Contraction and cyclic AMP-related relaxation of the intimal and medial smooth muscle layers of pig thoracic aorta.

Abstract

The contractile responses of an alpha-adrenoceptor agonist, phenylephrine, and of histamine were compared in the intimal and medial smooth muscle layers of the pig aortic arch. Further, the relaxant effects evoked by some compounds influencing the cyclic AMP system were compared in the two muscle layers, as well as their effects on the cyclic AMP content and phosphodiesterase activity. Phenylephrine and histamine induced contraction of the smooth muscle layers. The increase in tension was faster in the intimal than in the medial layer. The alpha-adrenoceptor agonist phenylephrine was a more potent contractile agent in the intimal than in the medial smooth muscle. With histamine, no significant difference in the dose-response curves between the two muscle layers was found. Histamine-contracted muscle preparations were relaxed in a dose-dependent manner by the phosphodiesterase-inhibiting compound 3-isobutyl-1-methylaxanthine (MIX) and by 8-bromo-cyclic AMP. The two substances were more potent relaxants in the medial than in the intimal smooth muscle layer. The content of cyclic AMP in the intimal and the medial smooth muscle was increased by MIX. Isoprenaline had no relaxing effect on the muscle preparations and did not change the content of cyclic AMP. There were no differences in the basal levels of cyclic AMP in the intima and media. Vmax of phosphodiesterase activities differed, however, between the two preparations. This study demonstrates that the intimal layer is characterized by a larger contractile responsiveness to phenylephrine and a lower relaxant response to compounds influencing the cyclic AMP-system than those of the medial layer.