Angiotensin II and myosin light-chain phosphorylation contribute to the stretch-induced slow force response in human atrial myocardium

Abstract

Aims: Stretch is an important regulator of atrial function. The functional effects of stretch on human atrium, however, are poorly understood. Thus, we characterized the stretch-induced force response in human atrium and evaluated the underlying cellular mechanisms. Methods and results: Isometric twitch force of human atrial trabeculae (n = 252) was recorded (37°C, 1 Hz stimulation) following stretch from 88 (L88) to 98% (L98) of optimal length. [Na +]i and pHi were measured using SBFI and BCECF epifluorescence, respectively. Stretch induced a biphasic force increase: an immediate increase [first-phase, Frank-Starling mechanism (FSM)] to ∼190% of force at L88 followed by an additional slower increase [5-10 min; slow force response (SFR)] to ∼120% of the FSM. FSM and SFR were unaffected by gender, age, ejection fraction, and pre-medication with major cardiovascular drugs. There was a positive correlation between the amplitude of the FSM and the SFR. [Na+]i rose by ∼1 mmol/L and pHi remained unchanged during the SFR. Inhibition of Na+/H+-exchange (3 μM HOE642), Na+/Ca2+-exchange (5 μM KB-R7943), or stretch-activated channels (0.5 μM GsMtx-4 and 80 μM streptomycin) did not reduce the SFR. Inhibition of angiotensin-II (AngII) receptors (5 μM saralasin and 0.5 μM PD123319) or pre-application of 0.5 μM AngII, however, reduced the SFR by ∼40-60%. Moreover, stretch increased phosphorylation of myosin light chain 2 (MLC2a) and inhibition of MLC kinase (10 μM ML-7 and 5 μM wortmannin) decreased the SFR by ∼40-85%. Conclusion: Stretch elicits a SFR in human atrium. The atrial SFR is mediated by stretch-induced release and autocrine/paracrine actions of AngII and increased myofilament Ca2+ responsiveness via phosphorylation of MLC2a by MLC kinase. © The Author 2008.