Sodium transport in triads isolated from rabbit skeletal muscle.
Triads and transverse tubules isolated from mammalian skeletal muscle actively accumulated Na+ in the presence of K+ and Mg-ATP. Active Na+ transport exhibited a fast single-exponential phase, lasting 2 min, followed by slower linear uptake that continued for 10 minutes. Valinomycin stimulated Na+ uptake, suggesting it decreased a pump-generated membrane potential gradient (Vm) that prevented further Na+ accumulation. At the end of the fast uptake phase transverse tubule vesicles incubated in 30 mM external [Na+] attained a ratio [Na+]in/[Na+]out=13.4. From this ratio and the transverse tubule volume of 0.35 microl/mg protein measured in this work, [Na+]in=400 mM was calculated. Determinations of active K+ transport in triads, using 86Rb+ as tracer, showed a 30% decrease in vesicular 86Rb+ content two minutes after initiating the reaction, followed by a slower uptake phase during which vesicles regained their initial 86Rb+ content after 10 minutes. Transverse tubule volume increase during active Na+ transport-as shown by light scattering changes of isolated vesicles--presumably accounted for the secondary Na+ and 86Rb+ uptake phases. These combined results indicate that isolated triads have highly sealed transverse tubules that can be polarized effectively by the Na+ pump through the generation of significant Na+ gradients.
Donoso
P
P
Instituto de Ciencias Biomédicas, Facultad de Medicina, Universidad de Chile, Casilla 70005, Santiago 7, Chile. pdonoso@machi.med.uchile.cl
Hidalgo
C
C
D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D017136:Ion Transport / Q000187:drug effects / Q000502:physiology; D018482:Muscle, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D014634:Valinomycin / Q000494:pharmacology
D000900:Anti-Bacterial Agents; D014634:Valinomycin; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium