Choline transport in collecting duct cells isolated from the rat renal inner medulla

Glycerophosphorylcholine (GPC) plays an important role in the osmoregulation of the renal inner medulla. Under hyperosmotic conditions, a striking increase in cellular GPC content is observed. In order to characterize the cellular events involved in GPC metabolism, we have studied the uptake of choline, a precursor of GPC, by freshly isolated rat inner medullary collecting duct (IMCD) cells at 300 mosmol/l. Choline uptake occurred by a single transport system with an apparent affinity (K _m) of 80 M and a maximal velocity (V _max) of 120 pmol/l cell water/min. Hemicholinium-3, ethanolamine and N,N-dimethylethanolamine were potent inhibitors, but betaine had no effect. Choline uptake was not altered by the replacement of Na⁺ with N-methylglucamine⁺, suggesting a sodium-independent process. Addition of 50 mM KCl to the incubation medium to reduce the cell membrane potential inhibited choline uptake by 19±4% after 10 min. Increasing the extracellular osmolarity to 600 or 900 mosmol/l had no effect on the kinetic parameters of choline uptake. These results suggest that choline uptake into IMCD cells occurs by a sodium-independent transport system driven by the inside negative cell membrane potential. Furthermore, the increase in the GPC content under hyperosmotic conditions is not associated with increased activity of the transport systems of biosynthetic precursors.     

Acid mucopolysa ccharides of the inner zone of the renal medulla in albino rats kept under different conditions

In albino rats kept without water reactions for acid mucopolysaccharides in the interstitial tissue of the distal part of the inner zone of the renal medulla remained highly stable in experiments carried out under conditions of higher relative atmospheric humidity.Department of Geographic Pathology, Research Institute of Human Morphology, Academy of Medical Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR A. P. Avtsyn.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 82, No. 10, pp. 1271–1274, October, 1976.     

Interstitial compliance and transcapillary fluid balance in renal hypertensive rats

Local interstitial fluid volume (IFV) and pressure (IFP) were used to estimate interstitial compliance (= delta IFV/delta IFP) in skin and skeletal muscle of normotensive (NT) and renal hypertensive rats (HT). The IFV was measured as the extravascular 51Cr-EDTA space, and IFP with micropipettes (tip diameter 2-4 microns) connected to a servocontrolled counter-pressure system. After control measurements, overhydration was induced by infusion of saline, 10% of body weight i.v. with or without venous stasis. Alternatively, dehydration was induced by peritoneal dialysis with 20% glucose or by furosemide infusion with or without 24 h fluid deprivation. Control ECV averaged 24.94 and 24.73 ml per 100 g in NT and HT, respectively (P greater than 0.05). Control PV averaged 2.81 and 3.28 ml per 100 g in NT and HT, respectively (P = 0.061), and control IFP was more positive in HT than in NT: 0.4 mmHg in skin (P less than 0.05) and 0.2 mmHg in skeletal muscle (P greater than 0.05). Dehydration changed PV significantly more in HT than in NT (P less than 0.05). The interstitial volume-pressure curve was linear in dehydration and the initial part of overhydration but gradually levelled off, and the maximal rise in IFP was 1-1.5 mmHg in skin and muscle. Interstitial compliance was calculated from the dehydration part of the volume-pressure curve and was in NT 14% per mmHg both in skin and skeletal muscle. In HT, compliance during dehydration was 10.2 and 20.7% reduction in IFV per mmHg fall in IFP in skin and muscle, respectively, not significantly different from corresponding values in NT (P greater than 0.05). We conclude that HT had unaltered ECV and a tendency to higher PV, and that interstitial compliance was not significantly different in normotensive and hypertensive rats.     

Regulation of cyclooxygenase-2 in renal medulla

AIM: Renal medulla is a major site for production and action of prostaglandins (PGs). The renal medullary functions as well as structural integrity are in part dependent on PGs under certain physiological or pathophysiological conditions. The two COX isoforms, COX-1 (constitutive form) and COX-2 (inducible form) are both abundantly expressed in renal inner medulla at basal state, raising a question of which COX isoform may mediate the known functions of PGs in the region. As in many other cell types, COX-1 expression in renal medulla is unlikely subject to robust regulation. In contrast, COX-2 expression in renal medulla is markedly stimulated by chronic salt loading, dehydration and endotoxaemia in vivo. At cellular levels, the signalling pathways responsible for the COX-2 stimulation in renal medullary cells seem to involve both the mitogen-activated protein kinases and NF-kappa B. It is likely that in response to various insults that are detrimental to renal medulla, the induction of PG synthesis may become more dependent on COX-2 than COX-1, and this phenomenon may be relevant to the cytoprotective response against the insults.     

Dynamic simulation of the renal medulla

The operation of the urinary concentrating system is intimately linked to the complex and distinctive anatomy of the renal medulla, and research into the concentrating mechanism has stimulated considerable endeavour in modelling of the process. The paper presents a dynamic model of the renal medulla which attempts to describe concentrations and axial flows in two sets of tubes, one comprising the loop of Henle and collecting duct and the other comprising the descending and ascending vasa recta, and in the interstitial space. Particular attention is given to modelling of the characteristics of the inner medulla. The model allows for the tapering of the inner medulla towards the papilla, the union of collecting ducts and the fact that nephrons and vascular loops turn back at various depths so that only a small portion of them reach the papilla. The inclusion of this detail in the model leads to a good representation of the concentrating mechanism and the model provides a good basis for the study of phenomena such as sodium reabsorption.     

Effects of some vasodilator drugs on transcapillary fluid exchange in renal cortex.

In 23 Munich-Wistar rats with surface glomeruli, the determinants of glomerular ultrafiltration and peritubular capillary uptake of proximal reabsorbate were studied before and during intra-arterial infusions of mildly vasodepressor doses of prostaglandin E1,acetylcholine, and bradykinin. For each drug single-nephron glomerular filtration rate remained unchanged from normal hydropenic values while glomerular plasma flow rate increased, resulting in declines in single-nephron filtration fraction (SNFF). Mean glomerular transcapillary hydraulic pressure difference (delta P) increased or remained unchanged on average. Declines in SNFF were accompanied by reductions in efferent arteriolar oncotic pressure (piE). Filtration pressure equilibrium, equality between pi E and delta P, obtained before but not during drug infusions. In the latter situation values for the glomerular capillary ultrafiltration coefficient were calculated and found to be significantly reduced from published control values. Despite marked falls in pi E during drug infusion, absolute proximal reabsorption was not reduced significantly, due, it is suggested, to the opposing effects of increases in efferent arteriolar plasma flow and interstitial hydraulic pressure.     

Contribution of the ventral structures of the medulla oblongata to the hemodynamics and transcapillary fluid exchange in the lungs

Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 116, N ^o 12, pp. 563–566, December, 1993     

Hydraulic and oncotic pressure measurements in inner medulla of mammalian kidney.

The vasa recta are thought to play an important role in the transfer of water andsolutes within the renal medulla. Hydraulic pressures were measured in vasa recta onthe surface of the exposed papilla in young Munich Wistar rats, and blood was collected from these microvessels for determination of total protein concentration and calculation of colloid oncotic pressure. In descending vasa recta at the base of the exposed papilla, mean hydraulic pressure was 9.2 plus or minus 0.4 (SE) mmHg and plasma protein concentration averaged 7.1 plus or minus 0.4 g/100 ml. Corresponding valuesin ascending vasa recta at the same level were 7.8 plus or minus 0.4 mmHg and 5.6 plusor minus 0.3 g/100 ml. respectively. The protein concentrations correspond to calculated oncotic pressures of 26 and 18 mmHg in descending and ascending vasa recta, respectively. We interpret these findings as evidence for net water uptake by the vasa recta in the renal inner medulla for which the driving forces are the transcapillary hydraulic and oncotic pressure differences.     

Effects of repeated deprivation of drinking water on the structure of renal medulla of rats.

Rats were exposed to repeated episodes of hydropenia for periods of 5 and 16 weeks, and structural alterations of renal medulla were investigated by light and electron microscopy. The hydropenia induced, within the inner medulla, focally severe structural changes in the tubular and endothelia and loss of interstitial cells. The findings present evidence that repeated episodes of hydropenia are capable of inducing cellular alterations of the renal medulla, particularly in the interstitial cells.     

Regulation of cyclooxygenase‐2 in renal medulla

Aim: Renal medulla is a major site for production and action of prostaglandins (PGs). The renal medullary functions as well as structural integrity are in part dependent on PGs under certain physiological or pathophysiological conditions. The two COX isoforms, COX‐1 (constitutive form) and COX‐2 (inducible form) are both abundantly expressed in renal inner medulla at basal state, raising a question of which COX isoform may mediate the known functions of PGs in the region. As in many other cell types, COX‐1 expression in renal medulla is unlikely subject to robust regulation. In contrast, COX‐2 expression in renal medulla is markedly stimulated by chronic salt loading, dehydration and endotoxaemia in vivo . At cellular levels, the signalling pathways responsible for the COX‐2 stimulation in renal medullary cells seem to involve both the mitogen‐activated protein kinases and NF‐ κ B. It is likely that in response to various insults that are detrimental to renal medulla, the induction of PG synthesis may become more dependent on COX‐2 than COX‐1, and this phenomenon may be relevant to the cytoprotective response against the insults.