In vivo specific uptake of labeled insulin by liver, adipose tissue, pituitary, and adrenals in the turtle Chrysemys dorbigni

Top-down Approaches to the Study of Natural Variation in Complex Physiological Pathways Using the White-footed Mouse (Peromyscus leucopus) as a Model

Variation in complex physiological pathways has important effects on human function and medical treatment. Complex pathways involve cells at multiple locations, which serve different functions regulated by many genes and include complex neuroendocrine pathways that regulate physiological function. One of two competing hypotheses regarding the effects of selection on complex pathways predicts that variability should be common within complex pathways. If this hypothesis is correct, then we should expect wide variation in neuroendocrine function to be typical within natural populations. To test this hypothesis, a complex neuroendocrine pathway that regulates photoperiod-dependent changes in fertility in a natural population of white-footed mice (Peromyscus leucopus) was used to test for natural genetic variability in multiple components of the pathway. After testing only six elements in the photoperiod pathway in P. leucopus, genetic variation in the following four of these elements was evident: the circadian clock, melatonin receptor abundance or affinity, sensitivity of the reproductive axis to steroid negative feedback, and gonadotropin-releasing hormone neuronal activity. If this result can be extended to humans, the prediction would be that significant variation at multiple loci in complex neuroendocrine pathways is common among humans, and that variation would exist even in human populations from a common genetic background. This finding could only be drawn from an "exotic" animal model derived from a natural source population, confirming the continuing importance of nontraditional models alongside the standard laboratory species.

In vitro effect of insulin on the uptake of glucose and alpha-aminoisobutyric acid in the thyroid gland of the turtle (Chrysemys dorbigni)

The presence of specific insulin binding sites in the thyroid gland of the turtle Chrysemys dorbigni has been previously reported. The purpose of the present work was to investigate the probable action of insulin on the uptake of [14C]deoxy-D-glucose ([14C]DG) and [14C]alpha-aminoisobutyric acid ([14C]AIB) in turtle (C. dorbigni) thyroid glands in vitro. Thyroid fragments (+/- 40 mg) were incubated at 25 degrees in Krebs-Ringer-bicarbonate buffer containing 0.2 microCi of [14C]DG or [14C]AIB without or with bovine insulin at different periods of time. The uptake of [14C]DG and [14C]AIB increased with incubation time. The presence of insulin (7 x 10(-6) M) in the incubation medium during 240 min did not modify the [14C]DG uptake. However if the thyroid fragments were previously incubated with insulin (60 min) and then incubated (240 min) with the same concentration of the hormone, the [14C]DG uptake was markedly increased. This stimulatory effect of insulin was dose-dependent. In similar experimental conditions, insulin also produced a significant increase in the uptake of [14C]AIB. Therefore, these findings strongly support the hypothesis that insulin might exert a direct action on the thyroid function.

Hyperinsulinemia and the aldosterone and pressor responses to angiotensin II

To determine whether hyperinsulinemia alters angiotensin II-mediated aldosterone secretion, the increase in plasma aldosterone after intravenous angiotensin II (5, 10, and 20 ng/kg/min for 15 minutes each) was measured before and after euglycemic hyperinsulinemia in seven chronically instrumented dogs. In a random sequence on 4 successive days, dogs received either 0, 2, 4, or 8 milliunits/kg/min insulin. Euglycemic hyperinsulinemia, at all insulin doses, resulted in a significantly greater (p less than 0.01) change in the angiotensin II-stimulated increments of plasma aldosterone than was observed when angiotensin II was administered alone. However, there was no dose-dependence of insulin's effect on angiotensin II-stimulated aldosterone. The effect of weight gain on the angiotensin II response was also evaluated in five dogs. After weight gain, euglycemic hyperinsulinemia augmented angiotensin II-stimulated aldosterone to the same magnitude that was observed before weight gain. Possible mechanisms whereby insulin could increase angiotensin II-stimulated aldosterone production include: increased intracellular potassium, reduced plasma free fatty acids, and a direct action of insulin to induce increased adrenal steroidogenesis. In addition to altering the angiotensin II-aldosterone dose-response curve, hyperinsulinemia also increased the pressor action of angiotensin II. In contrast to the angiotensin II-aldosterone response, progressive hyperinsulinemia resulted in a progressive increase in the pressor response to angiotensin II. The increased pressor response is probably due to an increased activation of the sympathetic nervous system by insulin.

Specific uptake, dissociation, and degradation of 125I-labeled insulin in isolated turtle (Chrysemys dorbigni) thyroid glands

Thyroid glands from turtles (Chrysemys dorbigni) pretreated with potassium iodide were incubated with 125I-insulin in the presence or absence of unlabeled insulin, in order to study its specific uptake. At 24 degrees, the specific uptake reached a plateau at 180 min of incubation. The dose of bovine insulin that inhibited 50% of the 125I-insulin uptake was 2 micrograms/ml of incubation medium. Most of the radioactive material (71%) extracted from the gland, after 30 min incubation with 125I-insulin, eluted in the same position as labeled insulin on Sephadex G-50. Only 24% eluted in the salt position. After 240 min incubation, increased amount of radioactivity appeared in the Na125I position. When bovine insulin was added together with the labeled hormone, a substantial reduction of radioactivity was observed in the insulin and Na125I elution positions. Dissociation studies were performed at 6 degrees in glands preincubated with 125I-insulin either at 24 or 6 degrees. The percentage of trichloroacetic acid (TCA)-soluble radioactive material in the dissociation medium increased with incubation time at both temperatures. However, the degradation activity was lower at 6 than at 24 degrees. The addition of bovine insulin to the incubation buffer containing 125I-insulin reduced the radioactive degradation products in the dissociated medium. Chloroquine or bacitracin inhibited the degradation activity. Incubation of thyroid glands with 125I-hGH or 125I-BSA showed values of uptake, dissociation, and degradation similar to those experiments in which an excess of bovine insulin was added together with the labeled hormone. Thus, by multiple criteria, such as specific uptake, dissociation, and degradation, the presence of insulin-binding sites in the turtle thyroid gland may be suggested.