Mechanisms of intracellular pH regulation in the hamster inner medullary collecting duct perfused in vitro

Effects of [Ca2+]i and pH on epithelial Na+ channel activity of cultured mouse cortical collecting ducts

[Ca2+]i and pH have been demonstrated to affect Na+ transport in epithelium mediated via the apical epithelial Na+ channel (ENaC). However, it still remains unclear whether the effects of [Ca2+]i and intracellular pH (pHi) on ENaC activity are direct. In this study, inside-out recording was employed to clarify the effects of pH(i) and [Ca2+]i on ENaC activity. We found that elevation of [Ca2+]i induced a significant inhibition of ENaC open probability without altering channel conductance. The inhibitory effect was due to a direct interaction between Ca2+ and ENaC, and is dependent on [Ca2+]i. pHi also directly regulated ENaC open probability. Lower pHi (<7.0) reduced the ENaC open probability as shown in shorter opening time, and higher pH(i) (>7.0) enhanced the ENaC open probability as shown in augmented opening time. pHi did not cause any alteration in channel conductance. The effects of pHi on ENaC open probability could be summarized as an S-shaped curve around pH 7.2.

Mechanism of iodide transport in the rabbit cortical collecting duct

BACKGROUND

Pendrin, an anion exchanger known to participate in iodide transport in the apical membrane of follicular cells of the thyroid gland, has recently been shown to exist in the apical membrane of the beta- and gamma-intercalated (beta/gamma-IC) cells of the cortical collecting duct (CCD). We examined mechanisms of iodide transport in the CCD.

METHODS

Rabbit CCD was perfused in vitro, and lumen-to-bath flux coefficients for both (125)I(-) (K(I (lb))) and (36)Cl(-) (K(Cl (lb))) were measured simultaneously. The intracellular pH (pHi) of beta/gamma-IC cells in the perfused CCD was measured by microscopic fluorometory, by loading 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein tetraacetoxy methylester (BCECF-AM), a fluorescent marker for pHi. The effects on pHi of the replacement of NaCl with Na cyclamate, NaI, or NaBr in the lumen or bath were observed.

RESULTS

K(I (lb)) was comparable to or slightly higher than K(Cl (lb)). Both iodide and chloride in the lumen caused self- and cross-inhibitions to both fluxes. The addition of 5-nitro-2-(-3-phenylpropylamino)-benzoate (NPPB), a Cl(-) channel inhibitor, to the bath significantly reduced K(Cl (lb)), but not K(I (lb)). Replacement of luminal fluid NaCl with Na cyclamate, NaI, or NaBr caused alkalization of pHi, no change in pHi, and slight acidification of pHi, respectively. Replacement of bath NaCl with Na cyclamate, NaI, or NaBr caused alkalization, alkalization, and acidification of pHi, respectively. Luminal NaI prevented the acidification of pHi caused by bath Na cyclamate.

CONCLUSIONS

The data are consistent with the model that iodide is transported via the Cl(-)/HCO(3) (-) exchanger in the apical membrane of beta/gamma-IC cells and exits the basolateral membrane via an electroneutral transporter that is distinct from the Cl(-) channel. We could not, however, identify which type of beta/gamma-IC cell was mainly responsible.

Polarized distribution of Na+/H+ exchanger isoforms in rabbit collecting duct cells

The present study describes two Na+/H+ exchanger (NHE) isoforms in an immortalized rabbit renal cortical collecting tubule cell line (RC.SV3). Na+/H+ exchange activity was assayed using fluorescence measurements of intracellular pH (pHi) in monolayers mounted in a cuvette containing two fluid compartments, making it possible to independently measure Na+/H+ exchange activity on either the apical or basolateral surface. RC.SV3 monolayers express Na+/H+ exchange activities in both the apical and basolateral membrane domains. The two exchangers have half-saturation constants (Km) for external sodium and sensitivities to dimethylamiloride, to HOE-694 and to cimetidine and clonidine consistant with the NHE-1 isoform on the basolateral cell surface and the NHE-2 isoform on the apical surface. Protein kinase A inhibition of basolateral exchanger activity was significantly higher than that of the apical exchanger. Protein kinase C significantly stimulated both exchangers equally. RT-PCR analysis found RNA for only NHE-1 and NHE-2, and immunofluorescence with an antibody against NHE-1 demonstrated a basolateral location for this isoform. The results suggest that RC.SV3 cells have two Na+/H+ exchange activities separated spatially to the two cellular membranes, with the NHE-1 and the NHE-2 isoforms located on the basolateral and the apical membranes, respectively.

Characterization of plasma membrane Na+/H+ exchange in eel (Anguilla anguilla) intestinal epithelial cells

The ability of eel intestinal epithelial cells to recover from an acute acid load was analysed using the fluorescent dye 2',7'-bis-carboxy-ethyl-5,6-carboxyfluorescein (BCECF) and cell suspensions. Under these experimental conditions (bicarbonate-free solutions) the resting pHi in cells prepared from sea-water (7.52 +/- 0.031) and fresh-water (7.50 +/- 0.094) adapted animals proved to be similar. The recovery rate (following an acid load) increases by increasing the Na ion concentration in the extracellular medium. This pHi recovery is competitively inhibited by the specific inhibitor dimethylamiloride (DMA) with a low Ki in sea- (1.2 microM) as well as in fresh-water (1.3 microM) adapted animals, indicating the presence of a specific Na/H exchange activity in these cells. Using basolateral membrane vesicles it could be demonstrated that this activity is located on the basolateral side of the enterocyte membrane. The kinetic parameters (Kapp and Jmax) of this exchanger are similar in fresh-water and sea-water adapted animals suggesting that no salinity adaptation occurs, thus excluding the involvement of the antiporter in the osmoregulatory processes. These results are in agreement with the presence in the plasma membrane of the eel enterocytes of a Na/H-1 (housekeeper) form of the antiporter.

Localization of the Na+/H+ exchanger isoform NHE-3 in rabbit and canine kidney

The distribution and subcellular localization of Na+/H+ exchanger isoform NHE-3 was studied in rabbit and canine kidney using polyclonal antibodies to an NHE-3 fusion protein. Western blot analyses were performed against microsomal, brush-border, and basolateral membranes isolated from rabbit kidney cortex, outer medulla, and inner medulla. Immunoblots indicated that NHE-3 antibody recognized a strong band with 95-100 kDa molecular mass in cortical microsomes. Subcellular localization studies showed that NHE-3 was expressed in brush-border membranes of kidney cortex. Expression of NHE-3 in the medullary regions was studied by immunoblot analysis of NHE-3 antibody against the microsomal membranes from the outer and inner medulla. NHE-3 antibody specifically labelled a 95-100 kDa protein in outer but not inner medulla. Subcellular localization studies demonstrated that NHE-3 is localized to the brush-border membranes of the outer medulla. Immunoblot analysis against brush-border membranes from canine kidney cortex and outer medulla demonstrated the presence of an 83-90 kDa protein. The above experiments suggest that NHE-3 in rabbit kidney is a 95-100 kDa protein and is expressed in brush-border membranes of the cortex and outer medulla. The canine kidney NHE-3 is a 83-90 kDa protein and is expressed in brush-border membranes of the cortex and outer medulla. Based on its subcellular localization, we conclude that NHE-3 may be involved in vectorial Na+ and HCO3- transport and pHo regulation.

Changes in pH affect Cl- removal and AVP action in collecting tubules

We examined what mechanisms are involved in the alteration by chloride (Cl-) removal of arginine vasopressin (AVP)-induced cellular cAMP production, and cellular free calcium ([Ca2+]i) mobilization in rat renal papillary collecting tubule cells in culture, using two buffer systems: bicarbonate and non-bicarbonate buffers. The first study was performed in the bicarbonate-supplemented buffer. Removal of Cl-, which was replaced by methylsulfonate or gluconate, increased cellular pH (pHi) from 7.19 to 7.26. AVP increased cellular cAMP production in a dose-dependent manner; 1 x 10(-9) and 1 x 10(-7) M AVP-induced increases in cellular cAMP production were significantly enhanced by the Cl- removal. Also, 1 x 10(-7) M AVP-mobilized [Ca2+]i was augmented by the Cl- removal (181.3 vs. 224.5 nM, P < 0.05). The second study was carried out with the Krebs-Ringer buffered saline (KRB). Removal of Cl- lowered pHi from 7.20 to 7.09. AVP-induced increases in cellular cAMP production were significantly reduced in the Cl(-)-free KRB compared to those in the KRB. The reduction was obtained with KRB containing less than 25 mM Cl-. Similar results were obtained with 2 x 10(-8) M forskolin, a diterpene activator of adenylate cyclase. 1 x 10(-7) M AVP-mobilized [Ca2+]i was also diminished by the Cl(-)-free KRB. These results indicate that Cl- depletion affects the cellular response to AVP mediated via the changes in pHi in renal papillary collecting tubule cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Transcellular sodium transport and basolateral rubidium uptake in the isolated perfused cortical collecting duct

The relation between transcellular Na+ absorption, intracellular Na+ concentration and Na+/K(+)-ATPase activity (the last estimated by the rubidium uptake across the basolateral cell membrane) was examined in the different cell types of the rabbit cortical collecting duct (CCD). Experiments were performed on isolated perfused CCD in which Na+ absorption was varied by perfusing the tubule with solutions containing different Na+ concentrations (nominally Na(+)-free, 30 mM and 144 mM). Experiments were terminated by shock-freezing the tubules during perfusion. Precisely 30 s before shock-freezing, the K+ in the bathing solution was exchanged for Rb+. Intracellular element concentrations, including Rb+, were determined in freeze-dried cryosections of the tubules using energy-dispersive X-ray analysis. Increasing Na+ concentration in the perfusion solution caused significant rises in intracellular Na+ concentration and Rb+ uptake of principal cells. Principal cell Na+ and Rb+ concentrations were 7.8 +/- 0.9 and 7.0 +/- 0.8 mmol/kg wet weight respectively, when the perfusion solution was Na(+)-free, 10.1 +/- 0.7 and 11.6 +/- 0.6 mmol/kg wet weight with 30 mM Na+ in the perfusion solution, and 14.5 +/- 1.5 and 14.9 +/- 0.9 mmol/kg wet weight with 144 mM Na+ in the perfusion solution. In contrast, a comparable relationship between lumen Na+ concentration, intracellular Na+ concentration and basolateral Rb+ uptake was not seen in intercalated cells. These results support the notion that principal, but not intercalated, cells are involved in transepithelial Na+ absorption. In addition, the data demonstrate that apical Na+ entry and basolateral Na+/K(+)-ATPase activity are closely coupled in principal cells of the rabbit CCD.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of calcium transport across the basolateral membrane of the rabbit cortical thick ascending limb of Henle's loop

Although net Ca2+ absorption takes place in the thick ascending limb of Henle's loop, detailed mechanisms are unknown. Because it has been reported that the Ca2+ entry step across the luminal membrane is mediated by Ca2+ channels inserted by stimulation with parathyroid hormone, we studied the mechanism of Ca2+ transport across the basolateral membrane of rabbit cortical thick ascending limb (CTAL) perfused in vitro by using microscopic fluorometry of cytosolic Ca2+ ([Ca2+]i) with fura-2. The resting [Ca2+]i in this segment was 49.8 +/- 4.5 nmol/l. Neither Na+ removal from the bathing solution nor addition of ouabain (0.1 mmol/l) to the bath increased [Ca2+]i, indicating that a Na+/Ca2+ exchanger in the basolateral membrane may not contribute in any major way to [Ca2+]i of CTAL. To confirm our technical accuracy, similar protocols were conducted in the connecting tubule, where the existence of a Na+/Ca2+ exchanger has been reported. In this segment, Na+ removal from the bath increased cell Ca2+ from 148.6 +/- 6.4 nmol/l to 647.6 +/- 132.0 nmol/l, confirming the documented fact. [Ca2+]i in the CTAL was markedly increased when 1 mmol/l NaCN was added to the bath in the absence of glucose. Calmodulin inhibitors (trifluoperazine or W-7) increased [Ca2+]i. When the bath pH was made alkaline, [Ca2+]i was also increased. This response was abolished when Ca2+ was eliminated from the bath, indicating that the Ca2+ entry across the basolateral membrane is dependent on bath pH. Increase in [Ca2+]i induced by an alkaline bath was inhibited by increased the bath K+ from 5 nmol/l to 50 mmol/l, suggesting that the Ca2+ entry system is voltage-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Cl-/base exchange and cellular pH regulation in enterocytes isolated from chick small intestine

Intracellular pH (pHi) and Cl-/base exchange activity have been examined in isolated chicken enterocytes, both in the presence and absence of 25 mM HCO3-/5% CO2. Intracellular pH was measured with BCECF, a pH-sensitive carboxyfluorescein derivative. Under resting conditions pHi was 7.17 in Hepes and 7.12 in HCO3(-)-buffered solutions. Cells became more alkaline upon withdrawal of Cl-. Cells depleted of Cl- acidified upon reinstatement of Cl-. These changes were faster in the presence of HCO3- than in its absence. After an alkaline load (removal of HCO3- from the medium) pHi decreases towards base line in the presence of Cl-, but not in its absence. The Cl(-)-dependent pHi changes were prevented by H2DIDS and were unaffected by Na+. The Cl(-)-induced recovery from an alkaline load exhibited simple saturation kinetics, with an apparent Km of 12.5 mM Cl- and maximum velocity of approximately 0.20 pH units min-1. The Cl-/base exchange is functional under resting conditions, as shown by cell alkalinization on exposure to 0.5 mM H2DIDS, both in the presence and in the absence of HCO3-. It is concluded that Cl-/base exchange participates in setting the resting intracellular pH in isolated chicken enterocytes and helps recover from alkaline loads. The exchange operates both in the presence and in the absence of bicarbonate.

Regulation of acidification in the rat inner medullary collecting duct

The inner medullary collecting duct (IMCD) is the most distal portion of the nephron and plays an important role in urinary net acid excretion. The terminal or distal two thirds of the IMCD is lined by a single cell type, now termed the IMCD cell, which not only secretes protons, but transports sodium and potassium and responds to many hormones. The IMCD may account for greater than 50% of the excreted acid under control conditions and, during acidosis, absolute acid secretion may increase fivefold. Conversely, during alkalemia, acid secretion by this segment is abolished. Thus, the IMCD responds appropriately to perturbations in systemic acid-base balance. Furthermore, models of renal tubular acidosis have been demonstrated along this nephron segment. Three transporters that are important in acid-base control, the Na+/H+ and the Cl-/HCO3- exchanger and an active proton pump, presumably an H(+)-adenosine phosphatase (ATPase), have been demonstrated in IMCD cells. The former two are situated in the basolateral membrane, while the latter is situated in the apical membrane. Only the proton pump is responsible for actual acid addition to the urine. The intracellular mechanisms that modulate the proton pump are just beginning to be defined. It is likely that acid secretory activity involves exocytic insertion of additional pumps, and is dependent on cell pH changes, which are the primary signal, and on changes in intracellular calcium concentration and calmodulin activity, which are the second messengers.