Factors influencing the steady-state distribution and exchange of thyroid hormones between red blood cells and plasma of rainbow trout, Oncorhynchus mykiss

The relationship between ingested thyroid hormones, thyroid homeostasis and iodine metabolism in humans and teleost fish

Oral l-thyroxine (T4) therapy is used to treat human hypothyroidism but T4 fed to teleost fish does not raise plasma thyroid hormone (TH) levels nor induce growth, even though oral 3,5,3'-triiodo-l-thyronine (T3) is effective. This suggests a major difference in TH metabolism between teleosts and humans, often used as a starting thyroid model for lower vertebrates. To gain further insight on the proximate (mechanistic) and ultimate (survival value) factors underlying this difference, the several steps in TH homeostasis from intestinal TH uptake to hypothalamic-hypophyseal regulation were compared between humans and teleosts, and following dietary TH challenges. A major proximate factor limiting trout T4 uptake is a potent constitutive thiol-inhibited intestinal complete T4 deiodination that is ineffective for T3. At the hepatic level, T4 deiodination, conjugation and extensive biliary excretion with negligible T4 enterohepatic recycling can further block teleost T4 uptake to plasma. Such protection of plasma T4 from dietary T4 may be particularly critical for piscivorous fish consuming thyroid tissue, rich in T4 but not T3. It would prevent disruption by unregulated ingested T4 of the characteristic acute and transient changes in teleost plasma T4 due to diel rhythms, food intake and stress-related factors. These marked natural short-term fluctuations in teleost plasma T4 levels are enabled by the relatively small and rapidly-cleared plasma T4 pool, stemming largely from properties of the plasma T4-binding proteins. Humans, however, due mainly to plasma T4-binding globulin, have a relatively massive circulating pool of T4 and an extremely well-buffered free T4 level, consistent with the major TH role in regulating basal metabolic rate. Furthermore, this large well-buffered and slowly-cleared plasma T4 pool, in conjuction with enterohepatic recycling and relaxation of hypothalamic-hypophyseal negative feedback, allows humans to temporarily 'store' ingested T4 in plasma, thereby sparing endogenous TH secretion and conserving thyroidal iodine reserves. Indeed, iodine conservation is likely the key ultimate factor determining the divergent evolution of the human and teleost systems. For humans, ingested iodine in the form of I^-, or TH and their derivatives, is the sole iodine source and may be limiting in many environments. However, most freshwater teleosts, in addition to their ability to assimilate dietary I^-, can derive sufficient I^- from their copious gill irrigation, with no selective advantage in absorbing dietary T4 which would disrupt their natural acute and transient changes in plasma T4. Thus T4 may act also as a vitamin (vitamone) in humans but not in teleosts; in contrast, T3, naturally ingested at much lower levels, may act as a vitamone in both humans and teleosts.

The effect of handling and blood removal on plasma levels and hepatic deiodination of thyroid hormones in adult male and female rainbow trout, Oncorhynchus mykiss

We studied the effects of handling and blood removal (0.4% of body mass) on the thyroid system of fasted ~1000-g adult male and female rainbow trout, Oncorhynchus mykiss, over a 3-day period. Relative to undisturbed controls, bleeding lowered the mean hematocrit (HCT) by 19% and on day 1 increased the hepatic inner-ring deiodination of 3,5,3'-triiodothyronine (T3) (T3IRD), but did not alter thyroxine (T4) outer-ring deiodination (T4ORD) or inner-ring deiodination (T4IRD) activity or plasma T4 or T3 levels. Regardless of treatment, hepatic T4ORD activity and HCT were lower in females than in males, and over the 3 days following bleeding, the plasma T3 level increased and the plasma T4 level decreased progressively for both sexes. We conclude that handling and bleeding induce a temporary increase in hepatic T3 conversion to 3,3'-diiodothyronine with no change in T4 conversion to reverse T3. This is consistent with independent control of T3IRD and T4IRD activities. Furthermore, HCT and hepatic T4ORD activity are lower in adult females, and 10 days after transfer of trout to smaller tanks, plasma T4 and T3 levels may still be adjusting.

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.

Plasma membrane transport of thyroid hormones and its role in thyroid hormone metabolism and bioavailability

Although it was originally believed that thyroid hormones enter target cells by passive diffusion, it is now clear that cellular uptake is effected by carrier-mediated processes. Two stereospecific binding sites for each T4 and T3 have been detected in cell membranes and on intact cells from humans and other species. The apparent Michaelis-Menten values of the high-affinity, low-capacity binding sites for T4 and T3 are in the nanomolar range, whereas the apparent Michaelis- Menten values of the low-affinity, high-capacity binding sites are usually in the lower micromolar range. Cellular uptake of T4 and T3 by the high-affinity sites is energy, temperature, and often Na+ dependent and represents the translocation of thyroid hormone over the plasma membrane. Uptake by the low-affinity sites is not dependent on energy, temperature, and Na+ and represents binding of thyroid hormone to proteins associated with the plasma membrane. In rat erythrocytes and hepatocytes, T3 plasma membrane carriers have been tentatively identified as proteins with apparent molecular masses of 52 and 55 kDa. In different cells, such as rat erythrocytes, pituitary cells, astrocytes, and mouse neuroblastoma cells, uptake of T4 and T3 appears to be mediated largely by system L or T amino acid transporters. Efflux of T3 from different cell types is saturable, but saturable efflux of T4 has not yet been demonstrated. Saturable uptake of T4 and T3 in the brain occurs both via the blood-brain barrier and the choroid plexus-cerebrospinal fluid barrier. Thyroid hormone uptake in the intact rat and human liver is ATP dependent and rate limiting for subsequent iodothyronine metabolism. In starvation and nonthyroidal illness in man, T4 uptake in the liver is decreased, resulting in lowered plasma T3 production. Inhibition of liver T4 uptake in these conditions is explained by liver ATP depletion and increased concentrations of circulating inhibitors, such as 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid, indoxyl sulfate, nonesterified fatty acids, and bilirubin. Recently, several organic anion transporters and L type amino acid transporters have been shown to facilitate plasma membrane transport of thyroid hormone. Future research should be directed to elucidate which of these and possible other transporters are of physiological significance, and how they are regulated at the molecular level.

Efflux of T4 from the in situ perfused liver of rainbow trout: effect of T4, dithiothreitol and cysteine in the perfusate

A Cortland saline-perfused rainbow trout (Oncorhynchus mykiss) liver model was used to study aspects of T4 efflux from the intact organ system. There was a consistent efflux of T4 in the absence of T4 in the perfusate, and the T4 efflux was increased in the presence of T4 in the perfusate, but the efflux was not T4-dose dependent. The addition of the thiol-containing compound dithiothreitol (DTT, 2 mM) to the perfusate had no significant effect on the flux of T4 from the liver, whereas the addition of cysteine (2 mM), a thiol-containing amino acid suppressed T4 efflux. The results are consistent with the known mechanisms of thyroid hormone trafficking across cell membranes, and suggest that organ systems, such as the liver, may act as a major reserve of hormone, thus participating in plasma thyroid hormone homeostasis.