The force-frequency relationship is an intrinsic modulator of cardiac contractility and relaxation. Force of contraction increases with frequency, while simultaneously a frequency-dependent acceleration of relaxation occurs. While frequency dependency of calcium handling and sarcoplasmic reticulum calcium load have been well described, it remains unknown whether frequency-dependent changes in myofilament calcium sensitivity occur. We hypothesized that an increase in heart rate that results in acceleration of relaxation is accompanied by a proportional decrease in myofilament calcium sensitivity. To test our hypothesis, ultrathin right ventricular trabeculae were isolated from New Zealand White rabbit hearts and iontophorically loaded with the calcium indicator bis-fura 2. Twitch and intracellular calcium handling parameters were measured and showed a robust increase in twitch force, acceleration of relaxation, and rise in both diastolic and systolic intracellular calcium concentration with increased frequency. Steady-state force-intracellular calcium concentration relationships were measured at frequencies 1, 2, 3, and 4 Hz at 37 degrees C using potassium-induced contractures. EC(50) significantly and gradually increased with frequency, from 475 +/- 64 nM at 1 Hz to 1,004 +/- 142 nM at 4 Hz (P < 0.05) and correlated with the corresponding changes in half relaxation time. No significant changes in maximal active force development or in the myofilament cooperativity coefficient were found. Myofilament protein phosphorylation was assessed using Pro-Q Diamond staining on protein gels of trabeculae frozen at either 1 or 4 Hz, revealing troponin I and myosin light chain-2 phosphorylation associated with the myofilament desensitization. We conclude that myofilament calcium sensitivity is substantially and significantly decreased at higher frequencies, playing a prominent role in frequency-dependent acceleration of relaxation.

Cardiac contraction-relaxation coupling is determined by both the free intracellular calcium concentration ([Ca2+]i) and myofilament properties. We set out to develop a technique where we could assess these parameters (twitch and steady-state force [Ca2+]i) under near physiological conditions. Bis-fura-2 was iontophorically introduced into ultrathin rat trabeculae preparations to monitor the [Ca2+]i, and steady-state contractures were achieved by using a modified Krebs-Henseleit solution containing high K+. During K+ contractures, the very slow changes in [Ca2+]i and force development were in equilibrium and allowed for the construction of a steady-state, force-[Ca2+]i relationship. Twitch contractions before and after this myofilament calcium sensitivity assessment were unaltered, and this protocol could be repeated several times. For the first time, this novel protocol allows us to measure myofilament calcium sensitivity under physiological temperature. Not only do the data so obtained allow us to assess myofilament calcium sensitivity, the data also will allow us, in the same preparation under nearly identical conditions, to compare the dynamic to the steady-state, force-calcium relationship. To test whether the steady-state relationship between force and calcium in our novel protocol reproduces expected changes, we determined this relationship in the presence of isoproterenol and under acidosis and alkalosis. As expected, beta-adrenergic stimulation resulted in an increase of calcium amplitude and twitch force and a desensitization of the myofilaments as indicated by a rightward shift of the obtained steady-state, force-calcium relationship. An increase in pH shifted the curve leftward, whereas a decrease in pH resulted in the expected rightward shift.

Background-Ventricular hypertrophy is a physiological response to pressure overload that, if left untreated, can ultimately result in ventricular dysfunction, including diastolic dysfunction. The aim of this study was to test the hypothesis that frequency-dependent myofilament desensitization, a physiological response of healthy myocardium, is altered in hypertrophied myocardium. Methods and Results-New Zealand white rabbits underwent a pulmonary artery banding procedure to induce pressure overload. After 10 weeks, the animals were euthanized, hearts removed, and suitable trabeculae harvested from the free wall of the right ventricle. Twitch contractions, calibrated bis-fura-2 calcium transients, and myofilament calcium sensitivity (potassium contractures) were measured at frequencies of 1, 2, 3, and 4 Hz. The force frequency response, relaxation frequency response, and calcium frequency relationships were significantly blunted, and diastolic tension significantly increased with frequency in the pulmonary artery banding rabbits compared with sham-operated animals. Myofilament calcium sensitivity was virtually identical at 1 Hz in the treatment versus sham group (pCa 6.11Ϯ0.03 versus 6.11Ϯ0.06), but the frequency-dependent desensitization that takes place in the sham group (⌬pCa 0.14Ϯ0.06, PϽ0.05) was not observed in the pulmonary artery banding animals (⌬pCa 0.02Ϯ0.05). Analysis of myofilament protein phosphorylation revealed that the normally observed frequency-dependent phosphorylation of troponin-I is lost in pulmonary artery banding rabbits. Conclusions-The frequency-dependent myofilament desensitization is significantly impaired in right ventricular hypertrophy and contributes to the frequency-dependent elevation of diastolic tension in hypertrophy. (Circ Heart Fail. 2009;2:472-481.)Key Words: hypertrophy Ⅲ calcium sensitivity Ⅲ heart rate Ⅲ EC-coupling Ⅲ myofilaments V entricular hypertrophy can occur as a result of sustained pressure overload on the ventricles arising from hypertension, valvular stenosis, or ventricular dysfunction. The hypertrophic response is thought to be compensatory at first, but in later stages can result in ventricular dysfunction and eventually pump failure. 1 The normal myocardial responses to increases in heart rate can begin to change during the transition from compensatory hypertrophy to decompensation. The force frequency relationship (FFR), normally positive in healthy myocardium, is usually severely blunted or even negative in cases of decompensated hypertrophy and becomes worse as the heart approaches failure. 2,3 Clinical Perspective on p 481Although it is incompletely understood how the FFR changes with disease, it is clear that alterations in calcium handling play a major role. [3][4][5] The role (if any) the myofilaments play in the contractile dysfunction of decompensating ventricular hypertrophy remains unresolved, in particular as it relates to changes in heart rate. Myofilament calcium sensitivity has been reported to be unaltered in LV myocytes from rapid paced d...

It is well known that the rate of intracellular calcium ([Ca2+]i) decline is an important factor governing relaxation in unloaded myocardium. However, it remains unclear to what extent, under near physiological conditions, the intracellular calcium transient amplitude and kinetics contribute to the length-dependent increase in force and increase in duration of relaxation. We hypothesize that myofilament properties rather than calcium transient decline primarily determines the duration of relaxation in adult mammalian myocardium. To test this hypothesis, we simultaneously measured force of contraction and calibrated [Ca2+]i transients in isolated, thin rabbit trabeculae at various lengths at 37 degrees C. Time from peak tension to 50% relaxation (RT50(tension)) increases significantly with length (from 49.8+/-3.4 to 83.8+/-7.4 ms at an [Ca2+]o of 2.5 mM), whereas time from peak calcium to 50% decline (RT50(calcium)) was not prolonged (from 124.8+/-5.3 to 107.7+/-11.4 ms at an [Ca2+]o of 2.5 mM). Analysis of variance revealed that RT50(tension) is significantly correlated with length (P<0.0001). At optimal length, varying the extracellular calcium concentration increased both developed force and calcium transient amplitude, but RT50(tension) remained unchanged (P=0.90), whereas intracellular calcium decline actually accelerated (P<0.05). Thus, an increase in muscle length will result in an increase in both force and duration of relaxation, whereas the latter is not primarily governed by the rate of [Ca2+]i decline.

Cardiorenal syndrome (CRS) encompasses various disorders of the heart and kidneys; dysfunction of one organ leads to acute or chronic dysfunction of the other. It incorporates the intersection of heart-kidney interactions across several mediums, hemodynamically, through the alterations in neurohormonal markers, and increased venous and renal pressure, all of which are hallmarks of its clinical phenotypes. This article explores the epidemiology, pathology, classification and treatment of each type of CRS.