STUDIES ON THE INTESTINAL RELAXATION PRODUCED BY DOPAMINE

¹ Department of Pharmacology, Marquette School of Medicine, Milwaukee, Wisconsin

The intestinal relaxation produced by dopamine was compared to that produced by other sympathomimetic amines in the isolated rabbit jejunum. The drug is similar to norepinephrine in that it can stimulate both alpha and beta receptors to produce intestinal relaxation. Evidence which supports the latter conclusion includes the observations that 1) dopamine and norepinephrine were less susceptible than methoxamine to blockade by phentolamine, 2) dopamine and norepinephrine were less susceptible than isoproterenol to blockade by MJ-1999 and 3) blockade of the responses to dopamine and norepinephrine was enhanced when alpha and beta blocking agents were used in combination. Cold-stored intestinal preparations, in which alpha receptor activity is impaired or abolished, could still respond maximally to dopamine. Additionally, the intestinal relaxation produced by dopamine appears to be due in part to a catecholamine-releasing action. Thus, pretreatment with reserpine or administration of cocaine reduced the response to dopamine. Sheep jejunum differed from rabbit jejunum in that methoxamine stimulated the former and relaxed the latter and in that dopamine was the major catecholamine present in the former but not the latter. In both species, dopamine was about 100 times or more less potent than norepinephrine in producing intestinal relaxation.