Seasonal changes in plasma levels of thyroid hormones and the effects of the hormones on cellular ATP content in common bottlenose dolphin.

In general, thyroid hormones (THs) stimulate cellular metabolism by inducing ATP utilization that collaterally leads to thermogenesis. However, in cetaceans, TH functions and the contribution of THs to cold adaptation are not fully understood. To investigate the role of THs in metabolism of cetaceans, seasonal changes in circulating levels in THs were investigated in the common bottlenose dolphin Tursiops truncatus that were monitored under two different conditions for two years, with routine measurements of body temperature (BT), water temperature (WT) and air temperature (AT). The effects of THs on ATP synthesis were determined using cultured cells. Blood samples were collected from the species kept in different conditions at the Taiji Whale Museum located in the temperate zone and at Okinawa Expo Park in the subtropical zone. Circulating levels in total thyroxine (T4 ) for the dolphins at both aquaria and total 3,5,3'-tri-iodothyronine (T3 ) levels in dolphins at Taiji were measured by enzyme-linked immunoassay methods, respectively, and average concentrations were compared among seasons. To confirm the effects of THs on ATP synthesis, T3 or T4 was administrated to cultured kidney cells from the same species and cellular ATP contents were quantified at 0, 24, 48, 96 and 192 h after administration. BT of common bottlenose dolphins in each aquarium was measured for health check by chance in Taiji and every morning in Okinawa. WT in pools and AT were also measured every morning. Circulating T4 levels in autumn and winter were lower than those in spring and summer in dolphins in Taiji where WT and AT varied greatly from season to season. T4 levels showed a small difference between spring and autumn in dolphins in Okinawa with warmer WT and AT in smaller amplitude ranges than in Taiji. Total T3 level in Taiji was highest in spring and lowest in autumn as T4 levels, but not significant. The BT of dolphins in Taiji was also lower in autumn and winter compared with those in spring and summer, whereas the BT of dolphins in Okinawa fell in autumn but rose in summer, albeit to a lesser extent than in Taiji. Cellular ATP was increased by administration of both T3 and T4 compared to control. Collectively, these results suggest that the cellular metabolic activities regulated by THs may be enhanced in dolphins exposed to increasing surrounding temperature for lipolysis and reduced in dolphins exposed to colder conditions for fat accumulation.