Renal effects of uroguanylin and guanylin in vivo

Uroguanylin and guanylin are newly discovered endogenous heat-stable peptides that bind to and activate a membrane bound guanylyl cyclase signaling receptor (termed guanylyl cyclase C; GC-C). These peptides are not only found in blood but are secreted into the lumen of the intestine and effect a net secretion of electrolytes (Na+, K+, Cl-, HCO3-) and fluid into the intestine via a cyclic guanosine-3',5'-monophosphate (cGMP) mechanism. GC-C is also the receptor for Escherichia coli heat-stable enterotoxin (STa) and activation by STa results in a diarrheal illness. Employing mouse renal in vivo models, we have demonstrated that uroguanylin, guanylin, and STa elicit natriuretic, kaliuretic, and diuretic effects. These biological responses are time- and dose-dependent. Maximum natriuretic and kaliuretic effects are observed within 30-40 min following infusion with pharmacological doses of the peptides in a sealed-urethra mouse model. Our mouse renal clearance model confirms these results and shows significant natriuresis following a constant infusion of uroguanylin for 30 min, while the glomerular filtration rate, plasma creatinine, urine osmolality, heart rate, and blood pressure remain constant. These data suggest the peptides act through tubular transport mechanisms. Consistent with a tubular mechanism, messenger RNA-differential display PCR of kidney RNA extracted from vehicle- and uroguanylin-treated mice show the message for the Na+/K+ ATPase g-subunit is down-regulated. Interestingly, GC-C knockout mice (Gucy2c -/-) also exhibit significant uroguanylin-induced natriuresis and kaliuresis in vivo, suggesting the presence of an alternate receptor signaling mechanism in the kidney. Thus, uroguanylin and guanylin seem to serve as intestinal and renal natriuretic peptide-hormones influencing salt and water transport in the kidney through GC-C dependent and independent pathways. Furthermore, our recent clinical probe study has revealed a 70-fold increase in levels of urinary uroguanylin in patients with congestive heart failure. In conclusion, our studies support the concept that uroguanylin and guanylin are endogenous effector peptides involved in regulating body salt and water homeostasis.

The lysine-1 analog of guanylin induces intestinal secretion and natriuresis in the isolated perfused kidney

Guanylin is an endogenous peptide synthesized by several mammalian species that mimics the effects of a thermostable enterotoxin of Escherichia coli (STa: NTFYCCELCCNPACAGCY) in the gut. We have cloned a lysine-1 derivative of rat guanylin (Lys-1-NTCEICAYAACTGC) and tested its effects on ileal tissue membranes in Ussing chambers and in the isolated perfused rat kidney. Rabbit ileal mucosa membranes were mounted into a Ussing chamber and the effects of Lys-1 guanylin (Lys-1 G) and STa enterotoxin peptide on chloride secretion were determined by changes in short-circuit current (Isc). Lys-1 G (10 to 100 nM) showed a dose-dependent effect on chloride secretion with a maximal response estimated to be 52 microA/cm2. Lys-1 G mimics the effect of STa peptide, but the enterotoxin elicited a greater maximal effect of 120 microA/cm2 (p<0.01). Lys-1 G (2.5 microg/ml) promoted an increase in both urine flow (from 0.13 +/- 0.07 to 0.40 +/- 0.01 ml g(-1) min(-1), N = 4; P<0.05) and glomerular filtration rate (from 0.68 +/- 0.02 to 0.85 0.00 ml g(-1) min(-1), N = 4; P<0.01) in the isolated perfused kidney and a reduction of the fractional reabsorption of sodium (from 76.0 +/- 0.03 to 59.5 +/- 0.85 percent, N = 4; P<0.01). These maximal effects were accompanied by intense natriuretic effect observed 30 and 60 min after drug administration. The Lys-1 G analog similar to STa enterotoxin elicited intestinal chloride secretion and a natriuretic effect. These data demonstrate that the cloned peptide analog retains the biological activity of the native hormone and presents activity similar to STa. The properties of Lys-1 G resemble those of a factor formed during perfusion of the hypoxic rabbit kidney and named by us factor natriureticus similis (FNS).