Inhibition of Na+-K+ pump alleviates the shortening of action potential duration caused by metabolic inhibition via blockade of KATP channels 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+ (K-ATP) channels and prolong the action potential duration (ATD) 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 mu M carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (a mitochondrial uncoupler) shortened the APD in a time-dependent manner. When dihydroouabain (DHO, 5 mu M) was introduced 5 min but not 10 min after introduction of MI, the APD shortening was significantly attenuated. Application of glibenclamide (1 mu 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 mu M), a K-ATP-channel opener. In separate experiments using whole-cell patch-clamp techniques, we found that this concentration of DHO (5 mu M) depressed the Na+-K+ pump current of the guineapig ventricular myocytes from 210 to 100 pA (at 0 mV) or by 49.5%. We conclude that, during early phase (5 5 min) of MI, the APD shortening mostly results from the activation of K-ATP channels, and that even a similar to 50% inhibition of the Na+-K+ pump by DHO leads to the blockade of K-ATP channels and eventual lengthening of the APD, (C) 1999 Academic Press.