Salt-sensitive hypertension: contribution of chloride.

The effect of the anion associated with sodium loading on the development of hypertension in the Dahl salt-sensitive rat was determined. For 5 weeks rats were fed a diet containing normal or high concentrations of sodium chloride or high concentrations of sodium provided as a mixture of sodium bicarbonate, phosphate, and amino acids. After 1 week on these diets and until the end of the study the rats receiving high concentrations of sodium chloride had higher systolic blood pressures than the rats in the other two groups. There were no statistically significant group differences in plasma volume, arterial pH, or plasma concentrations of Na+, K+, Cl-, Ca2+, or creatinine, or in renomedullary prostaglandin E2 production. Compared to the animals receiving normal concentrations of sodium chloride, those receiving high concentrations of sodium chloride or amino acids showed decreased plasma renin activity and plasma aldosterone concentrations. Thus, the anion ingested with sodium alters the development and severity of hypertension in the Dahl salt-sensitive rat.

A microduplication of the long range SHH limb regulator (ZRS) is associated with triphalangeal thumb-polysyndactyly syndrome.

BACKGROUND: Sonic hedgehog (SHH) plays an important role in defining the anterior-posterior axis in the developing limbs. A highly conserved non-coding sequence about approximately 1 Mb upstream from the sonic hedgehog gene (SHH) was shown to be a long range regulator for SHH expression in the limb bud. Point mutations within this non-coding regulatory region designated ZRS lead to ectopic expression of Shh in the anterior margin of the limb bud, as shown in mice, and cause the human triphalangeal thumb and polysyndactyly (TPT-PS) phenotype. Even though this association is well established, its molecular mechanism remains unclear. METHODS AND RESULTS: We investigated a large pedigree with variable TPT-PS. A single nucleotide exchange within the SHH limb regulator sequence was excluded, but locus specific microsatellite marker analyses confirmed a linkage to this region. Subsequently, array comparative genomic hybridisation (array CGH) was carried out using a submegabase whole human genome tiling path bacterial artificial chromosome (BAC) array revealing a microduplication in 7q36.3 in affected individuals. A duplicated region of 588,819 bp comprising the ZRS was identified by quantitative real-time polymerase chain reaction (qPCR) and direct sequencing. CONCLUSION: A novel microduplication in 7q36.3 results in a similar TPT-PS phenotype as caused by single nucleotide alterations in the ZRS, the limb specific SHH regulatory element. Duplications can be added to the growing list of mechanisms that cause abnormalities of long range transcriptional control.

Effect of increased peritubule protein concentration on proximal tubule reabsorption in the presence and absence of extracellular volume expansion.

The effect of increased peritubule capillary oncotic pressure on sodium reabsorption by the proximal tubule of the dog was investistigated after extracellular volume expansion (ECVE) with Ringer's solution or during continued hydropenia. Control measurements were made after ECVE or during hydropenia and again during renal arterial infusion with hyperoncotic albumin solution. Absolute reabsorption by the proximal tubule was calculated from fractional reabsorption and single nephron filtration rates as determined by micropuncture. Direct measurements of efferent arteriole protein were used to determine efferent arteriolar oncotic pressure. Albumin infused into the renal artery after ECVE significantly increased efferent oncotic pressure by 17.6 plus or minus 5.3 mm Hg. Fractional and absolute reabsorption by the proximal tubule increased from 20 plus or minus 6 to 37 plus or minus 5% and from 22 plus or minus 6 to 36 plus or minus 7 nl/min, respectively. During hydropenia, the albumin infusion significantly increased efferent oncotic pressure by 15.0 plus or minus 4.4 mm Hg. However, in contrast to the effect seen during ECVE, neither fractional nor absolute reabsorption was changed, delta equals 0.3 plus or minus 1.5% and 3 plus or minus 5 nl/min, respectively. Single nephron filtration rates were not significantly different between the groups and were unchanged by the albumin infusion. Peritubule capillary hydrostatic pressures, measured with a null-servo device, were not changed by the albumin infusion in either group. Renal interstitial hydrostatic pressure, measured from chronically implanted polyethylene capsules, was decreased significantly from 7.2 plus or minus 0.9 to 3.4 plus or minus 0.6 mm Hg in the hydropenic group and from 0.6 plus or minus 0.6 to 4.8 plus or minus 0.7 mm Hg in the Ringer's expanded group. In the hydropenic group, the increase in efferent oncotic pressure was nearly compensated for by changes in interstitial forces so that the calculated net force for capillary uptake was almost unchanged, 17.8 mm Hg before vs. 21.4 mm Hg during the albumin infusion. The increased efferent oncotic pressure in the Ringer's expanded group was not compensated, so that the calculated net force for uptake was increased, 11.9 mm Hg before to 22.2 mm Hg during the albumin infusion. Thus, while the increase in efferent oncotic pressure during albumin infusion was not significantly different between the groups, absolute and fractional reabsorptions were increased only in the animals in which the extracellular volume was expanded. The results suggest that ECVE alters the effect of increased peritubule oncotic pressure on sodium reabsorption by the proximal tubule.

Effect of volume expansion on sodium excretion in the presence and absence of increased delivery from superficial proximal tubules.

The role of the proximal tubule in the natriuresis after volume expansion was investigated by evaluating sodium excretion both in the presence and absence of increased delivery from the proximal tubule. Proximal delivery was calculated from fractional reabsorption in superficial proximal tubules determined by micropuncture and glomerular filtration rate of the micropunctured kidney. Infusion of Ringer's solution in six dogs increased delivery from the proximal tubule 4.7+/-1 ml/min (P < 0.01) and increased fractional sodium excretion 3.6+/-1.1% (P < 0.025). Infusion of hyperoncotic albumin into the renal artery during sustained volume expansion decreased delivery from the proximal tubule 6.5+/-0.9 ml/min (P < 0.01). Although proximal delivery was restored to below control levels, fractional sodium excretion was significantly increased 2.5+/-0.5% (P < 0.01) as compared with the hydropenic control period. Fractional phosphate excretion was increased 15.5+/-3.7% (P < 0.01) after Ringer's infusion and was decreased 10.5+/-1.6% (P < 0.005) after intrarenal albumin infusion, suggesting that changes in superficial nephron reabsorption were paralleled by changes in reabsorption in deeper nephrons. Similar results were found in six additional dogs in which other factors known to affect phosphate reabsorption were controlled; however, these studies cannot completely eliminate a role for deep nephrons in the natriuresis after intrarenal albumin infusion. Since 70% of the natriuresis after volume expansion was present without increased delivery from superficial proximal tubules, it is likely that increased delivery from the proximal tubule contributes a relatively minor fraction to the natriuresis of volume expansion.

Proximal tubule reabsorption after hyperoncotic albumin infusion. Role of parathyroid hormone and dissociation from plasma volume.

Preferential expansion of the plasma volume by infusion of salt-poor hyperoncotic albumin solution decreases sodium reabsorption by the proximal tubule. The present micropuncture studies test the thesis that albumin infusion depresses proximal reabsorption by an effect unrelated to expansion of the plasma volume, perhaps due to an effect of parathyroid hormone (PTH) on proximal sodium reabsorption. Infusion of salt-poor hyperoncotic albumin significantly decreased plasma ionized calcium, increased immunoreactive PTH (iPTH) in plasma, decreased sodium reabsorption by the proximal tubule, and increased phosphate clearance. In contrast, infusions of albumin, in which the ionized calcium was restored to normal plasma levels, had no significant effect on ionized calcium, iPTH, proximal reabsorption, or phosphate clearance in intact dogs. Similarly, in parathyroidectomized animals given a constant replacement infusion of PTH, albumin infusion had no significant effect on proximal reabsorption or phosphate clearance. Plasma volume was markedly expanded following albumin infusion in all groups of dogs. These findings (a) indicate that PTH plays a significant role in the decrease in sodium reabsorption by the renal proximal tubule after salt-poor hyperoncotic albumin infusion, and (b) dissociate preferential expansion of the plasma volume from decreases in sodium reabsorption by the proximal tubule.

Effect of adrenalectomy on renin release in the rat.

The purpose of this study was to evaluate the mechanism of increased renin secretion in the adrenalectomized (Adx) rat. Plasma renin concentration (PRC) responses to acute infusion of 0.9% NaCl (5% of body weight) were compared in three groups of rats: Adx, Adx rats treated with dexamethasone (Adx + Dex), and sham controls. During the 7 days after adrenalectomy and before acute infusion. Adx animals drank 0.9% NaCl; sham animals drank water. Despite a more positive sodium balance over the 7 days, preinfusion PRC was higher in Adx than in the other two groups (P less than 0.01) and did not decrease with NaCl infusion [31.2 +/- 9.6 (+/-SE) U/30 min to 30.4 +/- 9.5]. PRC was suppressed by NaCl infusion in Adx + Dex (10.2 +/- 2.4 to 4.1 +/- 1.2) and sham controls (9.7 +/- 0.9 to 2.6 +/- 0.5). In separate groups of animals, PRC decreased (P less than 0.01) in response to volume expansion with 25% albumin (1% of body weight) in both Adx (42.6 +/- 8.9 to 23.1 +/- 6.1) and sham controls (10.2 +/- 1.2 to 2.1 +/- 0.7). These results are consistent with the hypothesis that altered responsiveness to NaCl, in the absence of volume contraction, contributes to increased renin release in adrenal insufficiency. Glucocorticoid replacement restored renin responsiveness to NaCl.

Failure of salt loading to inhibit tissue norepinephrine turnover in prehypertensive Dahl salt-sensitive rats.

To determine if alterations of electrolyte balance or sympathetic nervous system activity are present in Dahl salt-sensitive rats (DS) before the onset of hypertension, we compared electrolyte balances, extracellular fluid volume (inulin space), plasma volume (radiolabeled albumin), and norepinephrine turnover in peripheral tissues (heart and interscapular brown fat) in prehypertensive DS and Dahl salt-resistant rats (DR). Animals were maintained for 5 to 7 days on either a "normal" or high NaCl diet. Tissue norepinephrine turnover was evaluated by measuring the rate at which norepinephrine content decreased following tyrosine hydroxylase inhibition with alpha-methyl-p-tyrosine. Blood pressure was higher (p less than 0.05) in DS (135 +/- 2 [SE] mm Hg) than in DR (129 +/- 2 mm Hg) and was not affected by the diets. Extracellular fluid volume and net Na+ and Cl- balances did not differ between DS and DR. However, plasma volume was greater in DS than in DR (p less than 0.05). In both fat and heart, norepinephrine turnover was decreased by dietary NaCl loading in DR (p less than 0.01), but not in DS. Thus, the tendency of the DS to become hypertensive with high NaCl intake may be related to the combined effects of an increased plasma volume and the failure of high dietary NaCl to inhibit peripheral sympathetic nervous system activity.

Importance of chloride for deoxycorticosterone acetate-salt hypertension in the rat.

Selective dietary sodium loading (without chloride) fails to produce hypertension in the Dahl salt-sensitive rat. This study attempted to evaluate the effect of selective sodium loading on blood pressure in another NaCl-dependent model of hypertension--deoxycorticosterone acetate (DOCA)-salt hypertension. Three groups of uninephrectomized rats were studied for 32 days on one of the following regimens: (1) high NaCl diet plus DOCA, (2) high dietary sodium intake without chloride plus DOCA, and (3) high NaCl diet without DOCA. Both indirect and direct arterial pressure were higher (p less than 0.01) in the DOCA-NaCl group than in the other two groups. In the two DOCA-treated groups, net sodium and potassium balance and total carcass sodium and potassium content did not differ. In the DOCA-NaCl group, higher blood pressures were associated with a more positive chloride balance and total carcass chloride content (p less than 0.01), an expanded extracellular fluid volume (p less than 0.05), and increased renal vascular resistance (p less than 0.01). Higher renal vascular resistance in DOCA-NaCl animals suggests that chloride contributes to NaCl-induced vasoconstriction.

Mechanism of increased renin release in the adrenalectomized rat. Adrenal insufficiency and renin.

We have previously suggested that inhibition of renin release by sodium chloride is related to absorptive chloride transport in the loop of Henle. Infusion of sodium chloride fails to inhibit renin release in the adrenalectomized (Adx) rat, and dexamethasone restores renin responsiveness to sodium chloride. The purpose of the present study was to evaluate the relationship between loop function (urinary diluting and concentration capacity) and plasma renin concentration (PRC) in the Adx rat. After hypotonic sodium chloride infusion, free water clearance (CH2O) of Adx rats (0.56 ml/hr/100 g +/- 0.17 SE) was decreased (p less than 0.01) compared to controls (2.86 ml/hr/100 g +/- 0.29 SE); PRC of Adx rats (61.9 units/ml +/- 11.2 SE) was increased (p less than 0.01) above controls (6.0 units/ml +/- 1.7 SE). These differences persisted after administration of d(CH2)5Tyr(Et)VAVP, a potent ADH antagonist. In separate groups of animals, after water deprivation, urine concentration of Adx rat (1,401 mOsm/kg +/- 45 SE) was less (p less than 0.01) than that of controls (2,117 mOsm/kg +/- 169 SE). Dexamethasone normalized both CH2O and urinary concentrating ability and also decreased PRC in Adx rats. Thus, in the glucocorticoid deficient rat, increased renin release is associated with impaired loop function. The loop defect may account for high PRC that is not suppressed by sodium chloride.

Effect of dietary chloride on salt-sensitive and renin-dependent hypertension.

We have previously reported that 1) selective dietary sodium loading (without chloride) does not produce hypertension in rats of the Dahl salt-sensitive strain (DS) and 2) selective chloride loading (without sodium) lowers plasma renin activity in the intact Sprague-Dawley rat maintained on a low NaCl diet. The present study examined the effect of selective dietary chloride loading on two models of hypertension: the DS and the renin-dependent one-kidney, one clip Sprague-Dawley rat. The DS were pair-fed (n = 7/group) a "normal" NaCl, a high NaCl (4%), or a "normal" sodium-high chloride diet for 11 weeks. From Week 7 until the end of the experiment, the high NaCl-fed animals had higher (p less than or equal to 0.05) blood pressures than animals fed either the normal NaCl or normal sodium-high chloride diet, which were not different from each other. Thus, in the DS, hypertension depends on high dietary intakes of both sodium and chloride. In one-kidney, one clip hypertensive rats, selective chloride loading failed to lower plasma renin activity (9 +/- 1 vs 7 +/- 1 ng angiotensin I/ml/hr) or to prevent hypertension (160 +/- 10 vs 166 +/- 9 mm Hg). Thus, selective dietary chloride loading (without sodium) does not alter blood pressure in either salt-sensitive or renin-dependent hypertension.

Thin section arrays for I-FISH analysis of chromosome-specific imbalances in squamous cell carcinomas of the head and neck.

Thin section arrays of 20 head and neck squamous cell carcinomas were studied by I-FISH for gains (including amplification) and losses of specific genomic segments. These arrays allow the examination not only of a number of tumor sections but also of the surrounding margins and of inconspicuous control tissue in one experiment. All tumor sections examined significantly differed from the inconspicuous control tissues by containing more or less extensive cell populations with aberrant signal constitutions. In no case, however, did the aberrant population constitute the whole area of the section. Gains of signals were strikingly more frequent than were losses. All tumors showed significant gains of the segments examined, the highest differences between tumor and control sections were found for the segments 9q34 and 8q24, followed by 5p15.3 and 11q13. Amplifications were most frequently found of 11q13: 8 of the 20 tumors showed amplifications in more than 20% of the nuclei, while no nucleus with more than four signals was found in any of the control tissues (control: 0%). Amplifications of the target sequences on chromosomes 8 (14 tumors) and 9 (8 tumors) were observed in low but significant percentages of nuclei, no significant cell population was detected with an amplification of 5p15.3. Fourteen tumors exhibited a significant loss of 13q14, and only 8 tumors a significant loss at any other site. In the tumor margin sections, in most cases, the margins apparently were also affected by the one or the other of the genomic changes of the pertinent primary tumor. Nevertheless, there were, in some cases, also large differences depending on the way of analysis, but also on the specific signal constitution considered. Tumor stages T3 and T4 tended to have higher frequency of nuclei with gains of 5p15.3, 8q24, and 11q13 as compared to T2 tumors and less gains of 9q34 and loss of 13q14. With the exception of 8q24 and 13q14 alterations there was also a trend to higher percentages of aberrant nuclei in the margin of T3-4 tumors vs. T2 tumors.

Calcium, parathyroid hormone, and vitamin D in the "prehypertensive" Dahl salt-sensitive rat.

The purpose of this study was to determine if alterations of calcium and calcium regulating hormones precede the onset of NaCl-induced hypertension in the Dahl salt-sensitive (S) rat. After a 5-day balance study, serum ionized calcium, parathyroid hormone (PTH), and 1,25-dihydroxy vitamin D concentrations were measured in Dahl-S and salt-resistant (R) rats that had been maintained on a "normal" (1%) or high (7%) NaCl intake. Blood pressure was higher in Dahl-S than Dahl-R (P less than .01), but was not affected by 5 days of high NaCl. On both NaCl intakes, urine calcium excretion was increased, serum calcium was decreased, and serum PTH and 1,25 dihydroxy vitamin D were increased in Dahl-S compared to Dahl-R (P less than .01). On the high NaCl intake, fecal calcium was greater in Dahl-S than in Dahl-R, and net 5-day calcium balance was less positive in Dahl-S (P less than .05). Thus, alterations of calcium, PTH, and vitamin D precede NaCl-induced hypertension in Dahl-S. These alterations may contribute to the development of hypertension in this animal model.

Calcium and calcium regulating hormones in the "prehypertensive" Dahl salt sensitive rat (calcium and salt sensitive hypertension).

The purpose of this study was to determine if alterations of calcium and calcium regulating hormones precede the onset of NaCl induced hypertension in the Dahl salt sensitive (S) rat. After a 5 day balance study, serum ionized calcium, PTH, and 1,25 dihydroxy vitamin D concentrations were measured in Dahl-S and salt resistant (R) rats that had been maintained on a "normal" (1%) or high (7%) NaCl intake. Blood pressure was higher in Dahl-S than R (P less than .01), but was not affected by 5 days of high NaCl. On both NaCl intakes, urine calcium excretion was increased, serum calcium was decreased, and serum PTH and 1,25 dihydroxy vitamin D were increased in Dahl-S compared to Dahl-R (P less than .01). On the high NaCl intake, fecal calcium was greater in Dahl-S than in Dahl-R, and net 5 day calcium balance was less positive in Dahl-S (P less than .05). In contrast to NaCl, a high dietary intake of sodium with anions other than chloride (NaAA) fails to produce hypertension in the Dahl-S rat. NaAA loading resulted in decreased urine calcium excretion (P less than .01), and after 5 days of the high NaAA diet, serum calcium and PTH did not differ in Dahl-S and Dahl-R. Thus, alterations of calcium, PTH, and vitamin D precede NaCl-induced hypertension in Dahl-S. These alterations may contribute to the development of hypertension in this animal model.

Site-specific effects of dietary salt intake on guanylin and uroguanylin mRNA expression in rat intestine.

Guanylin and uroguanylin are newly discovered intestinal peptides that have been shown to affect NaCl transport in both the intestine and kidney. The present study tests the hypothesis that guanylin and uroguanylin mRNA expression in each major region of the intestine is regulated by NaCl intake. Semiquantitative multiplex RT-PCR analysis was used to determine the molecular expression of guanylin and uroguanylin in the duodenum, jejunum, ileum, and colon in rats maintained on low (LS), normal (NS), or high (HS) NaCl intake for 4 days. LS intake reduced the expression of uroguanylin, and to a lesser degree, guanylin mRNA in all intestinal segments compared to NS intake. The duodenum was the site of the greatest decrease for both. In contrast, HS intake significantly increased the expression of guanylin mRNA only in the duodenum and jejunum and had minimal effect on uroguanylin mRNA. The minimum time required for altered gene expression was determined by delivering an oral NaCl challenge directly to the gastrointestinal tract by oro-gastric administration to LS or NS animals. In LS rats, NaCl oro-gastric administration significantly increased mRNA expression of both peptides in all intestinal segments. Furthermore, the increases in guanylin and uroguanylin mRNA were detected within 4 h and plateaued by 8 h. Conversely, acute oro-gastric administration of the same NaCl solution to NS rats caused elevations of guanylin mRNA only in the duodenum and jejunum, and of uroguanylin mRNA only in the ileum and colon. In conclusion, the data demonstrate that variations in NaCl intake lead to intestinal segment-specific changes in guanylin and uroguanylin mRNA expression.

Guanylyl cyclase-C receptor mRNA distribution along the rat nephron.

Guanylin (GN) and uroguanylin (UGN) are two recently identified peptides that have been shown to affect water and electrolyte transport in both the intestine and the kidney. Mechanistically, the effects of both peptides are thought to be mediated by intracellular cGMP which results from ligand binding to a plasma membrane guanylyl cyclase-C (GC-C) receptor. To date, the specific intrarenal site(s) of GN and UGN action have not been established. To begin to address this issue, the present studies utilized semi-quantitative RT-PCR to assess the distribution of GC-C mRNA in specific microdissected segments of the rat nephron. GC-C mRNA expression was highest in the cortical collecting tubule, followed by the proximal convoluted tubule, medullary thick ascending limb and collecting tubule, and thin limbs of Henle's loop. Expression levels were significantly lower in all other segments tested, including the glomerulus. The renal tubular expression pattern for cGMP-dependent protein kinase II (cGK-II) mRNA, which is activated in response to GN/UGN-dependent cGMP accumulation, was similar to that for GC-C. Notably, both GN and UGN mRNAs were also expressed along the nephron. The highest levels of expression for both peptides were detected in the medullary collecting tubule. Lower, but comparable levels of GN and UGN expression also occurred in the cortical collecting tubule, cortical and medullary thick ascending limb, and thin limbs of Henles loop. In the proximal convoluted tubule, GN mRNA expression was also quite high, while UGN mRNA was almost undetectable. The presence of renal GC-C and cGK-II in the kidney are consistent with a proposed endocrine function for GN and UGN. In addition however, the present data suggest that intrarenally synthesized GN and UGN may also contribute to the regulation of renal tubular transport.

Acute effects of gentamicin on thick ascending limb function in the rat.

It is well established that the aminoglycoside antibiotics can adversely affect proximal tubule function. Predominantly indirect evidence suggests that aminoglycosides may also affect function of more distal nephron segments. The present study utilized whole kidney clearance, in vivo micropuncture and in vitro microperfusion to directly determine whether acute gentamicin treatment affects sodium chloride transport in the thick ascending limb of the loop of Henle. Gentamicin (25 mg/kg) significantly increased urine flow, as well as sodium, potassium and chloride excretion within 15 min of intravenous injection. Glomerular filtration rate and proximal tubule fluid reabsorption were not altered by acute gentamicin treatment. In contrast, both fractional and absolute loop chloride transport was significantly decreased. In the in vitro microperfused medullary thick ascending limb, luminal but not basolateral administration of gentamicin (1 mM) significantly decreased chloride reabsorption when compared to time controls. These data suggest that the increased urine and electrolyte excretion associated with acute gentamicin treatment is, at least in part, a consequence of decreased transport in the thick ascending limb of Henle's loop.

Ultrasonic characterization of acute renal failure.

The attenuation coefficient, propagation velocity and backscattering coefficient were measured in vitro, on freshly excised and functionally impaired rabbit kidneys. subcutaneous glycerol treatment was used to introduce acute renal failure. Elevated plasma creatinine levels, measured prior to the excision of kidneys, were used as an index of the degree of renal functional impairment. Propagation velocity for the ten kidneys ranged between 1538-1575 m/s with that for the normals being 1540 +/- 4 m/s. Velocity was found to increase with increasing renal damage. The attenuation coefficient for all ten kidneys exhibited a linear frequency dependence over the range 3.5-6.5 MHz. The slope of the attenuation coefficient for the glycerol treated kidneys (0.723 dB/cm/MHz) was found to be higher than the slope for the normals (0.499 dB/cm/MHz). The frequency dependence of the backscattering coefficient was not altered by glycerol treatment leading to the postulate that modification of frequency dependent behavior of the attenuation coefficient in this experimental model is primarily due to absorption.

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 gamma-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.