The effect of heat-stable escherichia coli enterotoxin, theophylline and forskolin on cyclic nucleotide levels and mucosal surface (acid microclimate) pH in rat proximal jejunum in vivo

H+-coupled nutrient, micronutrient and drug transporters in the mammalian small intestine

The H(+)-electrochemical gradient was originally considered as a driving force for solute transport only across cellular membranes of bacteria, plants and yeast. However, in the mammalian small intestine, a H(+)-electrochemical gradient is present at the epithelial brush-border membrane in the form of an acid microclimate. Over recent years, a large number of H(+)-coupled cotransport mechanisms have been identified at the luminal membrane of the mammalian small intestine. These transporters are responsible for the initial stage in absorption of a remarkable variety of essential and non-essential nutrients and micronutrients, including protein digestion products (di/tripeptides and amino acids), vitamins, short-chain fatty acids and divalent metal ions. Proton-coupled cotransporters expressed at the mammalian small intestinal brush-border membrane include: the di/tripeptide transporter PepT1 (SLC15A1); the proton-coupled amino-acid transporter PAT1 (SLC36A1); the divalent metal transporter DMT1 (SLC11A2); the organic anion transporting polypeptide OATP2B1 (SLC02B1); the monocarboxylate transporter MCT1 (SLC16A1); the proton-coupled folate transporter PCFT (SLC46A1); the sodium-glucose linked cotransporter SGLT1 (SLC5A1); and the excitatory amino acid carrier EAAC1 (SLC1A1). Emerging research demonstrates that the optimal intestinal absorptive capacity of certain H(+)-coupled cotransporters (PepT1 and PAT1) is dependent upon function of the brush-border Na(+)-H(+) exchanger NHE3 (SLC9A3). The high oral bioavailability of a large number of pharmaceutical compounds results, in part, from absorptive transport via the same H(+)-coupled cotransporters. Drugs undergoing H(+)-coupled cotransport across the intestinal brush-border membrane include those used to treat bacterial infections, hypercholesterolaemia, hypertension, hyperglycaemia, viral infections, allergies, epilepsy, schizophrenia, rheumatoid arthritis and cancer.

Effects of guanylin and uroguanylin on rat jejunal fluid and electrolyte transport: comparison with heat-stable enterotoxin

The effects of rat guanylin, human guanylin, human uroguanylin and STa on net fluid and electrolyte transport in the closed jejunal loop were compared in anesthetized rats. STa administered into the lumen caused a concentration-dependent (10(-8) to 10(-6) M) inhibition of net fluid and NaCl absorption in the jejunal loop. Uroguanylin had a similar but weaker effect than STa. Both rat and human guanylin inhibited fluid and NaCl absorption only at 10(-6) M. Their order of potency was STa > human uroguanylin > rat guanylin = human guanylin. Changing the luminal pH from 5 to 8 failed to affect the action of guanylin on fluid absorption. Both STa and uroguanylin, but not guanylin, increased the luminal pH by stimulating bicarbonate secretion. Pretreatment of the jejunal loop with guanylin (10(-6) M) 5 min before the instillation of STa (10(-7) M) significantly reduced the inhibitory effect of STa on fluid absorption. It is concluded that guanylin and uroguanylin administered into the rat jejunal lumen have an STa-like action on fluid and electrolyte transport. Guanylin may act as an endogenous antagonist of STa in the rat jejunum and prevent excessive fluid loss by STa.

Atrial natriuretic peptide and cGMP inhibit Na+/H+ antiporter in vascular smooth muscle cells in culture

The aim of the present paper was to study the mechanisms of the inhibitory effect of atrial natriuretic peptide (ANP) on the sustained contraction phase of vascular smooth muscle cells (VSMC). Specifically, the potential role of ANP on the Na+/H+ antiporter and Na+ transport systems was investigated. Both ANP and 8-bromo cGMP inhibited 22Na+ uptake and decreased intracellular Na ([Na+]i) in VSMC, an effect that was mimicked by the specific Na+/H+ antiporter inhibitor, hexamethylen amiloride (HMA). The effect of ANP was not additive with HMA, therefore suggesting that both inhibit the same 22Na+ transport pathway. On the other hand, the inhibition of 22Na+ accumulation by ANP was additive with the inhibition by furosemide or bumetanide, thus suggesting that both drugs act on different Na+ exchange systems. In HEPES-buffered medium, ANP, cGMP, and HMA significantly inhibited the AVP-induced intracellular alkalinization, an effect which was associated with significant inhibition of the AVP-induced shape change. In bicarbonate buffered medium, ANP and cGMP decreased the pH level below the baseline after application of AVP, and an inhibition by ANP and cGMP of AVP-induced VSMC shape change was also observed. The recovery of cellular pH after three different types of acid load, namely, ammonium chloride pulse, nigericin clamp and lowering of extracellular pH, was significantly decreased by ANP and cGMP. Taken together, these results indicate that ANP/cGMP inhibit the activity of the Na+/H+ antiporter in VSMC, either in hormone- or pH-stimulated conditions.