Seasonality in plasma thyroxine binding in turtles

Abstract

The relation between seasonal changes in plasma thyroxine (T,) and T, binding activity was studied in three species of turtle previously shown to differ in the nature of their binding proteins; additional direct measurements of a high affinity T, binding protein (TBP) were made in one species using RIA. In the painted turtle, Chrysemyspicta (Family Emydidae), a species with large seasonal changes in plasma T,, the binding protein measured by activity and RIA showed significant seasonality. Binding activity correlated with direct estimates of TBP by RIA. TBP was minimal upon emergence from hibernation and maximal in late summer (July-August), about a month after the peak in plasma T,. In contrast, two other nonemydid species, the green turtle (Chelonia mydas) and Mexican tortoise (Gopherus flauomarginatus) showed relatively low plasma T, and binding activity with no significant seasonality in either. Electrophoretic studies (PAGE) confirmed that T, binding in C. picta is almost wholly associated with a high affinity, low capacity TBP; whereas, it is confined t o high capacity sites (probably albumin) in the other species. The transport of T, is clearly handled differently in these three turtles. In the green turtle and tortoise, one would expect relatively constant free T, (fT4) throughout the year. Marked seasonal fluctuations (an order of magnitude) in the ratios of TBP to total T, in C. picta due to the temporal lag between the two may cause profound changes in fT, with peaks in late spring.

A comparison of thyroid hormone levels and plasma capillary zone electrophoresis in red‐eared sliders (Trachemys scriptaelegans) and map turtles (Graptemysspp.) depending on season and sex

“…The results of the present study showed that not all thyroid hormones were correlated with TP and albumin levels in regard to either sex or season. This is in contrast to the results of previous studies, 10‐13,20 in which the binding proteins (TBP and albumin) and tT4 levels were highly correlated. Licht et al 12 reported that TBP in turtles ( T scripta ) was near the albumin fraction on polyacrylamide‐gel electrophoresis, which correlated with the α1‐fraction in the present study.…”

Thyroxine is Permissive to Seasonal Transitions in Reproductive Neuroendocrine Activity in the Ewe1

“…It is known that only a small portion of the total circulating thyroxine is in the "active" or "free" form; the remainder is bound to highaffinity plasma proteins [22]. Further, the serum concentrations of these binding proteins appear to vary seasonally in some species [23,24], although this has not yet been examined in the ewe. Thus, the amount of thyroid hormone available for exerting action might change with season despite an unaltered total serum thyroxine concentration.…”

Metabolic depression in Cyclorana alboguttata and the potential role of the endogenous opioid system and thyroid axis as regulators of aestivation

“…The highest T 4 levels were found during metabolic depression in the lizard Cnemidophorus sexilineatus (Sellers et al, 1982) and T 3 levels were highest during torpor in woodchucks (M. monax) (Young et al, 1979a). However, a large number of studies have found thyroid hormones decrease during or just prior to the hibernation period in a wide variety of animals, including black bears (Ursus americanus) (Azizi et al, 1979;Lundberg et al, 1976;Tomasi et al, 1998), the Finish brown bear (U. arctos arctos) (Hissa et al, 1994), hedgehogs (E. europaeus) (Fowler, 1988), iguanas (Dipsosaurus dorsalis) (John-Alder, 1984), painted turtles (Chrysemys picta) (Licht et al, 1991), thirteen-lined ground squirrels (S. tridecemlineatus) (Tomasi and Stribling, 1996) and echidnas (Tachyglossus aculeatus) (Nicol et al, 2000). Other studies have shown that levels of thyroid hormone reach their lowest levels during the hibernation period, but levels begin to increase towards the end of the hibernation period.…”