Skeletal muscle fibres from the toad were mechanically skinned under paraffin oil and then bathed in a potassium HDTA solution (HDTA: hexamethylenediamine-tetraacetate) which mimicked the ionic composition of the myoplasm. 2. Rapid transient contractions could be triggered by substitution of K+ with Na+ (with no change of anion), which should have virtually no direct effect on the electrical polarization of the sarcoplasmic reticulum (SR) membrane. Up to thirty or more contractions could be evoked by repeated substitutions if there was sufficient 'repriming' time (about 30 s) between them; these rapid contractions were analagous to potassium contractures in intact fibres. 3. When the SR was not heavily loaded, substitution of potassium HDTA with choline chloride also produced a rapid, brief contraction. 4. All treatments designed to 'inactivate' the voltage sensor in the T-system invariably abolished the rapid contractions. Thus, rapid contractions were absent if (i) the T-system was permanently depolarized by pre-soaking the muscle in a high potassium solution with ouabain before skinning, (ii) a fibre was split rather than skinned, (iii) the T-system was temporarily depolarized by Na+ substitution immediately before choline chloride substitution, or vice versa, (iv) a skinned fibre was briefly exposed to saponin (50 micrograms/ml) to selectively disrupt the T-system membrane or (v) the muscle was pre-soaked in a solution with 1 mM-EGTA and no Ca2+ or Mg2+ before skinning. In contrast to (v), if 10 mM-Mg2+ was present in the EGTA solution before skinning, rapid contractions could be elicited, presumably because the presence of Mg2+ prevented the inactivation of the T-system voltage sensor in low [Ca2+]. 5. These results unequivocally demonstrate that (a) the T-system reseals and repolarizes after mechanical skinning under oil and (b) the fast contractions are produced by activation of the voltage sensor in the T-system. 6. When the SR had been heavily loaded, choline chloride substitution (but not Na+ substitution) could also induce an unphysiological, slow contraction ('second component'). In total contrast to the fast contraction, this slow component was unaffected by any of the treatments (i-v) above, indicating that it did not depend on activation of the voltage sensor in the T-system but resulted from a direct action of choline chloride on the SR.(ABSTRACT TRUNCATED AT 400 WORDS)
