DIURNAL AND SEASONAL-CHANGES IN SYMPATHETIC SIGNAL-TRANSDUCTION IN CARDIAC VENTRICLES OF EUROPEAN HAMSTERS

This investigation of the relationship between cardiac beta-adrenocept ors and adenosine 3',5'-cyclic monophosphate (cAMP) formation in cardi ac ventricles of the nocturnally active European hamster both during e uthermia under a 12:12-h dark-light cycle and during hibernation under constant-darkness conditions showed that neither the densities, affin ities, nor distribution of the beta(1)- and beta(2)-receptor subtypes differed between the dark phase, light phase, and hibernation. Basal f ormation of cAMP by the cardiac adenylyl cyclase of euthermic hamsters was higher in ventricles obtained at night [core temperature (T-core) 37.8 degrees C] than in ventricles obtained during the day (T-core = 36.4 degrees C). Basal formation of cAMP was also significantly lower in hibernating hamsters (T-core = 7.0 degrees C) than in euthermic ham sters. When adenylyl cyclase activity was stimulated by isoprenaline, guanylylimidodiphosphate [Gpp(NH)p], or forskolin, the rank order of p otency was the same in euthermic hamsters and hibernating hamsters: is oprenaline < Gpp(NH)p < forskolin. Functional competition curves indic ated that in the euthermic hamsters beta(1)-receptors were responsible for 67% of the response to isoprenaline at night and 62% of the respo nse during the day. In hibernating hamsters, in contrast, most of the response to isoprenaline (58%) was mediated via beta(2)-receptors. Thi s shift in the relative importance of the receptor subtypes may facili tate arousal from hibernation by making the heart more sensitive to ci rculating epinephrine.