| Background and aims: Naturally occurring temporal variability of action potential duration (APD) in isolated myocytes has been noted. Most of the studies have been focusing on analyzes of the differences in ionic channels and currents among the epicardial-, mid-myocardial-(M) and endocardial myocytes, and the rate-dependent (adaptation) characteristics of APD. We have found that the change in APD during a change in frequency of stimulation mostly reflects a change in rate of repolarization at distinct membrane potential levels.We assumed that in the myocytes, there is balancing mechanism, which is constantly adjusting the various ionic currents accommodating to the changing conditions. This intrinsic ability of adaptation is important and may offer some of the consequences of the transmural heterogeneity in adaptation of APD. This adaptive behaviors maybe equally important in maintaining the normal electrophysiological properties and in induction of arrhythmia in a case of error in normal adaptation. Though most studies of Na+/Ca2+ exchange (NCX) has been emphasized on its reverse activaty during pathyological condition. Our hypothesis is that reverse activaty of NCX also plays an important role in adjusting the repolarization of AP during a physiological condition. A mismatch between action potential (AP) repolarization and relaxation of the contraction can be caused by intracellular Ca2+ transport abnormalities. Ca2+ influx via reverse activation of NCX can load the sarcoplasmic recticulum (SR), which has arrhythmogenic effect.Methods: We studied the single myocytes from the heart of adult mongrel dogs. During the cell separation, Ca2+ -free Tyrode, low Ca2+ (60#,M) BS containing collagenase was perfused through LAD by Langandorff system. The tissue perfused by the LAD was then minced into a suspension, filtered , washed and resuspended 5 times in BS. The myocytes were stored in a minimum essential medium with Hank's salt at 17癈. Penicillin (100,000 u/L) inhibited bacterial growth in the storage medium. We determinded AP in current clamp mode, we verified the origin of each myocyte by examining the characteristics of the APs under baseline conditions. By this method we managed to separate the three types of myocytes very effectively. Myocyte contraction was imaged by a video camera, shortening of unloaded myocytes was detected by a video edgemotion detector, using changes in light intensity at the edges of themyocyte.INcx was recorded in voltage clamp mode and activated by Ca2+loading protocol..Re8uItst From 60 consecutive recorded APs at a constant 1.0 Hzstimulation under steady state conditions we found there is avariance in the repolarization between l0mV and -40mV. We alsofound the variance in the APD during the rate adaptation range ofrepolarization. Fluctuation in the transient may contribute to theAPD variability. To test this hypothesis we block the transient byintracellular dialysis with l0 mM EGTA(n=l9), this caused asignificant reduction in the coeffcient variability (CV=SD/meanAPD%) from 2.3t0.8 to l .3t0.3 p <0.0l. During a rate change ofthe stimulation from 0.6Hz to 1.0Hz. The AP duration increasedfrom 278l8msec to 320f9msec (MeanlSD, n=5, 50 APs, P<0.05),contraction is accompanied by an after-contraction(A-CON). There1axation of contraction precedes the repolarization of the AP. Weassumed that the enhancemcnt of repolarization and the productionof after-contraction can be possibly induced by reverse mode ofNCX. Reducing [Na+l. by substitution of 40mM Na+ with Li+ favorsNCX actiyating the reverse mode, which significantly decreased thedome of the AP from 4.8 i 0.3 to -- 10.6 i l .2mV(P < 0.05) andincreased the APD from 330 I l3 to 368 f l4msec(P < 0.05). Whenthe SR function was abolished by 10 u M ryanodine and 3 ll Mthapsigargin, contraction was significantly decreased (from 2.l l0.2 tol .2 l 0. l, P r 0.05) o 10nM isopretorenol (ISO) can onlyincrease the amplitude of the contraction (from 2.l I 0.2 .to 3 .8 i0.4, P wt 0.0l), but the rate of rise decreased (542 I 234 to 354...
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