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

Isometric tension was recorded by force displacement transducers in ring se gments of the inferior branch of the renal artery from newborn hooded seals (Cystophora cristata, n = 6), harp seals (Phoca groenlandica, n = 3) and d omestic pigs (Sus scrofa f. domestica, n = 5). Arterial segments were mount ed in heated organ baths and exposed to graded concentrations of noradrenal ine (NA) and adrenaline (A), alone or together with melatonin. The melatoni n concentration in the bath was similar to the plasma concentration normall y found in each experimental animal. Melatonin alone did not affect the ten sion in any of the species, but melatonin potentiated the contraction induc ed by NA in hooded and harp seal arteries to a maximum of about 25% above t he 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 melat onin specifically increases the NA-sensitivity of smooth muscles in renal a rteries from newborn seals. It. is presumed that similar effects may operat e in foetal and maternal seals and may influence their circulation during m aternal diving.