Apparent thermogenic effect of injected glucagon is not due to a direct effect on brown fat cells

To examine the significance of brown adipose tissue for the thermogenic response to glucagon, we injected glucagon intraperitoneally into rats (that have glucagon-sensitive brown fat cells) and into hamsters (that have glucagon-insensitive brown fat cells). Although a thermogenic response to glucagon injection was apparently observed in rats, this response was not augmented by cold acclimation and was not dose dependent. Similar observations were made in hamsters. The thermogenic response could be fully blocked by prior injection of the beta-adrenergic blocker propranolol. Thus no direct thermogenic response to injected glucagon could be demonstrated, and the thermogenic response observed was fully due to vehicle injection. However, glucagon injection was able to unmask mitochondrial [3H]GDP binding. As expected, isolated brown fat cells from rats and mice responded thermogenically to glucagon but brown fat cells from hamsters were unresponsive. The EC50 of the rat brown fat cells was high (5 nM); these cells also responded to secretin, with an EC50 of 22 nM. It was concluded that, in contrast to earlier observations, no thermogenic response to injected glucagon could be observed; this may be related to differences in glucagon preparations. Brown fat cells from certain species are, however, glucagon sensitive. It is uncertain whether glucagon is the endogenous agonist for these receptors, but the presence of the glucagon-responsive receptor indicates alternative means to norepinephrine for stimulation of brown adipose tissue thermogenesis and, probably, of recruitment.