Changes in electrical and mechanical activities of rabbit papillary muscle during hypoxic perfusion

The influence of moderate hypoxia (p02 : 70 mmHg) on action potential and contraction of isolated rabbit papillary muscles was analyzed. 1. Action potential duration, maximum twitch tension and time-to-peak tension decreased during 60 min of hypoxic perfusion, while resting tension increased and resting potential, overshoot and fast depolarization of the action potential remained constant. 2. MnCl2 (5 mM), after 60 min of hypoxia, abolished overshoot increased resting potential, and further reduced action potential duration. "Slow"action potentials induced by isoproterenol (2.5 X 10(-6) M) in KCl (20 mM) depolarized muscles, were highly resistant to hypoxic perfusion. This resistance indicates that the inflow of cations through slow channels persists under these hypoxic conditions. 3. CsCl (2-4 mM) increased action potential duration in hypoxic fibers to control (O2) values. These results suggest that an increase in membrane background K conductance could account for the reduction in action potential duration under hypoxic incubation. A possible relationship between changes in internal free calcium concentration and background K conductance during hypoxia is considered.