AJP - Regulatory, Integrative and Comparative Physiology, Vol 270, Issue 1 304-R309, Copyright © 1996 by American Physiological Society
Diurnal and seasonal changes in sympathetic signal transduction in cardiac ventricles of European hamsters
K. Pleschka, A. Heinrich, K. Witte and B. Lemmer
Max-Planck-Institut fur Physiologische und Klinische Forschung, W. G. Kerckhoff-Institut, Bad Nauheim, Germany.
This investigation of the relationship between cardiac beta-adrenoceptors
and adenosine 3',5'-cyclic monophosphate (cAMP) formation in cardiac
ventricles of the nocturnally active European hamster both during euthermia
under a 12:12-h dark-light cycle and during hibernation under
constant-darkness conditions showed that neither the densities, affinities,
nor distribution of the beta 1- and beta 2-receptor subtypes differed
between the dark phase, light phase, and hibernation. Basal formation of
cAMP by the cardiac adenylyl cyclase of euthermic hamsters was higher in
ventricles obtained at night [core temperature (Tcore) = 37.8 degrees C]
than in ventricles obtained during the day (Tcore = 36.4 degrees C). Basal
formation of cAMP was also significantly lower in hibernating hamsters
(Tcore = 7.0 degrees C) than in euthermic hamsters. When adenylyl cyclase
activity was stimulated by isoprenaline, guanylylimidodiphosphate
[Gpp(NH)p], or forskolin, the rank order of potency was the same in
euthermic hamsters and hibernating hamsters: isoprenaline < Gpp(NH)p
< forskolin. Functional competition curves indicated that in the
euthermic hamsters beta 1-receptors were responsible for 67% of the
response to isoprenaline at night and 62% of the response during the day.
In hibernating hamsters, in contrast, most of the response to isoprenaline
(58%) was mediated via beta 2-receptors. This shift in the relative
importance of the receptor subtypes may facilitate arousal from hibernation
by making the heart more sensitive to circulating epinephrine.
