Sites of interactions of surfactants with beta-adrenergic responses in trout (Salmo gairdneri) gills.

The aims of this study were to investigate the effects of surfactants on the beta-adrenergic responses of the arterio-arterial (lamellar) vascular pathway of isolated gills from the trout Salmo gairdneri. The resistance of the arterio-arterial vasculature was monitored by measuring the input pressure of isolated gills perfused at a constant flow and at constant efferent pressure. Specific beta-adrenergic stimulation with isoprenaline produced a dose-dependent reduction in vascular resistance which was mimicked by the presence of cyclic 8-(4-chlorophenylthio) adenosine-3':5'-monophosphate (8-cpt cyclic AMP). These results suggest that cyclic AMP is involved as an intracellular second messenger of beta-adrenergic regulatory responses of branchial haemodynamics. Surfactants in the perfusate at nominal concentrations of 1.0 mumol 1(-1) (0.3-0.5 mg 1(-1)) inhibited the response of the branchial tissue to isoprenaline but did not affect the response to 8-cpt cyclic AMP. This suggests that the surfactant effects are the result of events occurring within the cell membrane. Furthermore because the surfactants used affected the efficacy, rather than the affinity, of the isoprenaline response then the mechanism of action is unlikely to be an effect on agonist/receptor interaction but may be further along the stimulus response chain, perhaps affecting receptor/regulatory nucleotide/adenylate cyclase interaction or via direct inhibition of the cyclase itself.