Cardiac Cyclic Nucleotide Phosphodiesterase (PDE) Activity in the European Hamster during Hibernation
Being aware that not only the termination but in particular the modulation of the transduced cAMP signals by the PDE is decisive for the extent to which cardiac contractility can be adjusted, led us to determine the activities of both, cAMP and cGMP hydrolyzing PDEs in cardiac ventricles (c.v.) of hamsters during states of nocturnal activity, daytime rest and during hibernation and arousal. We also evaluated the temperature dependency of the PDEs by running the analysis at 25°C and 7°C. PDE activity was measured using HPLC with on-line liquid scintillation spectrometry. During summer, PDE activities for the analysis of c[3H]AMP and of c[H]GMP in c.v. collected from euthermic hamsters during day were significantly higher than during night. During winter, PDE activities in c.v. from hamsters during hibernation and arousal were significantly increased in comparison with euthermic summer values. Lowering the incubation temperature (Ti) at which PDE activities were determined from 25°C to 7°C decreased significantly PDE activities in c.v. collected during day and night as well as in c.v. collected from hamsters during hibernation and arousal. The corresponding Q10-values calculated for both, the cAMP and cGMP hydrolyzing PDEs were relatively low and did not differ between day and night, nor between summer and winter animals. Regarding the increase of PDE activities during hibernation one has to consider that even in deep hypothermia, when cardiac action is drastically reduced, cAMP formation continues and that each stimulation of the adrenergic signal transduction cascade causes in increase in cAMP formation. In this condition increase in PDE activity could be advantageous in avoiding cAMP accumulation and thereby counteracting its stimulatory action. The increased PDE activity also prevents a “luxury perfusion” of the hypothermic and thus only little oxygen consuming heart by the enhanced hydrolysis of cGMP. The unchanged Q10 values speak against substantial day-night or seasonal adjustments of PDE activities. Considering the wide range in which core temperature in these animals is switching a certain thermostability of the enzyme can be assumed.
KeywordsNocturnal Activity Deep Hypothermia cAMP Formation Cardiac Ventricle Seasonal Adjustment
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