Activation of reverse Na+-Ca2+ exchanger by skeletal Na+ channel isoform increases excitation-contraction coupling efficiency in rabbit cardiomyocytes

Our prior work has shown that Na⁺ current (I_Na) affects sarcoplasmic reticular (SR) Ca²⁺ release by activating early reverse of the Na⁺-Ca²⁺ exchanger (NCX). The resulting Ca²⁺ entry primes the dyadic cleft, which appears to increase Ca²⁺ channel coupling fidelity. It has been shown that the skeletal isoform of the voltage-gated Na⁺ channel (Na_v1.4) is the main tetrodotoxin (TTX)-sensitive Na_v isoform expressed in adult rabbit ventricular cardiomyocytes. Here, I tested the hypothesis that it is also the principal isoform involved in the priming mechanism. Action potentials (APs) were evoked in isolated rabbit ventricular cells loaded with fluo-4, and simultaneously recorded Ca²⁺ transients before and after the application of either relatively low doses of TTX (100 nM), the specific Na_v1.4 inhibitor μ-Conotoxin GIIIB or the specific Na_v1.1 inhibitor ICA 121430. Although APs changes after the application of each drug reflected the relative abundance of each isoform, the effects of TTX and GIIIB on SR Ca²⁺ release (measured as the transient maximum upstroke velocity) were no different. Furthermore, this reduction in SR Ca²⁺ release was comparable with the value that we obtained previously when total I_Na was inactivated with a ramp applied under voltage clamp. Finally, SR Ca²⁺ release was unaltered by the same ramp in the presence of TTX or GIIB. In contrast, application of ICA had no effect of SR Ca²⁺ release. These results suggest that Na_v1.4 is the main Na_v isoform involved in regulating the efficiency of excitation-contraction coupling in rabbit cardiomyocytes by priming the junction via activation of reverse-mode NCX.NEW & NOTEWORTHY A number of studies suggest that the Na⁺-Ca²⁺ exchanger (NCX) activated by Na⁺ currents is involved in the process of excitation-contraction (EC) coupling in cardiac ventricular myocytes. Although insufficient to trigger sarcoplasmic Ca²⁺ release alone, the Ca²⁺ entering through reverse NCX during an action potential can prime the dyadic cleft and increase the Ca²⁺ current coupling fidelity. Using specific Na⁺ inhibitors in this study, we show that in rabbit ventricular cells the skeletal Na⁺ channel isoform (Na_v1.4) is the main isoform responsible for this priming. Our study provides insights into a mechanism that may have an increased relevance where EC coupling is remodeled.

The Degree of t-System Remodeling Predicts Negative Force-Frequency Relationship and Prolonged Relaxation Time in Failing Human Myocardium

The normally positive cardiac force-frequency relationship (FFR) becomes flat or negative in chronic heart failure (HF). Here we explored if remodeling of the cardiomyocyte transverse tubular system (t-system) is associated with alterations in FFR and contractile kinetics in failing human myocardium. Left-ventricular myocardial slices from 13 failing human hearts were mounted into a biomimetic culture setup. Maximum twitch force (F), 90% contraction duration (CD₉₀), time to peak force (TTP) and time to relaxation (TTR) were determined at 37°C and 0.2–2 Hz pacing frequency. F_1Hz/F₀._5Hz and F_2Hz/F₀._5Hz served as measures of FFR, intracellular cardiomyocyte t-tubule distance (ΔTT) as measure of t-system remodeling. Protein levels of SERCA2, NCX1, and PLB were quantified by immunoblotting. F_1Hz/F₀._5Hz (R² = 0.82) and F_2Hz/F₀._5Hz (R² = 0.5) correlated negatively with ΔTT, i.e., samples with severe t-system loss exhibited a negative FFR and reduced myocardial wall tension at high pacing rates. PLB levels also predicted F_1Hz/F₀._5Hz, but to a lesser degree (R² = 0.49), whereas NCX1 was not correlated (R² = 0.02). CD₉₀ correlated positively with ΔTT (R² = 0.39) and negatively with SERCA2/PLB (R² = 0.42), indicating that both the t-system and SERCA activity are important for contraction kinetics. Surprisingly, ΔTT was not associated with TTP (R² = 0) but rather with TTR (R² = 0.5). This became even more pronounced when interaction with NCX1 expression was added to the model (R² = 0.79), suggesting that t-system loss impairs myocardial relaxation especially when NCX1 expression is low. The degree of t-system remodeling predicts FFR inversion and contraction slowing in failing human myocardium. Moreover, together with NCX, the t-system may be important for myocardial relaxation.

Myosin-binding protein C corrects an intrinsic inhomogeneity in cardiac excitation-contraction coupling

The beating heart exhibits remarkable contractile fidelity over a lifetime, which reflects the tight coupling of electrical, chemical, and mechanical elements within the sarcomere, the elementary contractile unit. On a beat-to-beat basis, calcium is released from the ends of the sarcomere and must diffuse toward the sarcomere center to fully activate the myosin- and actin-based contractile proteins. The resultant spatial and temporal gradient in free calcium across the sarcomere should lead to nonuniform and inefficient activation of contraction. We show that myosin-binding protein C (MyBP-C), through its positioning on the myosin thick filaments, corrects this nonuniformity in calcium activation by exquisitely sensitizing the contractile apparatus to calcium in a manner that precisely counterbalances the calcium gradient. Thus, the presence and correct localization of MyBP-C within the sarcomere is critically important for normal cardiac function, and any disturbance of MyBP-C localization or function will contribute to the consequent cardiac pathologies.

Endocardial endothelium is a key determinant of force-frequency relationship in rat ventricular myocardium

We tested the hypothesis that removing endocardial endothelium (EE) negatively impacts the force-frequency relationship (FFR) of ventricular myocardium and dissected the signaling that underlies this phenomenon. EE of rat trabeculae was selectively damaged by brief (<1 s) exposure to 0.1% Triton X-100. Force, intracellular Ca(2+) transient (iCa(2+)), and activity of protein kinase A (PKA) and protein kinase C (PKC) were determined. In control muscles, force and iCa(2+) increased as the stimulation frequency increased in steps of 0.5 Hz up to 3.0 Hz. However, EE-denuded (EED) muscles exhibited a markedly blunted FFR. Neither isoproterenol (ISO; 0.1-5 nmol/l) nor endothelin-1 (ET-1; 10-100 nmol/l) alone restored the slope of FFR in EED muscles. Intriguingly, however, a positive FFR was restored in EED preparations by combining low concentrations of ISO (0.1 nmol/l) and ET-1 (20 nmol/l). In intact muscles, PKA and PKC activity increased proportionally with the increase in frequency. This effect was completely lost in EED muscles. Again, combining ISO and ET-1 fully restored the frequency-dependent rise in PKA and PKC activity in EED muscles. In conclusion, selective damage of EE leads to significantly blunted FFR. A combination of low concentrations of ISO and ET-1 successfully restores FFR in EED muscles. The interdependence of ISO and ET-1 in this process indicates cross-talk between the β1-PKA and ET-1-PKC pathways for a normal (positive) FFR. The results also imply that dysfunction of EE and/or EE-myocyte coupling may contribute to flat (or even negative) FFR in heart failure.

Structural factors that determine the ability of adenosine and related compounds to activate the cardiac ryanodine receptor

The effects of adenosine and adenine on the gating of native sheep cardiac ryanodine receptor (RyR) channels were investigated. By examining the mechanisms underlying channel activation and by using comparative molecular field analysis (CoMFA) we have investigated the structural features of adenine-based ligands involved in channel activation. In the presence of 10 microM cytosolic Ca(2+), adenosine and adenine both activate the channel but only to a level approximately 10 and 20% respectively of that of ATP indicating that both are partial agonists of low efficacy. Adenosine was able to antagonize the ATP-induced increase in open probability (Po) as expected for a partial agonist of low efficacy at the ATP sites on the cardiac RyR. GTP (100 microM - 10 mM) had no effect on channel gating indicating that the adenine ring structure is important for agonist activity at the ATP-sites on RyR. CoMFA revealed an extremely strong correlation between the structural features of the five ATP analogues and the ability to increase (Po). Our model indicates that the high efficacy of ATP results primarily from the large electrostatic field established by the ionized phosphate groups. Reducing the number of phosphate groups lowers the strength of this field, leading to ligands with lower efficacy. In addition, steric interactions between the alpha-phosphate and ribose moieties and the RyR are correlated with low Po.