Inactivation of Ca²⁺ transients in amphibian and mammalian muscle fibres

Abstract

MagFluo-4 fluorescence (Ca²⁺) transients associated with action potentials were measured in intact muscle fibres, manually dissected from toads (Leptodactylus insularis) or enzymatically dissociated from mice. In toads, the decay phase of the Ca²⁺ transients is described by a single exponential with a time constant (τ) of about 7 ms. In mice, a double exponential function with τ's of 1.5 and 15.5 ms, respectively gives a better fit. In both species the amplitude of Ca²⁺ transients diminished during repetitive stimulation: in amphibian muscle fibres, the decrease was about 20% with 1 Hz stimulation and 55% at 10 Hz. In mammalian fibres, repetitive stimulation causes a less conspicuous decrease of the transient amplitude: 10% at 1 Hz and 15 % at 10 Hz. During tetanic stimulation at 100 Hz the transient amplitude decays to 20 % in toad fibres and 40 % in mouse fibres. This decrease could be associated with the phenomenon of inactivation of Ca²⁺ release, described by other authors. Recovery from inactivation, studied by a double stimuli protocol also indicates that in toad fibres the ability to release Ca²⁺ is abolished to a greater extent than in mouse fibres. In fact the ratio between the amplitudes of the second and first transient, when they are separated by a 10 ms interval, is 0.29 for toad and 0.58 for mouse fibres. In toad fibres, recovery from inactivation, to about 80 % of the initial value, occurs with a τ of 32 ms at 22 °C; while in mouse fibres recovery from inactivation is almost complete and occurs with a τ of 36 ms under the same conditions. The results indicate that Ca²⁺ release in enzymatically dissociated mammalian muscle fibres inactivates to a smaller extent than in intact amphibian muscle fibres.