BAY K 8644-induced enhancement of 45Ca uptake by rabbit aortic rings

BAY K 8644, a Ca2+ channel activator, enhances uptake of 45Ca by rabbit aortic rings. This effect depends on the concentration of K+ in the medium: at 20 mM K+ the effect of BAY K 8644 was more pronounced than at 5 mM, whereas at 80 mM, no significant enhancement of 45Ca uptake by BAY K 8644 was found. In the medium containing 5 mM K+, BAY K 8644 was effective in experiments involving 10 or 30 (but not 3) min exposure of aortic rings to 45Ca. The dose-response curve for BAY K 8644 was established in 5 mM K+-containing medium and for 30 min exposure to 45Ca. BAY K 8644 was effective at 0.01 microM and higher concentrations. In the presence of norepinephrine (0.1 or 10 microM), BAY K 8644 had no greater effect on 45Ca uptake than in control 5 mM K+ medium. Our observation that the presence of norepinephrine in 5 mM K+ did not enhance BAY K 8644-induced 45Ca uptake suggests that activation of alpha-adrenergic receptor does not depolarize aortic membranes to the same extent as an increase in K+ concentration to 20 mM or that BAY K 8644 does not enhance Ca2+ entry through receptor-operated channels.