Effects of pre- and postnatal polychlorinated biphenyl exposure on metabolic rate and thyroid hormones of white-footed mice.

Energy budgets have proven to be a valuable tool for predicting life history from physiological data in terrestrial vertebrates, yet these concepts have not been applied to the physiological effects of contaminants. Contaminants might affect energy budgets by imposing an additional metabolic cost or by reducing the overall amount of energy taken in; either process will reduce the energy available for production (i.e., growth or reproduction). This study examined whole animal energetic effects of polychlorinated biphenyl (PCB) exposure in white-footed mice (Peromyscus leucopus). Exposure to PCBs is known to reduce concentrations of plasma thyroid hormones, and thyroid hormones exert strong control over the rate of energy metabolism in mammals. Peromyscus leucopus that were proven breeders were fed PCBs in their food at 0, 10, and 25 ppm. Through lactation, offspring were exposed to PCB from conception and were maintained on the maternal diet to adulthood. No effects were seen on energy metabolism (O2 consumption, measured in adulthood) or on growth, but there were large dose-dependent decreases in thyroid hormone concentrations, particularly T4. The apparent disparity in our data between unchanged metabolic rates and 50% reductions in T4 concentrations can be rationalized by noting that free T3 (the fraction not bound to plasma protein) in treated mice was not significantly different from controls and that metabolism is most strongly influenced by free T3. Overall, this study did not demonstrate any energetic consequences of PCB exposure in P. leucopus at dietary concentrations up to 25 ppm.

Effects of fungicides on thyroid function, metabolism, and thermoregulation in cotton rats.

Among the myriad of recent studies on endocrine-disrupting chemicals, relatively few involve thyroid disruption, and most of these address exposure/disruption during embryonic life. Of those involving adult vertebrates, the endpoints examined are thyroid measurements. Even though thyroid disruption could potentially interfere with energy metabolism and thermoregulation such that over-winter survival might be compromised, the possible energetic consequences of these thyroid perturbations have not been investigated. We assessed thyroid function and measured resting metabolic rates of cotton rats chronically exposed to the fungicides vinclozolin or mancozeb. In addition, we measured norepinephrine-induced nonshivering thermogenesis and cold-induced thermogenesis and then cold-acclimated the mancozeb animals. Although thyroid hormone concentrations generally decreased, this was compensated for by an increase in thyroxine turnover (vinclozolin study only) such that thyroxine utilization rate was not different. In addition, there was no difference between the treated and control animals in any of the metabolic parameters measured. It is concluded that wild rodents exposed to these compounds are not energetically compromised.

Seasonal variations in thyroid hormone levels in free-living echidnas (Tachyglossus aculeatus).

Hulbert and Augee (1982) have suggested that the thyroid has little effect on energy metabolism in the echidna. In order to investigate whether thyroid status changes during hibernation, when metabolism drops dramatically, we measured levels of thyroid hormones in 31 free-living echidnas at various times during the year. Unlike eutherian hibernators, in which thyroid hormone levels may rise to seasonally high values in late hibernation, total and free T(4) and T(3) were all significantly depressed throughout hibernation. TT(4) from nonhibernating echidnas was 11.8 +/- 0.9 ng/ml (n = 23), confirming previously published values, but fell to half this level (5.9 +/- 0.7 ng/ml, n = 8) during hibernation. By contrast to the low TT(4) values, nonhibernating FT(4), TT(3), and FT(3) values were similar to normal values for eutherian mammals. Differences in the seasonal pattern of variation in thyroid hormones between echidnas and hibernating eutherians may be due to differences in thyroid hormone transporting proteins.

Thyroid hormone concentrations in black bears (Ursus americanus): hibernation and pregnancy effects.

Previous studies on thyroid hormones in hibernating bears have used very few sampling periods, so that the time course of any change is poorly understood. In this study, plasma sampled from pregnant and nonpregnant black bears before and during hibernation (16 samples each at 10-day intervals) was assayed by radioimmunoassay for concentrations of thyroxine (T4) and triiodothyronine (T3). Only free T4 showed a difference (P = 0.019) between females that produced cubs and those that did not, but this appeared to be due to higher preimplantation values. Free T3, total T3, and free T4 varied (P = 0.001, 0.038, 0.002, respectively) among sampling periods: during December, bears had depressed concentrations. These lowered concentrations were maintained during hibernation for the free hormones. Our data confirm previous work showing that food restriction and/or physiological preparation for hibernation is coincident with depressed plasma concentrations of thyroid hormones. Hormonal changes associated with pregnancy were minor.

Utilization rates of thyroid hormones in mammals.

1. Thyroxine utilization rates (T4U: N = 37 species) and triiodothyronine utilization rates (T3U: N = 7 species) scale with body mass to the 0.81 and 0.74 power respectively. 2. T4U rates tend to be lower in summer relative to other seasons, vary unpredictably during pregnancy and lactation, increase with regular physical activity, and generally decrease with age. 3. Both T4U and T3U increase with cold exposure, decrease with heat exposure and during fasting, and increase/decrease with hyperthyroidism/hypothyroidism. 4. Since these T4U and T3U changes are qualitatively similar but quantitatively different, the T3U/T4U ratio varies, suggesting a variable deiodination rate from thyroxine to triiodothyronine.

Thyroid function and cold acclimation in the hamster, Mesocricetus auratus.

Basal metabolic rate (BMR), thyroxine utilization rate (T4U), and triiodothyronine utilization rate (T3U) were measured in cold-acclimated (CA) and room temperature-acclimated (RA) male golden hamsters, Mesocricetus auratus. Hormone utilization rates were calculated via the plasma disappearance technique using 125I-labeled hormones and measuring serum hormone levels via radioimmunoassay. BMR showed a significant 28% increase with cold acclimation from 4.50 +/- 0.05 to 5.77 +/- 0.10 ml O2 X h-1 X g-2/3. The same cold exposure also produced a 32% increase in T4U (10.75 +/- 0.51 vs. 14.19 +/- 0.75 ng X day-1 X g-2/3), and a 204% increase in T3U (5.51 +/- 0.53 vs. 16.77 +/- 1.35). The much greater increase in T3U implies that previous assessments of the relationship between cold acclimation and thyroid function may have been underestimated and that cold exposure induces both quantitative and qualitative changes in thyroid function. It is concluded that in the cold-acclimated state, T3U more accurately reflects thyroid function than does T4U. A mechanism for the cold-induced change in BMR is proposed, for which alterations in four aspects of thyroid function are required: a decrease in plasma T4 binding, an elevation of the pituitary T4 "set point," a preferential shift in deiodinase activity from reverse T3 to T3 production, and an increase in the thyroidal secretion of T3.