Hyaluronan Production by Renomedullary Interstitial Cells: Influence of Endothelin, Angiotensin II and Vasopressin.

The content of hyaluronan (HA) in the interstitium of the renal medulla changes in relation to body hydration status. We investigated if hormones of central importance for body fluid homeostasis affect HA production by renomedullary interstitial cells in culture (RMICs). Simultaneous treatment with vasopressin and angiotensin II (Ang II) reduced HA by 69%. No change occurred in the mRNA expressions of hyaluronan synthase 2 (HAS2) or hyaluronidases (Hyals), while Hyal activity in the supernatant increased by 67% and CD44 expression reduced by 42%. The autocoid endothelin (ET-1) at low concentrations (10-10 and 10-8 M) increased HA 3-fold. On the contrary, at a high concentration (10-6 M) ET-1 reduced HA by 47%. The ET-A receptor antagonist BQ123 not only reversed the reducing effect of high ET-1 on HA, but elevated it to the same level as low concentration ET-1, suggesting separate regulating roles for ET-A and ET-B receptors. This was corroborated by the addition of ET-B receptor antagonist BQ788 to low concentration ET-1, which abolished the HA increase. HAS2 and Hyal2 mRNA did not alter, while Hyal1 mRNA was increased at all ET-1 concentrations tested. Hyal activity was elevated the most by high ET-1 concentration, and blockade of ET-A receptors by BQ123 prevented about 30% of this response. The present study demonstrates an important regulatory influence of hormones involved in body fluid balance on HA handling by RMICs, thereby supporting the concept of a dynamic involvement of interstitial HA in renal fluid handling.

Inhibition of mTOR activity in diabetes mellitus reduces proteinuria but not renal accumulation of hyaluronan.

OBJECTIVES: Accumulation of extracellular matrix (ECM) components is an early sign of diabetic nephropathy. Also the glycosaminoglycan hyaluronan (HA) is elevated in the renal interstitium during experimental diabetes. The mammalian target of rapamycin (mTOR) pathway participates in the signaling of hyperglycemia-induced ECM accumulation in the kidney, but this has not yet been investigated for HA. We hypothesized that interstitial HA accumulation during diabetes may involve mTOR activation. METHODS: Diabetic rats (6 weeks post-streptozotocin (STZ)) were treated with rapamycin to inhibit mTOR or vehicle for 2 additional weeks. Kidney function (glomerular filtration rate, renal blood flow, urine output) and regional renal HA content were thereafter analyzed. The ability of the animals to respond to desmopressin was also tested. RESULTS: Diabetic animals displayed hyperglycemia, proteinuria, hyperfiltration, renal hypertrophy, increased diuresis with reduced urine osmolality, and reduced weight gain. Cortical and outer medullary HA was elevated in diabetic rats. Urine hyaluronidase activity was almost doubled in diabetic rats compared with controls. The ability to respond to desmopressin was absent in diabetic rats. Renal blood flow and arterial blood pressure were unaffected by the diabetic state. In diabetic rats treated with rapamycin the proteinuria was reduced by 32%, while all other parameters were unaffected. CONCLUSION: Regional renal accumulation of the ECM component HA is not sensitive to mTOR inhibition by rapamycin, while proteinuria is reduced in established STZ-induced diabetes. Whether the diabetes-induced renal accumulation of HA occurs through different pathways than other ECM components, or is irreversible after being established, remains to be shown.

Inhibition of hyaluronan synthesis in rats reduces renal ability to excrete fluid and electrolytes during acute hydration.

BACKGROUND: Hyaluronan (HA) is the dominant glycosaminoglycan in the renomedullary interstitium. Renomedullary HA has been implicated in tubular fluid handling due to its water-attracting properties and the changes occurring in parallel to acute variations in the body hydration status. METHODS: HA production was inhibited by 4-methylumbelliferone (4-MU in drinking water for 5 days, 1.45 ± 0.07 g/day/kg body weight) in rats prior to hydration. RESULTS: Following hypotonic hydration for 135 min in control animals, diuresis and osmotic excretion increased while sodium excretion and glomerular filtration rate (GFR) remained unchanged. The medullary and cortical HA contents were 7.85 ± 1.29 ng/mg protein and 0.08 ± 0.01 ng/mg protein, respectively. Medullary HA content after 4-MU was 38% of that in controls (2.98 ± 0.95 ng/g protein, p < 0.05), while the low cortical levels were unaffected. Baseline urine flow was not different from that in controls. The diuretic response to hydration was, however, only 51% of that in controls (157 ± 36 versus 306 ± 54 µl/g kidney weight/135 min, p < 0.05) and the osmolar excretion only 47% of that in controls (174 ± 47 versus 374 ± 41 µOsm/g kidney weight/135 min, p < 0.05). Sodium excretion, GFR, and arterial blood pressure were similar to that in control rats and unaltered during hydration. CONCLUSIONS: Reduction of renomedullary interstitial HA using 4-MU reduces the ability of the kidney to respond appropriately upon acute hydration. The results strengthen the concept of renomedullary HA as a modulator of tubular fluid handling by changing the physicochemical properties of the interstitial space.

Renal interstitial hyaluronan: functional aspects during normal and pathological conditions.

The glycosaminoglycan (GAG) hyaluronan (HA) is recognized as an important structural component of the extracellular matrix, but it also interacts with cells during embryonic development, wound healing, inflammation, and cancer; i.e., important features in normal and pathological conditions. The specific physicochemical properties of HA enable a unique hydration capacity, and in the last decade it was revealed that in the interstitium of the renal medulla, where the HA content is very high, it changes rapidly depending on the body hydration status while the HA content of the cortex remains unchanged at very low amounts. The kidney, which regulates fluid balance, uses HA dynamically for the regulation of whole body fluid homeostasis. Renomedullary HA elevation occurs in response to hydration and during dehydration the opposite occurs. The HA-induced alterations in the physicochemical characteristics of the interstitial space affects fluid flux; i.e., reabsorption. Antidiuretic hormone, nitric oxide, angiotensin II, and prostaglandins are classical hormones/compounds involved in renal fluid handling and are important regulators of HA turnover during variations in hydration status. One major producer of HA in the kidney is the renomedullary interstitial cell, which displays receptors and/or synthesis enzymes for the hormones mentioned above. During several kidney disease states, such as ischemia-reperfusion injury, tubulointerstitial inflammation, renal transplant rejection, diabetes, and kidney stone formation, HA is upregulated, which contributes to an abnormal phenotype. In these situations, cytokines and other growth factors are important stimulators. The immunosuppressant agent cyclosporine A is nephrotoxic and induces HA accumulation, which could be involved in graft rejection and edema formation. The use of hyaluronidase to reduce pathologically overexpressed levels of tissue HA is a potential therapeutic tool since diuretics are less efficient in removing water bound to HA in the interstitium. Although the majority of data describing the role of HA originate from animal and cell studies, the available data from humans demonstrate that an upregulation of HA also occurs in diabetic kidneys, in transplant-rejected kidneys, and during acute tubular necrosis. This review summarizes the current knowledge regarding interstitial HA in the role of regulating kidney function during normal and pathological conditions. It encompasses mechanistic insights into the background of the heterogeneous intrarenal distribution of HA; i.e., late nephrogenesis, its regulation during variations in hydration status, and its involvement during several pathological conditions. Changes in hyaluronan synthases, hyaluronidases, and binding receptor expression are discussed in parallel.

Effects of proinsulin C-peptide on oxygen transport, uptake and utilization in insulinopenic diabetic subjects--a review.

Exogenous C-peptide administration has beneficial effects in many of the tissues commonly affected by diabetic complications. Diabetes-induced circulatory impairments such as decreased blood flow are prevented by C-peptide, possibly via Ca2+-mediated effects on nitric oxide release. C-peptide also improves diabetes-induced erythrocyte deformability, which likely improves oxygen availability and uptake in affected tissues. Furthermore, C-peptide prevents diabetic neuropathy via improvements of endoneural blood flow and by preventing axonal swelling. In the kidney, C-peptide normalizes the diabetes-induced increase in oxygen consumption via inhibition of the Na+/K+-ATPase. Surprisingly, C-peptide has also been shown to prevent complications in patients with type II diabetes. Taken together, these results may indicate that C-peptide treatment has the potential to reduce the prevalence of diabetic complications. In this paper, the current knowledge regarding these beneficial effects of C-peptide administered to diabetic subjects will be reviewed briefly.

C-peptide normalizes glomerular filtration rate in hyperfiltrating conscious diabetic rats.

Tubular electrolyte transport accounts for a major part of the oxygen consumed by the normal kidney. We have previously reported a close association between diabetes and increased oxygen usage, partly due to increased tubular electrolyte transport secondary to glomerular hyperfiltration during the early onset of diabetes. Several studies have shown that acute administration of C-peptide to diabetic rats with glomerular hyperfiltration results in normalized glomerular filtration rate (GFR). In this study, we validated a novel method for precise and repetitive GFR measurements in conscious rats and used C-peptide injection in diabetic rats for evaluation. First, GFR was determined in normoglycemic control rats before and after C-peptide administration. Thereafter, all rats were made diabetic by an i.v. streptozotocin injection. Fourteen days later, GFR was again determined before and after C-peptide administration. GFR was estimated from plasma clearance curves using a single bolus injection of FITC-inulin, followed by serial blood sampling over 155 min. FITC-inulin clearance was calculated using non-compartmental pharmacokinetic data analysis. Baseline GFR in normoglycemic controls was 2.10 +/- 0.18 ml/min, and was unaffected by C-peptide (2.23 +/- 0.14 ml/min). Diabetic rats had elevated GFR (3.06 +/- .034 ml/min), which was normalized by C-peptide (2.35 +/- 0.30 ml/min). In conclusion, the used method for estimation of GFR in conscious animals result in values that are in good agreement with those obtained from traditional GFR measurements on anaesthetized rats. However, multiple measurements from the same conscious subject can be obtained using this method. Furthermore, as previously shown on anaesthetized rats, C-peptide also normalizes GFR in hyperfiltrating conscious diabetic rats.