Na-Ca Exchange Studies in Frog Phasic Muscle Cells (original) (raw)
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Sodium-calcium exchange in transverse tubules isolated from frog skeletal muscle
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1989
Transverse tubule vesicles isolated from frog skeletal muscle display sodium-calcium exchange activity, which was characterized measuring 4SCa influx in vesicles incubated with sodium. The initial rates of exchange varied as a function of the membrane diffusion potentials imposed across the membrane vesicles, increasing with positive intravesicular potentials according to an electrogenic exchange with a stoichiometry greater than 2 sodium ions per calcium ion transported. The exchange activity was a saturable function of extravesicular free calcium, with an apparent Ko. s value of 3 tzM and maximal rates of exchange ranging from 3 to 5 nmol/mg protein per 5 s. The exchange rate increased when intravesicular sodium concentration was increased; saturation was approached when vesicles were incubated with concentrations of 160 mM sodium. The isolated transverse tubule vesicles, which are sealed with the cytoplasmic side out, had a luminal content of 112 + 39 nmol calcium per mg protein. In the absence of sodium, the exchanger carried out electroneutral calcium-calcium exchange, which was stimulated by increasing potassium concentrations in the intravesicular side. Calcium-calcium exchange showed an extravesicular calcium dependence similar to the calcium dependence o1~ the sodium-calcium exchange, with an apparent Ko. s of 6 p M. Sodium-calcium and calcium-calcium exchange were both inhibited by amiloride. The sodium-calcium exchange system operated both in the forward and in the reverse mode; sodium, as well as calcium, induced calcium efflux from 4SCa-ioaded vesicles. This system may play an important role in decreasing the intracellular calcium concentration in skeletal muscle following electrical stimulation.
muscle fibers and isolated transverse tubules Sodium/calcium exchange in amphibian skeletal
2000
Hidalgo. Sodium/calcium exchange in amphibian skeletal muscle fibers and isolated transverse tubules. Am J Physiol Cell Physiol 279: C89-C97, 2000.-The Na ϩ /Ca 2ϩ exchanger participates in Ca 2ϩ homeostasis in a variety of cells and has a key role in cardiac muscle physiology. We studied in this work the exchanger of amphibian skeletal muscle, using both isolated inside-out transverse tubule vesicles and single muscle fibers. In vesicles, increasing extravesicular (intracellular) Na ϩ concentration cooperatively stimulated Ca 2ϩ efflux (reverse mode), with the Hill number equal to 2.8. In contrast to the stimulation of the cardiac exchanger, increasing extravesicular (cytoplasmic) Ca 2ϩ concentration ([Ca 2ϩ ]) inhibited this reverse activity with an IC 50 of 91 nM. Exchangermediated currents were measured at 15°C in single fibers voltage clamped at Ϫ90 mV. Photolysis of a cytoplasmic caged Ca 2ϩ compound activated an inward current (forward mode) of 23 Ϯ 10 nA (n ϭ 3), with an average current density of 0.6 A/F. External Na ϩ withdrawal generated an outward current (reverse mode) with an average current density of 0.36 Ϯ 0.17 A/F (n ϭ 6) but produced a minimal increase in cytosolic [Ca 2ϩ ]. These results suggest that, in skeletal muscle, the main function of the exchanger is to remove Ca 2ϩ from the cells after stimulation.
Sodium/calcium exchange in amphibian skeletal muscle fibers and isolated transverse tubules
American Journal of Physiology-Cell Physiology, 2000
The Na+/Ca2+ exchanger participates in Ca2+ homeostasis in a variety of cells and has a key role in cardiac muscle physiology. We studied in this work the exchanger of amphibian skeletal muscle, using both isolated inside-out transverse tubule vesicles and single muscle fibers. In vesicles, increasing extravesicular (intracellular) Na+ concentration cooperatively stimulated Ca2+ efflux (reverse mode), with the Hill number equal to 2.8. In contrast to the stimulation of the cardiac exchanger, increasing extravesicular (cytoplasmic) Ca2+ concentration ([Ca2+]) inhibited this reverse activity with an IC50 of 91 nM. Exchanger-mediated currents were measured at 15°C in single fibers voltage clamped at −90 mV. Photolysis of a cytoplasmic caged Ca2+ compound activated an inward current (forward mode) of 23 ± 10 nA ( n = 3), with an average current density of 0.6 μA/μF. External Na+ withdrawal generated an outward current (reverse mode) with an average current density of 0.36 ± 0.17 μA/μF (...
The Journal of General Physiology, 1988
In this paper we investigate the effects of caffeine (5-20 mM) on ferret papillary muscle. The intracellular Ca 2+ concentration ([Ca~+]i) was measured from the light emitted by the photoprotein aequorin, which had previously been microinjected into superficial cells. Isometric tension was measured simultaneously. The rapid application of caffeine produced a transient increase of [Ca2+]i, which decayed spontaneously within 2-3 s and was accompanied by a transient contracture. The removal of extracellular Na + or an increase in the concentration of intracellular Na + (produced by strophanthidin) increased the magnitude of the caffeine response. Cessation of stimulation for several minutes or stimulation at low rates decreased the magnitude of the stimulated twitch and Ca ~+ transient. These maneuvers also decreased the size of the caffeine response. These results are consistent with the hypothesis that the caffeine-releasable pool of Ca 2+ (sarcoplasmic reticulum) is modulated by maneuvers that affect contraction. Ryanodine (10 tzM) decreased the magnitude of the caffeine response as well as that of the stimulated twitch. In contrast, the rapid removal of external Ca ~+ abolished the systolic Ca ~+ transient within 5 s, but had no effect on the caffeine response. From this we conclude that the abolition of twitch by Ca2+-free solutions is not due to depletion of the sarcoplasmic reticulum of Ca ~+, but may be due to a requirement of Ca ~+ entry into the cell to trigger Ca ~+ release from the sarcoplasmic reticulure.
Na(+)-Ca2+ exchange in locust striated muscles
General physiology and biophysics, 1990
High Na+ + Ca2+ exchange rates comparable with those reported for crayfish striated muscle, rat heart and rat brain, were observed in locust striated muscle homogenates and membrane preparations. The Na(+)-Ca2+ exchange followed the 1st order kinetics with a Km value of 18 mumol.l-1 for Ca, the pH optimum was at 8, the temperature optimum at 30 degrees C, and the exchange was inhibited in the presence of sodium in the incubation medium, with a KiNa of approx. 25 mmol.l-1. The present results suggest a high Na(+)-Ca2+ exchange in locust striated muscles which operate on the calcium electrogenesis principle.
Effect of external sodium and calcium on calcium efflux in frog striated muscle
The Journal of Membrane Biology, 1978
The effect of media with different ionic composition on calcium efflux from the dorsal head of semitendinosus muscles of Rana pipiens was studied. The reduction in the fractional loss of 45Ca, when going from normal Ringer's solution to an ONa-OCa medium, was 60 ~. Withdrawal of only Na or Ca from the external medium also caused a significant drop in the fractional loss (33 ~o and 34 ~, respectively). The effect of different concentrations of Ca (studied in the absence of the external Na) was also studied. It was found that a linear function could describe the relationship between the calciumdependent calcium efflux and the external calcium concentration. These results indicate that calcium efflux from frog muscle fibers consists of three major components: one that is dependent on the presence of calcium in the external medium, one that is dependent on the presence of sodium in the external medium, and one that persists in the absence of these two cations.