IMMUNOCYTOCHEMICAL LOCALIZATION OF NA,K-ATPASE IN RAT MUSCLE-SPINDLES

In the muscle spindle, one of the major sensory receptors in the verte brate skeletal muscle, it was demonstrated that stretching caused a co nductance increase of the sensory terminal membrane mainly to Na+ (Hun t, Wilkinson and Fukami, 1978 (6)). Since the muscle spindle is a slow ly adapting stretch receptor, and even at rest some spindles are activ e, a vigorous Na, K-pump activity is expected to counteract the incess ant inflow of Na+ into the terminal. To test this assumption, rat musc le spindles were examined by immunofluorescence microscopy as well as by the electron microscopic immunogold technique using antibody agains t rat alpha-subunit of Na, K-ATPase. The results indicate that the sen sory ending has the highest density of the enzyme among the other cell ular components examined, and that the enzyme density appears to be hi gher in the plasma membrane of the sensory ending facing the intrafusa l muscle fiber (synaptic membrane) than the rest of the membrane (extr a-synaptic membrane). The functional significance of the above finding s was discussed.