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

Abstract

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.