Comparison of the effects of the vasoconstrictive dihydropyridines BAY K 8644 and CGP 28392 on isolated rat aorta: different sensitivities to ultraviolet radiation.

The effect of BAY K 8644 was compared to that of CGP 28392 in isolated rat thoracic aorta. In low concentration both dihydropyridines had a direct concentration dependent contractile effect that could be eliminated in Ca-free medium and by the Ca-antagonist nifedipine. Readdition of Ca to the Ca-free medium restored the contractile response to both agents. In high concentrations (greater than or equal to 10(-5) M), both agents depressed active tone. In physiological salt solution containing 15.8 mM, potassium BAY K 8644 was far more potent in contracting rat aorta than CGP 28392. The responses to BAY K 8644 and CGP 28392 were not affected by alpha- and beta-adrenoceptorblockade. Both compounds shifted the noradrenaline- and potassium-concentration response curve to the left and increased the maximal response. Ultraviolet radiation (UVR) had a reversible and marked effect on BAY K 8644 and no effect on CGP 28392 contractions. The UVR-mediated relaxation of BAY K 8644-induced contractions could partly be eliminated by raising extracellular Ca. The results showed that differences according to potency and mode of action exist between BAY K 8644 and CGP 28392 in rat aorta. To explain the effect of UVR on BAY K 8644-induced contractions, a modification of a Ca channel model is proposed that implies the existence of "photosensitive proteins" located to "Ca antagonist" binding sites. According to this hypothesis, UVR may induce a reversible binding of BAY K 8644 to "Ca antagonist receptors."