Na+-K+-ATPase-dependent sodium flux in cortical collecting tubule

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Abstract
The instantaneous rate of efflux of intracellular Na was studied in rabbit isolated cortical collecting tubules (CT) as a function of temperature and intracellular Na concentration ([Na]i). [Na]i of microdissected CCT was increased by cold and K-free exposure in the presence of 22Na and the extracellular tracer [3H] sorbitol. [Na]i rose rapidly to 40 mM at 30 min, after which it rose more slowly, reaching 120-140 mM at 6 h. Kinetics of Na efflux were studied after rapid rewarming, using a special device allowing measurements at 20-s intervals. Under control conditions, the total Na load was extruded in <8 min, whereas, in the presence of 10-4 M ouabain, only 50% of the load was extruded during this period of time. Ouabain-sensitive Na efflux was first evident at 13.degree. C and gradually increased between 13 and 35.degree. C. At 37.degree. C, Na+-K+-ATPase-dependent Na efflux was dependent on [Na]i. This efflux gradually increased, from 0.05 to 0.5 peq .cntdot. nl tubular volume-1 .cntdot. s-1 as a function of [Na]i and reached a plateau at 70 mM [Na]i. It is concluded that [Na]i is a major modular of the pump activity in CCT; at normal levels of [Na]i, the pump is operating at only a small fraction of its total capacity.