Effects of the Ca agonists Bay K 8644 and CGP 28392 on vascular smooth muscle tone

1. Nifedipine (5 x 10(-9) M) reduced and the Ca agonists CGP 28392 (10(-6) M) and Bay K 8644 (10(-7) M) increased the contractions elicited by K+ in segments of rat aorta, whereas those caused by noradrenaline (NA) were unaffected. 2. CGP 28392 and Bay K 8644 evoked concentration-dependent contractions in segments moderately depolarized with 15 mM K+; these responses were reduced by nifedipine. 3. 45Ca uptake induced by 50 mM K+ or NA (10(-7) M) was reduced by nifedipine (10(-6) M). Bay K 8644 (10(-6) M) only increased the 45Ca uptake induced by K+, which was inhibited by nifedipine (10(-6) M). 4. These results indicate that in this vascular preparation: (a) Ca agonists only activate voltage-dependent Ca channels (VDCs), the potency of Bay K 8644 being higher than of CGP 28392, and (b) VDCs and receptor-operated Ca channels have different pharmacological properties.