Effects of moderate hypoxia on action potential and contraction of rabbit ventricular muscle

Experiments were performed on rabbit right ventricular strips in order to investigate the effects of moderate hypoxia on electrical and mechanical activities of this preparation. The results indicate that a slow inward current (Isi) is responsible for overshoot (OS) and the plateau phase of the action potential (AP). Hypoxia (pO2 = 65-70 mmHg; 8.66-10.0 kPa) induced a rapid shortening of AP and an increase in the phase-3 slope. It also markedly reduced twitch tension and time to peak tension, and increased resting tension. The resting potential (RP), OS and maximum rate of depolarization of phase O were not affected during 60 min of hypoxic perfusion. Isoproterenol under this condition had a positive inotropic effect and also increased AP duration (APD) measured at -60 mV, having a less pronounced effect on the plateau phase. MnCl2 in hypoxic conditions drastically reduced OS indicating that even in this condition, Isi was present. These results suggest that an increase in gK is responsible for the hypoxia-induced reduction in APD. This also partially explains the maintenance of RP during hypoxic incubation.