An electrophysiological study of chelonian skeletal muscle

Abstract

Membrane properties of the fibers of the retractor-capitis-muscles of the tortoise, Testudo graeea, and of the terrapin, Pseudemys elegans scripta, were investigated by electrophysiological techniques. The features studied were qualitatively and quantitatively similar in both. Fibers are electrically excitable and many extend through 90% of the length of the muscle. In the tortoise, muscle condition velocity was 0.8 m/sec. in hibernating animals, and 1.3 m/sec. in active animals. Resting potentials averaged -80 m V and action potentials about 115 mV. The critical membrane potential in the tortoise was -38.5 mV. Neurally initiated action potentials, recorded at the end-plate, were reduced in amplitude by 14 m V in both forms, and the end-plate "step" from which the action potential was -39 mV. Resting potentials at the end-plate did not differ from those recorded at non-junctional sites. The rise time of the action potential at non-junctional sites was 1.1 msec, and the time for repolarization to 50% was 2 msec, in both forms. The rise time was 3 times longer and the fall time doubled in hibernating tortoises. The average calculated fiber diameter was ca [about] 50 [mu] in the tortoise muscle, and membrane constants were: [lambda] -1.5 mm; [tau] [long dash]34 msec; Rm[long dash]4860 [OMEGA]cm2; Cm[long dash]7.9 [mu]F/cm2. Most of the superficial fibers in both tortoise and terrapin muscles were multiply innervated, but end-plates were focal rather than diffuse. Junctional membranes in both types of preparations were highly sensitive to inotophoretically applied ACh [acetylcholine] but sensitivity fell off sharply with distance from the end-plate. The maximum ACh sensitivity averaged 18.8 mV/nC in the tortoise and 15.5 mV/nC in the terrapin. The distance over which this sensitivity fell to 1/10 was ca. 150 [mu] in the tortoise and ca. 100 /j in the terrapin. In non-junctional regions, ACh reactivity was not detectable in the terrapin muscle. Tortoise fibers were demonstrably reactive to ACh everywhere, but the sensitivity in the non-junctional membrane was 10-3 to 10-5 of the sensitivity of the junctional membrane. Miniature end-plate potentials (M.e.p.p.s.) could be recorded at junctional regions in either tortoise or terrapin muscles. They occurred at random intervals and had a normal amplitude distribution. The a verage amplitude was 0.78 mV in the tortoise and 0.54 mV in the terrapin. These amplitudes were approximately doubled with neostigmine, and were greatly decreased by (+)-tubocurarine. The average frequency was 0.2/sec in the tortoise and 0.4/sec in the terrapin. In the tortoise very low frequency m.e.p.p.s we?e encountered at a number of junctions (< 1/min). These did not show a skewed amplitude distribution and could be increased in frequency with hypertonic solutions. They were not associated with junctions at which transmission had failed. The quantum content of the normal end-plate potential was determined by 2 methods, and was between 100 and 200 units, for both tortoise and terrapin. Thus, the retractor capitis muscles, of both terrapin and tortoise appear to be composed largely of "twitch" fibers. The presence of a very low-level sensitivity of ACh in non-junctional sites of tortoise fibers is regarded as a quantitative rather than a qualitative difference from terrapin fibers. It is suggested that it reflects a condition in which the regulating influence of the motor nerve does not suppress the receptor density in the non-junctional membrane below the level of detectability.