The data are consistent with the hypothesis that SFR results from increases in [Ca(2+)](i) secondary to altered flux via NCX in part resulting from increases in [Na(+)](i) mediated by NHE1.

Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice. Indirect evidence from clinical trials demonstrates that chronic inhibition of the renin-angiotensin-system (RAS) significantly reduces the incidence of AF. Since mechanisms of this protective effect of RAS-blockade are poorly understood, we directly tested proarrhythmic effects of angiotensin II (Ang II) in human atrial myocardium. Methods: Isolated trabeculae from human atrial appendages (n = 80) were electrically stimulated. We assessed isometric force and incidence of arrhythmic extra contractions (AECs) with and without increasing concentrations of Ang II (1-1000 nmol/L) in the absence or presence of receptor-blockade by saralasin (non-specific ATR-antagonist), irbesartan (AT1R-antagonist) or PD123319 (AT2R-antagonist). Results: Twitch force and AECs concentration-dependently increased with Ang II. Effects became significant at concentrations N 1 nmol/L Ang II and were maximal at 1000 nmol/L (increase in twitch force to 157 ± 14% and AECs from 0 to 80%) saralasin and irbesartan partially prevented the inotropic effect of 100 nmol/L Ang II (by 45 ± 12% and 68 ± 6%; p b 0.05), and completely prevented the occurrence of AECs. Conclusion: Ang II exerts direct pro-arrhythmic effects in human atrial myocardium. These effects are mediated by AT1-receptors and can be prevented by AT1R-blockade. This mechanism may contribute to the beneficial effects of RAS-blockade on AF in clinical trials.

Background-Stretch elicits an immediate, followed by a delayed, inotropic response in various animal models and failing human myocardium. This study aimed to characterize functional differences in the stretch response between failing and nonfailing human myocardium. Methods and Results-Experiments were performed in muscle tissue from 86 failing and 16 nonfailing human hearts.Muscles were stretched from 88% to 98% of optimal length.