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 (V-m) 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 mul/mg protein measured in this work, [Na+](in)=400 mM was calculated. Determinations of active K+ transport in triads, using Rb-86(+) as tracer, showed a 30% decrease in vesicular Rb-86(+) content two minutes after initiating the reaction, followed by a slower uptake phase during which vesicles regained their initial Rb-86(+) 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 Rb-86(+) 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.