26601
title and abstract
The effects of labetalol and dilevalol on isolated cardiovascular preparations of the guinea-pig and rat.
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 (> or = 10(-8) M) and of dilevalol (> or = 10(-7) M) inhibited to a small extent the responses to electrical cardiac stimulation, which is indicative of membrane stabilizing activity. Labetalol (> or = 3 x 10(-8) M) and dilevalol (> or = 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 beta 1-adrenoceptors of the rat left atria and 7.90 and 8.31, respectively, on the guinea-pig left atria which has functional beta 1- and beta 2-adrenoceptors. Labetalol and dilevalol (both at > or = 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 beta 2-adrenoceptor mediated. The isoprenaline attenuation responses of the portal vein were inhibited by labetalol and dilevalol (both at > or = 10(-7) M) and the pA2 value for the labetalol at beta 2-adrenoceptors was 7.59. It is concluded that labetalol and dilevalol are beta 1-adrenoceptor selective antagonists.