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Dear Sir: Costantin (1970) controlled the potential across the surface membrane of a skeletal muscle fiber and studied the radial spread of shortening as this surface membrane potential was varied. He found that the surface depolarization necessary to elicit shortening of axial myofibrils could be less than that necessary to produce movement of the entire cross-section, a result which implies that the depolarization of axially located T tubules was greater than that of the more superficial tubules. The reversal of the expected potential gradient within the T system was taken as evidence for a net membrane current in the T system in the opposite direction from that which would occur in a passive system, and thus for the presence of an action potentiallike mechanism in the T system. This note provides a more formal basis for that conclusion. We assume that the resting transmembrane potential is uniform throughout the T system. Depolarization of the T system will be considered as a positive displacement of tubular transmembrane potential and will result, in a passive membrane, in the flow of positive current from the sarcoplasm to the T system lumen. For a fiber of radius a in which depolarization of the axially located T tubules, V(O), is greater than the depolarization of the tubules at the fiber surface, V(a), we have:

The Radial Variation of Potential in the Transverse Tubular System of Skeletal Muscle