After myocardial infarction (MI), the noninfarcted myocardium undergoes significant hypertrophy as part of the post-MI structural remodeling. Electrophysiological changes associated with the hypertrophied remodeled myocardium may play a key role in arrhythmia generation in the post-MI heart. We investigated the cellular and ionic basis of arrhythmias in remodeled left ventricular (LV) myocardium 3 to 4 weeks after MI in the rat. We analyzed (1) the incidence of induced ventricular tachyarrhythmias (VTs) in the in vivo heart, (2) action potential characteristics and arrhythmia mechanisms in multicellular preparations and isolated remodeled LV myocytes, and (3) the density and kinetics of the L-type Ca2+ current (ICa-L) and the fast and slow components of transient outward K+ currents (Ito-f and Ito-s, respectively). The results were compared with those from sham-operated rats. In vivo, programmed stimulation induced sustained VT in 80% of post-MI rats but not in sham-operated rats. The capacitance of post-MI hypertrophied myocytes was significantly increased compared with myocytes from sham-operated rats. Post-MI myocytes had prolonged action potential duration (APD) with marked heterogeneity of the time course of repolarization. The prolongation of APD could be explained by the significant decrease of the density of both Ito-f and Ito-s. There was no change in the kinetics of both currents compared with control. Both the density and kinetics of ICa-L were not significantly different in post-MI remodeled myocytes compared with control. The cellular studies showed that reentrant excitation secondary to dispersion of repolarization and triggered activity from both early and delayed afterdepolarizations are potential mechanisms for VT in the post-MI remodeled heart.

Three weeks after myocardial infarction (MI) in the rat, remodeled hypertrophy of noninfarcted myocardium is at its maximum and the heart is in a compensated stage with no evidence of heart failure. Our hemodynamic measurements at this stage showed a slight but insignificant decrease of +dP/dt but a significantly higher left ventricular end-diastolic pressure. To investigate the basis of the diastolic dysfunction, we explored possible defects in the beta-adrenergic receptor-G(s/i) protein-adenylyl cyclase-cAMP-protein kinase A-phosphatase pathway, as well as molecular or functional alterations of sarcoplasmic reticulum Ca(2+)-ATPase and phospholamban (PLB). We found no significant difference in both mRNA and protein levels of sarcoplasmic reticulum Ca(2+)-ATPase and PLB in post-MI left ventricle compared with control. However, the basal levels of both the protein kinase A-phosphorylated site (Ser16) of PLB (p16-PLB) and the calcium/calmodulin-dependent protein kinase-phosphorylated site (Thr17) of PLB (p17-PLB) were decreased by 76% and 51% in post-MI myocytes (P<0.05), respectively. No change was found in the beta-adrenoceptor density, G(salpha) protein level, or adenylyl cyclase activity. Inhibition of phosphodiesterase and G(i) protein by Ro-20-1724 and pertussis toxin, respectively, did not correct the decreased p16-PLB or p17-PLB levels. Stimulation of beta-adrenoceptor or adenylyl cyclase increased both p16-PLB and p17-PLB in post-MI myocytes to the same levels as in sham myocytes, suggesting that decreased p16-PLB and p17-PLB in post-MI myocytes is not due to a decrease in the generation of p16-PLB or p17-PLB. We found that type 1 phosphatase activity was increased by 32% (P<0.05) with no change in phosphatase 2A activity. Okadaic acid, a protein phosphatase inhibitor, significantly increased p16-PLB and p17-PLB levels in post-MI myocytes and partially corrected the prolonged relaxation of the [Ca(2+)](i) transient. In summary, prolonged relaxation of post-MI remodeled myocardium could be explained, in part, by altered basal levels of p16-PLB and p17-PLB caused by increased protein phosphatase 1 activity.

T-type Ca2+ channel gene and current are reexpressed in rat post-MI remodeled LV myocytes. Its functional significance in the post-MI remodeling process remains to be defined.

ATP-dependent decay and recovery of the inward rectifier and ATP-sensitive K+ channels were investigated using inside-out patch recording in cardiac myocytes. The solution facing the inner side of the membrane was instantaneously changed with the oil-gate concentration jump method. Both channels were decayed by removing ATP and were recovered by reapplying ATP. The coexistence of Mg2+ was required for the recovery. 5'-Adenylylimidodiphosphate failed to reverse the ATP-dependent decay. The cumulative histograms of survival time and recovery time, obtained from the inward rectifier K+ channel, showed a single exponential distribution, time constants of which were 55 and 43 s, respectively. The time-dependent nature of decay and recovery was also confirmed in the ATP-sensitive K+ channel. The findings indicated that intracellular ATP is one of the factors that determines the activity of the K+ channels. It is most probable that phosphorylation of channel molecules is essential for maintaining the K+ channel in an operative state.

Our findings raise the possibility that the increase in the slow component of I(Na) in post-MI remodeled myocytes is secondary to the increased expression of NaCh I. Additional studies are required to address these questions and to characterize the functional role of the NaCh I subtypes in cardiac myocytes.

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Kinetics for gating the ATP-sensitive K+ channel was studied by exposing the inside-out patch to instantaneous changes in the intracellular concentration of ATP ( [ATP]i) using the oil-gate concentration jump technique in guinea pig ventricular cells. The closing time course of the channel after increasing [ATP]i was exponential with a time constant (tau), which decreased with increasing [ATP]i. The linear 1/tau - [ATP]i relation revealed two different binding (closing) rate constants (mu) of 51.7 and 5.6 mM-1.s-1 and predicted a common unbinding (opening) rate constant (lambda) of 3.2 s-1. A variable latent period was observed before channel opening when [ATP]i was decreased. The mechanism of latency is not clear. Once the channel started to open at the change lowering [ATP]i, the opening time course was exponential. Measurements of the exponential tau obtained at 0 mM [ATP]i were divided into two groups with corresponding lambda of 2.8 and 20.1 s-1, respectively. The former agrees with the predicted value of 3.2 s-1, but in the latter case, tau for opening increased as [ATP]i was increased. This increase in tau was attributed to a decrease of lambda, which approached an asymptotic value of 3.2 s-1. We conclude that binding and unbinding of one molecule of ATP determine the gating of ATP-sensitive K+ channel. Different pairs of mu and lambda result in four types of gating patterns and practically two states of sensitivities to ATP.

A new method was developed to instantaneously replace the solution on the inner side of an inside-out membrane patch in order to measure time courses with which active substances acted on single ionic channels. Inside-out membrane patches were isolated from single ventricular cells of the guinea pig heart. The recording bath consisted of two chambers separated by a partition having a narrow slit. Mixing of two test solutions through this slit was prevented by filling it with paraffin oil. The pipette tip with a tightly sealed inside-out membrane patch was moved through the oil from one solution to the other so that the pipette tip was instantaneously exposed to a new solution. When the pipette tip was jumped between different K+ concentrations, the leak current through the membrane patch increased or decreased with a half time of 6.3 +/- 3.0 ms (n = 15). The amplitude of single K+ channel currents changed to a new steady level within approximately 20 ms. These time courses were well explained by diffusion of K+ in the dead space between the pipette tip opening and the membrane patch. An application of this method to the ATP-regulated K+ channel revealed a latent period of 1-2 s before the channel started its activity after the instantaneous removal of ATP, whereas no obvious latency was observed in the rapid suppression of the channel, which was completed in 100-300 ms after reapplying ATP.