Inhibition of Na+-K+Pump Alleviates the Shortening of Action Potential Duration caused by Metabolic Inhibition via Blockade of KATPChannels in Coronary Perfused Ventricular Muscles of Guinea-pigs

Abstract The Na + -K + pump is a consumer of intracellular ATP. We therefore examined whether blockade of the Na + -K + pump by cardiac glycosides could inhibit ATP-sensitive K + (K ATP ) channels and prolong the action potential duration (APD) of the guinea-pig ventricular muscles perfused with Tyrode's solution via the coronary artery and stimulated at 3 Hz. The metabolic inhibition (MI) achieved by application of 0.1 μ m carbonyl cyanide p -(trifluoromethoxy) phenylhydrazone (a mitochondrial uncoupler) shortened the APD in a time-dependent manner. When dihydroouabain (DHO, 5 μ m) was introduced 5 min but not 10 min after introduction of MI, the APD shortening was significantly attenuated. Application of glibenclamide (1 μ m), a blocker of K ATP channels, introduced both 5 and 10 min after MI also alleviated the APD shortening; DHO did not alleviate the APD shortening effect produced by cromakalim (5 μ m), a K ATP -channel opener. In separate experiments using whole-cell patch-clamp techniques, we found that this concentration of DHO (5 μ m) depressed the Na + -K + pump current of the guinea-pig ventricular myocytes from 210 to 100 pA (at 0 mV) or by 49.5%. We conclude that, during early phase (5 min) of MI, the APD shortening mostly results from the activation of K ATP channels, and that even a 50% inhibition of the Na + -K + pump by DHO leads to the blockade of K ATP channels and eventual lengthening of the APD