A study of store dependent Ca²⁺ influx in frog skeletal muscle

Ca²⁺ influx across the plasma membrane upon drastic reduction of the sarcoplasmic reticulum Ca²⁺ content was studied in voltage clamped frog skeletal muscle fibers. Depletion was produced by the application of 30 μM cyclopiazonic acid (CPA) in Ca²⁺-free, [Mg²⁺] = 8 mM external salines and produced an increase in resting free myoplasmic [Ca²⁺]. Once depletion was attained the external solution was changed to one containing the same concentration of the drug but with Ca²⁺ instead of Mg²⁺. Of 27 fibers studied only nine showed a secondary increase in free myoplasmic [Ca²⁺] upon readmitting Ca²⁺ in the external perfusate. In the presence of CPA the resting myoplasmic [Ca²⁺] in Ca²⁺-free external saline was 0.08 ± 0.01 μM (Mean ± SEM), and in Ca²⁺-containing external saline 0.10 ± 0.02 μM when the intracellular solution contained [EGTA] = 5 mM (n = 18). In cells with lower (0.5 mM) intracellular [EGTA] resting [Ca²⁺] went from 0.35 +/- 0.08 μM in Ca²⁺-free external solution to 0.42 +/- 0.12 μM upon reapplication of Ca²⁺(n = 9). In both cases the differences between means were not statistically significant (paired t test, p = 0.13 in high EGTA and p = 0.25 in low EGTA). In the nine fibers that showed a secondary increase of resting [Ca²⁺] the holding current measured at -90 mV did not significantly change. These results suggest the Ca²⁺ entry secondary to store depletion is a labile mechanism in frog skeletal muscle and when present does not have an obvious electrical manifestation.