Regulation of seasonal changes in circulating thyroid hormones in channel catfish

Abstract

Channel catfish, Ictalurus punctatus, were sampled at hics. morning, midday, and dusk in January, April, and July to determine if apparent seasonal differences in circulating thyroid hormones were due to seasonal changes in the phase of daily rhythms in circulating levels, or to seasonal alterations in peripheral hepatic deionization and blood protein binding. Plasma thyroxine (T4) and triiodothyronine (T3) were lowest in January, intermediate in April and highest in July. Except for plasma T3 levels in April, the pattern of significantly lower thyroid hormone concentrations in the morning which peaked later in the photophase was retained throughout all three months providing no evidence for seasonal phase shifting in diurnal thyroid hormone cycles. To examine seasonal changes in hepatic deionization, I developed an assay to measure hepatic outer ring deiodinase (011.I)). Channel catfish hepatic 011.13 was chemically similar to mammalian Type 11 and fish denominates. Hepatic [] were identical in January and July, at times of minimal and maximal food intake, respectively. The summer increase in circulating Ta therefore appears not to be due to an elevation in hepatic 011.13 activity. Hepatic ORD Vmax and Km, were highest in April, possibly a response to elevated reproductive steroids or increased food consumption, whereas high circulating Ts in July may inhibit ORD activity. Lastly, T4 binding by the plasma was highest in January whereas To binding was highest in July, coincident with highest blood protein levels. Circulating T3 levels may thus be more directly dependent on the centrally regulated provision of T4 and seasonal alterations in plasma binding proteins than on peripheral regulation of hepatic deiodinase activity.