Melatonin reduces noradrenaline-induced vasoconstriction in the uterine artery of pregnant hooded seals (Cystophora cristata).

In pregnant seals the dive-associated constriction of the uterine artery is inhibited for unknown reasons. The seal fetus has an extremely large and active pineal gland, not found in any other mammals. We have investigated if the pineal hormone melatonin affects fetal blood supply during diving. Using isolated ring segments of the uterine artery from pregnant hooded seals (Cystophora cristata), we measured the change in isometric tension caused by noradrenaline (NA) with and without physiological concentrations of melatonin. Melatonin alone had no effects while NA increased the tension in a dose-dependent manner. The NA-induced tension was about 70% reduced by melatonin, but was completely recovered after washout of melatonin. These results indicate that the large and active pineal gland of the fetal seal may be involved in upholding maternal uterine blood flow during diving.