SUMMARY1. Double-barrelled Cl--sensitive microelectrodes have been used to measure the intracellular Cl-activity (a',) and membrane potential (Em) in rat lumbrical muscles. The mean Cl-equilibrium potential (Ecl), calculated from the measured aC1 in sixty fibres, was 29 + 25 mV (S.D. of an observation) less negative than Em. The value of a', was higher than would be expected for a passive distribution, by a mean 1P4 + 1 2 mm. The mean Em was -59-5 + 8-2 mV. 3. If lack of selectivity of the Cl--sensitive ion exchanger and the intracellular presence of interfering anions, assumed to be responsible for the apparent al1 recorded in Cl--depleted fibres, were also responsible for the apparently non-passive Cl-distribution recorded under normal conditions, the difference between the calculated EC1 and Em would increase at more negative potentials. This was not observed over a range of Em values between -46 and -84 mV. 4. Inhibition of the Cl-permeability by application of 9-anthracene carboxylic acid (9-AC) resulted in an immediate rise in ai1 and hyperpolarization. An ai1 up to 40 mm higher, or eleven times higher, than that predicted by a passive distribution was recorded. Application of 9-AC after depletion of intracellular Cl-in Cl--free solution had no effect on either the apparent ai1 or Em.5. It is concluded that Cl-ions are actively accumulated by the skeletal muscle fibre and that the Cl-distribution therefore normally exerts a depolarizing influence.6. In the presence of 9-AC and nominal absence of CO2 and HC03-, readdition of Cl-to Cl--depleted fibres resulted in a substantial rise in a', and a small, maintained 7. Neither application of CO2 and HCQ3-nor application of DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) had any effect on the accumulation of Cl-ions. This suggests that Cl--HC03-exchange is not involved.8. Removal of Na+ or K+ resulted in a limited rise in a'1 such that EC1 approximated to Em. Removal of both cations had no greater effect than the absence of only one. Readdition of the cations caused an immediate accumulation of Cl-ions and divergence of Ec1 from Em. This was largely inhibited by the presence of frusemide. These results suggest that a Na+, K+, Cl-co-transport mechanism is responsible for the inward transport of Cl-.

Intraceilular CI-activity (aict) was measured with Cl--sensitive microelectrodes in normal and denervated rat lumbrical muscle. In normal muscle bathed in normal Krebs solution, a~l lay close to that predicted by the Nernst equation• The addition of 9-anthracene carboxylic acid, which blocks CI-conductance, caused alcl to increase far above that predicted by a passive distribution. Furosemide (10 vM) reversibly blocked this accumulation• After muscle denervation, a~l progressively increased for 1-2 wk. The rise occurred in two stages. The initial stage (1-3 d after denervation) reflected passive CIaccumulation owing to membrane depolarization. At later times, a~l continued to increase, with no further change in membrane potential, which suggests an active uptake mechanism. This rise approximately coincided with the natural reduction in membrane conductance to CI-that occurs several days after denervation. Na + replacement, K + replacement, and furosemide each reversibly blocked the active CI-accumulation in denervated muscle. Quantitative estimates suggested that there was little difference between Ci-flux rates in normal and denervated muscles. The results can be explained by assuming that, in normal muscle, an active accumulation mechanism operates, but that CI-lies close to equilibrium owing to the high membrane conductance to CI-. The rise• i m acl after denervation can be accounted for by the membrane depolarization, the reduction in membrane Cl-conductance, and the nearly unaltered action of an inwardly directed CI-"pump."

Recent investigations have established that many of the normal properties of muscle fibres are maintained, at least in part, by muscle activity. Thus, a fall in resting membrane potential, an increase in input resistance, and spread of acetylcholine receptors to extrajunctional sites can all be induced by abolishing muscle activity and prevented by direct stimulation of denervated muscle fibres. Muscle activity also exerts a trophic influence on the innervating motoneurones; furthermore it may be a factor in the regulation of sprouting. Brown and Ironton found fine, "ultra-terminal sprouts" emanating from the endplates of muscles rendered inactive by chronic conduction block of the muscle nerve. Pestronk and Drachman saw increased branching of the motor nerve terminal and a consequent increase in endplate size in similar conditions. If these sprouts at the endplates of inactive muscles were functional, one might expect more transmitter to be released in response to nerve stimulation. We report here that both quantum content and spontaneous miniature endplate potential (m.e.p.p) frequency are increased at the terminals of inactive (disused) muscles.

SUMMARY1. An electric current flows continuously out of the synaptic region of rat lumbrical muscle fibres. It is generated apparently as a result of a non-uniform Cl-conductance (GC1), with GC, being lowest at the end-plate.2. We investigated the effects of denervation on this current. The current persisted with little change after denervation. This was somewhat unexpected, since GC, falls dramatically after denervation, and in acute experiments on normal muscles, the steady current is greatly reduced by agents which block GC1.3. The steady current was blocked in denervated muscle, as in normal muscle, by low-Cl-solutions, Na+-free and K+-free solutions, and treatment with furosemide and 9-anthracene-carboxylic acid. The current in denervated muscle appears to be generated by the same general mechanism as in normal muscle. This has the effect of moving the Cl-equilibrium potential (EC,) in a positive direction, so that the driving force for passive Cl-efflux is increased. The increased driving force compensates for the reduced GC1, allowing the steady current to persist in denervated fibres.

The aim of this study is to compare the effects of simulated microgravity in tail suspended (TS) as well as in pelvic suspended (PS) rats mimicking aspects of prolonged bed rest and sub‐clinical insulinopenia.Materials and MethodsAdult male Sprague‐Dawley rats divided into suspended and control‐non‐suspended groups (6 each per group), were subjected to two separate methods of suspension and maintained with regular food and water for two weeks. Body weights, food and water consumption, soleus muscle weights, malodialdehyde levels of soleus muscle and brain tissues were measured. Pressure‐pain threshold (PPT) was measured using Randall‐Selitto technique at regular 2–3 days intervals during one week before and 2 weeks after HLS.ResultsBody weights in suspended rats from day 10 are significantly different between groups. Food and water intakes are not different between control and suspended groups. Suspended rats had smaller soleus muscle weights and increased MDA levels (p <0.05). PPT were decreased below control after 5 days of PS and 10 days of TS and continued to drop till day 14. X‐ray images show less spinal deformity in PS rats as compared to TS rats.ConclusionsThese data suggest that both procedures develop peripheral neuropathy. Pelvic suspension appears to provide a suitable alternative to tail suspension for chronic hindlimb unloading alleviating excess stress.