Increased beta-adrenergic receptors in brown fat of winter-acclimatized Alaskan voles

To assess a possible mechanism for the enhanced thermogenesis of cold-acclimated and winter-acclimatized red-backed voles (Clethrionomys rutilus), beta-adrenergic receptors of brown fat were characterized by specific binding of (-)-[3H]-dihydroalprenolol [( 3H]DHA) to isolated brown fat membranes from 23 degrees C-acclimated controls, cold-acclimated (5 wk or 5 mo at 5 degrees C), wild summer, and winter-acclimatized voles. Scatchard analysis to determine the equilibrium dissociation constant (Kd) and the maximum number of binding sites (Bmax) for control brown fat membranes gave a Kd of 4.45 nM [3H]DHA and Bmax of 249 fmol [3H]DHA bound per milligram of protein. beta-Adrenergic agonists competed for specific binding sites with an order of potency typical of the beta 1 subtype of adrenergic receptors: (-)-isoproterenol greater than (-)-norepinephrine greater than or equal to (-)-epinephrine. After cold acclimation for 5 wk or 5 mo, the Kd and Bmax for adrenergic binding sites were similar to those of controls. Brown fat mass was 1.5 times greater than that of controls after 5 wk cold acclimation but similar to controls after 5 mo cold acclimation. Winter voles had 1.7 times higher Bmax and 1.6 times more brown fat than summer voles. Thus seasonal acclimatization to winter in red-backed voles appears to involve an increase in beta-adrenergic receptors in brown fat, but cold acclimation does not. The results suggest quantitative and possibly qualitative differences in neural and hormonal stimulation of brown fat between cold acclimation and winter acclimatization in voles.