| Abstract: | Although thyroid hormones play a central role in the regulation of metabolism in poikilothermic vertebrates, little information is available on the mechanisms of thyroid hormone transport and delivery to target tissues via the blood. Variable body termperature would be expected to alter this transport through effects on thyroid hormone binding to plasma proteins. In channel catfish, however, temperature has little effect on the percentage of free (unbound) thyroid hormones in plasma. Saturation binding analysis shows that this is because catfish plasma lacks high affinity binding sites for thyroid hormones. Thus, metabolically-active hormone levels in fish blood may be maintained during temperature fluctuations because the hormone binding to available low-affinity binding sites is insensitive to temperature. |
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