A study of the phasic response of arterioles of the guinea pig small intestine to prolonged exposure to norepinephrine.

Arterioles of the guinea pig small intestine constricted in response to norepinephrine, but the constriction was not maintained. The duration of constriction was reduced after pretreatment by theophylline, 3-isobutyl-1-methyl-xanthine, bromo-cAMP, or bromo-cGMP, suggesting that the relaxation was related to an increase in cyclic nucleotide levels in the cell. Forskolin also reduced the duration of constriction, suggesting the involvement of cAMP. The duration of constriction was not affected by propranolol or isoprenaline, indicating no involvement of beta adrenoceptors. A scheme to explain these observations, in which alpha 1-adrenoceptor activation stimulates adenylate cyclase, leading to a rise in cAMP and an increased rate of intracellular calcium sequestration is proposed. The resulting fall in intracellular calcium leads to repolarization and relaxation.