Characterization by cyproheptadine of the dopamine-induced contraction in canine isolated arteries.

Spirally cut strips of isolated canine femoral and carotid arteries were used to characterize the vasoconstrictor effect of dopamine. A serotonin antagonist, cyproheptadine, 10- minus 7, 3 X 10- minus 7 and 10-minus 6 M, inhibited dopamine (pA2 = 7.46 plus or minus 0.11), tryptamine (pA2 = 7.38 plus or minus 0.09) and serotonin-induced contractions (pA2 = 7.59 plus or minus 0.09). Cyproheptadine shifted the threshold concentration of dopamine 2 log units to the right without altering norepinephrine response. In protection experiments, preincubation with serotonin (10-minus 5 M) for 10 minutes protected serotonin and tryptamine receptors from cyproheptadine (3 X 10-minus 7 M) and phenoxybenzamine (10-minus 6 M) blockade. Serotonin cross-protected dopamine receptors also. Reciprocally dopamine (10-minus 4 M) protected its own receptors as well as those for serotonin and tryptamine. Norepinephrine did not afford any protection of dopamine, serotonin or tryptamine receptors but did protect its own receptors from phenoxybenzamine blockade. The attempt to characterize the receptor subserving dopamine-induced contraction failed to confirm a relationship between dopamine and alpha adrenoceptors. Since dopamine, tryptamine and serotonin, unlike norepinephrine, were sensitive to cyproheptadine blockade, dopamine-induced contraction appeared to be mediated by a receptor closely related to serotonin receptors. Therefore, alpha adrenoceptors may not be exclusively involved in the vasoconstriction evoked by dopamine on canine vasculature.