Juxta-medullary nephrons are consistently found to have higher filtration rates than superficial nephrons. Is this due to the underlying anatomy of the cortical radial artery, from which the juxta-medullary afferent arterioles branch before the superficial arterioles? Is it due to anatomical differences in the glomeruli, such as length, surface area or permeability? Or is it perhaps a matter of differences in the regulation of glomerular blood flow?
Due in part to the physical difficulty of observing juxta-medullary glomeruli without damaging the kidney, this is not an easy question to answer. And indeed, I have been unable to find any experimental results that directly address this issue. The most relevant papers that I have been able to find detail studies of the filtration rates in superficial and juxta-medullary nephrons, without investigation of the underlying cause(s).
- Glomerular filtration in single nephrons (Wright and Giebisch, 1972) is a great review article with a wealth of references to experimental data. Perhaps the most pertinent sentence is: "At this time, therefore, a role for redistribution of SNGFR within the kidney during natriuretic states is not established. Natriuresis can occur without redistribution; however, the disproportionate increases in superficial SNGFR seen in some rat experiments seem to be real changes."
- Heterogeneity of Nephron Function (1977) is another great review article with a lot of useful data and critical evaluations of a wide range of experimental studies. They make a key observation: "In addition, all direct measurements of juxta-medullary nephron SNGFR have been obtained from young rats or desert rodents since the extrarenal part of the papilla is more readily accessible to micropuncture in the immature animal than in the adult. Whether these values are similar in adult rats has been evaluated with more indirect estimates of SNGFR."
- Glomerular blood flow (1990) is another great source of experimental references and data (such as Table 3B).
- Functional Heterogeneity of Nephrons II. Filtration Rates, Intraluminal Flow Velocities and Fractional Water Reabsorption (1969) provides mean SNGFRs for superficial, mid- cortical and juxta-medullary nephrons.
- The companion article, Functional Heterogeneity of Nephrons I. Intraluminal Flow Velocities (1969) is also interesting, and points to yet another interesting study: Renal hemodynamic factors in congestive heart failure (1966).
The (non-definitive) answer seems to be that redistribution of SNGFR is rarely and inconsistently observed in adult models, and so the safest modelling assumption is to assume a constant (or near-constant) ratio of juxta-medullary to superficial SNGFR in the range 1.5–2.0.
Other papers that consider the distribution of SNGFR between the superficial and deep cortex include:
- Micropuncture studies on the filtration rate of single superficial and juxtamedullary glomeruli in the rat kidney (1968), which describes the bewildering observation that in response to a high-sodium diet, the rats under observation showed almost a two-fold increase in superficial SNGFR and almost a four-fold decrease in juxta-medullary SNGFR.
- Function of juxtamedullary nephrons in normotensive and chronically hypertensive rats (1969) observes that unilateral Goldblatt hypertension produced a slight increase in total kidney GFR, but without a change in superficial SNGFR.
- Sodium Metabolism and Intrarenal Distribution of Nephron Glomerular Filtration Rates in the Unanesthetized Rat (1974) addresses an earlier study where "anesthesia may have blunted a physiological response" and concludes that "this study supports the view that dietary sodium does not affect the intrarenal distribution of nephron filtration rates."
- A micropuncture study of renal salt and water retention in chronic bile duct obstruction (1975), in contrast, states "single nephron filtration fraction, calculated from measurements of efferent arteriolar and arterial hematocrits, was significantly elevated in the cortical nephrons, even though whole kidney filtration fraction was the same as in normal rats."
- Renal handling of sodium and water in the hypothyroid rat: Clearance and micropuncture studies (1972) showed that "superficial single nephron filtration rate was reduced proportionately to the decrease in total filtration rate in the hypothyroid rats. The data also suggest that the observed decrease in glomerular filtration rate in the hypothyroid animals is not caused by a decrease in the number of functioning nephrons and that the observed increase in sodium and water excretion is not caused by a redistribution of filtrate from juxtamedullary to superficial nephrons."
- Effect of Dietary Sodium Intake on the Intrarenal Distribution of Nephron Glomerular Filtration Rates in the Rat (1973) comes to the same conclusion: "These variations in dietary sodium intake appeared to have no detectable effect on the intrarenal SNGFR distribution."
- Abnormal relationship between sodium excretion and hypertension in spontaneously hypertensive rats (1975) likewise agrees: "... indicating that no significant intrarenal GFR redistribution occurs in SH [spontaneous hypertension] following an acute hypertonic saline load."
- The influence of tubulo-glomerular feedback on the autoregulation of filtration rate in superficial and deep glomeruli (1984) has great data and figures, except that the results were obtained from young Sprague Dawley rats, and as noted in several review articles, it appears likely that structural changes with age modify the response of SNGFR distribution.
- Redistribution of glomerular filtration and renal plasma flow in CNS-induced natriuresis (1986) shows an increase in the ratio between inner-cortical and outer- cortical SNGFR with the infusion of hypertonic sodium chloride solution into the third cerebral ventricle (Table 2 and Figure 2). I could not determine if these results were obtained from young or adult rats.
- Nephron functional heterogeneity in the postobstructive kidney (1975) showed evidence of "redistribution of nephron function after relief of chronic, rather than acute, obstruction", which may be due to severe structural damage in the chronic condition.
- Isotonic saline loading and intrarenal distribution of glomerular filtration in dogs (1972) indicates "that a redistribution of glomerular filtration towards SUP [superficial] nephrons is not responsible for the natriuresis observed in these conditions. This finding is also in agreement with the data which demonstrate that the vasodilatation occurring in the kidney following isotonic saline loading is observed throughout the whole cortex."
- Evidence for redistribution of filtrate among nephrons after beta-adrenergic stimulation and blockade (1974) is a study that does show evidence of SNGFR redistribution: "Total kidney GFR did not change after both substances, whereas superficial nephron GFR increased after propranolol by 35% and decreased after isoprenaline by 35%. A redistribution of glomerular filtration rates among nephrons therefore must have occured with a shift of the GFR to deep nephrons after isoprenaline and to superficial nephrons after propranolol."
- Détermination du taux individuel de filtration glomérulaire des néphrons accessibles et inaccessibles à la microponction (1970) demonstrates a careful refinement to Hanssen's technique of estimating SNGFRs in nephrons that are inaccessible to micropuncture. Superficial and juxta-medullary SNGFRs were measured in non- diuretic rats (29.1 and 40.1 nL/min, respectively).
- Étude chez le rat des variations du débit individuel de filtration glomérulaire des néphrons superficiels et profonds en fonction de l'apport sodé (1970) is another study by the same authors, who determined that "the increase of the GFR of the whole kidney during salt loading was due to an increase of SGFR of all the nephrons. However, the increase of SGFR was more important for the superficial ones and the ratio SGFR superficial nephrons/SGFR juxtamedullary nephrons which was 0.7 in the normal rats reached 1.0 in the salt-loaded rats, indicating that there was no more functional heterogeneity of nephrons during our salt loadings." This variation is compared to the sodium-dependent intra-renal distribution of renin that has been observed in other studies.
- Micropuncture study of superficial and juxtamedullary nephrons in the rat (1970) found juxta- medullary SNGFRs to be about double the superficial SNGFRs (60.2 and 25.6 nL/min, respectively).
- Effect of saline infusion on superficial and juxtamedullary nephrons in the rat (1971) observes that superficial nephrons appeared to participate to a greater degree than juxtamedullary nephrons in the response to acute volume expansion.
- Effect of sodium balance on intrarenal distribution of blood flow in normal man (1970) is a rare study of the intra-renal distribution of blood flow in humans, but I cannot say for sure whether it is directly relevant, since I do not know what the "most rapid and second most rapid flow compartments" are.
On a side note, I'm happy to finally cite two papers by Christian de Rouffignac, which were published in French.