Immunocytochemical localization of Na, K-ATPase in rat muscle spindles

In the muscle spindle, one of the major sensory receptors in the vertebrate skeletal muscle, it was demonstrated that stretching caused a conductance increase of the sensory terminal membrane mainly to Na+ (Hunt, Wilkinson and Fukami, 1978 (6)). Since the muscle spindle is a slowly adapting stretch receptor, and even at rest some spindles are active, a vigorous Na, K-pump activity is expected to counteract the incessant inflow of Na+ into the terminal. To test this assumption, rat muscle spindles were examined by immunofluorescence microscopy as well as by the electron microscopic immunogold technique using antibody against rat alpha-subunit of Na, K-ATPase. The results indicate that the sensory ending has the highest density of the enzyme among the other cellular components examined, and that the enzyme density appears to be higher in the plasma membrane of the sensory ending facing the intrafusal muscle fiber (synaptic membrane) than the rest of the membrane (extra-synaptic membrane). The functional significance of the above findings was discussed.