Evaluation of thyroid hormone economy in elasmobranch fishes, with measurements of hepatic 5′‐monodeiodinase activity in wild dogfish

Abstract

This paper reviews the current understanding of thyroid hormone economy and homeostasis in elasmobranch fishes and considers those measures of the activity of the hypothalamic-pituitary gland-thyroid gland-peripheral tissue axis that are necessary for adequate assessment of thyroid hormone physiology. In particular, we focus on the value of measuring hepatic 5′-monodeiodinase (5′-MDA) activity as an indicator of the animal's cellular production rate of the active thyroid hormone, triiodo-L-thyronine (T3). We also examine the characteristics of hepatic 5′-MDA activity, in vitro, in adult female dogfish (Squalus acanthias) collected from Passamaquoddy Bay, New Brunswick, Canada, and in the embryos that they were carrying. T3 production from T4 by hepatic homogenates in vitro was time- and temperature-dependent, and was enhanced by the presence of a thiol donor. Michaelis constant (Km) and maximum reaction velocity (Vmax) values were 3.8 × 10−7 M and 0.29 nM T3/mg protein/hr, respectively. The inclusion of trimethylamine-N-oxide (TMAO) or a mixture of urea, TMAO, betaine and sarcosine significantly enhanced T3 production. Hepatic 5′-MDA activity was depressed in fish fasted for 7 days. J. Exp. Zool. 284:492–499, 1999. © 1999 Wiley-Liss, Inc.