Regulation of Na+-H+ exchange in cultured opossum kidney cells by parathyroid hormone, atrial natriuretic peptide and cyclic nucleotides

Acute regulation of Na+/H+ exchanger NHE3 by parathyroid hormone via NHE3 phosphorylation and dynamin-dependent endocytosis

Parathyroid hormone (PTH) is a potent inhibitor of mammalian renal proximal tubule Na(+) transport via its action on the apical membrane Na(+)/H(+) exchanger NHE3. In the opossum kidney cell line, inhibition of NHE3 activity was detected from 5 to 45 min after PTH addition. Increase in NHE3 phosphorylation on multiple serines was evident after 5 min of PTH, but decrease in surface NHE3 antigen was not detectable until after 30 min of PTH. The decrease in surface NHE3 antigen was due to increased NHE3 endocytosis. When endocytic trafficking was arrested with a dominant negative dynamin mutant (K44A), the early inhibition (5 min) of NHE3 activity by PTH was not affected, whereas the late inhibition (30 min) and decreased surface NHE3 antigen induced by PTH were abrogated. We conclude that PTH acutely inhibits NHE3 activity in a biphasic fashion by NHE3 phosphorylation followed by dynamin-dependent endocytosis.

Dual mechanisms of regulation of Na/H exchanger NHE-3 by parathyroid hormone in rat kidney

Parathyroid hormone (PTH) is a potent inhibitor of mammalian renal proximal tubule sodium absorption via suppression of the apical membrane Na/H exchanger (NHE-3). We examined the mechanisms by which PTH inhibits NHE-3 activity by giving an acute intravenous PTH bolus to parathyroidectomized rats. Parathyroidectomy per se increased apical membrane NHE-3 activity and antigen. Acute infusion of PTH caused a time-dependent decrease in NHE-3 activity as early as 30 min. Decrease in NHE-3 activity at 30 and 60 min was accompanied by increased NHE-3 phosphorylation. In contrast to the rapid changes in NHE-3 activity and phosphorylation, decrease in apical membrane NHE-3 antigen was not detectable until 4-12 h after the PTH bolus. The decrease in apical membrane NHE-3 occurred in the absence of changes in total renal cortical NHE-3 antigen. Pretreatment of the animals with the microtubule-disrupting agent colchicine blocked the PTH-induced decrease in apical NHE-3 antigen. We propose that PTH acutely cause a decrease in NHE-3 intrinsic transport activity possibly via a phosphorylation-dependent mechanism followed by a decrease in apical membrane NHE-3 antigen via changes in protein trafficking.

Endothelial cells regulate proximal tubule epithelial cell sodium transport

BACKGROUND

Proximal tubule epithelial cells are in close contact with the renal microvasculature, but the effect of endothelial cells (ECs) on proximal tubule epithelial cell (PTEC) function is not known.

METHODS

To determine if ECs regulate PTECs, we coincubated ECs with PTECs in a system that permitted cross-talk between the two cell types and the vectorial transport of sodium.

RESULTS

In the presence (but not absence) of ECs, adding bradykinin or acetylcholine increased cGMP and decreased sodium transport, as well as Na,K-ATPase in PTECs. Interleukin (IL)1B preconditioning of ECs also increased cGMP and decreased sodium transport and Na,K-ATPase in PTECs. Bradykinin, acetylcholine, and IL1B EC-dependent effects were reversed with the nitric oxide (NO) synthase inhibitor L-NNA. In the absence of ECs, the addition of NO donors to PTECs increased cGMP and decreased sodium transport and Na,K-ATPase. 8Br-cGMP also decreased PTEC sodium transport and Na,K-ATPase.

CONCLUSION

Endothelial cells regulate PTEC function. This effect is mediated by NO synthase-dependent up-regulation of cGMP in PTECs.

Both IgM and IgG anti-VSG antibodies initiate a cycle of aggregation-disaggregation of bloodstream forms of Trypanosoma brucei without damage to the parasite

Bloodstream forms of Trypanosoma brucei, when aggregated in the presence of either acute immune plasma, acute immune serum, purified IgM anti-VSG antibodies or purified IgG anti-VSG antibodies, subsequently disaggregated with a t1/2 for disaggregation of 15 min at 37 degrees C as long as the trypanosomes were metabolically active at the beginning of the experiment and maintained during the experiment in a suitable supporting medium. The t1/2 for disaggregation was found to be directly dependent upon temperature and inversely proportional to the antibody concentration. The trypanosomes were always motile and metabolically active during aggregation and after disaggregation and were fully infective for a mammalian host following disaggregation as well as able to grow and divide normally during axenic culture. The disaggregation was strictly energy dependent and was inhibited when intracellular ATP levels were reduced by salicylhydroxamic acid or following addition of oligomycin while respiring glucose. In addition the process of disaggregation was dependent upon normal endosomal activity as evidenced by its sensitivity to a wide variety of inhibitors of various endosomal functions. Disaggregation was not due to separation of immunoglobulin chains by either disulphide reduction or disulphide exchange reactions and gross proteolytic cleavage of the immunoglobulins attached to the surface of the parasite was not detected. In addition, gross cleavage or release of the VSG from the surface of the cell did not occur during disaggregation but proteolytic cleavage of a small proportion of either the VSG or the immunoglobulins could not be eliminated from consideration. Finally the mechanism of disaggregation was found to be a regulated process, independent of Ca2+ movements but dependent upon the activity of protein kinase C or related kinases and inhibited by the activity of protein kinase A as evidenced by the effects of a panel of inhibitors and cAMP analogues on the process of disaggregation. The mechanism of disaggregation displayed by trypanosomes aggregated by anti-VSG antibody is proposed to form part of the parasite's defence against the host immune system and functions to aid survival of trypanosomes in the presence of antibody in the host prior to the occurrence of a VSG switching event.

Potassium transport in opossum kidney cells: effects of Na-selective and K-selective ionizable cryptands, and of valinomycin, FCCP and nystatin

The effects of two ionizable cryptands, the Na-selective (221)C10 and the K-selective (222)C10, and of valinomycin, FCCP and nystatin on K+ fluxes in opossum kidney (OK) cells have been quantified. The Na,K-ATPase (ouabain-sensitive 86Rb influx) was stimulated by nystatin (> or = 20%), and inhibited by the other ionophores (50-80%), by barium (K-channel blocker) (61%) and by amiloride (Na entry blocker) (34%). The Vmax of the Na,K-ATPase phosphatase activity was unmodified by the ionophores, indicating the absence of direct interaction with the enzyme. The ATPi content was unmodified by the inhibitors and nystatin, but was lowered by (221)C10 (47%), (222)C10 (75%), valinomycin (72%) and FCCP (88%). Amiloride was found to partially remove the inhibition caused by (222)C10 (51%) and valinomycin (49%). Rb efflux was stimulated by nystatin (32%), unmodified by valinomycin, and was inhibited by (221)C10 (19%), (222)C10 (19%) and FCCP (10%). Barium (39%) and amiloride (32%) inhibited this efflux and, in their presence, the nystatin effect persisted, whereas that of the other ionophores vanished. At pH 6.4, the Rb efflux decreased by 14% of its value at pH 7.4, with no additional inhibition by cryptands. Cryptands are shown to inhibit the pH-sensitive K+-conductance, probably by inducing a K+-H+ exchange at the plasma membrane, and by uncoupling oxidative phosphorylation by inducing the entry of K+ and H+ (and possibly Ca2+) ions into the mitochondria.

pH modulates cAMP-induced increase in Na+ transport across frog skin epithelium

Apical membrane potential (Va), fractional apical membrane resistance (FRa), and/or intracellular pH (pHi) were measured in principal cells of isolated frog (Rana pipiens) skin with microelectrodes under short-circuit conditions. Apical exposure to 0.33 mM 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (cAMP) depolarized Va, decreased FRa and increased short-circuit current (Isc). cAMP-induced 50% larger effects on Va and Isc at external pH (pHo) of 8.0 than at pHo 6.4. Increasing pHo from 6.4 to 8.0 in presence of cAMP further depolarized Va and increased Isc. cAMP-induced effects on Va and Isc were observed in the absence of Cl- and HCO3- and in the presence of 1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or 10 microM 5-(N-ethyl-N-isopropyl)amiloride (EIPA) or 1 microM 5-(N-methyl-N-isobutyl)amiloride (MIA). These data indicate that Na(+)-H+ exchange, Cl(-)-HCO3- exchange, and electrogenic Na(+)-(HCO3-)n cotransport are not involved in cAMP-induced increase in Isc. Apical exposure to 2 mM Cd2+ or Zn2+ depolarized Va, decreased FRa, increased Isc and increased pHi. In HCO(3-)-free solutions containing DIDS, unilateral replacement of apical Cl- by NO3- induced a fast transient depolarization of Va and an increase in Isc. These data suggest that potential-dependent changes in pHi are involved in increases in Isc. However, when changes in Va were minimized by pretreating the basolateral membrane with 25 or 75 mM K+, the cAMP-induced increase in Isc was not blocked. These data indicate that changes in pHi do not play a strict regulatory role but are only permissive in cAMP-induced effects on Isc.

Regulation of apical Na+ conductive transport in epithelia by pH

Alterations in extracellular (pHo) and/or intracellular pH (pHi) have significant effects on the apical Na+ conductive transport in tight epithelia. They influence apical membrane Na+ conductance via a direct effect on amiloride-sensitive apical Na+ channel activity and indirectly through effects on the basolateral Na+/K(+)-ATPase. Changes in pH also modulate the hormonal regulation of apical Na+ conductive transport. The pH sensitive steps in hormone action include: (i) hormone-receptor binding, (ii) increase in intracellular cyclic 3',5'-adenosine monophosphate (cAMP), (iii) mobilization of intracellular free Ca2+ ([Ca2+]i), and (iv) incorporation of new channels into the apical membrane or recruitment of existing channels. Alternately, changes in pH induce secondary effects via alterations in [Ca2+]i. A reciprocal relationship between pHi and [Ca2+]i has been demonstrated in renal epithelial cells. Natriferic hormones induce a significant increase in pHi. There is a strong temporal relation between hormone-induced increase in pHi and overall increase in transepithelial Na+ transport. This suggests that changes in pHi act as an intermediate in the second messenger cascade initiated by the hormones. Several natriferic hormones activate Na(+)-H+ exchanger, H(+)-ATPase, H+/K(+)-ATPase, H+ conductive pathways in cell membranes or potential-induced changes in pHi. However, changes in pHi do not seem to be essential for the hormone effect on Na+ conductive transport. It is suggested that the role of pHi changes during hormone action is permissive rather than strictly obligatory.

Cloning and expression of a cAMP-activated Na+/H+ exchanger: evidence that the cytoplasmic domain mediates hormonal regulation

The ubiquitous plasma membrane Na+/H+ exchanger (termed NHE1) is activated by diverse hormonal signals, with the notable exception of hormones acting through cAMP as second messenger. Therefore, the Na+/H+ exchanger found in the nucleated trout red cell is of particular interest since it is activated by catecholamines, forskolin, and cAMP analogues. We report here that a cloned cDNA encoding the red cell exchanger restores functional Na+/H+ activity when transfected into Na+/H+ antiporter-deficient fibroblasts (i.e., it regulates intracellular pH in a Na-dependent and amiloride-sensitive manner). This red cell exchanger represents an additional form of Na+/H+ exchanger (termed beta NHE), which is characterized by a specific cytoplasmic domain involved in activation by the cAMP-dependent signaling pathway. After transfection in the same cellular context, beta NHE, but not NHE1, is activated by cAMP or by hormones that increase cAMP levels. Comparison of the amino acid sequences of exchangers shows that beta NHE, but not NHE1, contains two clustered consensus motifs for phosphorylation by a cAMP-dependent protein kinase (protein kinase A; PKA). A deletion mutant devoid of the C-terminal region of the cytoplasmic loop containing the two PKA sites restores Na+/H+ activity but is no longer activated by cAMP analogues or catecholamines. In red blood cells, the Na+/H+ exchanger is also activated by another pathway involving protein kinase C (PKC). Expression of beta NHE in fibroblasts shows that these two independent signaling pathways impinge on two distinct domains of the exchanger. The cytoplasmic segment containing PKA consensus sites, which is crucial for cAMP activation, is unnecessary for stimulation by PKC activators.

pHi-dependent membrane conductance of proximal tubule cells in culture (OK): differential effects on K(+)- and Na(+)-conductive channels

Confluent monolayers of the established opossum kidney cell line were exposed to NH4Cl pulses (20 mmol/liter) during continuous intracellular measurements of pH, membrane potential (PDm) and membrane resistance (R'm) in bicarbonate-free Ringer. The removal of extracellular NH4Cl leads to an intracellular acidification from a control value of 7.33 +/- 0.08 to 6.47 +/- 0.03 (n = 7). This inhibits the absolute K conductance (gK+), reflected by a decrease of K+ transference number from 71 +/- 3% (n = 28) to 26 +/- 6% (n = 5), a 2.6 +/- 0.2-fold rise of R'm, and a depolarization by 24.2 +/- 1.5 mV (n = 52). In contrast, intracellular acidification during a block of gK+ by 3 mmol/liter BaCl2 enhances the total membrane conductance, being shown by R'm decrease to 68 +/- 7% of control and cell membrane depolarization by 9.8 +/- 2.8 mV (n = 17). Conversely, intracellular alkalinization under barium elevates R'm and hyperpolarizes PDm. The replacement of extracellular sodium by choline in the presence of BaCl2 significantly hyperpolarizes PDm and increases R'm, indicating the presence of a sodium conductance. This conductance is not inhibited by 10(-4) mol/liter amiloride (n = 7). Patch-clamp studies at the apical membrane (excised inside-out configuration) revealed two Na(+)-conductive channels with 18.8 +/- 1.4 pS (n = 10) and 146 pS single-channel conductance. Both channels are inwardly rectifying and highly selective towards Cl-. The low-conductive channel is 4.8 times more permeable for Na+ than for K+. Its open probability rises at depolarizing potentials and is dependent on the pH of the membrane inside (higher at pH 6.5 than at pH 7.8).

Parathyroid hormone regulation of Na+/H+ exchange in opossum kidney cells: polarity and mechanisms

In previous work we have shown that parathyroid hormone (PTH) inhibits Na+/H+ exchange in cellular suspensions of OK (opossum kidney) cells (an established renal epithelial cell line) in a dose-dependent manner. PTH effects could be mimicked by pharmacological activation of both protein kinase A and protein kinase C (Helmle-Kolb et al. 1990). In the present paper we extend these observations and analyze the PTH-dependent control of Na+/H+ exchange in OK cells kept in epithelial configuration (monolayer). Na+/H+ exchange activity is examined by microfluorometry using the intracellularly trapped pH-sensitive dye 2'7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein. Cells recovered from an acid load (NH4Cl prepulse) after addition of apical Na+. Ethylisopropylamiloride inhibits Na(+)-dependent pHi recovery at micromolar concentrations. PTH leads to an inhibition of apical Na+/H+ exchange activity; inhibition is observed even at a concentration of 5 pM PTH. PTH given at maximally effective concentrations (24 nM) reduces the total Na+/H+ exchange capacity by 60%-70%. Apical as well as basolateral hormone additions elicit an inhibitory response at low (5 pM) or high (24 nM) concentrations. Forskolin (activation of protein kinase A) and phorbol esters (activation of protein kinase C) lead to an inhibition of Na+/H+ exchange activity (60%-70% inhibition). These observations suggest that Na+/H+ exchange activity is preferentially located in the apical membranes of OK cells kept in monolayer configuration.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of intracellular pH by cultured opossum kidney cells

Opossum kidney (OK) cells (an epithelial cell line) were examined by flame photometry of cellular Na+ and K+ and by microfluorometric measurements of the intracellular pH (pHi) of single cells loaded with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF). The work concentrates on defining resting pHi values under different experimental conditions and examines factors that contribute to the maintenance of resting pHi. To use nigericin to calibrate the intracellular response of BCECF, cellular K+ levels were measured by a null point analysis, and the stability and magnitude of cellular Na+ and K+ levels were determined vs. time. Resting pHi in medium without added CO2 was high when measured by null point analysis of the population (pHi 7.6) and from measurements of single cells that have recovered from an acid load caused by NH4 prepulse (pHi 7.76 +/- 0.03, n = 20 cells). In single-cell measurements, addition of CO2-HCO3- to the medium results in cellular acidification of the steady-state pHi by 0.35 +/- 0.04 pH units. In medium equilibrated with room air, the resting pHi is shown to be a dynamic steady state composed of net flux due to apical Na(+)-dependent transport (Na(+)H+ exchange) plus acidifying processes. It is concluded that although 5-[N-ethyl-N-isopropyl]amiloride (EIPA) inhibits the forward reaction of Na(+)-H+ exchange, EIPA is either ineffective as an inhibitor of the reverse reaction of Na(+)-H+ exchange or Na(+)-H+ exchange does not reverse measurably in the OK cells.

Polarity and kinetics of Na(+)-H+ exchange in cultured opossum kidney cells

Opossum kidney (OK) cells (an established cell line) were loaded with 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF; a fluorescent dye with a pH-sensitive spectrum), and intracellular pH (pHi) was examined by microfluorometry. Single cells, within a confluent monolayer and grown on a permeant support, were examined for the mechanism of recovery from an acid load as imposed by exposure to ammonium chloride (NH4 prepulse). The Na(+)-dependent recovery of pHi from an acid load (Na(+)-H+ exchange) is examined in terms of the Na+ activation kinetics of the recovery and the polarity of the response. In 80% of the cells examined (33/41), both apical and basolateral Na+ cause recovery from an acid load. The response of cells to apical Na+ is well fit by Michaelis-Menten kinetics [Kt(Na) = 35 mM], but the response to basolateral Na+ is not. The response to basolateral Na+ addition is modeled in terms of variable transepithelial leak of Na+ and variable amounts of basolateral Na(+)-H+ exchange. Despite an average response to basolateral (145 mM) Na+ that is 34% of the response to apical Na+, modeling suggests that basolateral Na(+)-H+ exchange must be less than 10% of the cellular total to fit the basolateral Na+ activation kinetics. The model, and experiments using ordered addition of Na+ from the apical vs. basolateral medium, also suggest that transepithelial leak (of basolateral Na+ to the apical compartment) is required to explain the pHi recovery observed due to addition of basolateral Na+. Direct estimation of (basolateral to apical) transepithelial leak demonstrates that the response due to basolateral Na+ addition is explained by transepithelial leak and a Na(+)-H+ exchange that is expressed solely in the apical membrane.

Regulation of Na+/H+ exchange in opossum kidney cells by parathyroid hormone, cyclic AMP and phorbol esters

Parathyroid hormone (PTH) controls two proximal tubular brush border membrane transport systems, Na+/phosphate co-transport and Na+/H+ exchange. In OK cells, a cell line with proximal tubular transport characteristics, PTH acts via kinase C and kinase A activation to inhibit Na+/phosphate co-transport [6, 8, 9, 19, 22]. In the present study, we show that PTH inhibits Na+/H+ exchange and that this effect can be mimicked by pharmacological activation of kinase A and kinase C. Ionomycin-dependent increases in cytoplasmic Ca2+ concentration do not induce inhibition of Na+/H+ exchange; PTH-dependent inhibition of Na+/H+ exchange is not prevented by ionomycin or by the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (Ca2+ clamping). Detailed dose-response curves for the different agonists, given either alone or in combination, suggest that the two regulatory cascades (kinase A and kinase C) are operating independent of each other and reach a common final target, resulting in 40-50% inhibition of Na+/H+ exchange. An analysis of intracellular pH sensitivity of Na+/H+ exchange suggests that inhibition is not related to a shift in set point, but is rather explained by a reduced Vmax of Na+/H+ exchange and/or reduced affinity for protons at the internal membrane surface. It is suggested that kinase A as well as kinase C can mediate PTH inhibition of renal proximal tubular Na+/H+ exchange and that the relative importance of a particular regulatory cascade is determined by the PTH-concentration-dependent rates in the liberation of diacylglycerol (phospholipase C/kinase C) and cAMP (adenylate cyclase/kinase A).

A functional parameter to study heterogeneity of glial cells in rat brain slices: cyclic guanosine monophosphate production in atrial natriuretic factor (ANF)-responsive cells

Stimulation of guanylate cyclase in vitro by atrial natriuretic factor (ANF) or sodium nitroprusside was studied in rat brain tissue slices biochemically as well as by means of cyclic guanosine monophosphate (cGMP) immunocytochemistry. The ANF-responsive, cGMP-producing cells were studied in the olfactory bulb, the septal area, the hippocampus, the medial amygdala, and the medial preoptic area. These cells, having the ANF-stimulated particulate guanylate cyclase, were characterized as astroglial cells on the basis of their glial fibrillary acidic protein (GFAP) immunostaining, although not all astroglial cells in these areas could be identified as cGMP-immunoreactive cells. Sodium nitroprusside-stimulated soluble guanylate cyclase activity was demonstrated in neuronal cell bodies and varicose fibers and was associated with blood vessel walls. Upon maturation, a significant decrease in cGMP production was found after stimulation by 100 nM ANF-(103-126) in the olfactory bulb, the medial amygdala, and the hippocampus, but not in the septal area; no change was found in these areas in cGMP content after stimulation of cGMP production by 10 microM sodium nitroprusside. Via cGMP immunocytochemistry, no qualitative differences were seen in the ANF-responsive, cGMP-producing cells upon maturation.

Electrical properties of cultured renal tubular cells (OK) grown in confluent monolayers

OK cells grown to confluent monolayers were investigated by microelectrode techniques and microinjection. Cell membrane potential difference (PDm) in bicarbonate-free solution is -61.8 +/- 0.6 mV (n = 208), cell membrane resistance (Rm) amounts to 1.4 +/- 0.2 k omega. cm2 (n = 8). The apparent transference number for potassium (t'k+) is 71 +/- 3% (n = 28) and can be reduced by 3 mmol/l BaCl2 to 7.5 +/- 4.0%; (n = 8). In the presence of extracellular CO2 and HCO3- (pH 7.4) the cells acidify by 0.34 +/- 0.05 pH units (n = 12). This leads to a depolarization of PDm by 8.4 +/- 1.8 mV (n = 8), an increase in Rm by 49 +/- 10% (n = 10), and a reduction of K+-conductance to 63 +/- 5% (n = 13). Intracellular acidification by the NH4Cl-prepulse technique also inhibits K+-conductance and depolarizes the membrane. Recovery from an intracellular acid load is reflected by cell membrane repolarization. This recovery can be inhibited by amiloride (10(-3) mol/l). Na+- and Cl- -conductances could not be detected. The transepithelial resistance (Rte) of OK cell monolayers 1 day after plating is 41 +/- 6 omega.cm2 and decreases with time after plating. Intercellular communication (electrical or dye coupling) was not observed.

CONCLUSIONS

1. The membrane potential of OK cells is largely determined by a pH-sensitive, barium-blockable K+-conductance. 2. Amiloride-blockable Na+/H+-exchange is reflected by membrane potential changes via this K+-conductance. 3. Monolayers of OK cells are electrically leaky.

Flow cytometric analysis of intracellular pH in cultured opossum kidney (OK) cells

Suspensions of OK cells (a continuous renal epithelial cell line originating from the opossum kidney) were examined by flow cytometry. Three parameters were evaluated simultaneously; cell integrity as assayed by propidium iodide fluorescence, cell size as measured by time-of-flight, and intracellular pH as measured by fluorescence of 2',7'-bis-(2-carboxyethyl)-5,6 carboxyfluorescein (BCECF). The suspension was shown to be composed of both intact singlets and doublets of cells, and no difference was noted in the behavior of these two populations with respect to the resting intracellular pH, or of the response of intracellular BCECF to changes in pH. Evidence suggests that using NH4 prepulses to create an acid load broadens the intracellular pH distribution. The population of OK cells demonstrates a recovery from this acid load which is very homogeneous with respect to its sensitivity to Na+ removal of EIPA (ethylisopropyl-amiloride), suggesting that virtually all cells utilize Na+/H+ exchange for this recovery. The data also suggest heterogeneity in the cellular pH recovery from an acid load with respect to the observed rates of Na+/H+ exchange. Despite this heterogeneity, the Na+/H+ exchanger is observed to focus the resting intracellular pH of the population to approximately pH 7.4-7.5. The response of the population to PTH suggests that the majority of cells respond to the hormone, and that the total Na+/H+ exchange in individual cells is only partially inhibited even in the presence of saturating PTH concentrations.

Transport of sulfate and phosphate in small intestine and renal proximal tubule: methods and basic properties

1. Isolated brush border membrane vesicles, basolateral membrane vesicles, and cultured renal epithelial cells provide good material for studying transport systems. 2. The vesicle systems have been used to study the transport of labeled phosphate, sodium/phosphate cotransport, sodium/sulfate cotransport, basolateral transport of sulfate and basolateral transport of phosphate via anion exchange. 3. Cultured renal cells show sodium/phosphate cotransport and parathyroid dependent inhibition of phosphate transport.