Melatonin potentiates the vasoconstrictive effect of noradrenaline in renal artery from newborn hooded seals (Cystophora cristata) and harp seals (Phoca groenlandica)

Abstract.

Isometric tension was recorded by force displacement transducers in ring segments of the inferior branch of the renal artery from newborn hooded seals (Cystophora cristata, n=6), harp seals (Phoca groenlandica, n=3) and domestic pigs (Sus scrofa f. domestica, n=5). Arterial segments were mounted in heated organ baths and exposed to graded concentrations of noradrenaline (NA) and adrenaline (A), alone or together with melatonin. The melatonin concentration in the bath was similar to the plasma concentration normally found in each experimental animal. Melatonin alone did not affect the tension in any of the species, but melatonin potentiated the contraction induced by NA in hooded and harp seal arteries to a maximum of about 25% above the resting, pre-stretch level. The selective melatonin receptor antagonist luzindole reduced this potentiation by 80%. Species-specific concentrations of melatonin did not potentiate the NA-effect in the domestic pig or the A-induced contraction in any of the species. The results indicate that melatonin specifically increases the NA-sensitivity of smooth muscles in renal arteries from newborn seals. It is presumed that similar effects may operate in foetal and maternal seals and may influence their circulation during maternal diving.