Modification of sensory-terminal responses and membrane structure of frog muscle spindle by collagenase

A preparation of decapsulated muscle spindles with intact sensory innervation has been developed to allow direct access to the sensory terminal for the application of drugs or to alter the extracellular ionic composition. Muscle spindles were isolated from semitendinosus muscles of the frog Rana catesbeiana, and were incubated in a calcium-free Ringer's solution containing 0.2% collagenase. Following optimal incubation at 34 degrees C for 30 min the response pattern of the spindles during stretch could not be distinguished from that of intact spindles, although the duration of individual afferent spikes was prolonged about 4 times normal. The spikes disappeared immediately after the Ringer's solution was replaced with an isotonic choline chloride solution, in contrast to those of intact spindles which remained for 30 min after the replacement. Electron microscopy showed that the outer and inner capsules were partially disrupted. No significant change was observed in the size or packing density of intramembrane particles in freeze-fracture replicas of spindles decapsulated under optimum conditions. More prolonged treatment with the enzyme resulted in abolition of the static component of the response during stretch, and also in an aggregation of the particles, whose size decreased and packing density increased.