ryanodine and Hypertrophy--Left-Ventricular

Hypertension affects about 5% of western populations and in the majority of cases it is of unknown aetiology. It exposes the heart to greater levels of myocardial stretch as a result of increased systolic pressure and peripheral resistance. Under certain circumstances myocardial stretch may trigger arrhythmias but the mechanisms and clinical importance of this phenomenon are unclear. This article outlines the risks of sudden cardiac death conferred by hypertension and left ventricular hypertrophy, presents the results of experiments using an animal model of myocardial stretch and discusses some possible mechanisms underlying stretch-induced arrhythmias which may be important in hypertensive patients.

Topics: Animals; Arrhythmias, Cardiac; Calcium; Calcium Channel Blockers; Cardiotonic Agents; Death, Sudden, Cardiac; Dogs; Electrocardiography; Gadolinium; Heart; Heart Ventricles; Humans; Hypertension; Hypertrophy, Left Ventricular; Isoproterenol; Meta-Analysis as Topic; Models, Cardiovascular; Myocardial Contraction; Myocardium; Nifedipine; Ouabain; Quinolines; Randomized Controlled Trials as Topic; Rats; Rats, Inbred SHR; Rats, Wistar; Ryanodine; Thiadiazines; Vasodilator Agents

Left ventricular hypertrophy (LVH) is frequently associated with clinical atrial arrhythmias, but little is known about how it causes those arrhythmias. Our previous studies have shown that LVH increases the late sodium current (I(Na-L)) that plays an important role in the genesis of ventricular arrhythmias. We hypothesize that LVH may also induce an upregulation of the I(Na-L) in atrial myocytes, leading to atrial electrical abnormalities. The renovascular hypertension model was used to induce LVH in rabbits. Action potential and membrane current recordings were performed in single myocytes. At a pacing cycle length of 2,000 ms, spontaneous phase-2 early afterdepolarizations (EADs) could be recorded from the left atrial myocytes in 10 of 12 LVH rabbits, whereas no EADs could be elicited in right atrial myocytes of LVH rabbits or atrial myocytes from any of the 12 control rabbits. Spontaneous automaticity (SA) from left atrial myocytes was observed in 9 out of 12 LVH rabbits, but none in right atrial myocytes of LVH rabbits or control rabbits, at a pacing rate of 8,000 ms. The left atrial myocytes of LVH rabbits had a significantly higher density of the I(Na-L) compared with those of control rabbits (0.90 +/- 0.12 in LVH vs. 0.50 +/- 0.08 pA/pF in control, n = 8, P < 0.01). Tetrodotoxin, an I(Na-L) blocker, abolished all atrial EADs and SA at 10 microM. Our results demonstrate that LVH induction results in a significant increase of I(Na-L) in the left atrial myocytes that may render these cells susceptible to the genesis of EADs and SA. The I(Na-L) may serve as a potentially useful ionic target for antiarrhythmic drugs for the treatment of atrial arrhythmias in the setting of LVH.

Topics: Action Potentials; Animals; Arrhythmias, Cardiac; Calcium Channel Blockers; Cell Separation; Electrophysiology; Heart Atria; Hypertrophy, Left Ventricular; In Vitro Techniques; Male; Membrane Potentials; Myocytes, Cardiac; Organ Size; Rabbits; Ryanodine; Sodium Channel Blockers; Sodium Channels; Tetrodotoxin

T-wave alternans, characterized by a beat-to-beat change in T-wave morphology, amplitude, and/or polarity on the ECG, often heralds the development of lethal ventricular arrhythmias in patients with left ventricular hypertrophy (LVH). The aim of our study was to examine the ionic basis for a beat-to-beat change in ventricular repolarization in the setting of LVH. Transmembrane action potentials (APs) from epicardium and endocardium were recorded simultaneously, together with transmural ECG and contraction force, in arterially perfused rabbit left ventricular wedge preparation. APs and Ca(2+)-activated chloride current (I(Cl,Ca)) were recorded from left ventricular myocytes isolated from normal rabbits and those with renovascular LVH using the standard microelectrode and whole cell patch-clamping techniques, respectively. In the LVH rabbits, a significant beat-to-beat change in endocardial AP duration (APD) created beat-to-beat alteration in transmural voltage gradient that manifested as T-wave alternans on the ECG. Interestingly, contraction force alternated in an opposite phase ("out of phase") with APD. In the single myocytes of LVH rabbits, a significant beat-to-beat change in APD was also observed in both left ventricular endocardial and epicardial myocytes at various pacing rates. APD alternans was suppressed by adding 1 microM ryanodine, 100 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), and 100 microM 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS). The density of the Ca(2+)-activated chloride currents (I(Cl,Ca)) in left ventricular myocytes was significantly greater in the LVH rabbits than in the normal group. Our data indicate that abnormal intracellular Ca(2+) fluctuation may exert a strong feedback on the membrane I(Cl,Ca), leading to a beat-to-beat change in the net repolarizing current that manifests as T-wave alternans on the ECG.

Topics: 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; 4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; Action Potentials; Animals; Arrhythmias, Cardiac; Calcium Signaling; Cardiac Pacing, Artificial; Chloride Channels; Disease Models, Animal; Electrocardiography; Endocardium; Hypertrophy, Left Ventricular; Myocardial Contraction; Myocytes, Cardiac; Patch-Clamp Techniques; Pericardium; Rabbits; Ryanodine; Time Factors

Cardiac hypertrophy is associated with contractile dysfunction, a feature of which is a slowing of the time to reach peak contraction. We have examined the main mechanisms involved in the initiation of contraction and investigated if their functions are changed during cardiac hypertrophy. Cardiac hypertrophy was induced by constriction of the ascending aorta in the rabbit. After 6 weeks left ventricular myocytes were isolated or left ventricular and septal mixed membrane preparations were produced for electrophysiological and radioligand binding studies, respectively. Aortic constriction resulted in a 24% and 23% increase in heart weight to body weight ratio and cell capacitance, respectively. Action potential duration and time-to-reach 50% and 90% peak contraction (TTP(50)and TTP(90), respectively) were significantly prolonged in myocytes from hypertrophied hearts. The prolongation of TTP(50)and TTP(90)could not be explained by altered peak calcium current density or SR calcium content which were unchanged in hypertrophy. Radioligand binding studies performed on tissue preparations from the same hearts, revealed a 34% reduction in ryanodine receptor (RYR) density with no change in dihydropyridine receptor (DHPR) density. This resulted in a reduction in the ratio of RYR to DHPR from 4.4:1 to 3.3:1 in hypertrophy. Ryanodine receptor Ca(2+)-sensitivity was unchanged between sham operated and hypertrophied groups. A reduction in the ratio of RYRs to DHPRs may result in a degree of "functional uncoupling" causing defective release of Ca(2+)from the SR. These findings may underlie the slowed TTP of myocyte contraction in hypertrophy.

Topics: Animals; Antihypertensive Agents; Calcium; Calcium Channel Blockers; Calcium Channels, L-Type; Cells, Cultured; Disease Models, Animal; Electrophysiology; Heart Ventricles; Hypertrophy, Left Ventricular; Isradipine; Myocardial Contraction; Rabbits; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum

The objective of this study was to determine the primary event that occurs in Ca2+-regulatory sarcoplasmic-reticular (SR) proteins during subacute transition from concentric/mechanically-compensated left ventricular (LV) hypertrophy to eccentric/decompensated hypertrophy. Using Dahl salt-sensitive rats with hypertension, changes of myocardial contraction, intracellular Ca2+ transients, SR Ca2+ uptake, protein levels of SR Ca2+ ATPase (SERCA2), phospholamban, and calsequestrin (CSQ), and mRNA levels of SERCA2 and CSQ were serially determined and compared between the established stage of LV hypertrophy (LVH) and the subsequent stage of overt LV dysfunction (CHF). In LVH, isolated LV papillary muscle preparations showed an equal peak-tension level and a mild prolongation of the isometric tension decay compared to those of age-matched controls. The Ca2+ transients as measured by aequorin were unchanged. The Ca2+ uptake of isolated SR vesicles and the protein/mRNA levels of SR proteins were also equivalent to those of the controls. In contrast, in CHF, the failing myocardium showed a further prolongation of the contraction time course and a 39% reduction of the peak-tension development. The Ca2+ transients showed changes consisting of a decrease in the peak level and a prolongation of the time course. In addition, the SR Ca2+ uptake was decreased by 41%. Despite these functional changes, the protein and mRNA levels of the SR components remained equivalent to those of the age-matched controls. Thus, in this hypertensive animal, 1) at the LVH stage, myocardial contractility and intracellular capability to regulate Ca2+ remained normal; 2) at the CHF stage, impaired SR Ca2+ handling and the subsequent reduction of myocardial contraction were in progress; and 3) impairments of SR function occurred at the post-translational protein level rather than at the transcriptional/translational levels. Our findings support the role of SR proteins as the primary determinant of the contractile dysfunction that occurs during the heart-failure transition; however, post-translational modulators of these SR elements may also be critical.

Topics: Aequorin; Animals; Atrial Natriuretic Factor; Calcium; Calcium-Binding Proteins; Calcium-Transporting ATPases; Calsequestrin; Heart Failure; Hemodynamics; Hypertension; Hypertrophy, Left Ventricular; In Vitro Techniques; Male; Myocardial Contraction; Myocardium; Papillary Muscles; Rats; Rats, Inbred Dahl; RNA, Messenger; Ryanodine; Sarcoplasmic Reticulum; Sarcoplasmic Reticulum Calcium-Transporting ATPases; Ventricular Dysfunction, Left

The goal of this study was to examine the transmural distribution of ryanodine receptors in left ventricular (LV) hypertrophy (LVH) and its in vivo consequences. Dogs were chronically instrumented with an LV pressure gauge, ultrasonic crystals for measurement of LV internal diameter and wall thickness, and a left circumflex coronary blood flow velocity transducer. Severe LVH was induced by chronic banding of the aorta (12+/-1 months), which resulted in a 78% increase in LV/body weight. When ryanodine was infused directly into the circumflex coronary artery, it did not affect LV global function or systemic hemodynamics; however, it reduced LV wall thickening and delayed relaxation in the posterior wall in control dogs but was relatively ineffective in dogs with LVH. In LV sarcolemmal preparations, [3H]ryanodine ligand binding revealed a subendocardial/subepicardial gradient in normal dogs. In LVH there was a 45% decrease in ryanodine receptor binding and a loss in the natural subendocardial/subepicardial gradient, which roughly correlated inversely with the extent of LVH and directly with regional wall motion. Both mRNA and Western analyses revealed similar findings, with a reduction of the transmural mRNA levels and a loss in the natural gradient between subendocardial and subepicardial layers in LVH. Thus, ryanodine receptor message and binding in LVH is reduced preferentially in the subendocardium with consequent attenuation of the action of ryanodine in vivo. The selectively altered ryanodine regulation subendocardially in LVH could reconcile some of the controversy in this field and may play a role in mediating decompensation from stable LVH.

Topics: Animals; Coronary Vessels; Dogs; Endocardium; Female; Heart; Hemodynamics; Hypertrophy, Left Ventricular; Injections, Intra-Arterial; Male; RNA, Messenger; Ryanodine; Ryanodine Receptor Calcium Release Channel; Ventricular Function, Left

The number of dihydropyridine and ryanodine receptors (DHP-R and RyR) has been measured in control and hypertrophied ventricles from rats, guinea pigs and ferrets to determine whether these two channels contribute to the alterations in excitation-contraction coupling (ECC), and in Ca2+ transient during compensated cardiac hypertrophy. We found that ventricular hypertrophy did not change the density of DHP-R. Mild hypertrophy did not alter the density of RyR in the rat but decreased it in the guinea-pig and in the ferret (30% and 36%, respectively). Severe hypertrophy decreased the density of RyR by 20% in the rat and by 34% in the guinea-pig. Therefore, the decrease is greater in ferret and guinea-pig hearts than in rat heart. We conclude that the sarcoplasmic reticulum (SR) Ca2+ release channels but not the L-type Ca2+ channels could contribute to the slowing of intracellular Ca2+ movements and to the reduced velocity of shortening of the hypertrophied hearts. We suggest that, in the guinea pig and ferret hearts which express only the beta myosin heavy chain (MHC) isoform, the reduced velocity of shortening during hypertrophy is related to the decrease in RyR density, whereas in the rat, it is regulated primarily via a shift in the MHC isoform, except in severe hypertrophy in which the moderate decrease in RyR would also be involved.

Topics: Adaptation, Physiological; Animals; Calcium; Calcium Channels; Calcium Channels, L-Type; Female; Ferrets; Guinea Pigs; Hypertrophy, Left Ventricular; Hypertrophy, Right Ventricular; Male; Microsomes; Muscle Proteins; Myocardial Contraction; Myocardium; Myosins; Rats; Ryanodine; Ryanodine Receptor Calcium Release Channel; Sarcolemma; Sarcoplasmic Reticulum