Effects of selective adrenergic agonists and antagonists on gastric tone in the rat

The aim of the present in vivo study was to investigate in anaesthetized rats, the effects of selective adrenergic agonists and antagonists on basal gastric tone and phasic contractions by the use of a volumetric method. L-phenylephrine, an alpha-1 agonist, induced hypertension, bradycardia and a significant gastric relaxation. Clonidine, an alpha-2 agonist, caused hypotension, bradycardia and a significant gastric contraction and a reduction of the amplitude of phasic contractions. Salbutamol, a beta-2 agonist, induced a dose-dependent tachycardia and a significant inhibition of gastric tone whereas prenalterol, a beta-1 agonist, induced tachycardia without any significant influence on gastric basal tone. Yohimbine, an alpha-2 blocker, significantly decreased gastric basal tone and reversed the inhibition of phasic contractions induced by clonidine. Prazocine, a selective alpha-1 blocker, and propranolol, a non-selective beta-blocker, had no significant influence on gastric basal tone or phasic contractions. It is concluded that sympathetic inhibition of basal gastric tone in the rat is mediated by alpha-1 and beta-2 adrenergic receptors. Activation of alpha-2 adrenergic receptors significantly increased basal gastric tone and reduced the amplitude of phasic contractions. A blockade of alpha-2 receptors significantly decreased basal gastric tone and restored the amplitude of phasic contractions.