Nancy E. Henderson

Serum T4 and T3 in active and torpid ground squirrels, Spermophilus richardsoni.

Abstract Serum titres of l -thyroxine (T 4 ) and 3,5,3′-triiodo- l -thyronine (T 3 ) were measured by radioimmunoassay at monthly intervals over the annual cycle of Richardsons ground squirrel, Spermophilus richardsoni . Animals were sampled in the field between March and October and from laboratory colonies of hibernating and nonhibernating ground squirrels between October and February. In nonhibernating animals, T 4 concentrations ranged from mean values of 10–30 ng/ml and T 3 titres varied from 1 to 2 ng/ml over the year. In hibernators, killed on the second day of a dormancy bout, hormone levels increased to 45–75 ng of T 4 /ml and 5–6 ng of T 3 /ml. Within the dormancy phase, T 4 concentrations fell by 50% during hibernation bouts and T 3 titres decreased sharply within a few hours of arousal. Levels of both iodothyronines increased before or on reentry into torpor. The results indicate continued thryoid function during dormancy in S. richardsoni .

Thyroid hormone resistance in hibernating ground squirrels, Spermophilus richardsoni. I: Increased binding of triiodo-L-thyronine and L-thyroxine by serum proteins

Thyroid function was examined during the annual cycle of Richardsons ground squirrel, Spermophilus richardsoni. A number of facets were measured to facilitate comparison of thyroid function in active, dormant, and aroused animals. This report discusses changes in the serum thyroid hormone and binding as assessed by radioimmunoassay, equilibrium dialysis, and competitive binding assays. During the hibernation phase (both dormant and aroused), total serum T3 (trioodo-L-thyronine) and T4 (L-thyroxine) are elevated over active levels, two- to fivefold and four- to sixfold, respectively. However, in dormant squirrels, both free T3 and free T4 are reduced compared with both active and aroused phases of the annual cycle, while in aroused squirrels there is an increase in free T3 but no change in free T4 compared with active squirrels. The difference between changes in total and free thyroid hormone levels in the three groups is due to changes in serum binding of thyroid hormone. There is a more than twofold increase in the capacity of a saturable T3-binding site in serum of both dormant and aroused squirrels, and there is an increase in serum binding affinity at the low core temperature of dormant squirrels (6 degrees). Therefore, even though serum total T3 and T4 are elevated during dormancy, free T3 and T4 levels are reduced to half of the levels in active squirrels as a consequence of increased serum binding capacity and affinity. In aroused animals, however, increased serum binding capacity only partially buffers the increase in total T3 and T4, so that free thyroid hormone levels exceed those of active squirrels.

Thyroxine metabolism in active and torpid ground squirrels, Spermophilus richardsoni.

Abstract Sereral aspects of thyroxine (T 4 ) metabolism were examined in active and torpid ground squirrels, Spermophilus richardsoni . A [ 125 I]T 4 turnover study showed thyroxine half-time was increased while the total distribution volume and metabolic clearance rate were decreased in hibernators compared with active animals. However, T 4 degradation rates were similar in hibernators and nonhibernators due to increased serum titres of total and, more importantly, free T 4 (FT 4 ) in torpid animals. Percentage FT 4 was estimated by equilibrium dialysis, and serum total T 4 concentration, by radioimmunoassay. The estimated average FT 4 values were 1.5 ng/100 ml in hibernators and 0.7 ng/100 ml in active animals (when serum was dialysed at the respective body temperatures). Serum levels of 3,5,3′-triiodo- l -thyronine (T 3 ) and 3,3′,5′-triiodo- l -thyronine (rT 3 ) were measured by radioimmunoassay. The titres of both iodothyronines were increased in dormant animals, but the ratio of rT 3 to T 3 in hibernators was less than that in active ground squirrels. These results provide further evidence for continued thyroid function during dormancy in a mammalian hibernator.

Structural similarities between the neurohypophyses of brook trout and tetrapods

Abstract The vascularization and prcoptic innervation of the hypophysis of the brook trout ( Salvelinus fontinalis ) indicate that the rostral neurohypophysis is structurally similar to the median eminence of tetrapods and that the caudal neurohypophysis is comparable with the neural lobe of higher vertebrates. These conclusions are supported by three observations. 1. The vascular supply to the rostral hypophysis is largely independent of that to the caudal hypophysis. 2. Neurosecretory fibers of the preoptic tract appear to terminate on a rostral capillary plexus whose extensions form the chief blood supply of the pars distalis. 3. Stainable neurosecretory material accumulates in the extremities of the caudal neurohypophyseal processes which often abut on venous channels. It is suggested that some of this material passes directly into the systemic circulation.

Thyroid hormone resistance in hibernating ground squirrels, Spermophilus richardsoni: II. Reduction of hepatic nuclear receptors

Hepatic nuclear triiodo-L-thyronine (T3) receptors were studied to investigate the mechanism of thyroid hormone resistance in Richardsons ground squirrels, Spermophilus richardsoni, during the hibernation phase of the annual cycle. The cycle is divided into an active phase and a hibernation phase, the latter composed of alternating dormancy and arousal bouts. In addition to animals in these three states, a group of cold-exposed animals was also examined (those animals held at 6 degrees which showed no indications of entering hibernation). Binding of T3, to squirrel hepatic nuclei from all groups, was characterized as high affinity, Kd ranging from 111 to 267 pM, and low capacity, 50 to 314 fmol T3/mg DNA. Based on these data, other criteria examined, and models established in the literature for other species, this binding site has been tentatively identified as a T3 receptor. Receptor concentrations in nuclei from dormant and aroused squirrels were only 15-20% of the concentration in active animals. There were no differences in the affinity of the T3 receptor over the annual cycle estimated by the in vitro assay at 24-26 degrees. The reduction in nuclear T3 receptors, together with the previously reported increase in serum binding of thyroid hormone, provides an explanation for thyroid hormone resistance during the hibernation phase in S. richardsoni.

Thyroxine and protein-bound iodine concentrations in plasma of the pacific hagfish, Eptatretus stouti (Cyclostomata)

Abstract 1. 1. Plasma levels of thyroxine (T4) were determined by competitive protein-binding analysis and compared with plasma concentrations of protein-bound iodine (PBI). 2. 2. Measurements were made on samples from normal, hypophysial-Lesioned and sham-operated fish. 3. 3. In normal animals, which had been held in the laboratory for 3 weeks, the measured T4 values ranged from 2 to 3 μg/100 ml of plasma and PBI levels varied from 5 to 8 μg/100 ml. 4. 4. T4 concentrations were significantly lower in both the hypophysial-Lesioned and sham-operated fish which had been maintained in the laboratory for 18 weeks. 5. 5. PBI levels tended to be lower in the experimental fish than in normal animals, but the differences were not significant. 6. 6. In both normal and experimental hagfish, thyroxine iodine comprised less than one-third of the plasma content of organic iodine.

Hibernation in thyroidectomized ground squirrels, Spermophilus richardsoni

Abstract Ground squirrels, Spermophilus richardsoni, were thyroidectomized in late July toward the end of the preparative phase of the annual cycle. Animals were sampled in September, in November–December, and in January–February. Serum was assayed for total 3,5,3′-triiodo- l -thyronine (T3) and l -thyroxine (T4) by radioimmunoassay. Daily records were maintained of the hibernating behavior. All thyroidectomized animals had low or nondetectable levels of T3 and T4 while torpid sham and intact controls had high levels of both hormones. The temporal hibernating behavior of thyroidectomized ground squirrels was similar to that of sham and intact controls. Although characteristic of torpid animals, high circulating T3 and T4 concentrations are not necessary for hibernation in S. richardsoni.

Mitochondrial α-glycerophosphate dehydrogenase activity in active and torpid ground squirrels, Spermophilus richardsoni

1. Mitochondrial alpha-glycerophosphate dehydrogenase (GPD) activity was assayed in liver homogenates from active and torpid ground squirrels. 2. Arrhenius plots of GPD activity were linear in non-hibernating animals and discontinuous in hibernators. Compared with non-hibernators, the energy of activation in hibernators was reduced between 37 and 25 degrees C, but increased between 25 and 6 degrees C. 3. A dose-response relation between GPD activity and injected thyroxine was determined in active animals. No correlation was found between enzyme activity at 37 or 6 degrees C and circulating titres of thyroid hormones, in ground squirrels sampled during the preparative and hibernating phases.