In winter, hibernating mammals enter a long phase of
lethargy which is characterized by low body temperature, depressed metabolism and minimal release of metabolic substrates from endogenous fuel stores. Periodically, they spontaneously warm themselves to regain the euthermic state. These arousals are, by contrast, times of high release and consumption of endogenous substrates.
Insulin and
glucagon may contribute to the control of both contrasting metabolic periods. The secretion and metabolic effects of these two
hormones were investigated in two hibernators: the hedgehog (Erinaceus europaeus) and the edible dormouse (Glis glis). During
lethargy,
blood glucose,
insulin and
glucagon concentrations were low. In vivo and in vitro studies showed that the secretion of both
hormones was markedly depressed by low temperatures. Insulin secretion was not stimulated by
glucose, although
glucagon secretion remained reactive to
arginine.
Blood glucose was not regulated by
insulin but pharmacological doses of
glucagon increased
blood glucose concentrations. The tissues were found to be highly
insulin-resistant, preventing the fall of
blood glucose and consequently limiting the depletion of glucidic substrates during the long periods of
starvation. During arousal,
blood glucose,
insulin and
glucagon levels increased at the end of
rewarming while
glucose turnover gradually increased above a body temperature of 15 degrees C. The effects of
glucagon and
insulin on
glucose metabolism increased markedly beyond this stage. Thus the metabolic effect of both
hormones are temperature-dependent.
Insulin and
glucagon allow an increase in
glucose availability for the active metabolic processes which occur during arousal.