Studies of capsule and capsular space of cat muscle spindles.

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Abstract
1. The presence of hyaluronate in the capsular space of the cat muscle spindle was demonstrated using alcian blue staining at various pHs, the critical electrolyte concentration technique and hyaluronidase treatment. 2. In spindles with intact capsules an extracellular marker, the dye Ruthenium Red, gained access to the capsular space through the gap in the sleeve region, but for a limited distance. In muscle spindles with the capsule nicked, the marker diffused into the capsular space in the equatorial region, revealing a dense network in this space which consisted of globular structures interconnected by thin filaments. Based on their thickness, these filaments were inferred to be hyaluronic acid, and the globular structures were inferred to be protein molecules. Longitudinal diffusion of the dye into the capsular space through the nicked site was limited. The limited diffusion is probably due to electrostatic binding of the dye, which is a hexavalent cation, to negatively charged glycosaminoglycan hyaluronate that is present in the space. 3. The transcapsular potential was measured by use of glass micropipettes filled with a 3 M-KCl. The value was 15 mV .+-. 4 (average .+-. S.D., n = 12; range, 10-20 mV) inside negative. The input resistance and capacitance of the capsule, measured with two independent electrodes, varied widely (1.3-8.0 M.OMEGA. and 0.5-1.3 nF, n = 4) and the capsule showed marked delayed rectification to outward current pulses. [K+] in the space measured with K+-sensitive resin-filled glass micropipettes was a few millimolar higher than that in the bathing solution. 4. The effects of [K+] and [Ca2+] on impulse activities were examined in spindles with intact capsules or with partially resected capsules. In spindles with intact capsules the effects of [K+] and [Ca2+] were significantly less or negligible compared with those in spindles with the capsule opened. 5. Hyaluronidase (.apprx. 10-4 g/ml) added to the bathing solution around nicked capsules significantly reduced both resting and stretch-induced impulse activities in 40-50 min. By this time the capsular space was completely collapsed. An increase in [K+] of the bathing solution from 3.5 to 6 or 8 mM restored these impulse activities. A similar restoring effect was also observed when [Ca2+] in the bathing solution was reduced. Based on the above findings it is proposed that the negatively charged glycosaminoglycans or hyaluronate present in the capsular space are responsible, at least partially, for the transcapsular potential, and more significantly, for the decreased potential difference across the sensory terminal membrane, thus making the sensory ending sensitive to mechanical stimuli. 6. It was concluded that the spindle capsule and hyaluronate present in the capsular space play an important role in regulating the ionic milieu around the sensory endings and maintaining their sensitivity to mechanical stimuli.