Activation of Na+-H+ exchange and stretch-activated channels underlies the slow inotropic response to stretch in myocytes and muscle from the rat heart

We present the first direct comparison of the major candidates proposed to underlie the slow phase of the force increase seen following myocardial stretch: (i) the Na+-H+ exchanger (NHE) (ii) nitric oxide (NO) and the ryanodine receptor (RyR) and (iii) the stretch-activated channel (SAC) in both single myocytes and multicellular muscle preparations from the rat heart. Ventricular myocytes were stretched by approximately 7% using carbon fibres. Papillary muscles were stretched from 88 to 98% of the length at which maximum tension is generated (L-max). Inhibition of NHE with HOE 642 (5 muM) significantly reduced (P < 0.05) the magnitude of the slow force response in both muscle and myocytes. Neither inhibition of phosphatidylinositol-3-OH kinase (PtdIns-3-OH kinase) with LY294002 (10 muM) nor NO synthase with L-NAME (1 mm) reduced the slow force response in muscle or myocytes (P > 0.05), and the slow response was still present in the single myocyte when the sarcoplasmic reticulum was rigorously inhibited with 1 muM ryanodine and 1 muM thapsigargin. We saw a significant reduction (P < 0.05) in the slow force response in the presence of the SAC blocker streptomycin in both muscle (80 muM) and myocytes (40 muM). In fura 2-loaded myocytes, HOE 642 and streptomycin, but not L-NAME, ablated the stretch-induced increase in [Ca2+](i) transient amplitude. Our data suggest that in the rat, under our experimental conditions, there are two mechanisms that underlie the slow inotropic response to stretch: activation of NHE; and of activation of SACs. Both these mechanisms are intrinsic to the myocyte.