RELATIONSHIP BETWEEN DEPOLARIZATION-INDUCED FORCE RESPONSES AND CA2-MUSCLE FIBERS OF RAT AND TOAD( CONTENT IN SKELETAL)

1. The relationship between the total Ca2+ content of a muscle fibre a nd the magnitude of the force response to depolarization was examined in mechanically skinned fibres from the iliofibularis muscle of the to ad and the extensor digitorum longus muscle of the rat. The response t o depolarization in each skinned fibre was assessed either at the endo genous level of Ca2+ content or after depleting the fibre of Ca2+ to s ome degree. Ca2+ content was determined by a fibre lysing technique. 2 . In both muscle types, the total Ca2+ content could be reduced from t he endogenous level of similar to 1.3 mmol l(-1) (expressed relative t o intact fibre volume) to similar to 0.25 mmol l(-1) by either depolar ization or caffeine application in the presence of Ca2+ chelators, sho wing that the great majority of the Ca2+ was stored in the sarcoplasmi c reticulum (SR). Chelation of Ca2+ in the transverse tubular (T-) sys tem, either by exposure of fibres to EGTA before skinning or by permea bilizing the T-system with saponin after skinning, reduced the lower l imit of Ca2+ content to less than or equal to 0.12 mmol l(-1), indicat ing that 10-20% of the total fibre Ca2+ resided in the T-system. 3. In toad fibres, both the peak and the area (i.e. time integral) of the f orce response to depolarization were reduced by any reduction in SR Ca 2+ content, with both decreasing to zero in an approximately linear ma nner as the SR Ca2+ content was reduced to < 15% of the endogenous lev el. In rat fibres, the peak size of the force response was less affect ed by small decreases in SR content, but both the peak and area of the response decreased to zero with greater depletion. In partially deple ted toad fibres, inhibition of SR Ca2+ uptake potentiated the force re sponse to depolarization almost 2-fold. 4. The results show that in th is skinned fibre preparation: (a) T-system depolarization and caffeine application can each virtually fully deplete the SR of Ca2+, irrespec tive of any putative inhibitory effect of SR depletion on channel acti vation; (b) all of the endogenous level of SR Ca2+ must be released in order to produce a maximal response to depolarization; and (c) a subs tantial part (similar to 40%) of the Ca2+ released by a depolarization is normally taken back into the SR before it can contribute to force production.