title: A study of store dependent Ca²⁺ influx in frog skeletal muscle.
abstract: 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.
authors.affiliation: Departamento de Biofísica, Facultad de Medicina, Universidad de la República, Gral. Flores 2125, CP11800, Montevideo, Uruguay.
authors.forename: J F
authors.initials: JF
authors.lastname: Olivera
authors.forename: Gonzalo
authors.initials: G
authors.lastname: Pizarro
mesh_terms: D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling; D002462:Cell Membrane / Q000378:metabolism; D002463:Cell Membrane Permeability; D004533:Egtazic Acid / Q000378:metabolism; D055724:Electrophysiological Phenomena; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D008345:Manganese / Q000378:metabolism; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000378:metabolism; D018482:Muscle, Skeletal / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D018408:Patch-Clamp Techniques; D011892:Rana catesbeiana / Q000378:metabolism* / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism / Q000502:physiology; D013997:Time Factors
chemical_list: D007211:Indoles; D008345:Manganese; D004533:Egtazic Acid; D002118:Calcium; C000543:cyclopiazonic acid