Regional differentiation of gastric and of pyloric smooth muscle in the pig: mechanical responses to acetylcholine, histamine, substance P, noradrenaline and adrenaline.

1. Mechanical activity was recorded in isolated muscle preparations from the circular and longitudinal layers of different regions of porcine stomach and from the pyloric ring. 2. In all gastric strips acetylcholine (10(-7)-10(-4) mol l-1), histamine (10(-7)-10(-5) mol l-1) and substance P (10(-9)-10(-7) mol l-1) elicited contractions. 3. In fundic strips the catecholamines, noradrenaline and adrenaline (10(-8)-10(-5) mol l-1), produced inhibitory responses at lower concentrations and excitatory responses at high concentrations. After blockade of beta-adrenoceptors by propranolol, a pure excitatory response remained which could be inhibited by alpha-adrenoceptor blockade with phentolamine. No responses were observed in corpus and antrum. 4. Strong contractions were induced by these catecholamines in circular muscle strips of the pyloric ring, in contrast to previous findings reported for human and canine preparations. The contractile response of strips from the inner layer of the pyloric ring corresponded to an average shortening of 25-30% of resting length, at a concentration of 10(-5) mol l-1 of noradrenaline or adrenaline. The responses of preparations from the outer layer of the pyloric ring were similar to those of the inner layer but generally weaker. The excitatory responses to the catecholamines were totally suppressed by 10(-5) mol l-1 phentolamine or prazosin but remained unaltered after pretreatment with 10(-5) mol l-1 propranolol or 10(-6) mol l-1 yohimbine, which indicates that they are mediated by alpha 1-adrenoceptors. 5. Application of substance P (10(-7) mol l-1) in the pyloric preparations caused a contraction as strong as that induced by noradrenaline or adrenaline. However, histamine, which is known to induce strong contractions in canine pylorus, had--as in human pylorus--no effect on porcine pylorus at concentrations up to 10(-5) mol l-1.