Effects of dopamine and bromocriptine on colonic motility in dog

The effects of intravenous infusions of dopamine (0.1 to 1 mg kg-1h-1) and bromocriptine (10 to 40 micrograms kg-1h-1) on colonic motility were investigated in fasted dogs fitted with permanent strain gauges on the ascending, transverse and descending colon. Infused at rates of 0.5 and 1 mg kg-1h-1 during 1 h, dopamine immediately stimulated the motility of the descending colon; after a delay of 40 to 60 min this effect was balanced by an inhibition of the motility of the ascending and transverse colon. Bromocriptine infused intravenously at doses of 10 to 40 micrograms kg-1h-1 stimulated the motility of the whole colon but these effects were limited to the duration of the infusion (60 min). Both propranolol (0.5 mg kg-1) and tolazoline (2 mg kg-1) failed to block the effects of dopamine and bromocriptine whereas phentolamine (0.1 mg kg-1) and prazosin (0.2 mg kg-1) partially reduced the inhibitory effects of dopamine on the proximal colon. Haloperidol at doses higher than 0.2 mg kg-1 and domperidone blocked the bromocriptine-induced stimulation of colonic motility which was unaffected by previous treatment with alpha- and beta-adrenoceptor blocking agents. These results suggest that in the dog, dopamine and bromocriptine stimulate colonic motility through specific dopamine receptors. However, they suggest that the inhibitory effects of dopamine on the proximal colon which are blocked by dopamine antagonists are also partially due to an effect on alpha 1-adrenoceptors.