Influence of Bay k 8644 on Pressor Effects of B-HT 920 and Cirazoline in Pithed Cats

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Abstract
Interactions between the putative Ca entry promotor Bay k 8644 [1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoro methyl) phenyl) pyridine carboxylic acid methyl ester] and .alpha.1-adrenoceptor-mediated increases in diastolic pressure elicited by cirazoline as well as .alpha.2-adrenoceptor-mediated pressor responses induced by B-HT 920 [alefexole hydrochloride] in pithed cats. Bay k 8644 (0.01-1 mg/kg, i.a. [intraarterial]) did not affect the log dose-pressor response curve of cirazoline, but slightly potentiated the increase in diastolic pressure elicited by B-HT 920. After attenuation of the B-HT 920-induced pressor effects by the Ca blocker nifedipine (0.1 mg/kg i.a.), Bay k 8644 (0.1 mg/kg, i.a.) strongly enhanced the pressor response. The increase in diastolic pressure elicited by cirazoline was not affected by nifedipine (0.1 mg/kg, i.a.), and the addition of Bay k 8644 (0.1 mg/kg, i.a.) had no effect. In contrast to the increase in diastolic pressure elicited by B-HT 920, Ca channels evidently are not involved in the cirazoline-induced pressor responses in the pithed cat. The activation of the Ca channels by B-HT 920 is already so efficient that it cannot be further enhanced by Bay k 8644.