Simultaneous measurements of Ca2+ currents and intracellular Ca2+ concentrations in single skeletal muscle fibers of the frog.

The relationship between Ca2+ current amplitudes and myoplasmic Ca2+ transients was studied in single muscle fibers. Segments of muscle fibers were voltage-clamped in a double Vaseline gap chamber. Ca2+ transients were measured as an optical signal derived from the interaction between Ca2+ and the dye antipyrylazo III. The cells were maintained at -90 mV. Ca2+ currents were detected at pulse potentials to -50 mV, reached a maximum value at 0 mV, were reduced in size for larger depolarizations, and reversed at about 40 mV. Ca2+ transients were also detected at -50 Mv and progressively increased in size with larger pulse potentials up to 10 mV. Depolarizations to voltages greater than 10 mV did not further increase the size of the transient. The magnitude and time course of transients from 10 to 70 mV were almost identical Ca2+ fluxes into the myoplasm (Ca2+ input fluxes) were calculated from the Ca2+ transients applying a removal model. The size of the input fluxes increased with depolarization up to 0 mV. Between 0 and 70 mV the peak input flux slightly increased, while the flux measured at 200 ms remained unchanged. In conclusion, Ca2+ transients and input fluxes were not reduced during pulses to large positive potentials, even though a drastic reduction of Ca2+ current occurred at these potentials. These observations make it very unlikely that a voltage-dependent Ca2+ entry is the triggering signal for contraction.