Pharmacological properties of the adrenergic receptors regulating systemic vascular resistance in the rainbow trout

1. The adrenergic receptors in the systemic vasculature of the rainbow trout have been pharmacologically characterized using an isolated trunk preparation perfused at constant flow. 2. The dominant α-constrictory receptors in the trunk are similar to those of mammals in their adrenaline/noradrenaline potency ratio (3.2/1.0), and in the natures of their blockade by phenoxybenzamine and yohimbine. However they are more selective than mammalian α-receptors, responding directly to only adrenaline and noradrenaline, and not to phenylephrine, methoxamine, dopamine, or isoprenaline. 3. Tyramine and dopamine cause weak α-adrenergic constriction, apparently indirectly through the release of catecholamine stores. 4. The α-adrenergic response is susceptible to inhibition by competitive β-blocking agents, but this effect is due to non-competitive antagonism with a point of action beyond the adrenergic receptor. 5. β-dilatory receptors of the β₂, as in the homologous systemic vasculature of mammals, also apparently occur, but their dilatory actions cn only be demonstrated against a background of α-adrenergic vasoconstriction. 6. The racemate d,l-isoprenaline is a more potent vasodilator than the pure isomer l-isoprenaline during α-adrenergic tone because of the competitive α-blocking activity of the d-isomer.