Greater hydrogen ion‐induced depression of tension and velocity in skinned single fibres of rat fast than slow muscles.

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Abstract
1. The effects of variations in pH between 7.00 and 6.20 on Ca2+-activated tension development and maximum velocity of shortening (Vmax) were examined in skinned single skeletal fibres from rat slow-twitch soleus and fast-twitch superficial (s.v.l.) and deep (d.v.l.) regions of the vastus lateralis muscle. 2. At pH 6.50, Vmax was depressed to a similar degree in each of the soleus, d.v.l., and s.v.l. fibres. Lower pH to 6.20 resulted in a further decline in Vmax in all fibres; however, differences between the slow fibres, identified by SDS-polyacrylamide gel electrophoresis, and fast fibres were apparent, with soleus retaining a significantly greater proportion of its control Vmax (0.83 .+-. 0.03 in soleus vs. 0.69 .+-. 0.03 in s.v.l.; mean .+-. S.E.M.). 3. Maximum force production decreased significantly as pH was reduced. Peak force at pH 6.50, relative to that at pH 7.00, was significantly greater in soleus (0.80 .+-. 0.01) than in the s.v.l. (0.75 .+-. 0.01) fibres. At pH 6.20 these differences between slow and fast fibres were still greater, in that soleus fibres generated significantly greater relative forces (0.73 .+-. 0.01) than did d.v.l. (0.67 .+-. 0.02) or s.v.l. (0.63 .+-. 0.02) fibres. 4. As pH was lowered the tension-pCa relationship shifted to the right (i.e. to hgiher [Ca2+]), indicating a reduction in the Ca2+ sensitivity of tension development. The [Ca2+] necessary to achieve half-maximal tension in both the slow- and fast-twitch fibres increased approximately 5-fold when pH was lowered from 7.00 to 6.20. Furthermore, in the case of the soleus, the Ca2+ threshold for tension development was 45 times greater at pH 6.20 than at pH 7.00, while in the fast-twitch fibres, this increase was 4-fold. 5. Increased [H+] differentially affected the steepness of the tension-pCa relationship between slow and fast fibres. As pH was lowered, the steepness of the lower portion of the tension-pCa curve increased in the soleus and decreased in d.v.l. and s.v.l., suggesting that apparent positive co-operativity of tension development had increased in soleus and decreased in d.v.l. and s.v.l. fibres. 6. These results (1) demonstrate an increased resistance to H+ ion-mediated contractile dysfunction in slow-compared to fast-twitch single fibres, and (2) support the hypothesis that muscular fatigue resulting from short-term, intense muscular contraction may in part be related to elevated H+ ion concentration.