CIRCADIAN-RHYTHM OF THE VENTRICULAR-FIBRILLATION THRESHOLD IN FEMALE WISTAR RATS

The circadian rhythm of ventricular fibrillation threshold (VFT) and i ts relation to the heart rate (HR) and the rectal temperature (RT) was studied in female Wistar rats. The animals were exposed to daily ligh t-dark cycles of 12 h of light alternating with 12 h of darkness and w ere under pentobarbital anaesthesia (40mg/kg i.p.). The experiments we re performed on open chest animals and VFT was measured by direct stim ulation of the myocardium. VFT in female rats showed a circadian rhyth m with the acrophase -338 degrees (at 22.53 h), with the mesor 2.58 mA and the amplitude 0.33 mA. HR was not significantly changed during th e experiments and no dependence was found between VFT and HR during th e whole 24-hour period (r=0.08). The acrophase of the circadian rhythm of HR (on -47 degrees, i.e. at 03.08 h) was shifted to the acrophase of VFT. The circadian rhythms of RT before the application of the anae sthetic agent and under general anaesthesia before the operative inter ventions had a very similar course with the nearly corresponding acrop hases as the circadian rhythm of VFT. It is concluded that the electri cal stability of the rat heart measured by VFT shows the significant c ircadian rhythm in a parallel with the circadian rhythm of RT and prob ably without dependence on the changes of HR.