Effects of hypoxia and altered K0 on the membrane potential of rabbit ventricle

The upstroke of the ventricular action potential in the rabbit consists of two depolarizing components with different rates of rise. The effects of hypoxia on the resting potential (RP); the upstroke phases (I and II) and the maximum rate of rise of phase I (V max) were studied at different external K concentrations (K0). Perfused hearts were submitted to N2-equilibrated media containing 1.5 to 10 mM K0. Exposure of oxygenated hearts to different K0 changed the regenerative response from a fast rising action potential at 1.5 mM K0 to a depressed fast response at 7.5 and 10 mM K0. Hypoxia decreased the action potential amplitude (APA) at all K concentrations. In K0 less than or equal to 5 mM the reduction of APA was due to a decrease in the amplitude of phase II of the upstroke but the maximum rate of rise (V max) did not change. In contrast, phase I of the upstroke was markedly depressed by hypoxia in high K0, but phase II was unmodified and its V max compared well with values reported for other normoxic cardiac cells. Hyperkalemia per se did not slow conduction during normoxia but increased conduction time in hypoxia. The resting potential of hypoxic cells was closer to the K equilibrium potential than in the control. The RP v. Ko/Ki relation suggested that electrogenic Na extrusion persists in hypoxia. The electrogenic fraction of the resting potential as determined from pump inhibition with 10(-4) M ouabain amounted to -6 mV. Our results did not indicate whether the differential effects of hypoxia on the upstroke components were potential dependent or were related to direct effects of K+ on the ionic currents that determine the action potential. The persistence of phase II during hypoxia in partly depolarized cells may assure the maintenance of propagated electrical activity under conditions that are likely to be encountered in vivo during cardiac ischemia.