The transport of D-glucose was examined in isolated perfused segments of the rabbit proximal nephron. The total unidirectional lumen-to-bath flux of D-glucose in rabbit proximal convoluted tubules (PCT) and early and late segments of proximal straight tubules (PST) could be described as the sum of two independent fluxes: one due to an active saturable transport site and the other a parallel passive permeation pathway. Both fluxes increased with increasing perfusion rate as a result of increased average luminal glucose concentration. The maximal active transport rate for glucose decreased along the nephron from 83.2 pmol . min-1 . mm-1 in the PCT to 12.9 and 7.9 in the early and late PST, respectively. The Km value for the active site also decreased from 1.64 mM in the PCT to 0.70 and 0.35 in the early and late PST, respectively. The permeability value for the passive permeation pathway, which was assessed from the linear dependence of bath-to-lumen fluxes of L-glucose on bath concentration, decreased from 0.033 micrometers/s in the PCT to 0.015 and 0.009 in the early and late PST, respectively. These characteristics of a high transport capacity with moderate leak in the PCT and lower transport capacity with low leak in the PST allow the establishment of steep glucose concentration gradients in the PCT that are maintained and augmented in the late proximal nephron.

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Direct examination of the structure and function of the macula densa is compromised by the relative inaccessibility and small size of this cell plaque. We report the isolation and perfusion of rabbit nephron segments with attached glomeruli and the direct visualization of the macula densa with differential interference-contrast microscopy. We used this technique to examine the structural sensitivity of the macula densa to perturbations in luminal osmolality and NaCl concentration. Reducing luminal osmolality from 290 to 70 mosmol/kg by removing NaCl resulted in a dilation of the lateral intercellular spaces that was both reversible and specific to the region of the macula densa. Associated with the dilation of the intercellular spaces was a small (congruent to 10%), but statistically significant, increase in the height of the macula densa cells. These structural events were related to the reduction in luminal osmolality, since isosmotic replacement of NaCl with mannitol resulted in no detectable structural changes. Thus, the macula densa may represent a small water-permeable plaque of cells within the remaining water-impermeable thick ascending limb of Henle's loop.