In ventricular cardiac myocytes, T-tubule density is an important determinant of the synchrony of sarcoplasmic reticulum (SR) Ca ²⁺ release and could be involved in the reduced SR Ca ²⁺ release in ischemic cardiomyopathy. We therefore investigated T-tubule density and properties of SR Ca ²⁺ release in pigs, 6 weeks after inducing severe stenosis of the circumflex coronary artery (91±3%, N=13) with myocardial infarction (8.8±2.0% of total left ventricular mass). Severe dysfunction in the infarct and adjacent myocardium was documented by magnetic resonance and Doppler myocardial velocity imaging. Myocytes isolated from the adjacent myocardium were compared with myocytes from the same region in weight-matched control pigs. T-tubule density quantified from the di-8-ANEPPS (di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate) sarcolemmal staining was decreased by 27±7% ( P <0.05). Synchrony of SR Ca ²⁺ release (confocal line scan images during whole-cell voltage clamp) was reduced in myocardium myocytes. Delayed release (ie, half-maximal [Ca ²⁺ ] _i occurring later than 20 ms) occurred at 35.5±6.4% of the scan line in myocardial infarction versus 22.7±2.5% in control pigs ( P <0.05), prolonging the time to peak of the line-averaged [Ca ²⁺ ] _i transient (121±9 versus 102±5 ms in control pigs, P <0.05). Delayed release colocalized with regions of T-tubule rarefaction and could not be suppressed by activation of protein kinase A. The whole-cell averaged [Ca ²⁺ ] _i transient amplitude was reduced, whereas L-type Ca ²⁺ current density was unchanged and SR content was increased, indicating a reduction in the gain of Ca ²⁺ -induced Ca ²⁺ release. In conclusion, reduced T-tubule density during ischemic remodeling is associated with reduced synchrony of Ca ²⁺ release and reduced efficiency of coupling Ca ²⁺ influx to Ca ²⁺ release.

Rationale : Persistent atrial fibrillation (AF) has been associated with structural and electric remodeling and reduced contractile function. Objective : To unravel mechanisms underlying reduced sarcoplasmic reticulum (SR) Ca ²⁺ release in persistent AF. Methods : We studied cell shortening, membrane currents, and [Ca ²⁺ ] _i in right atrial myocytes isolated from sheep with persistent AF (duration 129±39 days, N=16), compared to matched control animals (N=21). T-tubule density, ryanodine receptor (RyR) distribution, and local [Ca ²⁺ ] _i transients were examined in confocal imaging. Results : Myocyte shortening and underlying [Ca ²⁺ ] _i transients were profoundly reduced in AF (by 54.8% and 62%, P <0.01). This reduced cell shortening could be corrected by increasing [Ca ²⁺ ] _i . SR Ca ²⁺ content was not different. Calculated fractional SR Ca ²⁺ release was reduced in AF (by 20.6%, P <0.05). Peak Ca ²⁺ current density was modestly decreased (by 23.9%, P <0.01). T-tubules were present in the control atrial myocytes at low density and strongly reduced in AF (by 45%, P <0.01), whereas the regular distribution of RyR was unchanged. Synchrony of SR Ca ²⁺ release in AF was significantly reduced with increased areas of delayed Ca ²⁺ release. Propagation between RyR was unaffected but Ca ²⁺ release at subsarcolemmal sites was reduced. Rate of Ca ²⁺ extrusion by Na ⁺ /Ca ²⁺ exchanger was increased. Conclusions : In persistent AF, reduced SR Ca ²⁺ release despite preserved SR Ca ²⁺ content is a major factor in contractile dysfunction. Fewer Ca ²⁺ channel–RyR couplings and reduced efficiency of the coupling at subsarcolemmal sites, possibly related to increased Na ⁺ /Ca ²⁺ exchanger, underlie the reduction in Ca ²⁺ release.

Background— Ventricular expression of phosphodiesterase-5 (PDE5), an enzyme responsible for cGMP catabolism, is increased in human right ventricular hypertrophy, but its role in left ventricular (LV) failure remains incompletely understood. We therefore measured LV PDE5 expression in patients with advanced systolic heart failure and characterized LV remodeling after myocardial infarction in transgenic mice with cardiomyocyte-specific overexpression of PDE5 (PDE5-TG). Methods and Results— Immunoblot and immunohistochemistry techniques revealed that PDE5 expression was greater in explanted LVs from patients with dilated and ischemic cardiomyopathy than in control hearts. To evaluate the impact of increased ventricular PDE5 levels on cardiac function, PDE5-TG mice were generated. Confocal and immunoelectron microscopy revealed increased PDE5 expression in cardiomyocytes, predominantly localized to Z-bands. At baseline, myocardial cGMP levels, cell shortening, and calcium handling in isolated cardiomyocytes and LV hemodynamic measurements were similar in PDE5-TG and wild-type littermates. Ten days after myocardial infarction, LV cGMP levels had increased to a greater extent in wild-type mice than in PDE5-TG mice ( P <0.05). Ten weeks after myocardial infarction, LV end-systolic and end-diastolic volumes were larger in PDE5-TG than in wild-type mice (57±5 versus 39±4 and 65±6 versus 48±4 μL, respectively; P <0.01 for both). LV systolic dysfunction and diastolic dysfunction were more marked in PDE5-TG than in wild-type mice, associated with enhanced hypertrophy and reduced contractile function in isolated cardiomyocytes from remote myocardium. Conclusions— Increased PDE5 expression predisposes mice to adverse LV remodeling after myocardial infarction. Increased myocardial PDE5 expression in patients with advanced cardiomyopathy may contribute to the development of heart failure and represents an important therapeutic target.

The extent and mechanism of the cardiac benefit of early exercise training following myocardial infarction (MI) is incompletely understood, but may involve blunting of abnormalities in Ca ²⁺ -handling and myofilament function. Consequently, we investigated the effects of 8-weeks of voluntary exercise, started early after a large MI, on left ventricular (LV) remodeling and dysfunction in the mouse. Exercise had no effect on survival, MI size or LV dimensions, but improved LV fractional shortening from 8±1 to 12±1%, and LVdP/dt _P30 from 5295±207 to 5794±207 mm Hg/s (both P <0.05), and reduced pulmonary congestion. These global effects of exercise were associated with normalization of the MI-induced increase in myofilament Ca ²⁺ -sensitivity (ΔpCa ₅₀ =0.037). This effect of exercise was PKA-mediated and likely because of improved β ₁ -adrenergic signaling, as suggested by the increased β ₁ -adrenoceptor protein (48%) and cAMP levels (36%; all P <0.05). Exercise prevented the MI-induced decreased maximum force generating capacity of skinned cardiomyocytes (F _max increased from 14.3±0.7 to 18.3±0.8 kN/m ² P <0.05), which was associated with enhanced shortening of unloaded intact cardiomyocytes (from 4.1±0.3 to 7.0±0.6%; P <0.05). Furthermore, exercise reduced diastolic Ca ²⁺ -concentrations (by ∼30%, P <0.05) despite the unchanged SERCA2a and PLB expression and PLB phosphorylation status. Importantly, exercise had no effect on Ca ²⁺ -transient amplitude, indicating that the improved LV and cardiomyocyte shortening were principally because of improved myofilament function. In conclusion, early exercise in mice after a large MI has no effect on LV remodeling, but attenuates global LV dysfunction. The latter can be explained by the exercise-induced improvement of myofilament function.

Block of Na/Ca exchange (NCX) has potential therapeutic applications, in particular, if a mode-selective block could be achieved, but also carries serious risks for disturbing the normal Ca ²⁺ balance maintained by NCX. We have examined the effects of partial inhibition of NCX by SEA-0400 (1 or 0.3 μmol/L) in left ventricular myocytes from healthy pigs or mice and from mice with heart failure (MLP ^−/− ). During voltage clamp ramps with [Ca ²⁺ ] _i buffering, block of reverse mode block was slightly larger than of forward mode (by 25±5%, P <0.05). In the absence of [Ca ²⁺ ] _i buffering and with sarcoplasmic reticulum (SR) fluxes blocked, rate constants for Ca ²⁺ influx and Ca ²⁺ efflux were reduced to the same extent (to 36±6% and 32±4%, respectively). With normal SR function the reduction of inward NCX current ( I _NCX ) was 57±10% (n=10); during large caffeine-induced Ca ²⁺ transients, it was larger (82±3%). [Ca ²⁺ ] _i transients evoked during depolarizing steps increased (from 424±27 to 994±127 nmol/L at +10mV, P <0.05), despite a reduction of I _CaL by 27%. Resting [Ca ²⁺ ] _i increased; there was a small decrease in the rate of decline of [Ca ²⁺ ] _i . SR Ca ²⁺ content increased more than 2-fold. Contraction amplitude of field-stimulated myocytes increased in healthy myocytes but not in myocytes from MLP ^−/− mice, in which SR Ca ²⁺ content remained unchanged. These data provide proof-of-principle that even partial inhibition of NCX results in a net gain of Ca ²⁺ . Further development of NCX blockers, in particular, for heart failure, must balance potential benefits of I _NCX reduction against effects on Ca ²⁺ handling by refining mode dependence and/or including additional targets.