Internephron heterogeneity for carbonic anhydrase-independent bicarbonate reabsorption in the rat

The glomerular basement membrane as a model system to study the bioactivity of heparan sulfate glycosaminoglycans

The glomerular basement membrane and its associated cells are critical elements in the renal ultrafiltration process. Traditionally the anionic charge associated with several carbohydrate moieties in the glomerular basement membrane are thought to form a charge selective barrier that restricts the transmembrane flux of anionic proteins across the glomerular basement membrane into the urinary space. The charge selective function, along with the size selective component of the basement membrane, serves to limit the efflux of plasma proteins from the capillary lumen. Heparan sulfate glycosaminoglycans are anionically charged carbohydrate structures attached to proteoglycan core proteins and have a role in establishing the charge selective function of the glomerular basement membrane. Although there are a large number of studies in the literature that support this concept, the results of several recent studies using molecular genetic approaches to minimize the anionic charge of the glomerular basement membrane would suggest that the role of heparan sulfate glycosaminoglycans in the glomerular capillary wall are still not yet entirely resolved, suggesting that this research area still requires new and novel exploration.

Carbonic anhydrase XIV: luminal expression suggests key role in renal acidification

BACKGROUND

Carbonic anhydrase (CA) plays a fundamental role in regulation of systemic acid-base homeostasis by facilitating urinary acidification. Four CA isozymes (CA II, IV, XII, XIV) have been identified in kidney. Until now, luminal CA IV, a GPI-anchored isozyme, was thought to mediate most bicarbonate absorption. Although CA XIV mRNA has been demonstrated in mouse and human kidney, the localization of this newly discovered CA has not been established.

METHODS

RT-PCR and Western blot analyses were used to demonstrate CA XIV mRNA and protein in extracts of cortex and medulla of mouse kidney. Polyclonal antibodies against mouse CA XIV were utilized for immunofluorescence to examine the pattern of expression of CA XIV in the nephron of both rat and mouse kidney.

RESULTS

Immunofluorescence staining showed abundant expression of CA XIV in apical plasma membranes of the S1 and S2 segments of proximal tubules, and weaker staining in the basolateral membranes. Also, strong staining was seen in the initial portion of the thin descending limb of Henle. These results show that luminal CA XIV is strongly expressed in regions of the rodent nephron that have been thought to be important in urinary acidification. Staining for CA XIV and CA IV in the same sections showed some areas of co-expression, but also some areas where each was expressed without the other.

CONCLUSIONS

Luminal CA XIV may account for a substantial fraction of the bicarbonate reabsorption previously attributed to CA IV. If so, CA XIV and CA IV may be functionally redundant.

The effects of respiratory alkalosis and acidosis on net bicarbonate flux along the rat loop of Henle in vivo

We have studied the effects of acute respiratory alkalosis (ARALK, hyperventilation) and acidosis (ARA, 8% CO2), chronic respiratory acidosis (CRA; 10% CO2 for 7-10 days), and subsequent recovery from CRA breathing air on loop of Henle (LOH) net bicarbonate flux (JHCO3) by in vivo tubule microperfusion in anesthetized rats. In ARALK blood, pH increased to 7.6, and blood bicarbonate concentration ([HCO3-]) decreased from 29 to 22 mM. Fractional urinary bicarbonate excretion (FEHCO3) increased threefold, but LOH JHCO3 was unchanged. In ARA, blood pH fell to 7.2, and blood [HCO3-] rose from 28 to 34 mM; FEHCO3 was reduced to < 0.1%, but LOH JHCO3 was unaltered. In CRA, blood pH fell to 7.2, and blood [HCO3-] increased to > 50 mM, whereas FEHCO3 decreased to < 0.1%. JHCO3 was reduced by approximately 30%. Bicarbonaturia occurred when CRA rats breathed air, yet LOH JHCO3 increased (by 30%) to normal. These results suggest that LOH JHCO3 is affected by the blood-to-tubule lumen [HCO3-] gradient and HCO3- backflux. When the usual perfusing solution at 20 nl/min was made HCO3- free, mean JHCO3 was -34.5 +/- 4.4 pmol/min compared with 210 +/- 28.1 pmol/min plus HCO3-. When a low-NaCl perfusate (to minimize net fluid absorption) containing mannitol and acetazolamide (2 x 10(-4) M, to abolish H(+)-dependent JHCO3) was used, JHCO3 was -112.8 +/- 5.6 pmol/min. Comparable values for JHCO3 at 10 nl/min were -35.9 +/- 5.8 and -72.5 +/- 8.8 pmol/min, respectively. These data indicate significant backflux of HCO3-along the LOH, which depends on the blood-to-lumen [HCO3-] gradient; in addition to any underlying changes in active acid-base transport mechanisms, HCO3- permeability and backflux are important determinants of LOH JHCO3 in vivo.

Studies on the mechanism of trimethoprim-induced hyperkalemia

We examined the effects of trimethoprim (TMP) on metabolic parameters and renal ATPases in rats after a 90 minute infusion (9.6 mg/hr/kg body wt, i.v.) and after 14 days (20 mg/kg body wt/day, i.p.). After one dose of TMP, plasma electrolytes, arterial pH and aldosterone levels were normal, but a natriuresis, bicarbonaturia, and decreased urinary potassium excretion occurred. Na-K-ATPase activity in microdissected segments from these animals was decreased by 36 +/- 0.9% in proximal convoluted tubule (PCT) (P < 0.005); decreases of 50 +/- 2.1% and 40 +/- 1.1% were seen in cortical and medullary collecting tubules (CCT and MCT), respectively (P < 0.005). Na-K-ATPase activity was unaffected in medullary thick ascending limb (MTAL). H-ATPase (in PCT and collecting duct) and H-K-ATPase (in CCT and MCT)-activities were not changed. Following chronic TMP administration, plasma potassium increased as compared to control (5.16 +/- 0.05 mEq/liter vs. 3.97 +/- 0.05 mEq/liter, P < 0.05), however, acid-base status and plasma aldosterone levels were normal. Na-K-ATPase activity was decreased by 45 +/- 2.6% in PCT (P < 0.005), 73 +/- 2.0% in CCT (P < 0.001), and 53 +/- 2.5% in MCT (P < 0.005). Na-K-ATPase, activity in MTAL and H-K-ATPase activity in CCT and MCT were unchanged. H-ATPase activity in PCT and MTAL was normal, but in the collecting tubule (CCT and MCT) it was decreased by approximately 25% (P < 0.05). TMP inhibited Na-K-ATPase activity in a dose-dependent fashion in PCT, CCT, and MCT when tubules from normal animals were incubated in vitro with the drug; TMP in vitro did not affect H-ATPase or H-K-ATPase activity. These results suggest that TMP-induced hyperkalemia may result from decreased urinary potassium excretion caused by inhibition of distal Na-K-ATPase, in the face of intact H-K-ATPase activity.

Electrophysiological identification of alpha- and beta-intercalated cells and their distribution along the rabbit distal nephron segments

By cable analysis and intracellular microelectrode impalement in the in vitro perfused renal tubule, we identified alpha- and beta-intercalated (IC) cells along the rabbit distal nephron segments, including the connecting tubule (CNT), the cortical collecting duct (CCD), and the outer medullary collecting duct in the inner stripe (OMCDi). IC cells were distinguished from collecting duct (CD) cells by a relatively low basolateral membrane potential (VB), a higher fractional apical membrane resistance, and apparent high Cl- conductances of the basolateral membrane. Two functionally different subtypes of IC cells in the CCD were identified based on different responses of VB upon reduction of the perfusate Cl- from 120 to 12 mM: the basolateral membrane of beta-IC cells was hyperpolarized, whereas that of alpha-IC cells was unchanged. This is in accord with the hypothesis that the apical membrane of beta-IC cells contains some Cl(-)-dependent entry processes, possibly a Cl-/HCO3- exchanger. Further characterization of electrical properties of both subtypes of IC cells were performed upon lowering bath or perfusate Cl- from 120 to 12 mM, and raising bath or perfusate K+ from 5 to 50 mM. A 10-fold increase in the perfusate K+ had no effect on VB in both subtypes of IC cells. Upon abrupt changes in Cl- or K+ concentration in the bath, a large or a small depolarization of the basolateral membrane, respectively, was observed in both subtypes of IC cells. The electrical properties of alpha- and beta-IC cells were similar among the distal nephron segments, but their distribution was different: in the CNT, which consists of IC cells and CNT cells, 97.3% (36/37) of IC cells were of the beta type. In the CCD, which consists of IC cells and CD cells, 79.8% (79/99) of IC cells were of the beta-type, whereas in the OMCDi 100% (19/19) were of the alpha type, suggesting that the beta type predominates in the earlier and the alpha type in the later segment.