Na and K transport across the cortical and outer medullary collecting tubule of the rabbit: evidence for diffusion across the outer medullary portion

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The rabbit collecting tubule displays functional axial heterogeneity with respect to Na ion transport. The present experiments compared cortical collecting tubule (CCT) and outer medullary collecting tubule (OMCT) Na and K transport. Na efflux across the CCT was inhibited by ouabain, whereas Na efflux across the OMCT was smaller and unaffected by ouabain. Assessment of the equivalent conductivities of Na and K across the CCT by imposition of a Na-K bi-ionic gradient demonstrated a higher K/Na conductivity across the CCT than would be predicted from their respective limiting equivalent conductivities in water. In contrast, the ratio of their conductivities across OMCT were not different than would be predicted by their ratio in water. The "selective" nature of the Na and K pathways across CCT was confirmed by measuring the tracer efflux rate coefficients. In the amiloride-treated CCT the K/Na rate coefficient ratio was 9.8 +/- 1.5; this ratio across the OMCT was 1.51 +/- 0.10. The latter value is not different from the ratio of the mobilities of these ions in water. The diffusional nature of Na and K transfer across OMCT was confirmed by the demonstration of the concentration-independent Na efflux rate coefficient and the demonstration of appropriate net Na and K transepithelial flows in response to imposition of oppositely directed chemical gradients. Although the permeability of the OMCT is low, the chemical gradients found in vivo might be sufficient to effect some K absorption and Na secretion without completely dissipating the steep gradients generated by the CCT. These transport characteristics might be important in the regulation of Na excretion and K recycling into the renal medulla.