Acidosis or inorganic phosphate enhances the length dependence of tension in rat skinned cardiac muscle

Open Access
Publisher Website
Google Scholar
Abstract
1 We investigated the effect of acidosis on the sarcomere length (SL) dependence of tension generation, in comparison with the effect of inorganic phosphate (Pi), in rat skinned ventricular trabeculae. The shift of the mid‐point of the pCa‐tension relationship associated with an increase in SL from 1.9 to 2.3 μm (ΔpCa50) was studied. 2 Decreasing pH from 7.0 to 6.2 lowered maximal and submaximal Ca2+‐activated tension and increased ΔpCa50 in a pH‐dependent manner (from 0.21 ± 0.01 to 0.30 ± 0.01 pCa units). The addition of Pi (20 mm) decreased maximal tension and enhanced the SL dependence, both to a similar degree as observed when decreasing pH to 6.2 (ΔpCa50 increased from 0.20 ± 0.01 to 0.29 ± 0.01 pCa units). 3 Further experiments were performed using 6 % (w/v) Dextran T‐500 (molecular weight ∼500 000) to osmotically reduce interfilament lattice spacing (SL, 1.9 μm). Compared with that at pH 7.0, in the absence of Pi the increase in the Ca2+ sensitivity of tension induced by osmotic compression was enhanced at pH 6.2 (0.18 ± 0.01 vs. 0.25 ± 0.01 pCa units) or in the presence of 20 mm Pi (0.17 ± 0.01 vs. 0.24 ± 0.01 pCa units). 4 H+, as well as Pi, has been reported to decrease the number of strongly binding cross‐bridges, which reduces the co‐operative activation of the thin filament and increases the pool of detached cross‐bridges available for interaction with actin. It is therefore considered that during acidosis, the degree of increase in the number of force‐generating cross‐bridges upon reduction of interfilament lattice spacing is enhanced, resulting in greater SL dependence of tension generation. 5 Our results suggest that the Frank‐Starling mechanism may be enhanced when tension development is suppressed due to increased H+ and/or Pi under conditions of myocardial ischaemia or hypoxia.