EFFECTS OF PH ON MYOFIBRILLAR ATPASE ACTIVITY IN FAST AND SLOW SKELETAL-MUSCLE FIBERS OF THE RABBIT

In permeabilized single fibers of fast (psoas) and slow (soleus) muscl e from the rabbit, the effect of pH on isometric myofibrillar ATPase a ctivity and force was studied at 15 degrees C, in the pH range 6.4-7.9 . ATPase activity was measured photometrically by enzymatic coupling o f the regeneration of ATP to the oxidation of NADH, present in the bat hing solution. NADH absorbance at 340 nm was determined inside a measu ring chamber. To measure ATP turnover in single soleus fibers accurate ly, a new measuring chamber (volume 4 mu l) was developed that produce d a sensitivity approximately 8 times higher than the system previousl y used. Under control conditions (pH 7.3), the isometric force was 136 and 115 kN/m(2) and the ATP turnover was 0.43 and 0.056 mmol per lite r fiber volume per second in psoas and soleus fibers, respectively. Ov er the pH range studied, isometric force increased monotonically by a factor 1.7 for psoas and 1.2 for soleus fibers. In psoas the isometric ATPase activity remained constant, whereas in soleus it slightly decr eased with increasing pH. The pH dependency of relative tension cost ( isometric ATPase activity divided by force) was practically identical for psoas and soleus fibers. In both cases it decreased by about a fac tor 0.57 as pH increased from 6.4 to 7.9. The implications of these fi ndings are discussed in terms of cross-bridge kinetics. For both fiber types, estimates of the reaction rates and the distribution of cross- bridges and of their pH dependencies were obtained. A remarkable simil arity was found between fast- and slow-twitch fibers in the effects of pH on the reaction rate constants.