Quantitation of 'junctional feet' content in two types of muscle fiber from hind limb muscles of the rat.

1. Transverse tubules in fibers from rat soleus and extensor digitorum longus (EDL) muscles of the rat were infiltrated with silver dichromate (black reaction of Golgi). This provides a faithful, high-contrast outline of the tubules, which allows distinction between segments involved in junction formation with the sarcoplasmic reticulum and segments that are free. 2. Electron micrographs of semithin transverse sections were used to quantitate T tubule parameters and to measure cross-sectional area and perimeter of individual fibers. Thin sections and data from the literature were used to obtain the contribution of caveolae to external surface area and the frequency of junctional feet along the junctional T tubule membrane. 3. From the above data we calculate the ratio of number of feet to total external surface area for a given fiber segment. The ratio is compared with data in the literature on the total amount of 'charge movement' (in nC/uF of total external surface area). 4. The average feet/surface area ratio is twice as large in EDL than in soleus fibers, while the charge movement is up to five-fold larger. Probably some of the total charge movement is not directly associated with events related to the turning on of the SR permeability to calcium.