Local anaesthetics inhibit tension development and Nile blue fluorescence signals in frog muscle fibres

Abstract

FLUORESCENT DYES have been increasingly used to measure fluorescence intensity changes associated with changes in transmembrane potential in various preparations^1,2. The use of these compounds is of particular interest in muscle fibres, as they can be used to detect potential changes occurring across membranes not directly accessible to electrical measurements, and possibly associated with excitation–contraction coupling processes. For example, muscle fibres stained with Merocyanine 540 and electrically stimulated, give an early fluorescent signal which has been thought to arise from surface and T-system membranes^3,4. More interestingly, fibres stained with Nile blue A and electrically stimulated have been reported to give fluorescence signals probably arising from the sarcoplasmic reticulum membrane^2,5–7. Optical retardation changes arising from this membrane system have also been reported^8,9. These results seem to indicate the existence of an electrical response across the sarcoplasmic reticulum membrane which might be associated with calcium release from these structures during contractile activation. To test this possibility, we investigated and report here the effects of the local anaesthetics, procaine and tetracaine, on tension development and their comparison with the effects on the fluorescence signals generated in muscle fibres stained with Nile blue A. Procaine, and to a greater extent tetracaine, are thought to inhibit calcium release from the sarcoplasmic reticulum^10,11.