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.

Effect of chronic angiotensin II inhibition on the nitric oxide synthase in the normal rat during aging

OBJECTIVE

To assess the effect on the cardiovascular system, of enalapril (E) or losartan (L) given since weaning during 6 or 18 months to normal rats.

METHODS

Animals were divided in three groups: control (C), E-treated and L-treated; treated rats received 10 mg/ kg per day of drug. Systolic blood pressure (SBP), body weight, water and food intake (WI, FI), cardiac, left ventricular and aortic weight as well as the length of the tail were recorded. NADPH-diaphorase activity was determined as a marker of nitric oxide synthase (NOS) activity in aorta, arterioles of small intestine, heart and kidney of normal rats. NOS activity was measured as optical density (OD) in the stained tissue. Nitrate + nitrite urinary excretion was measured in 24 h urine. Only significant differences (P < 0.05) are reported.

RESULTS

SBP, absolute cardiac, left ventricular and aortic weight increased with age. Both treatments delayed these increments. At 6 and 18 months, NOS activity was higher in aortic endothelium (Em) of L- and E-treated animals. Losartan treatment during 6 months also increased NOS activity in aortic smooth muscle (SM). Aortic Em NOS activity fell in the 18 months-treated and untreated animals. E increased NOS activity in the SM of intestinal arterioles at 6 months but reduced it at 18 months.

CONCLUSIONS

The fact that both E and L delayed cardiac hypertrophy/hyperplasia and aortic growth and raised aortic endothelium NOS activity indicates a protective effect on cardiovascular damage due to aging, exerted through inhibition of angiotensin II.

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.

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.

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

Atrial natriuretic peptide (ANP) decreases sodium absorption in small intestine of rats in vitro under sodium concentration-gradient conditions (SCG) and this effect may be mediated by the inhibition of the sodium/glucose cotransporter (SGLT). In order to assess this hypothesis, the effects of ANP, phloridzine (Phlz) and methylene blue (MB), added alone or together, using a voltage clamp technique in Ussing's chamber with SCG were studied. ANP and Phlz significantly decreased potential difference and short circuit current. Effects of Phlz and ANP were not additive. The addition of MB alone did not affect ion transport, whereas it abolished ANP effects. These data suggest that ANP blocks the SGLT through mechanisms mediated by cGMP and/or NO.