Alterations in action potential profile enhance excitation‐contraction coupling in rat cardiac myocytes

Abstract

Action potential (AP) prolongation typically occurs in heart disease due to reductions in transient outward potassium currents (Ito), and is associated with increased Ca2+ transients. We investigated the underlying mechanisms responsible for enhanced Ca2+ transients in normal isolated rat ventricular myocytes in response to the AP changes that occur following myocardial infarction. Normal myocytes stimulated with a train of long post‐myocardial infarction (MI) APs showed a 2.2‐fold elevation of the peak Ca2+ transient and a 2.7‐fold augmentation of fractional cell shortening, relative to myocytes stimulated with a short control AP. The steady‐state Ca2+ load of the sarcoplasmic reticulum (SR) was increased 2.0‐fold when myocytes were stimulated with trains of long post‐MI APs (111 ± 21.6 μmol l−1) compared with short control APs (56 ± 7.2 μmol l−1). Under conditions of equal SR Ca2+ load, long post‐MI APs still resulted in a 1.7‐fold increase in peak [Ca2+]i and a 3.8‐fold increase in fractional cell shortening relative to short control APs, establishing that changes in the triggering of SR Ca2+ release are largely responsible for elevated Ca2+ transients following AP prolongation. Fractional SR Ca2+ release calculated from the measured SR Ca2+ load and the integrated SR Ca2+ fluxes was 24 ± 3 and 11 ± 2 % following post‐MI and control APs, respectively. The fractional release (FR) of Ca2+ from the SR divided by the integrated L‐type Ca2+ flux (FR/∫FCa,L) was increased 1.2‐fold by post‐MI APs compared with control APs. Similar increases in excitation‐contraction (E‐C) coupling gains were observed establishing enhanced E‐C coupling efficiency. Our findings demonstrate that AP prolongation alone can markedly enhance E‐C coupling in normal myocytes through increases in the L‐type Ca2+ current (ICa,L) trigger combined with modest enhancements in Ca2+ release efficiency. We propose that such changes in AP profile in diseased myocardium may contribute significantly to alterations in E‐C coupling independent of other biochemical or genetic changes.

Inhibition of Calcineurin and Sarcolemmal Ca 2+ Influx Protects Cardiac Morphology and Ventricular Function in K v 4.2N Transgenic Mice

“…6 The increased APD 50 in K v 4.2N myocytes is consistent with the dominant contribution of I to,f to repolarization in mouse myocardium. 17 As expected from previous reports, 9,18 total Ca 2ϩ entry through L-type Ca 2ϩ channels (Q Ca,L ) was elevated (PϽ0.01) nearly 3-fold in transgenic myocytes (Q Ca,L ϭ14.8Ϯ1.7 picocoulombs, nϭ6) stimulated with typical K v 4.2N APs compared with control myocytes (Q Ca,L ϭ5.1Ϯ0.9 picocoulombs, nϭ7) driven by control APs ( Figure 1B). Despite these differences in integrated Ca 2ϩ entry per beat, I Ca,L densities evoked by voltage steps to ϩ10 mV did not differ significantly between control (15.9Ϯ1.9 pA/pF, nϭ6) and K v 4.2N (15.2Ϯ1.5 pA/pF, nϭ6) myocytes.…”

“…9 For AP and intracellular Ca 2ϩ transient measurements, the extracellular solution was composed of (mmol/L): 140 NaCl, 4 KCl, 10 HEPES, 1 MgCl 2 , 2 CaCl 2 , and 10 D-glucose, adjusted to pH 7.4 with NaOH. For I Ca,L measurements, the extracellular solution contained (mmol/L): 135 choline-Cl, 10 HEPES, 1 MgCl 2 , 2 CaCl 2 , and 10 D-glucose, adjusted to pH 7.4 with CsOH.…”

“…4 Indeed, reduced K ϩ currents in transgenic mice can cause AP prolongation, [5][6][7] lengthened QT intervals, and ventricular tachyarrhythmias. 5,7 AP prolongation as a result of I to reduction in rodent hearts also elevates intracellular Ca 2ϩ transients via increased L-type Ca 2ϩ influx (I Ca,L ), 8,9 thereby leading to enhanced myocardial contractility. The finding that I to downregulation and associated AP prolongation occur early in the disease process 10 suggests that this may represent a compensatory mechanism to enhance contractility of the compromised myocardium.…”

Transient outward potassium current and Ca2+ homeostasis in the heart: beyond the action potential

“…A decrease in I to density results in AP prolongation, especially of the plateau phase. It is expected that a longer AP is associated with greater total Ca 2+ influx, and this has been confirmed by experimental evidence (12,21,40). Paucity of I to and prolonged AP are observed in some conditions in which the SR function is depressed, such as immaturity and senescence, ventricular hypertrophy and heart failure.…”

Myocardial infarction in rat eliminates regional heterogeneity of AP profiles,ItoK+currents, and [Ca2+]itransients

“…Indeed, APD prolongation in rat myocytes is associated with increased [Ca 2ϩ ] i transient amplitudes (7,10,30,54), particularly when APD prolongation is caused by reductions in I to (46,54). The link between changes in I to density and [Ca 2ϩ ] i in rat myocytes can be traced to the profound impact that the relatively fast activating and inactivating I to has on the early repolarization period when I Ca,L are also prominent (30,46,54). In our studies, peak [Ca 2ϩ ] i was much lower in RVW versus SEP myocytes, which correlated strongly with higher I to densities and shorter APDs.…”