A study of store dependent Ca 2+ influx in frog skeletal muscle

摘要

Ca 2+ influx across the plasma membrane upon drastic reduction of the sarcoplasmic reticulum Ca 2+ content was studied in voltage clamped frog skeletal muscle fibers. Depletion was produced by the application of 30 μM cyclopiazonic acid (CPA) in Ca 2+-free, [Mg 2+] = 8 mM external salines and produced an increase in resting free myoplasmic [Ca 2+]. Once depletion was attained the external solution was changed to one containing the same concentration of the drug but with Ca 2+ instead of Mg 2+. Of 27 fibers studied only nine showed a secondary increase in free myoplasmic [Ca 2+] upon readmitting Ca 2+ in the external perfusate. In the presence of CPA the resting myoplasmic [Ca 2+] in Ca 2+-free external saline was 0.08 ± 0.01 μM (Mean ± SEM), and in Ca 2+-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 [Ca 2+] went from 0.35 +/- 0.08 μM in Ca 2+-free external solution to 0.42 +/- 0.12 μM upon reapplication of Ca 2+(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 [Ca 2+] the holding current measured at -90 mV did not significantly change. These results suggest the Ca 2+ entry secondary to store depletion is a labile mechanism in frog skeletal muscle and when present does not have an obvious electrical manifestation.