The vasorelaxant effects of amrinone on neonatal rabbit thoracic aortic strips

The present study was designed to compare the in vitro vasorelaxant responses to the bipyridine compound amrinone of prostaglandin F2 alpha (PGF2 alpha) precontracted aortic strips isolated from newborn (2 to 3 days old) and nonpregnant adult rabbits. Changes in forces were recorded isometrically. The effects of isoproterenol (beta-adrenergic receptor stimulator), forskolin (direct activator of adenylate cyclase) and theophylline (non-specific phosphodiesterase inhibitor) were also studied simultaneously. The vasodilator responses to amrinone, forskolin and theophylline in aortic strips from neonates were comparable to those of strips from adults. In contrast, the maximum relaxations to isoproterenol were significantly reduced in those from neonates as compared with those from adults, suggesting that beta-adrenergic receptor mediated responses are not fully developed in the neonate. The mean value for the ED50 concentration (half the maximum response) for amrinone, isoproterenol and theophylline was 1.10 +/- 0.10 x 10(-4) M, 3.47 +/- 1.31 x 10(-8) M and 1.53 +/- 0.10 x 10(-4) M, respectively, in the neonatal group and 1.17 +/- 0.19 x 10(-4) M, 3.75 +/- 1.11 x 10(-8) M and 1.75 +/- 0.06 x 10(-4) M, respectively, in the adult group; there were no significant differences between the neonatal and adult preparations. We concluded that the vasorelaxation produced by phosphodiesterase inhibitors demonstrated no maturation-related changes probably due to a stable cyclic AMP turnover through an unaltered phosphodiesterase activity in newborn and adult arteries.