, noradrenaline and 11,9-epoxymethano prostaglandin H2 (11,9-epoxymethano PGH2) were accompanied by a large, moderate and negligible stimulation of 45Ca uptake in rabbit aortic rings, respectively. Bay K 8644, 14 and 56 nM, enhanced both the contraction and the 45Ca uptake stimulated by all 3 agonists. 2 In the absence of agonists, Bay K 8644 (14 and 56 nM) caused a small contraction and increase in 45Ca uptake. 3 45Ca efflux was increased by noradrenaline, and Bay K 8644 augmented this. 4 In Ca-free solution, contractions induced by noradrenaline or 11,9-epoxymethano PGH2 were not augmented by Bay K 8644. 5 Nifedipine (0.1 μM) antagonized 45Ca uptake stimulated by K+ or noradrenaline. Nifedipine also reduced the stimulant effect of Bay K 8644 on 45Ca uptake in the presence of all three agonists. 6 It is concluded that, in the rabbit aorta, Bay K 8644 enhances the opening of Ca channels both during depolarization and in the presence of receptor-specific agonists and is also able to open Ca channels under basal conditions. Bay K 8644 appears not to reduce Ca efflux or enhance Ca release from intracellular stores.