ryanodine and Cardiac-Output--Low

Previous studies have demonstrated that cardiac function changes with development of pressure overload-induced hypertrophy. The present study was undertaken to discover the basis for the changes in sarcoplasmic reticulum (SR) functions: uptake, (as related to the SR Ca2+ pump properties) and release in isolated, perfused hypertrophied rat hearts. Our results demonstrated significant prolongation of the time-to-90%-relaxation, both during the period of compensation (8 weeks after banding the ascending aorta, group HR1), when systolic function was preserved, and later with progressive hypertrophy (20 weeks after banding, group HR2) and contractile failure (20-22 weeks after banding, group F). The initial rates of the oxalate-supported SR Ca2+ uptake and the maximum transport rate (Vmax) of the SR Ca2+ pump, measured in the left ventricular homogenates, during blockade of the SR Ca2+ release channels with ruthenium red, were preserved in group HR1. To correlate early relaxation abnormalities with SR function, the [Ca2+] required for half-maximal pump activation (EC50) was examined and increased significantly in HRI vs. Sham1 (0.95+/-0.06 vs. 0.81+/-0.04 microM, P<0.05) indicating that the affinity of the SR Ca2+ pump for Ca2+ was reduced. The same tendency was demonstrated in groups HR2 (0.94+/-0.06 vs. 0.79+/-0.05) and F (0.89+/-0.05 vs. 0.78+/-0.05). In addition, with progression of hypertrophy we observed a significant decline in the amount of SR Ca2+ pump, as assessed by the Vmax, from 31.22+/-1.20 (Sham2) to 26.47+/-1.58 HR2) nmol/mg protein per min (P<0.05), and from 33.81+/-1.23 (Sham3) to 25.15+/-1.57 (F) nmol/mg protein per min, (P<0.01). This decrease was accompanied by a parallel reduction in the number of SR Ca2+ release channels by 14% (HR2) and 23% (F), as determined by maximum [3H] ryanodine binding (Bmax). These results suggest that pressure overload-induced changes in SR Ca2+ uptake (as reflected by Vmax and EC50) and SR Ca2+ release (as reflected by Bmax), both leading to diminished Ca2+ sequestration, may contribute to impaired cardiac relaxation with compensatory hypertrophy and failure.

Topics: Algorithms; Animals; Calcium; Calcium-Transporting ATPases; Cardiac Output, Low; Cardiomegaly; Coloring Agents; Hemodynamics; Kinetics; Male; Myocardial Contraction; Myocardium; Perfusion; Rats; Rats, Wistar; Ruthenium Red; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum

In the pathogenesis of cardiac dysfunction in heart failure, a decrease in the activity of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase is believed to be a major determinant. Here, we report a novel mechanism of cardiac dysfunction revealed by assessing the functional interaction of FK506-binding protein (FKBP12.6) with the cardiac ryanodine receptor (RyR) in a canine model of pacing-induced heart failure.. SR vesicles were isolated from left ventricular muscles (normal and heart failure). The stoichiometry of FKBP12.6 per RyR was significantly decreased in failing SR, as assessed by the ratio of the B(max) values for [(3)H]dihydro-FK506 to those for [(3)H]ryanodine binding. In normal SR, the molar ratio was 3.6 ( approximately 1 FKBP12.6 for each RyR monomer), whereas it was 1.6 in failing SR. In normal SR, FK506 caused a dose-dependent Ca(2+) leak that showed a close parallelism with the conformational change in RyR. In failing SR, a prominent Ca(2+) leak was observed even in the absence of FK506, and FK506 produced little or no further increase in Ca(2+) leak and only a slight conformational change in RyR. The level of protein expression of FKBP12.6 was indeed found to be significantly decreased in failing SR.. An abnormal Ca(2+) leak through the RyR is present in heart failure, and this leak is presumably caused by a partial loss of RyR-bound FKBP12.6 and the resultant conformational change in RyR. This abnormal Ca(2+) leak might possibly cause Ca(2+) overload and consequent diastolic dysfunction, as well as systolic dysfunction.

Topics: Animals; Calcium; Cardiac Output, Low; Disease Models, Animal; Dogs; Female; Male; Pacemaker, Artificial; Protein Conformation; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum; Tacrolimus; Tacrolimus Binding Proteins; Tritium

1 Calcium transport activity of isolated cardiac sarcoplasmic reticulum (SR) including Ca2+ uptake and release is decreased in animals with chronic heart failure (CHF) following myocardial infarction. The present study was undertaken to determine whether an angiotensin converting enzyme (ACE) inhibitor, trandolapril, improves cardiac sarcoplasmic reticular function in animals with CHF following myocardial infarction. 2 CHF was induced by left coronary artery ligation in rats, which resulted in an infarction of approximately 45% of the left ventricle. Aortic flow and cardiac output index were decreased, and left ventricular end-diastolic pressure was increased 8 weeks after the operation, suggesting the development of CHF. 3 The developed force transients of cardiac skinned fibres of the rats with CHF were decreased when the skinned fibre was preloaded for 0.25-1 min with 10(-5) M Ca2+ (48-88%) and when preloaded with 10(-6) M Ca2+ and then exposed to 0.1-1 mM caffeine (45-93%). 4 The [3H]-ryanodine-binding activity in SR-enriched fractions was reduced by 23% in the CHF group. These results suggest that the amount of Ca2+ released from SR is decreased due to a reduced rate of SR Ca2+ uptake and a downregulation of the SR Ca2+-release channel. 5 Rats were treated orally with 3 mg kg(-1) day(-1) trandolapril from the 2nd to the 8th week after the coronary artery ligation. Treatment with trandolapril attenuated the reduction in aortic flow and cardiac output index and the increase in left ventricular end-diastolic pressure, and improved the developed force transients of the skinned fibre of the animal with CHF without causing a reduction of infarct size. Treatment with trandolapril also attenuated the reduction in ryanodine receptor density in the viable left ventricle of the rat with CHF. 6 It is concluded that long-term treatment with trandolapril attenuates cardiac SR dysfunction in rats with CHF and that the mechanism underlying this effect is, at least in part, attributed to prevention of downregulation of Ca2+ release channel.

Topics: Angiotensin-Converting Enzyme Inhibitors; Animals; Calcium; Cardiac Output, Low; Chronic Disease; Heart; Hemodynamics; Indoles; Male; Myocardial Infarction; Myocardium; Rats; Rats, Wistar; Ryanodine; Sarcoplasmic Reticulum

Ca2+ handling in excitation-contraction coupling requires considerable O2 consumption (VO2) in cardiac contraction. We have developed an integrative method to quantify total Ca2+ handling in normal hearts. However, its direct application to failing hearts, where futile Ca2+ cycling via the Ca2+-leaky sarcoplasmic reticulum (SR) required an increased Ca2+ handling VO2, was not legitimate. To quantify total Ca2+ handling even in such failing hearts, we combined futile Ca2+ cycling with Ca2+ handling VO2 and the internal Ca2+ recirculation fraction via the SR. We applied this method to the canine heart mechanoenergetics before and after intracoronary ryanodine at nanomolar concentrations. We found that total Ca2+ handling per beat was halved after the ryanodine treatment from approximately 60 micromol/kg left ventricle before ryanodine. We also found that futile Ca2+ cycling via the SR increased to >1 cycle/beat after ryanodine from presumably zero before ryanodine. These results support the applicability of the present method to the failing hearts with futile Ca2+ cycling via the SR.

Topics: Animals; Calcium; Calcium-Transporting ATPases; Cardiac Output, Low; Cardiology; Dogs; Energy Metabolism; Feasibility Studies; Heart; In Vitro Techniques; Models, Cardiovascular; Myocardial Contraction; Myocardium; Oxygen Consumption; Ryanodine; Sarcoplasmic Reticulum

Experiments were performed to determine the relative contributions of direct Ca(2+)-entry through the L-type Ca(2+)-current and of Ca(2+)-release from the sarcoplasmic reticulum (s.r.) to the intracellular [Ca2+]i-transient in isolated human atrial and ventricular myocytes from patients with severe heart failure and from non-failing controls. Cells were isolated from explanted hearts of patients undergoing transplantation because of severe heart failure due to dilated or ischemic cardiomyopathy or from donor hearts which could not be transplanted for technical reasons. Ca(2+)-current densities were -2.1 +/- 0.6 pA/pF in atrial cells, -4.8 +/- 0.5 pA/pF in cells from patients with heart failure and -3.2 +/- 0.5 pA/pF in non-failing controls. [Ca2+]i-transients were significantly smaller in heart failure (370 +/- 33 nM) compared to ventricular cells from non-failing hearts (760 +/- 69 nM, p < 0.05). Atrial myocytes had average [Ca2+]i-transients of 505 +/- 38 nM. After incubation in ryanodine the average [Ca2+]i-transients were not significantly different between different cell types. The results indicate that the relative contribution of Ca(2+) released from the sarcoplasmic reticulum to the [Ca2+]i-transient is significantly smaller in heart failure. The absolute contribution of the L-type Ca(2+)-current to the transient seemed to be comparable in all cell types investigated. As the [Ca2+]i-transient in the presence of ryanodine was comparable in size in all cells, changes of the intracellular [Ca2+]i-transient in heart failure are mainly due to alterations of s.r. function in these cells.

Topics: Calcium; Cardiac Output, Low; Electric Conductivity; Female; Heart Atria; Heart Ventricles; Humans; Intracellular Membranes; Male; Middle Aged; Myocardium; Osmolar Concentration; Ryanodine; Sarcoplasmic Reticulum

Force-frequency relations were studied in an isolated-perfused rabbit heart model. Heart failure was induced by a double volume plus pressure overload. Studies were performed at the early stage of heart failure when basal ventricular function was not decreased. The normal positive staircase induced by pacing in control hearts (CH) was replaced by a negative staircase in failing hearts (FH) with an increase in end-diastolic pressure for increased heart rates in FH. Postpacing potentiation and postextrasystolic potentiation (PESP) were significantly reduced in FH as compared with CH. Ventricular function decreased by 60% in both CH and FH under ryanodine with similar dose-response curves. Postpacing and PESP disappeared under ryanodine in CH and in FH with a reversal of the negative staircase in FH. The abnormal force-frequency relations observed in heart failure are thus attributed to sarcoplasmic reticulum dysfunction. Basal ventricular function during spontaneous heart rate may be normal in the early stage of heart failure, but sarcoplasmic reticulum dysfunction produces abnormalities in ventricular function when heart rate is abruptly modified, particularly during tachycardia.

Topics: Animals; Cardiac Complexes, Premature; Cardiac Output, Low; Cardiac Pacing, Artificial; Dose-Response Relationship, Drug; Electrophysiology; Heart; Heart Atria; Heart Rate; Homeostasis; In Vitro Techniques; Rabbits; Reference Values; Ryanodine; Ventricular Function, Left