Birefringence signal and early mechanical changes at normal and increased tonicities in frog skeletal muscle.

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RESUMO

Simultaneous measurements of the time course of early mechanical events and the early large birefringence signal were performed during activation of isolated frog skeletal muscle fibres bathed in iso- and hypertonic media. A piezo-electric transducer was used which had a resonance frequency of 3.3 kHz and a sensitivity of 740 mV/mN (0.5 mV noise peak to peak) and a compliance of 0.6 mu/mN. The delay from stimulus onset to the onset of the birefringence signal was estimated to be 0.78 +/- 0.03 ms in normal Ringer solution at room temperature (20-23 degrees C). The beginning of latency relaxation appeared subsequent to the birefringence signal after a delay of 0.36 +/- 0.03 ms. Increasing the tonicity retarded the time course of both birefringence signal and latency relaxation. In solutions of twofold greater tonicity an increase in tension (denoted pre-relaxation contraction) was observed to precede latency relaxation. Pre-relaxation contraction became more prominent with increasing tonicity and appeared to coincide with its onset with the onset of the birefringence signal. The pre-relaxation contraction was shown not to be a stimulus artifact and was not reduced in amplitude by D 600 or calcium-poor solutions. Lowering the temperature to 2.2 degrees C in normal Ringer solution delayed the onsets of the birefringence signal and latency relaxation, and increased the interval between the two signals but did not result in a tension increase before latency relaxation. The coincidence of the birefringence signal onset with the onset of pre-relaxation contraction suggests a common aetiology. The aetiology remains speculative but seems to require hypertonicity for expression of the mechanical signal.

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