The effects of the calcium channel agonist, Bay K-8644, on guinea-pig ileum and rat uterine horn.

Bay K-8644 (methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2- trifluoromethylphenyl)pyridine-5-carboxylate) concentration-dependently caused contractions of the partly depolarized ileum, but at higher concentrations (10(-6) to 10(-5) M) produced relaxation. 10(-8) to 4 X 10(-7) M nifedipine antagonized while 10(-9) and 4 X 10(-9) M potentiated Bay K-8644. On the partly depolarized uterus, Bay K-8644 (10(-9) to 10(-6) M) had only a spasmogenic effect whereas nifedipine (4 X 10(-9) to 10(-7) M) was spasmolytic. Verapamil and diltiazem (each at 10(-4) M) both reduced the maximal response to Bay K-8644, while other spasmolytics were ineffective. Thus, Bay K-8644 activates ileum and uterus by opening voltage-operated Ca2+ channels, but its relaxant action at high concentration and its potentiation by nifedipine is not seen in both organs. Such differences probably depend on the concentration of the compounds used and the polarization state of the cell membranes.