Peritubular Space Research Articles

Relationship between tubular net sodium reabsorption and peritubular potassium uptake in the perfused Necturus kidney.

1. K influx from peritubular space into renal tubular cells, varphi(i) (K), was measured in doubly perfused Necturus kidneys by studying tissue uptake of (42)K added exclusively to the portal circulation. Concomitantly, net tubular Na reabsorption, varphi(n) (Na), was measured by clearance techniques. varphi(n) (Na) and varphi(i) (K) were varied widely by replacing solutions of physiological composition (controls) with solutions containing high K, low K, low Na, cyclamate instead of Cl, ouabain (10(-7)-10(-4)M) or ethacrynic acid (10(-5)-10(-4)M).2. The ratio of varphi(n) (Na) to varphi(i) (K) was found to vary with the experimental conditions, the control value of about 2 was maintained over a threefold variation in absolute Na reabsorption. This ratio increased with low K or ouabain to values near 4. With high K, ethacrynic acid, low Na or cyclamate the relationship was one or lower. Thus, net Na reabsorption can be uncoupled from peritubular K influx.3. These results can be best explained if there are two Na pumps working in parallel: pump A transporting Na (with Cl) and pump B, a Na-for-K-exchange pump. The ratio of Na efflux to K influx could approach infinity if only pump A works (if B is inhibited) and could approach one if only B works. It should vary between these limits in controls when both pumps are active, or when neither of the two pumps is completely inhibited.4. Alternatively, the experimental findings could be explained by a Na pump with a coupling ratio that varies within two extreme values, from high Na-K ratios (with Na reabsorption at, or near, control values but with very low K influx values) to low ratios (with normal K influx values but with low Na reabsorption values).

Further studies on the rectal complex of mealwormtenebrio molitor, L. (Coleoptera, Tenebrioidae)

An electron microscopical study has been made of the rectal complex. The perinephric membrane is a complicated structure which, in the posterior region, comprises an inner and an outer sheath separated by a space containing tracheolar end cells. The outer sheath is formed of a single layer of cells covered by an external basement membrane. The inner sheath is a multi-laminate structure made up of many thin, cellular layers which in places are reduced to closely apposed plasma membranes. Anteriorly the cellular layers are reduced in number, but each layer is of greater thickness; they finally terminate where the perinephric membrane is applied to the intestine. Posteriorly the inner sheath makes contact with the rectal epithelium. An earlier description identified three spaces within the rectal complex: the perirectal, subepithelial and peritubular spaces. The first two are true intercellular spaces, bounded by basement membranes, but the so-called peritubular space is occupied by necrotic cells. The inner sheath of the perinephric membrane is interrupted by the leptophragmata. Each leptophragma is bounded by a prominent electron-dense ring into which the laminae of the inner sheath are inserted. The outer sheath forms a blister over the leptophragma and is completely noncellular in this region. At the base of the blister a basement membrane covers the leptophragma itself, and the body of the leptophragma cell projects into the lumen of the tubule, with a thin layer of cytoplasm lying beneath the basement membrane. Both this layer and the cell body itself bear microvilli. The cell has a normal complement of mitochondria, but these do not invade the microvilli. In this last respect the ordinary tubule cells differ from the leptophragma cells in that most of their microvilli contain mitochondria, with connexions between the outer mitochondrial m em brane and the plasma membrane. The tubule cells have a poorly developed endoplasmic reticulum but are filled with numerous small granules; basal infoldings are restricted to those parts of the cell which face the perirectal space. The permeability of the perinephric membrane has been re-investigated and it is shown that the m em brane is more permeable to water and solutes at the anterior end, as might be expected if the inner sheath were the main barrier. Using preparations isolated in small volumes of haemolymph or other external media it has been shown that the rectal complex takes up potassium against a gradient of concentration. The lumen of the perirectal tubule is some 50 m V positive with respect to the external medium, so the uptake of potassium must be active. The leptophragma is freely permeable to chloride and this ion appears to enter the tubule passively. A model of the mechanism of the rectal complex is proposed, whose main feature is that the high osmolarity of the fluids within the rectal complex is brought about by the inward secretion of potassium chloride, unaccompanied by water, at the leptophragmata. This should result in a fall in the osmolarity of the external medium. A substantial fall has been observed on occasion, but in most experiments a fall is barely detectable. It is believed that the impermeability of the leptophragmata to water is rapidly lost in a deteriorating preparation.

Absorption of ferritin by rat kidney proximal tubule cells: Electron microscopic observations of the initial uptake phase in cells of microperfused single proximal tubules

Ferritin was injected through micropipettes into single proximal tubules of the rat kidney. After different time-intervals the injected tubules were microperfusion-fixed with glutaraldehyde and analyzed by electron microscopy. Five minutes after start of ferritin perfusion most of the absorbed protein was located in small and large apical vacuoles. No ferritin was observed in the cytoplasm outside the vacuoles. In the tubule lumen ferritin molecules were associated with the outer electron dense coating of the apical cell membrane invaginations. Sixteen to 20 minutes after start of perfusion most small apical vacuoles were devoid of ferritin. The observations indicated that ferritin was transported to the large apical vacuoles via small apical vacuoles, which represented pinched off plasma membrane invaginations. There was no indication that ferritin crossed the wall of the proximal tubule by passing between the cells or that the ferritin-containing vacuoles emptied into the peritubular space.

Absorption of I 125-labeled homologous albumin by rat kidney proximal tubule cells : A study of microperfused single proximal tubules by electron microscopic autoradiography and histochemistry

Small amounts of I125-labeled rat albumin were injected through micropipettes into single proximal tubules of the rat kidney. After different time-intervals the injected tubules were microperfusion-fixed with glutaraldehyde and the tissue analyzed by electron microscopic autoradiography. No ultrastructural alterations were observed in the proximal tubule cells during the absorption of the labeled protein. Six to 10 minutes after start of I125-albumin perfusion most of the absorbed protein was located in the large apical vacuoles in the proximal tubule cells but some label was also associated with small apical vacuoles. The observations suggest that the labeled albumin was transferred to the large apical vacuoles via small apical vacuoles which most likely represented pinched-off apical cell membrane invaginations. After 30 minutes most of the label was present in cytoplasmic bodies, which were limited by a triple-layered membrane about 90 Å in thickness and had an electron-dense content, and which were located in the middle or apical regions of the cells. After 60 minutes the label was confined to similar cytoplasmic bodies located in all regions of the cells. Combined electron microscopic autoradiography and histochemistry demonstrated that the labeled cytoplasmic bodies were acid phosphatase positive. The latter observation indicates that the cytoplasmic bodies, in which the I125-albumin was located, were lysosomes. It is suggested that at least some of the absorbed albumin was degraded in these lysosomes. There was no autoradiographic or morphologic evidence that the labeled albumin crossed the wall of the proximal tubule by passing between the cells or that vacuoles or cytoplasmic bodies containing absorbed albumin emptied into the peritubular space.

Urea enhancement of water reabsorption in the renal medulla.

In the dog, osmotic diuresis produced by the infusion of urea results in a higher rate of abstraction of solute-free water, at any given osmolal clearance, than does mannitol or sodium chloride diuresis. It is proposed that the ratio of urea to nonpermeating solute within the fluid entering the collecting duct system will influence the degree to which the urine can be concentrated and to which solute-free water can be abstracted from an isotonic precursor fluid. Urea, as a result of its ability to permeate the collecting duct epithelium, is relatively ineffective in opposing the osmotic force resulting from the presence of a hypertonic sodium chloride solution in the peritubular space. The osmotic movement of water out of the collecting duct is consequently limited primarily by the concentration of nonpermeating solute within the tubular fluid. Since during the infusion of urea the relative concentration of nonpermeating solute within the tubular lumen is reduced, the potential for water abstraction from this fluid is accordingly enhanced.