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

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 pH(i) were measured using SBFI and BCECF epifluorescence, respectively. Stretch induced a biphasic force increase: an immediate increase [first-phase, Frank-Starting mechanism (FSM)] to similar to 190% of force at L88 followed by an additional slower increase [5-10 min; stow force response (SFR)] to similar 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 similar to 1 mmol/L and pH(i) remained unchanged during the SFR. Inhibition of Na(+)/H(+)-exchange (3 mu M HOE642), Na(+)/Ca(2+)-exchange (5 mu M KB-R7943), or stretch-activated channels (0.5 mu M, GsMtx-4 and 80 mu M streptomycin) did not reduce the SFR. Inhibition of angiotensin-II (AngII) receptors (5 mu M saralasin and 0.5 mu M PD123319) or pre-application of 0.5 mu M AngII, however, reduced the SFR by similar to 40-60%. Moreover, stretch increased phosphorylation of myosin tight chain 2 (MLC2a) and inhibition of MLC kinase (10 mu M ML-7 and 5 mu M wortmannin) decreased the SFR by similar to 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 Ca(2+) responsiveness via phosphorylation of MLC2a by MLC kinase.