The Ca2+ ATPase content of slow and fast twitch fibers of guinea pig.

The Ca ATPase content in the sarcoplasmic reticulum (SR) of fast and slow twitch skeletal fibers was estimated using two well-characterized muscles of the guinea pig: the white bundle of the vastus lateralis and the soleus. Ca ATPase surface density was determined by counting the projections of individual molecules revealed on the cytoplasmic surface of freeze-dried, rotary-shadowed microsomal vesicles isolated from the two muscles. The Ca ATPase densities were 32,000/micron 2 and 25,000/micron 2 for the vastus lateralis and soleus muscles, respectively. The percentage of membrane area occupied by Ca ATPase-free lipid patches was estimated using freeze-fractured, rotary-shadowed in situ SR. In soleus muscle the free SR of terminal cisternae and the longitudinal SR have 34.5 and 19.7% of their surface free of ATPase, respectively. In the white vastus less than 1% of the surface was not occupied by Ca ATPase. These values were combined with stereological data from the literature to give a ratio of total Ca ATPase content per unit fiber volume of 1:2, slow versus fast. This is considerably less than the approximately sixfold difference in the overall relaxation time and the half times to relaxation between the two fiber types. This suggests that other factor such as differences in enzyme kinetics or cytoplasmic Ca buffering proteins must also play a role in determining rate of relaxation.