EFFECTS OF ACIDOSIS AND ALKALOSIS ON MECHANICAL-PROPERTIES OF HYPERTROPHIED RAT-HEART FIBER-BUNDLES

Effects of alkalosis (pH 7.4) or acidosis (pH 6.8) on the intrinsic me chanical properties of control and pressure-overloaded rat hearts were studied in Triton X-100-treated left ventricular fiber bundles. In co ntrol bundles, Ca sensitivity [pCa required for one-half maximal respo nse (pCa(50))] was 5.520 +/- 0.012 at pH 7.1. Alkalosis increased it b y 0.357 +/- 0.018 pCa unit, whereas acidosis decreased it by 0.365 +/- 0.014 pCa unit with no change in Hill coefficient. Maximal tension wa s decreased by acidic pH and increased by alkaline pH. Stiffness was m easured by the response to quick length changes. Acidosis decreased ma ximal stiffness but increased the stiffness-to-force ratio, whereas al kalosis increased maximal stiffness but had no effect on stiffness-to- force ratio, suggesting that acidosis decreased the force generated pe r cross bridge. Alkalosis increased the time constant of tension recov ery following a quick stretch from 10.6 +/- 0.66 to 17.45 +/- 1.83 ms, suggesting a decreased cross-bridge cycling rate. Pressure overload i nduced by thoracic aortic stenosis for 4-6 wk led to a 200% cardiac hy pertrophy associated with a shift from fast to slow ventricular myosin . pCa(50) of hypertrophied bundles was not different from control (5.5 41 +/- 0.012). Ca sensitivity was increased by 0.383 +/- 0.008 in alka line medium and decreased by 0.325 +/- 0.009 in acidic medium. Stiffne ss-to-force ratio was decreased in acidic pH, and the time constant of tension recovery was increased from 31.0 +/- 0.4 to 34.9 +/- 0.25 ms by alkalosis. In hypertrophied bundles, maximal tension was decreased by acidic pH but not changed by alkalosis. These results show that in the small pH range of our study 1) pH changes have symmetrical effects on Ca sensitivity in both control and hypertrophied bundles, 2) a dec rease or an increase in H+ concentration does not have symmetrical eff ects on the mechanics of the cross bridges, and 3) changes in the phen otype of contractile proteins induced by aortic stenosis do not influe nce Ca sensitivity, only moderately influence the response to pH chang es, and mainly affect the cross-bridge cycling rate.