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Received May 28, 2004
Revised June 18, 2004
Accepted after revision June 29, 2004
1 University of Leeds
* To whom correspondence should be addressed. E-mail: s.c.calaghan{at}leeds.ac.uk.
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 and iii) the stretch-activated channel (SAC) in both single myocytes and multicellular preparations from the rat heart. Ventricular myocytes were stretched by approximately 7% using carbon fibres. Papillary muscles were stretched from 88 to 98% of Lmax. Inhibition of NHE with HOE 642 (5 µM) significantly reduced (P<0.05) the magnitude of the slow force response in both muscle and myocytes. Neither inhibition of PtdIns-3-OH kinase (LY294002, 10 µM) nor NO synthase (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 µM ryanodine and 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 µM) and myocytes (40 µM). 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 2 mechanisms that underlie the slow inotropic response to stretch: activation of NHE and of SACs. Both these mechanisms are intrinsic to the myocyte.
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