Adaptive changes of juxtamedullary glomerular filtration in the remnant kidney

Novel Mechanistic PBPK Model to Predict Renal Clearance in Varying Stages of CKD by Incorporating Tubular Adaptation and Dynamic Passive Reabsorption

Chronic kidney disease (CKD) has significant effects on renal clearance (CLr ) of drugs. Physiologically-based pharmacokinetic (PBPK) models have been used to predict CKD effects on transporter-mediated renal active secretion and CLr for hydrophilic nonpermeable compounds. However, no studies have shown systematic PBPK modeling of renal passive reabsorption or CLr for hydrophobic permeable drugs in CKD. The goal of this study was to expand our previously developed and verified mechanistic kidney model to develop a universal model to predict changes in CLr in CKD for permeable and nonpermeable drugs that accounts for the dramatic nonlinear effect of CKD on renal passive reabsorption of permeable drugs. The developed model incorporates physiologically-based tubular changes of reduced water reabsorption/increased tubular flow rate per remaining functional nephron in CKD. The final adaptive kidney model successfully (absolute fold error (AFE) all < 2) predicted renal passive reabsorption and CLr for 20 permeable and nonpermeable test compounds across the stages of CKD. In contrast, use of proportional glomerular filtration rate reduction approach without addressing tubular adaptation processes in CKD to predict CLr generated unacceptable CLr predictions (AFE = 2.61-7.35) for permeable compounds in severe CKD. Finally, the adaptive kidney model accurately predicted CLr of para-amino-hippuric acid and memantine, two secreted compounds, in CKD, suggesting successful integration of active secretion into the model, along with passive reabsorption. In conclusion, the developed adaptive kidney model enables mechanistic predictions of in vivo CLr through CKD progression without any empirical scaling factors and can be used for CLr predictions prior to assessment of drug disposition in renal impairment.

Fluid shear stress induces renal epithelial gene expression through polycystin-2-dependent trafficking of extracellular regulated kinase

BACKGROUND

The cilium and cilial proteins have emerged as principal mechanosensors of renal epithelial cells responsible for translating mechanical forces into intracellular signals. Polycystin-2 (PC-2), a cilial protein, regulates flow/shear-induced changes in intracellular Ca(2+) ([Ca(2+)](i)) and recently has been implicated in the regulation of mitogen-activated protein (MAP) kinases. We hypothesize that fluid shear stress (FSS) activates PC-2 which regulates MAP kinase and, in turn, induces MAP kinase-dependent gene expression, specifically, monocyte chemoattractant protein-1 (MCP-1).

METHODS

To test this, PC-2 expression was constitutively reduced in a murine inner medullary collecting duct (IMCD3) cell line, and the expression of FSS-induced MCP-1 expression and MAP kinase signaling compared between the parental (PC-2-expressing) and PC-2-deficient IMCD3 cells.

RESULTS

FSS induces MAP kinase signaling and downstream MCP-1 mRNA expression in wild-type IMCD3 cells, while inhibitors of MAP kinase prevented the FSS-induced MCP-1 mRNA response. In contradistinction, FSS did not induce MCP-1 mRNA expression in PC-2-deficient cells, but did increase activation of the upstream MAP kinases. Wild-type cells exposed to FSS augmented the nuclear abundance of activated MAP kinase while PC-2-deficient cells did not.

CONCLUSIONS

PC-2 regulates FSS-induced MAP kinase trafficking into the nucleus of CD cells.

Intratubular hydrodynamic forces influence tubulointerstitial fibrosis in the kidney

PURPOSE OF REVIEW

Renal epithelial cells respond to mechanical stimuli with immediate transduction events (e.g. activation of ion channels), intermediate biological responses (e.g. changes in gene expression), and long-term cellular adaptation (e.g. protein expression). Progressive renal disease is characterized by disturbed glomerular hydrodynamics that contributes to glomerulosclerosis, but how intratubular biomechanical forces contribute to tubulointerstital inflammation and fibrosis is poorly understood.

RECENT FINDINGS

In-vivo and in-vitro models of obstructive uropathy demonstrate that tubular stretch induces robust expression of transforming growth factor beta-1, activation of tubular apoptosis, and induction of nuclear factor-kappaB signaling, which contribute to the inflammatory and fibrotic milieu. Nonobstructive structural kidney diseases associated with nephron loss follow a course characterized by compensatory increases of single nephron glomerular filtration rate and tubular flow rate. Resulting increases in tubular fluid shear stress reduce tissue-plasminogen activator and urokinase enzymatic activity, which diminishes breakdown of extracellular matrix. In models of high dietary Na intake, which increases tubular flow, urinary transforming growth factor beta-1 concentrations and renal mitogen-activated protein kinase activity are increased.

SUMMARY

In conclusion, intratubular biomechanical forces, stretch, and fluid shear stress generate changes in intracellular signaling and gene expression that contribute to the pathobiology of obstructive and nonobstructive kidney disease.

Comparison of residual glomerular function in experimental papillary necrosis and renal infarction

The purpose of this study was to examine the course of residual glomerular function in two models of renal ablation both with equivalent degrees of nephron reduction (1 1/3 nephrectomy). The remnant group underwent right nephrectomy and infarction of one third of the left kidney, yielding constant proportions of remnant deep and superficial nephrons, while the bromoethylamine (BEA) group underwent right nephrectomy and chemical ablation by BEA (50 mg intravenously [IV]) of the deep nephrons of the left kidney, leaving superficial nephrons. Ten weeks following ablation, greater whole kidney permselective defects indicative of worse injury occurred in the remnant group compared with the BEA group, as manifested by increased excretion of total protein (.65 +/- .14 v .23 +/- .04 g/d [65 +/- 14 v 23 +/- 4 mg/24 h]; P less than 0.05) and greater fractional clearance of albumin (71.8 +/- 42.1 v 9.0 +/- 3.4 X 10(-4); P less than 0.05) and IgG (17.9 +/- 10.2 v 3.6 +/- 1.2 x 10(-4); P less than 0.05). Glomerular filtration rate (GFR) and renal plasma flow (RPF) were similar in the ablated groups, but lower than a sham-operated control group. Superficial single nephron GFR (SNGFR) and mean glomerular capillary pressure (PGC) were elevated similarly in the ablated groups compared with the control group. Based on the differences in the two models, the site of the increased proteinuria in the remnant group could be the deep nephrons and/or the nephrons adjacent to the scar.

Relation of glomerular injury to preglomerular resistance in experimental hypertension

A semiquantitative glomerular damage index (GDI) was determined for overall (O), superficial (S), and juxtamedullary (JM) glomeruli in four models of experimental hypertension in the rat to assess the severity and distribution of injury in light of present day knowledge of glomerular hemodynamics. After a four week period of similar hypertension, comparison of Group 1 (renal ablation) with Group 2 (aortic ligature) revealed OGDIs of 0.420 +/- 0.064 (SEM) vs. 0.062 +/- 0.019, P less than 0.0001, SGDIs of 0.250 +/- 0.071 vs. 0.035 +/- 0.007, P less than 0.0089, and JGDIs of 0.455 +/- 0.071 vs. 0.155 +/- 0.036, P less than 0.002. Within Group 1 the SGDI and JMGDI were not significantly different but within Group 2 the SGDI was less (P less than 0.005) than the JMGDI. Arterial/arteriolar damage was comparable in both groups. After an eight week period of similar hypertension, comparison of Group 3 (deoxycorticosterone-saline) with Group 4 (stroke-prone spontaneously hypertensive rats) showed OGDIs of 0.301 +/- 0.065 vs. 0.128 +/- 0.023, P less than 0.025, SGDIs of 0.289 +/- 0.096 vs. 0.072 +/- 0.015, P less than 0.044, and JMGDIs of 0.394 +/- 0.083 vs. 0.307 +/- 0.062, NS. Within Group 3 the SGDI and JMGDI were not significantly different, but within Group 4 the SGDI was less (P less than 0.002) than the JMGDI. Vascular damage in the two groups was comparable. Taking into account known physiologic data, the findings are consistent with the idea that increased preglomerular resistance is protective of glomeruli, whereas decreased resistance with increased pressure and/or flow is injurious.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of thromboxane synthesis ameliorates the progressive kidney disease of rats with subtotal renal ablation

Ablation of greater than 70% of renal mass in the rat results in hypertension, proteinuria, and glomerular sclerosis of the remnant kidney. Rats with a remnant kidney have increased excretion of thromboxane in the urine when compared with normal rats. Chronic oral administration of OKY 1581, an inhibitor of thromboxane synthesis, in rats with a remnant kidney increases renal blood flow and glomerular filtration rate (GFR), decreases protein and thromboxane excretion in the urine, lowers blood pressure and cardiac index, and improves renal histology. The degree of hypertrophy of the remnant kidney was unaffected by administration of OKY 1581. Calculated values for single nephron plasma flow and GFR were significantly greater in rats with remnant kidneys given OKY 1581 than in rats given saline. Acute i.v. administration of OKY 1581 increased renal plasma flow and GFR in rats with a remnant kidney but not in normal rats or rats with a remnant kidney previously treated with acetylsalicyclic acid. OKY 1581 markedly inhibited platelet aggregation. We suggest that in this model of renal disease platelet aggregation and intraglomerular thrombosis play a key role in the development of glomerulosclerosis. Inhibition of platelet aggregation prevents development of glomerulosclerosis, hypertension, and cardiac hypertrophy. We suggest that hyperperfusion and hyperfiltration per se occurring in remnant glomeruli are not directly responsible for the development of glomerulosclerosis.

Role of heparin as a protective agent following reduction of renal mass

To determine the mechanism by which heparin affords protection from damage to the remnant kidney following reduction of renal mass, either whole heparin or a low molecular weight fraction of heparin was administered to Munich-Wistar rats which had had 90% ablation of renal mass. The low molecular weight fraction differed from the whole heparin in that it had greatly decreased anticoagulant activity as measured by the United States Pharmacopoeia (USP) assay, and increased antifactor Xa activity. The morphological alterations were examined and various physiological features were studied. The administration of whole heparin to rats with reduction of renal mass resulted in prolonged clotting time, reduced BP elevation, elimination of albuminuria, and lessening of renal damage. No such differences were observed in the rats receiving the fraction of heparin or the control group to which no therapy was given. The inability of the fraction of heparin to prolong clotting time suggested that it was unable to inactivate the serine proteases of the coagulation cascade. It is probable that the protective effect of whole heparin in this model is due to its ability to inhibit serine proteases such as kallikrein and components of the coagulation cascade resulting in a beneficial alteration in glomerular hemodynamics, lowering of systemic BP, and anticoagulation.

Effect of reduced renal mass on ammonium handling and net acid formation by the superficial and juxtamedullary nephron of the rat. Evidence for impaired reentrapment rather than decreased production of ammonium in the acidosis of uremia

Papillary and surface micropuncture were used to study the handling of ammonium and the formation of net acid by surface nephrons, deep nephrons, and the terminal segment of collecting duct (CD) after renal mass was reduced by two-thirds. Net acid excretion by the remnant kidney (RK) was significantly reduced, averaging 794+/-81 neq/min (SE) compared with 1,220+/-105 neq/min after sham operation (P < 0.001), due to a decrease in ammonium excretion (494+/-54 vs. 871+/-79 nmol/min in controls, P < 0.001). Urinary pH and titratable acid excretion were not different in the two groups of animals. After RK formation, ammonium delivery to the end of the proximal tubule increased nearly threefold and averaged 66.2+/-5.6 compared with 18.4+/-2.9 pmol/min in controls, (P < 0.001). This greater delivery of ammonium was primarily due to renal tubule entry rather than to changes in the filtered load and was only partially related to the differences in flow rate. Ammonium processing by deep nephrons was profoundly affected by a reduction in renal mass. Although absolute delivery of ammonium was greater to the bend of Henle's loop (BHL), the difference could be accounted for on the basis of an increase in nephron size. Thus, fractional delivery (FD(NH+4)) to this site was not different for the two groups of animals, averaging 1,567+/-180% in controls and 1,400+/-181% in the group with the RK. Hydrogen secretion in the proximal segments of deep and surface nephrons did not increase in proportion to the decrease in renal mass and as a consequence bicarbonate delivery to the end of the proximal tubule of surface nephrons and to the BHL of deep nephrons was increased. When renal mass was reduced FD(NH+4) to the base of the terminal CD doubled but did not change by the tip. In both groups FD(NH+4) to the base of the CD was greater than to the end of the distal tubule. However, the increase was the same. On the other hand, the increase in the net acid index between the end of the distal tubule and the base of the CD was profoundly greater in rats with an RK. This difference was primarily due to bicarbonate reabsorption rather than enhanced ammonium reentry. Indeed, >400% of the fractional ammonium delivered to the end of the proximal tubule was lost from the tubule fluid. The data suggest that the decrease in acid excretion by the RK is due to two factors. First, hydrogen secretion in the proximal segments of both nephron populations fails to increase in the proportion to the reduction in renal mass. Second, a reduced reentrapment of ammonia, rather than its impaired production, causes ammonium excretion to decrease.

Sodium homeostasis in dogs with chronic renal insufficiency

Volume homeostasis in the fasting rate and 24-hr sodium balance are maintained in chronic renal insufficiency as a result of adaptations in the residual nephrons. This study evaluates the limitations to these adaptations and the dynamics of sodium excretion (UNaV) after an acute challenge with 100 mEq of sodium chloride in normal dogs (GFR 50 ml/min) and in dogs with one remnant kidney and moderate chronic renal insufficiency (GFR 15 ml/min). When food was administered with the sodium challenge, no or minimal changes in serum protein and hematocrit values occurred, and the natriuretic responses were small and equivalent in normal and remnant dogs. On the other hand, when the sodium challenge was given without food, the natriuretic response was large in normal dogs and markedly blunted in remnant. Within 5 hr of the sodium challenge, the various groups of normal dogs excreted 40 to 63% of the sodium load, but the remnant animals eliminated only 12 to 22% (P less than 0.001). The blunted natriuresis in remnant dogs was associated with a prolonged hemodilution of circulating proteins, indicating a longer lasting expansion of the intravascular volume. The blunted response was independent of sodium diet, of the administration route (p.o. vs. i.v.) or strength (isotonic vs. hypertonic) of the sodium load, and appears to result from an inability of the remnant kidney to rapidly excrete a sodium load. Thus, administration of sodium to dogs with chronic renal insufficiency leads to prolonged sodium retention, prolonged extracellular fluid (ECF) volume expansion, and requires a prolonged excretory cycle to restore 24-hr balance.