Lack of pre- and postjunctional beta adrenoceptor-mediated effects in the canine saphenous vein at birth.

The present study was undertaken to compare the relevance of alpha and beta adrenoceptor-mediated responses at pre- and postjunctional level in the canine saphenous vein of neonates and adults. To quantify prejunctional action, the effect of drugs on the neurogenic outflow of tritium from the vessel loaded previously with [3H]norepinephrine or [3H]epinephrine was measured. The selective alpha-2 adrenoceptor agonist UK-14,304 [5-bromo-6-(2-imidazoline-2-ylamino)-quinoxaline; 10-1000 nM reduced and the selective alpha-2 adrenoceptor antagonist yohimbine (30-300 nM) enhanced the overflow of tritium evoked by electrical stimulation (1 Hz; 2 msec; 100 V; 300 pulses) in both adult and neonate tissues. However, in strips preloaded with [3H]norepinephrine, the beta adrenoceptor agonist isoproterenol (50 nM) increased the overflow of tritium in strips of adults but had no effect in strips of neonates; and in the strips preloaded with [3H]epinephrine, the beta adrenoceptor antagonist propranolol (1 microM) reduced the overflow of tritium in adults but had no effect in neonates. Postjunctionally, phenylephrine (0.1-50 microM) caused concentration-dependent contractions of the saphenous vein rings from adults and neonates but isoproterenol, which caused concentration-dependent relaxations on rings contracted previously by phenylephrine in adults, had no effect in neonates. In contrast to isoproterenol, forskolin (0.05-5 microM), under the same conditions, caused concentration-dependent relaxations of rings of both adults and neonates.(ABSTRACT TRUNCATED AT 250 WORDS)