Effects of light and BAY K 8644, a new 1,4-dihydropyridine, on mechanical responses of rat thoracic aorta.

The effect of day light and ultraviolet radiation (360 nm) on mechanical responses to BAY K 8644 (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(-2-trifluoromethylphenyl)-py ridine- 5 -carboxylate), potassium (K+) and noradrenaline (NA) of rat aorta rings was investigated. The contractile response to BAY K 8644 (10(-6)M) obtained before and after exposure of the BAY K 8644 stock solution to ultraviolet radiation was unchanged and equal to that of K+, 125 mM. Ultraviolet radiation and day light did not affect responses evoked by K+ (125 mM) and NA (1.8 X 10(-5)M). In contrast to this both types of light relaxed vessels contracted by BAY K 8644 (10(-6)M). The light induced relaxations were reversible, unaffected by addition of propranolol (3 X 10(-6)M) and could not be eliminated by washing the preparations repeatedly with Krebs solution. In vessels contracted by K+ (125 mM) and NA (1.8 X 10(-5)M) ultraviolet radiation induced a reversible relaxation in the presence of BAY K 8644. BAY K 8644 (10(-4)M) and nifedipine (10(-8)M) relaxed preparations contracted by K+. Nifedipine (10(-6)M) totally relaxed preparations contracted by BAY K 8644 (10(-6)M). Ultraviolet radiation eliminated the relaxant effect of nifedipine and decreased the relaxant effect of BAY K 8644 (10(-4)M). The results indicate that BAY K 8644 is more light-stable than nifedipine and that BAY K 8644 sensitized the vascular smooth muscle to ultraviolet radiation as well as day light. Consequently this should be taken into account when BAY K 8644 is studied.