Control of vascular tone in notothenioid fishes is determined by phylogeny, not environmental temperature

We examined potential vasomotor control mechanisms in an Antarctic fish (Tr ematomus bernacchii; usual core temperature approximately -1 degreesC), com paring sensitivity to agonists by means of the cumulative dose response and potency with reference to depolarization by 50 mM KCl. In efferent branchi al arteries, norepinephrine (NE) produced similar to 20% of the maximal KCl tension and similar to 40% in the presence of 10(-3) M sotalol, suggesting a modest contribution of alpha- and beta -adrenergic tonus [half-maximal r esponse (pEC(50)) = 6.29 +/- 0.37 M]. Carbachol (CBC) and serotonin (5-HT) had different sensitivities (pEC(50) = 4.50 +/- 0.40 and 6.82 +/- 0.08 M, r espectively) but similar potencies (21.6 +/- 11.1 and 31.1 +/- 5.3% of KCl) . A related species from warmer waters around New Zealand, Paranotothenia a ngustata, had similar vascular reactivity for NE (pEC(50) = 5.48 +/- 0.31 M ), CBC (pEC(50) = 4.94 +/- 0.22 M), and methysergide-sensitive vasoconstric tion with 5-HT (pEC(50) = 6.22 +/- 0.40 M). Agonist potencies were 9, 65, a nd 45% that of KCl, respectively. Bovichtus variegatus, a member of the phy logenetic sister group to the notothenioids, also gave broadly similar resp onses. In contrast, Dissostichus mawsoni, a pelagic Antarctic notothenioid, showed a dominance of vasodilatation over vasoconstriction, with sensitive isoprenaline (pEC(50) = 6.66 +/- 0.05 M) but weak serotonergic (5.2 +/- 1. 5% KCl) responses. The unusual dominance of serotonergic control appears to be primarily a consequence of evolutionary lineage rather than low environ mental temperature, but the pattern may be modified according to functional demand.