The effect of harmaline on intestinal sodium transport and on sodium-dependentd-glucose transport in brush-border membrane vesicles from rabbit jejunum

Harmaline inhibition of sodium uptake and of sodium-dependentd-glucose transport was investigated using brush-border membrane vesicles from frozen rabbit jejunum. Under sodium-gradient conditions, initiald-glucose uptake (20 s) was inhibited by harmaline at concentrations above 0.5 mM, but at lower harmaline concentrationsd-glucose uptake was stimulated by 10–15%. When a similar potassium gradient was used, harmaline had no effect. At concentrations upt to 2 mM, harmaline did not alter the equilibrium uptake ofd-glucose ord-mannitol. After pre-equlibration with sodium (25 mM),d-glucose uptake was inhibited at harmaline concentrations ranging from 0.1 to 2 mM. Sodium (10 mM) uptake was also inhibited by harmaline. Increasing the sodium concentration reduced the inhibitory effect of harmaline on tracer sodium uptake as well as on sodium-dependentd-glucose uptake. Similar to phlorizin, harmaline (1 mM) was able to prevent glucose-induced sodium influx across the brush-border membrane.Sodium uptake into brush-border membrane vesicles seems to be inhibited at lower harmaline concentrations than sodium-dependentd-glucose uptake. At high (2 mM) inhibitor concentrations, however, sodium-dependent glucose uptake is more strongly inhibited than sodium uptake. These results suggest that harmaline inhibits both sodium and sodium-dependent transport across intestinal brush-border membranes by interacting with specific sodium-binding sites.     

Polarized expression of Na+/H+ exchange activity in LLC-PK1/PKE20 cells: II. Hormonal regulation

LLC-PK₁/PKE₂₀ cells (a continuous epithelial cell line) has two different Na/H exchange activities: Na/H-1 located in the basolateral membrane and Na/H-2 located in the apical membrane [Casavola et al. (1989) Biochem Biophys Res Commun 165:833–837; Haggerty et al. (1988) Proc Natl Acad Sci USA 86:6797–6801]. In the present report we have studied hormone regulation of these exchange activities by measuring Na-dependent recovery of pH_i from an acid load (by using microspectrofluorometry and 2,7-bis(carboxyethyl)-5,6-carboxyfluorescein) in response to activation of regulatory cascades by either pharmacological agents or by vasopressin or calcitonin. Agents leading to activation of protein kinase A (cAMP-dependent), such as forskolin (10 M), 8-Br-cAMP (0.25 mM), and isobutylmethylxanthine (0.5 mM), inhibited Na/H-2 and Na/H-1 by an average of 49%. Stimulation of protein kinase C by a phorbol ester (phorbol 12-myristate 13-acetate, TPA, 100 nM) inhibited Na/H-2 (by an average of 48%) and stimulated Na/H-1 (by an average of 38%); these effects of TPA were also observed in the presence of forskolin (100 M). Addition of either vasopressin (2 M) or calcitonin (0.3 M) onto both sides of the monolayer decreased the activity of Na/H-2 by an average of 26.3% and 27.7% respectively, and stimulated the activity of Na/H-1 by an average of 17.4% and 38.7% respectively; exposure of cells to either hormone stimulated production of cAMP and inositol trisphosphate, respectively. Separate hormone additions to either the apical or basolateral cell surface led to effects similar to those produced by simultaneous hormone additions onto both cell surfaces, although the relative response of Na/H exchangers to either agonist is variable. In summary, these results suggest that in LLC-PK ₁/PKE₂₀ cells, vasopressin and calcitonin can act via receptor systems coupled either to adenylate cyclase or to phospholipase C. Activation of these receptor systems can lead to inhibition of Na/H-2 and stimulation of Na/H-1.     

IGF-I and vanadate stimulate Na/Pi-cotransport in OK cells by increasing type II Na/Pi-cotransporter protein stability

 Insulin-like growth factor (IGF)-I and vanadate increase Na-dependent phosphate (Na/P_i) cotransport in opossum kidney (OK) cells. To gain more information about the mechanisms by which IGF-I and vanadate stimulate Na/P_i-cotransport, we measured type II Na/P_i-cotransporter (NaPi-4) protein abundance by Western blot analysis and investigated the effects of protein synthesis and tyrosine kinase inhibitors. The key findings in the present studies are as follows. First, incubation in IGF-I (10^–8 M) and/or vanadate (10^–3 M) for 3 h led to a non-additive 1.4-fold increase in Na/P_i-cotransport activity which was paralleled by a 1.5- to 2-fold increase in NaPi-4 protein. Second, actinomycin D did not abolish the increase in Na/P_i-cotransport and cycloheximide did not prevent the IGF-I-induced increase in Na/P_i-cotransport and NaPi-4 protein. Third, among the protein kinase inhibitors tested, only staurosporine substantially reduced the stimulation of Na/P_i-cotransport. In conclusion, the stimulatory effect of IGF-I on Na/P_i-cotransport is paralleled by an increased expression of NaPi-4 protein that is independent of protein synthesis and therefore results from increased protein stability. The observation that IGF-I and/or vanadate lead to similar increases in Na/P_i-cotransport and NaPi-4 protein abundance provides further evidence that the stimulation of Na/P_i-cotransport by IGF-I and vanadate involves protein tyrosine phosphorylation of the same signalling molecules. Received: 1 May 1998 / Received after revision: 25 August 1998 / Accepted: 1 September 1998     

Renal expression of Na+-phosphate cotransporter mRNA and protein: Effect of the Gy mutation and low phosphate diet

The X-linked Gy mutation is closely linked, but not allelic, to Hyp and is characterized by rickets, hypophosphatemia, decreased renal tubular maximum for phosphate (Pi) reabsorption (Tm_P) and a specific reduction in renal brush-border membrane (BBM) Na⁺-Pi cotransport. Gy mice, like their normal littermates, respond to a low-Pi diet with an increase in BBM Na⁺-Pi cotransport, but fail to show an adaptive increase in Tm_p. Using an antibody raised against the NH₂ terminal peptide of the rat renal-specific Na⁺-Pi cotransporter (NaPi-2) and a NaPi-2 cDNA probe, we examined the effect of the Gy mutation and low-Pi diet (0.03% Pi) on NaPi-2 protein and mRNA abundance. The reduction in BBM Na⁺-Pi cotransport in Gy mice (51 ± 5% of normal, P < 0.05) was associated with a decrease in NaPi-2 protein (46 ± 12% of normal, P < 0.05) and mRNA abundance (76 ± 5%, P < 0.05). The low-Pi diet elicited a two- to three-fold increase in Na⁺-Pi cotransport in both normal and Gy mice that was accompanied by a large increase in NaPi-2 protein (10.2-fold in normal and 16.9-fold in Gy mice) and a modest increase in NaPi-2 mRNA (1.3-fold in both mouse strains, P < 0.05). The present data demonstrate that (1) the renal defect in BBM Pi transport in Gy mice can be ascribed to a deficit in NaPi-2 protein and mRNA abundance, (2) both normal and Gy mice respond to low Pi with an adaptive increase in NaPi-2 protein that exceeds the increase in Na⁺-Pi cotransport activity and NaPi-2 mRNA, (3) the adaptive increase in NaPi-2 protein and mRNA are not sufficient for the overall increase in Tm_P following Pi restriction. Received: 27 October 1995 / Received after revision: 4 December 1995 / Accepted: 6 December 1995     

The sodium phosphate cotransporter family SLC34

This review summarizes the characteristics of the solute carrier family SLC34 that is represented by the type ll Na/P(i)-cotransporters NaPi-lla (SLC34A1), NaPi-llb (SLC34A2) and NaPi-llc (SLC34A3). Other Na/P(i)-cotransporters are described within the SLC17 and SLC20 families. Type ll Na/P(i)-cotransporters are expressed in several tissues and play a major role in the homeostasis of inorganic phosphate. In kidney and small intestine, type ll Na/P(i)-cotransporters are located at the apical sites of epithelial cells and represent the rate limiting steps for transepithelial movement of phosphate. Physiological and pathophysiological regulation of renal and small intestinal epithelial transport of phosphate occurs through alterations in the abundance of type ll Na/P(i)-cotransporters.     

Intestinal Depletion of NaPi‐IIb/Slc34a2 in Mice: Renal and Hormonal Adaptation

The Na⁺‐dependent phosphate‐cotransporter NaPi‐IIb (SLC34A2) is widely expressed, with intestine, lung, and testis among the organs with highest levels of mRNA abundance. In mice, the intestinal expression of NaPi‐IIb is restricted to the ileum, where the cotransporter localizes specifically at the brush border membrane (BBM) and mediates the active transport of inorganic phosphate (Pi). Constitutive full ablation of NaPi‐IIb is embryonically lethal whereas the global but inducible removal of the transporter in young mice leads to intestinal loss of Pi and lung calcifications. Here we report the generation of a constitutive but intestinal‐specific NaPi‐IIb/Slc34a2–deficient mouse model. Constitutive intestinal ablation of NaPi‐IIb results in viable pups with normal growth. Homozygous mice are characterized by fecal wasting of Pi and complete absence of Na/Pi cotransport activity in BBM vesicles (BBMVs) isolated from ileum. In contrast, the urinary excretion of Pi is reduced in these animals. The plasma levels of Pi are similar in wild‐type and NaPi‐IIb–deficient mice. In females, the reduced phosphaturia associates with higher expression of NaPi‐IIa and higher Na/Pi cotransport activity in renal BBMVs, as well as with reduced plasma levels of intact FGF‐23. A similar trend is found in males. Thus, NaPi‐IIb is the only luminal Na⁺‐dependent Pi transporter in the murine ileum and its absence is fully compensated for in adult females by a mechanism involving the bone‐kidney axis. The contribution of this mechanism to the adaptive response is less apparent in adult males. © 2015 American Society for Bone and Mineral Research.