Identifying renal medullary neighborhoods—when do distances matter?

renal anatomists are special: few in number, but capable of providing enduring scientific contributions. Memorable contributions include the comparative biology by Sperber, the ultrastructural anatomy of tubules by Maunsbach or Pfaller, and of glomeruli by Kriz, or the injection studies defining renal vascular anatomy by Beeuwkes. There is little doubt that each of these workers believed that their obsession with anatomic detail was fueled by the conviction that understanding function begins with delineating structure. To this roster, I would add Pannabecker and colleagues, who have, over the last decade, provided an extraordinary view of the renal inner medulla. That view derives from the combination of immunohistochemical staining of structures within medullary slices, as developed in the Wade laboratory (7), and computer reconstruction to derive a full axial image (8). I confess to showing our medical students their early images with this technique (9), simply because it is such a tour de force. There is also no doubt that Pannabecker and colleagues believed that their effort serves the cause of deciphering the urine concentrating mechanism (UCM). The classic view of the renal inner medulla recognizes steep axial concentration gradients, but relatively homogenous interstitial composition at any given level. This view was rooted in the micropuncture observations of Gottschalk (3), who documented osmotically identical fluids within blood vessels, descending Henle limbs (DHL), and collecting ducts (CD) at any single inner medullary level within the antidiuretic kidney. This observation provided a crucial simplifying assumption in the central core model of Stephenson (10), in which all tubule segments experienced the same environment, namely, a well-mixed vascular and interstitial compartment. This view propagated into subsequent medullary models from other workers. Unfortunately, the UCM field has been plagued by an inability to devise medullary simulations, which can rationalize the steep axial medullary gradient while also being true to measured tubule properties. Contemporaneous with the anatomic studies of Pannabecker et al., the Laytons began to develop medullary simulations that could accommodate this new level of detail. They fashioned an outer medullary model, in which each slice was comprised of four distinct regions, each containing different tubular and vascular structures, and with a distinct interstitial solute composition (4, 5). In collaboration with Pannabecker and Dantzler, they fashioned an inner medulla model with two distinct regions (6). However, with respect to the UCM, outer medullary regionalization provided only a 16% increase in CD osmolality, and inner medullary regionalization actually diminished concentrating ability. A notable turn in this story came when Edwards focused attention on microvascular oxygen transport. In the outer medullary model, it turned out that regionalization of oxygen was huge, predicting a 40- to 50-mmHg Po2 gradient between the vascular bundle and the tubules (1, 2). There is as yet no O2 transport model for the inner medulla. This is the context in which one may view the contribution of Westrick et al. (11) in a recent issue of the American Journal of Physiolgy-Renal Physiolgy, which provides new information on rat inner medullary architecture. Solute movement between regions can be hindered either by increasing the distance between the regions or by decreasing the area of the pathway (“abutment,” as used in this work). The study by Westrick et al. provides measurements of both of these aspects of medullary geometry. Specifically, the authors find that a long DHL, initially at a distance (14 μm) from its closest CD, ultimately moves into closer proximity (4 μm), stays in close proximity as an ascending Henle limb (AHL), and then ultimately distances itself from the CD. The authors hypothesize that this geometry contributes to the efficacy of the UCM, speculating that anatomically defined “intracluster” [CD, along with AHLs and ascending vasa rectae (AVR)] and “intercluster” regions (DHL and descending vasa rectae, along with AHL and AVR) may have different solute concentrations, and thus be functionally distinct. Beyond speculation, however, the authors offer estimates of the impact of the morphology on permeabilities between structures. To translate a distance into an effective permeability, one needs only an estimate of the solute diffusion coefficient, so, for example, a diffusivity of 10−5 cm2/s extending across a 10-μm interstitial distance, yields an effective permeability of 0.01 cm/s. For several barriers in series, large permeabilities are inconsequential, while the smallest is rate limiting. In view of the reference medullary permeabilities (6), it would seem that diffusion limitation is only of relevance to vasa rectae fluxes, since the permeability attributed to the interstitial diffusion barrier is at least an order of magnitude greater than tubule permeabilities. To decide whether a diffusion barrier within the inner medullary interstitium will sustain a regional concentration gradient, one needs to know the relevant solute flux across the barrier. Westrick et al. (11) do not go there, but despite the uncertainties it is not too difficult to consider estimates of CD fluxes for Na+, urea, and oxygen (Table 1). For the case of Na+, it is assumed that 1% of the filtered Na+ is reabsorbed by the inner medullary CD (IMCD). Total tubular length is 18,000 mm (7,200 tubules coalescing uniformly over 5 mm), and tubular diameter is 30 μm. With these assumptions, concentration differences across a 10-μm-thick interstitial space between tubule and blood vessel are trivial: much less than 1 mM using the free diffusion coefficient, and barely exceeding 1 mM when diffusion is hindered by 80% (as assumed by Westrick et al.). For the case of urea, it is assumed that 50% of the filtered urea is reabsorbed across the most distal 10% of the IMCD. In this case, the concentration difference would be negligible (2.7 mM) when there is free diffusion, and questionably detectable when diffusion is hindered by 80% (13.5 mM in ambient 600 mM inner medullary urea). For O2, the story is a bit different. One estimate of O2 consumption derives from the measurements of Zeidel et al. (12) of IMCD in suspension, and this number is about an order of magnitude larger than the value used by Chen et al. (1). An even larger estimate for O2 utilization comes from assuming that all of the Na+ transport is driven by oxidative metabolism, with a stoichiometry of 18 Na+ per O2. With such fluxes, a 10-μm interstitial barrier could support a substantial Po2 difference, especially with hindered diffusion. In short, it is difficult to make a case that the Henle limb-to-CD distances measured by Westrick et al. (11) will be critical to understanding the UCM, or medullary metabolism of K+, or even CO2, whose concentration is several mM in the inner medulla. It does seems likely, however, that the medullary structural data from this group will provide the foundation for modeling the inner medullary microcirculation, particularly in relation to medullary oxidative metabolism. Such models will provide a map of local Po2, which could be subject to experimental scrutiny. Table 1. Lateral concentration gradients required to drive representative fluxes across a 10-μm inner medullary interstitial layer