Insulin binding and glucose oxidation in edible dormouse (Glis glis) adipose tissue: seasonal variations

Glucose oxidation by adipose tissue of the edible dormouse (Glis glis) during hibernation and arousal: effect of insulin

1. The effect of insulin on U-14C-glucose oxidation by adipose tissue isolated from hibernating or arousing edible dormouse has been studied. 2. CO2 production derived from radioglucose was analysed point by point during in vitro rewarming (from 6 to 37 degrees C). 3. The rate of temperature increase was 2 degrees/5 min in order to mimic the rate of rewarming during the spontaneous arousal of the dormouse. 4. Insulin did not increase the glucose oxidation by the adipose tissue from hibernating dormouse whereas adipocytes from active animal present high insulin sensitivity. 5. These results suggest that insulin resistance occurs during hibernation.

Radioglucose metabolism by Richardson's ground squirrels in the weight-gain and weight-loss phases of the circannual cycle

Captive fed, starved, and refed Richardson's ground squirrels in the weight-gain and weight-loss phases of the circannual cycle were injected with radioglucose and the activity of the label in skeletal muscle proteins and white adipose tissue lipids four hours after injection was used to determine if lean body mass and white adipose tissue would be rapidly restored when starved animals were refed. Starvation for six days reduced carcass mass 27-31% and white adipose tissue mass 23-24% (Table 1). Activity of the label in both tissues of weight-gain and weight-loss animals was reduced by starvation. After four days of refeeding activities returned to levels similar to those in fed animals, with the exception of lower activity in skeletal muscle proteins of weight-gain animals. Furthermore, activity in each tissue fraction of starved and refed weight-gain animals was similar to that in weight-loss animals when expressed as per cent of activity in the respective fed state (Table 2). Radioglucose incorporation indicated that when skeletal muscle and adipose tissue are depleted by starvation, distribution of the label upon refeeding is similar to that in the fed state. Four days after refeeding weight-gain phase ground squirrels had restored 5.5 g of lean body mass and 7.5 g of adipose tissue, including 1.4 g (6 kcal) of protein and 7.0 g (66 kcal) of lipid, respectively. These results are also consistent with the fed state, in which weight-gain animals were depositing more lipid than lean body mass.

Hibernation depth influences the edible dormouse pancreatic B cell during the spring arousal

In order to determine the influence of hibernation depth upon the secretion and the effect of insulin, two groups of edible dormice were maintained in winter under different climatic and nutritional conditions, and their pancreatic B-cell function was tested during the spring arousal. The first group of animals was exposed to a moderate temperature and fed ad libitum. Their periods of hypothermia were short and irregular and the active periods sometimes lasted several days; their body weight increased during the winter months; in spring, the sensitivity of B cells to glucose was low, decreasing insulin secretion in vivo and in vitro, and the adipocytes were insulin resistant. The second group of fasting animals was exposed to a low and constant temperature (5 degrees). Their phases of lethargy were long and regular (about 15 days), separated by active periods (6-8 hr); their body weight decreased during the winter months; in spring the B-cell secretion was increased and the sensitivity of the tissues to insulin ensured a high peripheral glucose utilization. These data show that the winter climatic and nutritional conditions which influence the depth of hibernation modify the edible dormouse B-cell activity during the spring arousal.

Seasonal effect of catecholamines on glucose-induced insulin secretion in the edible dormouse (Glis glis L.): in vivo and in vitro studies

The results of the present study support the view that catecholamines are important for insulin secretion during the annual cycle of the edible dormouse exposed to seasonal changes in southwest France. Glucose tolerance tests and perfusion of the isolated pancreas have shown that in spring and summer, the B cell secretion mechanism is less sensitive to glucose than in other seasons. In spring and summer, insulin secretion induced by glucose is enhanced after in vivo and in vitro phentolamine treatment. The autumn and winter control insulin levels were not modified by phentolamine treatment. These results indicate that the inhibition of insulin secretion in spring and summer is due to noradrenaline liberated at the nerve endings adjacent to the B cells rather than to circulating catecholamines. This seasonal effect of noradrenaline may be attributed either to seasonal changes in the sensitivity of the alpha-adrenergic receptors of B cells or to an increase in noradrenaline release at the nerve endings in the islet during spring and summer.