Evidence against nonprostanoid endothelium-derived relaxing factor(s) in trout vessels.

Bradykinin (BK)- or acetylcholine (ACh)-mediated vasodilation has only rarely been observed in fish. This suggests that many fish vessels lack the endothelium-dependent relaxing mechanisms recently identified in mammals. To examine this hypothesis, isolated vascular rings were prepared from trout ventral aortas (VA), efferent branchial and celiacomesenteric (CM) arteries, and anterior cardinal veins (CV) and examined for endothelium-mediated responses. A doubly perfused trunk preparation was also used to evaluate the response of the microcirculation. ACh produced dose-dependent contractions in all vascular rings and increased vascular resistance when perfused into the CM but had no affect when perfused into the dorsal aorta. Neither ACh nor BK relaxed precontracted vessels or lowered resting tone. Removal of the endothelium did not affect ACh or BK responses. The calcium ionophore A23187 produced an endothelium-dependent relaxation of precontracted VA, CM, and CV. The A23187 response was abolished by indomethacin, indicating that a prostanoid was involved in the relaxation. ATP contracted and/or relaxed precontracted CM and CV. ATP effects were independent of an intact endothelium. Sodium nitroprusside and atrial natriuretic factor partially or completely relaxed all vessels, indicating the presence of soluble and particulate guanylate cyclases, respectively. These results suggest that nonprostanoid endothelium-derived relaxing factors (EDRFs) or EDRF-like vasodilator mechanisms are not present in trout vessels or, if they are, they are not released by classical agonists.