Increased Nav 1.5 channel late current underlies prolongation of action potential duration of ventricular myocytes from aged guinea pigs

Abstract

<h3>Background</h3> The action potential duration (APD) of aged hearts is prolonged. Because the late Na⁺ current (I_NaL) has been shown to play a significant role in modulating cardiac APD, we sought to determine whether the age-related prolongation of APD is due to an increase in endogenous I_NaL. <h3>Methods</h3> Experiments were performed on ventricular myocytes isolated from the hearts of young (one-month old) or aged (one-year old) guinea pigs. The APD and the amplitude of I_NaL were determined using the whole-cell patch-electrode technique at 36 ^oC. The APD was measured at 50% repolarization (APD₅₀). I_NaL was activated by 250-ms voltage-clamp pulses from -90 to -30 mV under conditions wherein K⁺ and Ca²⁺ currents were inhibited. The amplitude of I_NaL was calculated as the average current during the last 100-ms of the voltage clamp. <h3>Results</h3> The APD₅₀ of myocytes from aged hearts (323±9 ms, n=29) was significantly longer than those from young hearts (229±6 ms, n=43, p < 0.001). The selective I_NaL inhibitor GS967 (0.01 - 1 µmol/L, n=7) and the Na⁺ channel blocker tetrodotoxin (0.1 - 10 µmol/L, n=9) shortened the APD₅₀ of myocytes from aged hearts by 24±4 to 44±4% and by 32±4 to 45±4%, respectively (p < 0.001). Consistent with the results of APD measurements, the amplitude of I_NaL of aged myocytes (-0.51±0.05 pA/pF, n=17) was significantly greater than that of young myocytes (-0.22±0.02 pA/pF, n=27, p < 0.001). MTSEA (methanethiosulfonate ethylammonium, 2 mmol/L), a Na_v1.5 channel blocker, reduced the I_NaL of young and aged myocytes by 94±3% (n = 10) and 96±1% (n = 8), respectively, suggesting that the Na_v 1.5 channel is the major contributor to I_NaL of both young and aged guinea pig ventricular myocytes. <h3>Conclusion</h3> The present results suggest that 1) an increase in endogenous I_NaL is responsible for the prolongation of APD in aged guinea pig ventricular myocytes, and 2) Na_v 1.5 channels are the major contributor to the increased I_NaL.