Effect of atrial natriuretic peptide on sodium-glucose cotransport in the rat small intestine

Guanylyl cyclase/natriuretic peptide receptor-A: Identification, molecular characterization, and physiological genomics

The natriuretic peptides (NPs) hormone family, which consists mainly of atrial, brain, and C-type NPs (ANP, BNP, and CNP), play diverse roles in mammalian species, ranging from renal, cardiac, endocrine, neural, and vascular hemodynamics to metabolic regulations, immune responsiveness, and energy distributions. Over the last four decades, new data has transpired regarding the biochemical and molecular compositions, signaling mechanisms, and physiological and pathophysiological functions of NPs and their receptors. NPs are incremented mainly in eliciting natriuretic, diuretic, endocrine, vasodilatory, and neurological activities, along with antiproliferative, antimitogenic, antiinflammatory, and antifibrotic responses. The main locus responsible in the biological and physiological regulatory actions of NPs (ANP and BNP) is the plasma membrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), a member of the growing multi-limbed GC family of receptors. Advances in this field have provided tremendous insights into the critical role of Npr1 (encoding GC-A/NPRA) in the reduction of fluid volume and blood pressure homeostasis, protection against renal and cardiac remodeling, and moderation and mediation of neurological disorders. The generation and use of genetically engineered animals, including gene-targeted (gene-knockout and gene-duplication) and transgenic mutant mouse models has revealed and clarified the varied roles and pleiotropic functions of GC-A/NPRA in vivo in intact animals. This review provides a chronological development of the biochemical, molecular, physiological, and pathophysiological functions of GC-A/NPRA, including signaling pathways, genomics, and gene regulation in both normal and disease states.

The plasma levels of atrial natriuretic peptide and brain natriuretic peptide in type 2 diabetes treated with sodium-glucose cotransporter-2 inhibitor

PURPOSE

The aim of this study was to determine the levels of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) after treatment with sodium-glucose cotransporter-2 (SGLT2) inhibitor or dipeptidyl peptidase-4 (DPP4) inhibitor in patients with type-2 diabetes inadequately controlled by insulin, and to determine whether variation in ANP levels can explain favorable cardiovascular outcome.

METHODS

We enrolled 56 patients, aged 18-80years, with type-2 diabetes inadequately controlled by insulin: i.e., HbA1c level 7.5-10.5% despite at least 8weeks' injectable insulin at a stable mean dose of 20-150IU daily, with or without no more than two oral antidiabetic agents.

FINDINGS

The 56 patients were randomized between 3 treatment groups: SGLT2 inhibitor (n=18), DPP4 inhibitor (n=19) and placebo (n=19). Patients who received SGLT2 inhibitor or DPP4 inhibitor treatment all showed significantly lower HbA1c levels, fasting blood glucose (FBG) levels and systolic blood pressure at 24weeks than controls. SGLT2 inhibitor treatment decreased ANP levels, BNP levels, systolic blood pressure and weight compared with placebo. Compared to those receiving DPP4 inhibitor, patients receiving SGLT2 inhibitor showed lower HbA1c levels (7.01 vs. 7.58%; P=0.03), ANP levels (28.41 vs. 43.03 pg/mL; P=0.00) and weight (66.14 vs. 71.76 kg; P=0.04) at 24weeks after adjusting for baseline values. The SGLT2 inhibitor group showed higher sodium concentrations than the placebo and DPP4 inhibitor groups (145.89 vs. 143.89 and 144.79 mmol/L, respectively; P=0.00 and P=0.04) at 24 weeks. ANP and BNP levels did not significantly correlate with HbA1c and blood glucose levels.

IMPLICATIONS

These results indicated that SGLT2 inhibitors may be superior to DPP4 inhibitors in reducing risk of cardiovascular disease in diabetic patients. The major study limitation was the small number of patients per group, which should be enlarged in further research.

Sodium-glucose co-transporter-2 inhibitors suppress atrial natriuretic peptide secretion in patients with newly diagnosed Type 2 diabetes

AIMS

To observe changes in atrial natriuretic peptide levels after treatment with sodium-glucose co-transporter-2 inhibitors in patients with newly diagnosed type 2 diabetes.

METHODS

A total of 28 patients with newly diagnosed Type 2 diabetes and HbA_1c levels of 58 -91 mmol/mol (7.5-10.5%) were randomly selected to receive sodium-glucose co-transporter-2 inhibitor treatment (n = 18) or placebo (n = 10) for 24 weeks. We analysed atrial natriuretic peptide concentrations, using an enzyme-linked immunosorbent assay. In addition, sodium and HbA_1c levels were measured at baseline, 12 weeks and 24 weeks and blood lipid levels and insulin sensitivities at baseline and 24 weeks.

RESULTS

Compared with patients treated with placebo, patients who received sodium-glucose co-transporter-2 inhibitor treatment exhibited lower atrial natriuretic peptide levels (36.74 vs 56.90 pg/ml in the placebo group; P < 0.05) and higher sodium levels (144.3 vs 141.4 mmol/l in the placebo group; P < 0.01) at 24 weeks, after adjusting for baseline values. HbA_1c levels were lower after sodium-glucose co-transporter-2 inhibitor treatment compared with placebo (51 vs 60 mmol/mol; P < 0.01). No correlation was found between atrial natriuretic peptide and HbA_1c levels. Homeostatic model assessment of β-cell function values and lipid profiles were generally similar after 24 weeks of treatment with placebo or sodium-glucose co-transporter-2 inhibitor.

CONCLUSIONS

This study shows the ability of sodium-glucose co-transporter-2 inhibitors to lower atrial natriuretic peptide levels and improve glycaemic control, which may benefit the cardiovascular system.

Renal sodium-glucose cotransporter activity and aquaporin-2 expression in rat kidney during chronic nitric oxide synthase inhibition

BACKGROUND/AIMS

The renal sodium glucose cotransporter (SGLT2) and the water channel aquaporin-2 (AQP2) play a critical role in tubular sodium and water reabsorption and in the regulation of extracellular fluid volume both in physiologic and pathophysiologic conditions. However, there is little information about SGLT2 and AQP2 expression and/or activity in hypertension and there are no reports during hypertension induced by chronic nitric oxide synthase (NOS) inhibition.

METHODS

Hypertension was induced in rats by oral administration of N(G)-nitro-L-arginine methyl ester (L-NAME) (20 mg/kg/24 h) for 6 (H6) or 12 (H12) weeks. SGLT2 activity was measured using alpha-(14)C-methylglucose active uptake. The expression level of transporters was assessed by immunohistochemistry and/or immunoblotting.

RESULTS

SGLT2 activity was reduced in both H6 and H12; this was due neither to a decrease in SGLT2 expression nor to a change in membrane phospholipid composition. In H6, AQP2 expression diminished only in the inner medulla (IM), while in H12 it diminished in both outer (OM) and IM. This reduced expression of AQP2 may partially account for the increased urinary volume and decreased urinary osmolality in H12, since we obtained a strong correlation between AQP2 expression and these urinary parameters in both OM and IM.

CONCLUSION

We propose that in rats in which hypertension is induced by NOS inhibition, SGLT2 activity and AQP2 expression are modified to compensate for the elevated arterial pressure. However, we cannot discount the possibility that the observed changes are due to the decrease in NO production itself.

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Atrial natriuretic peptide and endothelin-3 target renal sodium-glucose cotransporter

Atrial natriuretic peptide (ANP) and endothelin (ET) are endogenous vasoactive factors that exert potent diuretic and natriuretic actions. We have previously shown that ANP and ET-3 act through an NO pathway to inhibit the sodium-glucose cotransporter (SGLT) in the intestine [Gonzalez Bosc LV, Elustondo PA, Ortiz MC, Vidal NA. Effect of atrial natriuretic peptide on sodium-glucose cotransport in the rat small intestine. Peptides 1997; 18: 1491-5; Gonzalez Bosc LV, Majowicz MP, Ortiz MC, Vidal NA. Effects of endothelin-3 on intestinal ion transport. Peptides 2001; 22: 2069-75.]. Here we address the role of ANP and ET-3 on SGLT activity in renal proximal tubules. In rat renal cortical brush border membranes (BBV), fluorescein isothiocianate (FITC) labeling revealed a specific 72-kD peptide that exhibits increased FITC labeling in the presence of Na+ and D-glucose. Using alpha-14C-methylglucose active uptake, rat BBV were shown to possess SGLT activity with an affinity constant (K(0.5) approximately 2.4 mM) that is consistent with the expression of the low-affinity, high-capacity SGLT2 isoform. SGLT2 activity in these preparations is dramatically inhibited by ANP and ET-3. This inhibition is independent of changes in membrane lipids and is mimicked by the cGMP analogue, 8-Br-cGMP, suggesting the involvement of cGMP/PKG pathways. These results are the first demonstration that both ANP and ET-3 inhibit rat cortical renal SGLT2 activity, and suggest a novel mechanism by which these vasoactive substances modulate hydro-saline balance at the proximal tubular nephron level.

Effects of endothelin-3 on intestinal ion transport

We investigated the effects of endothelin 3 (ET-3) on electrolyte transport in rat small intestine using a voltage clamp technique in Ussing's chamber. ET-3 diminished potential difference (PD) and short circuit current (Isc). ET-3 did not affect PD or Isc in low Na(+) and/or D-glucose-free medium. Phloridzine (an inhibitor of sodium-glucose cotransporter [SGLT1]) pretreatment abolished the effect of ET-3 on Isc. Methylene blue (a soluble guanylate cyclase inhibitor) or N-nitro-L-arginine methyl ester (a NOS inhibitor) pretreatment delayed the effect of ET-3 on PD and Isc. ET-3 enhanced NOS activity on enterocytes and systemic NO production. Then, ET-3 could inhibit SGLT1 with the participation of NO.

Effects of atrial natriuretic peptide in the gut

The intestinal tract is a target organ for atrial natriuretic peptide (ANP), characterized by various biologic activities, immunoreactivity, as well as specific binding sites for ANP. A review of previous studies reveals that ANP is an important regulator of water and nutrient intake, which acts via multiple signaling pathways including activation of guanylyl cyclase to produce its biologic responses. As a regulator, the peptide locally controls hydrosaline balance and acute systemic effects. Therefore, ANP could also act as a local mediator or paracrine effector of intestinal function.

Role of natriuretic peptides in ion transport mechanisms

Natriuretic peptides (NP) act as ligands on the guanylyl cyclase family of receptors. The NP binding site on these receptors is extracellular and the guanylyl cyclase and protein kinase domains are intracellular. The guanylyl cyclase receptor catalyzes the synthesis of the second messenger molecule, cGMP, which activates protein kinase. This in turn is involved in the phosphorylation of various ion transport proteins. Ion transport proteins, which are modulated by NP and are thought to underlie the natriuretic and diuretic actions of NP, include: (a) calcium-activated K+ channels; (b) ATP-sensitive K+ channels; (c) inwardly-rectifying K+ channels; (d) outwardly-rectifying K+ channels; (e) L-type Ca2+ channels; (f) Cl- channels including cystic fibrosis transmembrane conductance regulator Cl- channels; (g) Na+- K+ 2Cl- co-transporter; (h) Na+- K+ ATPase; (i) Na+ channels; (j) stretch-activated channels; and (k) water channels. It appears that NP modulate the kinetics, rather than the conductance, of ion channels. Some of these channels, like the Ca2+, ATP-sensitive K+ and stretch-activated channels, are also involved in NP secretion. In addition, the structural properties of the NP, e.g., ovCNP-22 and ovCNP-39, appear to confer on them the ability to form ion channels. These CNP-formed ion channels can modify the trans-membrane signal transduction and second messenger systems underlying NP-induced pathological effects.

Atrial natriuretic peptide effect on NADPH-diaphorase in rat intestinal tract

Histochemical reaction of NADPH-diaphorase (NOS-NADPH-d) was used to identify NO synthesis. A 30-min 0.1 microg microg/kg/min ANP infusion led to about a 10% and 35% increase in small and large intestine enterocytes stain respectively. This increase was abolished by a bolus of 1 mg/kg L-NAME before ANP infusion in small intestine, and partially abolished it in colon. Incubation of small and large intestine with 0.5 microM ANP increased stain at about 20%. In both tissues the preincubation with 0.1 mM L-NAME abolished the ANP effect. Incubation with 0.1 mM 8-Br-cGMP enhanced staining about 70% and 30% in small and large intestine respectively. Our results show that ANP enhances NOS-NADPH-d activity, suggesting that ANP stimulates NO synthase in enterocytes by L-arginine-NO pathway. 8-Br-cGMP mimicked the effect of ANP described above. Therefore, the guanylate cyclase-coupled natriuretic receptors, NPR-A and NPR-B, probably mediate this ANP effect.

Effect of atrial natriuretic peptide on alpha-methyl-D-glucoside intestinal active uptake in rats

In vivo, atrial natriuretic peptide (ANP) inhibits water and sodium absorption by the intestine. In addition, ANP inhibits glucose (re)absorption at the level of both the intestine and kidney. ANP also decreases sodium absorption in the rat small intestine in vitro, but only if glucose is present on the luminal side of the tissue. These findings suggest that ANP inhibits the sodium-glucose cotransporter (SGLT) of enterocytes. In the present study the inhibitory effect of 1 microM ANP on SGLT1 in rat small intestine and colon was tested. For this purpose, the apparent kinetic constants of SGLT1 were determined using radioactive alpha-methyl-D-glucoside (alpha-MG), a non-metabolizable glucose analogue that selectively serves the luminal Na+-dependent intestinal uptake, but not the serosal-facilitated diffusion sugar carrier. In both tissues, incubation with ANP increased Km without modifying the Vmax. In addition, Vmax in the small intestine was found to be higher than in the colon. The evidence presented here suggests that ANP, through its second messenger, may be a competitive inhibitor of SGLT1. Since SGLT1 is also expressed in the brush-border membrane of the renal proximal tubule, we suggest that this peptide might regulate the hydro-saline balance at intestinal and proximal tubular nephron levels.