Disintegration of junctional feet in crayfish muscle fibres kept in vitro.

The significance of junctional feet (Franzini-Armstrong 1971) for excitation-contraction (E-C) coupling is suggested by their location in the gap separating the T-system from the sarcoplasmic reticulum (SR). Until now, however, no specific treatment for selectively disrupting junctional feet has been known and the direct evidence relating the presence of feet to the transfer of signals across the T-SR gap is thus still lacking. The importance of morphological integrity of tubulo-reticular (T-SR) junction for E-C coupling was invoked by Oota and Nagai (1973) and Frank and Treffers (1977) who showed widening of the junctional gap following urea-removal treatment and by Eisenberg and Eisenberg (1982) reporting an increased number of pillars spanning the T-SR gap in fibres activated by potassium-rich solution. On the other hand, E-C decoupling in crayfish after glycerol-removal (Zacharová and Uhrík 1978) or urea-removal (Krolenko et al. 1980) treatments could be satisfactorily explained by the detachment of T-tubules from the sarcolemma. An interesting type of E-C decoupling was described by Rýdlová et al. (1984) on crayfish (Astacus fluviatilis) muscle fibres kept in vitro. When the fibres were maintained in a culture medium consisting of a mixture (2:1) of crayfish saline with calf serum supplemented with penicillin and streptomycin (Daig and Spindler 1979; Rýdlová et al. 1983) the dissociation of the E-C link was developing gradually. A significant decrease in the mechanical response upon depolarisation (induced either by potassium or electrical stimulation) could be recorded on the 4th day after the isolation of fibres. Prolonged maintenance of fibres in the culture medium (for 8 days or more) mostly resulted in nearly complete E-C decoupling. At the same time the membrane potential, action potentials in strontium solution or inward calcium currents under voltage clamp were not significantly different from controls. Caffeine contractures were normal or higher and ultrastructurally intact T-system connected to sarcolemma and SR with normal appearance could be seen.