Thyroid of lake sturgeon, Acipenser fulvescens. I. Hormone levels in blood and tissues.

The authors measured thyroid hormone (TH) levels in plasma, whole carcass, and tissues of cultured 2-year-old immature lake sturgeon held in fresh water and in serum of adults at spawning time from the Winnipeg River. Circulating thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels were low (T4 approximately 0.3 ng/ml, T3 approximately 0.2 ng/ml) in all cultured fish and most adults, but a few of the latter had exceptionally high T4 and T3 levels. The percentages of blood TH found in erythrocytes were 19.5% (T4), 6.1% (T3) and 6.9% (reverse T3 = rT3). Equilibrium dialysis showed much higher percentages of plasma free (F) FT4 (1.1%), FT3 (0.4%), and FrT3 (3,3',5'-triiodothyronine = rT3, 3.0%) for sturgeon than for rainbow trout, indicating more limited TH binding to sturgeon plasma sites. However, concentrations of FT4 and FT3 were close to those reported for salmonids. T3 levels exceeded T4 levels in most extrathyroidal tissues of cultured sturgeon but in most cases were less than 0.1 ng/g and 10 to 100 times lower than reported for salmonids; only the whole brain T3 concentration (5.6 ng/g) approached that of salmonids. The digested thyroid contained 21.3 ng T3/g and 2.4 ng T4/g. The authors conclude that lake sturgeon have a low circulating reserve of bound TH but have FT4 and FT3 concentrations close to those of salmonids. The high thyroidal T3:T4 ratio and low tissue T4 levels suggest that, in contrast to teleosts studied to date, the thyroid may be a significant direct source of T3, the primary TH in sturgeon tissues. High serum T4 and T3 levels in some sturgeon at spawning time may suggest a thyroid role in reproduction.

Thyroid of lake sturgeon, Acipenser fulvescens. II. Deiodination properties, distribution, and effects of diet, growth, and a T3 challenge.

The authors studied the properties and tissue distribution of thyroid hormone (TH) deiodination activities measured in vitro at subnanomolar substrate levels for cultured 2-year-old lake sturgeon held at 12 to 15 degrees. We also studied the deiodination responses to an exogenous 3,5,3'-triiodothyronine (T3) challenge and to a diet-induced growth suppression. Thyroxine (T4) outer-ring deiodination (T4ORD), T4 inner-ring deiodination (T4IRD), T3IRD, and 3,3',5'-triiodothyronine (rT3)ORD activities were evident in liver and intestine. Their properties resembled those of teleosts. T3IRD and T4IRD activities predominated in brain. Low or negligible deiodination in any form occurred in gill, skeletal muscle, kidney, notochord, or immature gonad. Only T4ORD activity was evident in the thyroid, suggesting that it secretes some T3. T3ORD and rT3IRD activities were undetectable in any tissues. Hepatic T4ORD activity varied during the photophase and was highest during late morning. A dietary T3 challenge that doubled plasma T3 levels decreased hepatic T4ORD activity without altering any other deiodination pathways in liver, intestine, or brain. A diet change from trout pellets to ocean zooplankton reduced somatic growth and plasma T3 levels and increased hepatic and intestinal T3IRD activities and hepatic rT3ORD activity but did not alter hepatic or intestinal T4ORD activity. The authors conclude that plasma T3 in lake sturgeon can be derived both from the thyroid and from hepatic (and intestinal) T4ORD activity, which varies with sampling time and downregulates in response to a T3 challenge. However, a reduction in plasma T3 accompanying a change in diet and reduced growth was not due to a decrease in T4ORD activity; rather, it was due to an increase in hepatic and intestinal T3IRD activities. These results suggest a difference in emphasis in thyroidal regulation between sturgeon and certain teleosts.