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The Effects of Labetalol and Dilevalol on Isolated Cardiovascular Preparations of the Guinea‐pig and Rat *

Abstract— Differing effects of labetalol and dilevalol on cardiovascular preparations have been reported. I have studied the effects of labetalol and dilevalol on the contractile responses of the rat and guinea‐pig left atria and rat portal vein. On the guinea‐pig left atria low concentrations of labetalol (≥ 10−8 m) and of dilevalol (≥ 10−7 m) inhibited to a small extent the responses to electrical cardiac stimulation, which is indicative of membrane stabilizing activity. Labetalol (≥ 3× 10−8 m) and dilevalol (≥ 10−8 m) caused surmountable antagonism of the isoprenaline responses of the atria and the pA2 values were 8·60 and 8·98 at the β1‐adrenoceptors of the rat left atria and 7·90 and 8·31, respectively, on the guinea‐pig left atria which has functional β1 and β2‐adrenoceptors. Labetalol and dilevalol (both at ≥ 10−7 m) attenuated the spontaneous contractile activity of the rat portal vein and the attenuation to labetalol at 10−6 m was abolished by ICI 118,551 which illustrates that the labetalol‐induced attenuation is β2‐adrenoceptor mediated. The isoprenaline attenuation responses of the portal vein were inhibited by labetalol and dilevalol (both at ≥ 10−7 m) and the pA2 value for the labetalol at β2‐adrenoceptors was 7·59. It is concluded that labetalol and dilevalol are β1‐adrenoceptor selective antagonists.