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

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+(KATP) channels and prolong the action potential duration (APD) of the guinea-pig ventricular muscles perfused with Tyrodeʹs solution via the coronary artery and stimulated at 3 Hz. The metabolic inhibition (MI) achieved by application of 0.1μm carbonyl cyanidep-(trifluoromethoxy) phenylhydrazone (a mitochondrial uncoupler) shortened the APD in a time-dependent manner. When dihydroouabain (DHO, 5μm) was introduced 5 min but not 10 min after introduction of MI, the APD shortening was significantly attenuated. Application of glibenclamide (1μm), a blocker of KATPchannels, introduced both 5 and 10 min after MI also alleviated the APD shortening; DHO did not alleviate the APD shortening effect produced by cromakalim (5μm), a KATP-channel opener. In separate experiments using whole-cell patch-clamp techniques, we found that this concentration of DHO (5μm) 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 KATPchannels, and that even a 50% inhibition of the Na+-K+pump by DHO leads to the blockade of KATPchannels and eventual lengthening of the APD