Sodium-calcium exchange in transverse tubules isolated from frog skeletal muscle.
Transverse tubule vesicles isolated from frog skeletal muscle display sodium-calcium exchange activity, which was characterized measuring 45Ca 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 K0.5 value of 3 microM 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 of the sodium-calcium exchange, with an apparent K0.5 of 6 microM. 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 45Ca-loaded vesicles. This system may play an important role in decreasing the intracellular calcium concentration in skeletal muscle following electrical stimulation.
Donoso
P
P
Departamento Preclinicas, Facultad de Medicina, Universidad de Chile, Santiago.
Hidalgo
C
C
D000818:Animals; D001001:Anura; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002132:Calcium Radioisotopes; D002352:Carrier Proteins / Q000378:metabolism*; D004563:Electrochemistry; D008564:Membrane Potentials; D008870:Microtubules / Q000378:metabolism; D009132:Muscles / Q000378:metabolism / Q000648:ultrastructure*; D011188:Potassium / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger; D013054:Spectrophotometry, Atomic
D002132:Calcium Radioisotopes; D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D011188:Potassium; D002118:Calcium