Cardiac cyclic nucleotide phosphodiesterase (PDE) activity in the Europeanhamster 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 whi ch cardiac contractility can be adjusted, led us to determine the activitie s of both, cAMP and cGMP hydrolyzing PDEs in cardiac ventricles (c.v.) of h amsters during states of nocturnal activity, daytime rest and during hibern ation :Ind arousal. We also evaluated the temperature dependency of the PDE s by running the analysis at 25 degreesC and 7 degreesC. PDE activity was m easured using HPLC with on-line liquid scintillation spectrometry. During s ummer, PDE activities for the analysis of c[H-3]AMP and of c[H-3]GMP in c.v . collected from euthermic hamsters during day were significantly higher th an during night. During winter, PDE activities in c.v. from hamsters during hibernation and arousal were significantly increased in comparison with eu thermic summer values. Lowering the incubation temperature (Ti) at which PD E activities were determined from 25 degreesC to 7 degreesC decreased signi ficantly PDE activities in c.v. collected during day and night as well as i n c.v. collected from hamsters juring hibernation and arousal. The correspo nding Q(10)-values calculated fur both, the cAMP and cGMP hydrolyzing PDEs were relatively low and did nor differ between day and night, nor between s ummer and winter animals. Regarding the increase of PDE activities during h ibernation one has to consider that even in deep hypothermia, when cardiac action is drastically reduced, cAMP formation continues and that each stimu lation of the adrenergic signal transduction cascade causes in increase in cAMP formation. In this condition increase in PDE activity could be advanta geous in avoiding cAMP accumulation and thereby counteracting its stimulato ry action. The increased PDE activity also prevents a "luxury perfusion" of the hypothermic and thus only little oxygen consuming heart by the enhance d hydrolysis of cGMP. The unchanged Q(10) values speak against substantial day-night or seasonal adjustments of PDE activities. Considering the wide r ange in which core temperature in these animals is switching a certain ther mostability of the enzyme can be assumed.