Hypoxic enhancement of K conductance

pp 31-39

Abstract

Metabolic inhibition shortens the cardiac action potential (AP) and induces loss of cellular potassium. Despite the latter. the resting potencial remains at control levels during high flow hypoxia. All these effects can be attributed to an increased membrane conductance to K+ but the channels involved have not been identified yet. The discovery of A TP.sensitive K+ channels in cardiac muscle led to the proposition that these channels are involved in the AP shortening induced by hypoxia but this view remains controversial because of a time lag observed between the appearance of electrical changes and the onset at A TP depletion. The aim of this work was to elucidate the role that different K+ conductances may play in the myocardial response to metabolic depression. This was inferred from the effects of several K+ channel blockers on action potential parameters and myocardial Na+ and K+ contents in perfused rabbit hearts both in normoxia and hypoxia. The hearts were paced at 2.5 Hz. The diffusion component of the resting potential was evaluated by elimination of the Na+-pump related fraction with 10^-4 M ouabain. The action potential duration at 25 Oft and 95010 repolarization (APD-25 and APD-95) was reduced by 75 % and 60 % respectively by hypoxia. Steady state value were reached within 15 min. Tetraethylammonium (10 mM) delayed but did not prevent AP shortening whereas 30 μ glibenclamide (specific blocker of the ATP-sensitive K+ channels) significantly reduced the APD.25 shortening to 55 % and the APD-95 shortening to 40 % after 30 min of hypoxia. A low -concentration of BaCl₂ (40  μM) specifically blocks the inward rectifying. current I_k1 in a time and voltage- dependent manner. Consequently, a slight depolarization and an increase in APD-9S were produced by'Ba2+ in normoxia. In hypoxia, Ba²⁺ further depolamed the resting potential Vr, reduced by 50 % the APD-95 shortening but only slightly lengthened APD-25. The combination of 0.2 mM Ba2+ and 4 mM Cs+ (complete block of gkl) prevented APD.95 shortening but did not modify APD-25 shortening during hypoxia. Ex- posure to 10-4 M ouabain produced an initial depo- lari~ation that lasted about 5 min reflecting the aboli- tion of electrogenic Na+ extmsion. The membrane potential reoorded under these condition represents the diffusion component of Vr,-Vd generated by the ionic gradients and the membrane background conductance (mainly gkl)' The K+ dependence of these potential fractions was studied under several experimental conditions. In normoxia, the .Na + - pump related potential decreased as external K+ (Kg) increased which probably reflected the decrease in membrane resistance with Kg. The diffusion potential increased in hypoxia and approached the K+ equilibrium potential. Barium (40  μM) decreased Vd in normoxia and considerably flattened the curve relating Vr to Ko in hypoxia. Moreover, Vd became insensitive to changes in Kg. Tetraethyl- ammonium and Ba2+ also reduced the myocardial K+ loss from 27 Oft to 16 Oft without altering Na + accumulation. Our results indicate that an increase in Ik1 underlies the maintenance of Vr and hence of propagated electrical activity as well as the acceleration of the late repolarization in the hypoxic heart. The involvement of Ik1 in the APD-95 shortening was demonstrated by the abolition of this effect in the presence of complete Ik1 blockade with Ba2+ and Cs+ whose specificity is supported by the depolarization observed. Glibenclamide and TED at the concentrations used are not supposed to alter Ik1 and did not affect Vr. (...)