Sites of cholinergic vasoconstriction in trout gills.

Sites of cholinergic vasoconstriction were investigated in isolated saline-perfused holobranchs of trout (Salmo gairdneri and S. trutta). Acetylcholine (ACh) always increased overall branchial vascular resistance (BVR) and, in addition, decreased the proportion of the total inflow appearing at the outflow cannula from the efferent arch artery. Since this was observed in both constant pressure and constant flow situations, it was concluded that ACh exerted most of its effect at a site downstream from the secondary lamellae, probably at the bases of the efferent filament arteries. Prussian blue dye injections indicated that, in addition, ACh caused a marked reduction in flow to the distal halves of the filaments and that flow within the proximal secondary lamellae was restricted during ACh administration to the inner and outer marginal channels of the lamellae. The results are discussed in terms of recent findings concerning the vascular anatomy of teleost gills.