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Hepatic and circulating endothelin-1 (ET-1) are increased in patients with cirrhosis and in cirrhotic animals. However, the distinct roles of ET receptor subtypes [ET.sub.A] and [ET.sub.B] in cirrhosis and portal hypertension (PHT) have not been clearly elucidated. Thus, we studied the effects of selective ET-1 antagonists ([ET.sub.A]-ant or [ET.sub.e]-ant) and nonselective ET-1 antagonist ([ET.sub.A/R]-ant) on hepatic hemodynamics in cirrhotic rats. Liver fibrosis and PHT were induced by complete bile duct ligation (BDL) in rats. Two weeks after BDL or sham surgery, hemodynamic responses were measured during intraportal infusion of incremental doses of the following ET-ants: (i) BQ-123, (ii) BQ-788, and (iii) bosentan. After equilibration with vehicle, doses of ET-ants were infused for 30 min periods, and steady-state systemic and hepatic hemodynamics, portal venous pressure (PVP), and hepatic blood flow (HBF) were measured. BDL induced significant PHT and elevated concentrations of plasma ET-1 compared with sham. [ET.sub.A]- ant decreased PVP of cirrhotic rats but had no effect on sham, whereas ETis-ant increased PVP in sham but had no effect in BDL. Nonselective [ET.sub.A/R]-ant decreased PVP of BDL similarly to [ET.sub.A]-ant. Both [ET.sub.A]-ant and [ET.sub.A/R]-ant decreased local HBF, whereas a nonselective antagonist did not change HBF in sham; however no significant changes were observed in HBF of BDL rats with any of the antagonists. These findings suggest [ET.sub.A] activation contributes to PHT in cirrhotic rats, whereas ETH-mediated portal depressor effects are attenuated in cirrhotic rats compared with noncirrhotic rats. Key words: cirrhosis, hepatic hemodynamics, hepatic flow, endothelin. Les taux d'endothelines-1 (ET-1) circulantes et hepatiques augmentent chez les animaux et les patients cirrhotiques. Toutefois, les roles specifiques des sous-types de recepteurs de l'ET, [ET.sub.A] et [ET.sub.B], dans la cirrhose et l'hypertension portale (HTP) n'ont pas encore ete totalement elucides. Nous avons examine les effets d'antagonistes selectifs ([ET.sub.A] et [ET.sub.B]) et non selectif de l'ET-1 (ant-[ET.sub.A/B]) sur l'hemodynamique hepatique de rats normaux et cirrhotiques. Nous avons induit une Fibrose hepatique et une HTP par une ligature complete du canal choledoque (LCC) chez des rats. Deux semaines apres la LCC ou une operation simulee, nous avons mesure les reponses dynamiques durant l'administration intraporte de doses graduelles des ant-ET suivants: (i) BQ-123, (ii) BQ-788, et (iii) bosentan. Apres une mise a l'equilibre avec un vehicule, nous avons perfuse les doses d'ant-ET pendant des periodes de 30 min, puis mesure les reponses du debit sanguin hepatique (DSH), de la pression veineuse porte (PVP) et de l'hemodynamique systemique a l'equilibre. La LCC, par rapport a l'operation simulee, a induit une forte HTP et une augmentation significative des concentrations plasmatiques d'ET-1. Les ant-[ET.sub.A] ont diminue la PVP des rats cirrhotiques, mais n'ont pas eu d'effet sur celle des rats soumis a l'operation simulee, alors que les ant-[ET.sub.B] ont eu les effets inverses. Les ant-ET non selectifs et les ant-[ET.sub.A] ont diminue de maniere analogue la PVP des LCC. Chez les rats soumis a une operation simulee, les ant-[ET.sub.A] et [ET.sub.B] ont diminue le DSH local, alors qu'un antagoniste non selectif ne l'a pas modifie; cependant, aucun des antagonistes n'a modifie significativement le DSH chez les rats LCC. Ces resultats donnent a penser que l'activation de l'[ET.sub.A] contribue a l'HTP chez les rats cirrhotiques, alors que les effets hypotenseurs vehicules par l'[ET.sub.B] pourraient etre attenues chez les rats cirrhotiques comparativement a ceux observes chez les rats normaux. Mots-des : cirrhose, hemodynamique hepatique, debit hepatique, endotheline. [Traduit par la Redaction], Introduction Portal hypertension (PHT) and its complications are among the main causes of morbidity and mortality in patients suffering of liver fibrosis. The development of PHT in animals and patients [...]

A study was conducted to test the hypothesis that endogenously produced nitric oxide (NO) directly reduces NaCl transport by the thick ascending limb of the loop of Henle (TALH). Results indicated a significant role for locally produced NO, which may act through an autocrine mechanism to directly affect TALH sodium chloride transport. Hence, TALH NO synthesis and inhibition of chloride transport may contribute to the diuretic and natriuretic effects of NO seen in vivo.

Stimulation of [[Alpha].sub.2]-adrenergic receptors inhibits transport in various nephron segments, and the thick ascending limb of the loop of Henle (THAL) expresses [[Alpha].sub.2]-receptors. We hypothesized that selective [[Alpha].sub.2]-receptor activation decreases NaCl absorption by cortical THALs through activation of NOS and increased production of NO. We found that the [[Alpha].sub.2]-receptor agonist clonidine (10 nM) decreased chloride flux ([J.sub.Cl]) from 119.5 [+ or -] 15.9 to 67.4 [+ or -] 13.8 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1] (43% reduction; P [is less than] 0.02), whereas removal of clonidine from the bath increased [J.sub.Cl] by 20%. When NOS activity was inhibited by pretreatment with 5 mM [N.sup.G]-nitro-L-arginine methyl ester, the inhibitory effects of clonidine on THAL [J.sub.Cl] were prevented (81.7 [+ or -] 10.8 vs. 71.6 [+ or -] 6.9 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]). Similarly, when the NOS substrate L-arginine was deleted from the bath, addition of clonidine did not decrease THAL [J.sub.Cl] from control (106.9 [+ or -] 11.6 vs. 132.2 [+ or -] 21.3 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]). When we blocked the [[Alpha].sub.2]-receptors with rauwolscine (1 [micro] M), we found that the inhibitory effect of 10 nM clonidine on THAL [J.sub.cl] was abolished, verifying that [[Alpha].sub.2], rather than [I.sub.1], receptors mediate the effects of clonidine in the THAL. We investigated the mechanism of NOS activation and found that intracellular calcium concentration did not increase in response to clonidine, whereas pretreatment with 150 nM wortmannin abolished the clonidine-mediated inhibition of THAL [J.sub.Cl], indicating activation of phosphatidylinositol 3-kinase and the Akt pathway. We found that pretreatment of THALs with 10 [micro] M LY-83583, an inhibitor of soluble guanylate cyclase, blocked clonidine-mediated inhibition of THAL [J.sub.cl]. In conclusion, [[Alpha].sub.2]-receptor stimulation decreases THAL [J.sub.Cl] by increasing NO release and stimulating guanylate cyclase. These data suggest that [[Alpha].sub.2]-receptors act as physiological regulators of THAL NO synthesis, thus inhibiting chloride transport and participating in the natriuretic and diuretic effects of clonidine in vivo. nitric oxide synthase; clonidine; kidney

Plato, Craig F. [Alpha]-2 And [Beta]-adrenergic receptors mediate NE's biphasic effects on rat thick ascending limb chloride flux. Am J Physiol Regulatory Integrative Comp Physiol 281: R979-R986, 2001.--The sympathetic neurotransmitter norepinephrine (NE) influences renal sodium excretion via activation of adrenergic receptors. The thick ascending limb (THAL) possesses both [Alpha]-2 and [Beta]-adrenergic receptors. However, the role(s) different adrenergic receptors play in how isolated THALs respond to NE are unclear. We tested the hypothesis that both [Alpha]-2 and [Beta]-adrenergic receptors are responsive to NE in the isolated THAL, with [Alpha]-2 receptors inhibiting and [Beta]-receptors stimulating chloride flux ([J.sub.Cl]). THALs from male Sprague-Dawley rats were perfused in vitro, and the effects of 1) incremental NE, 2) the [Alpha]-2 agonist clonidine, and 3) the [Beta]-agonist isoproterenol on [J.sub.Cl] were measured. Low concentrations (0.1 nM) of NE decreased [J.sub.Cl] from a rate of 114.2 [+ or -] 8.1 to 93.5 [+ or -] 14.6 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1] (P [is less than] 0.05), with the nadir occurring at 1 nM (67.7 [+ or -] 8.8 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]; P [is less than] 0.05). In contrast, greater concentrations of NE significantly increased [J.sub.Cl] from the nadir to a maximal rate of 131.0 [+ or -] 28.5 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1] at 10 [micro]M (P [is less than] 0.05). To evaluate the adrenergic receptors mediating these responses, the THAL [J.sub.Cl] response to NE was measured in the presence of selective antagonists of [Beta]- and [Alpha]-2 receptors. A concentration of NE (1 [micro]M), which alone tended to increase [J.sub.Cl], decreased THAL [J.sub.Cl] (from 148.9 [+ or -] 16.4 to 76.2 [+ or -] 13.6 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]; P [is less than] 0.01) in the presence of the [Beta]-antagonist propranolol. In contrast, a concentration of NE (0.1 [micro]M), which alone tended to decrease [J.sub.Cl], increased THAL [J.sub.Cl] (from 85.5 [+ or -] 20.1 to 111.8 [+ or -] 20.1 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]; P [is less than] 0.05) in the presence of the [Alpha]-2 antagonist rauwolscine. To further clarify the role of different adrenergic receptors, selective adrenergic agonists were used. The [Alpha]-2 agonist clonidine decreased [J.sub.Cl] from 102.4 [+ or -] 9.9 to 54.0 [+ or -] 15.7 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1], a reduction of 49.1 [+ or -] 11.0% (P [is less than] 0.02). In contrast, the [Beta]-agonist isoproterenol stimulated [J.sub.Cl] from 95.3 [+ or -] 11.6 to 144.1 [+ or -] 15.0 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1], an increase of 56 [+ or -] 14% (P [is less than] 0.01). We conclude that 1) the sympathetic neurotransmitter NE exerts concentration-dependent effects on [J.sub.Cl] in the isolated rat THAL, 2) selective [Alpha]-2 receptor activation inhibits THAL [J.sub.Cl], and 3) selective [Beta]-receptor activation stimulates THAL [J.sub.Cl]. These data indicate the response elicited by the isolated rat THAL to NE is dependent on the neurotransmitter concentration, such that application of NE in vitro biphasically modulates [J.sub.Cl] via differential activation of [Alpha]-2 and [Beta]-adrenergic receptors in a concentration-dependent manner. kidney tubule; clonidine; isoproterenol; phenylephrine; sympathetic nervous system; norepinephrine

Plato, Craig F., David M. Pollock, and Jeffrey L. Garvin. Endothelin inhibits thick ascending limb chloride flux via [ET.sub.B] receptor-mediated NO release. Am J Physiol Renal Physiol 279: F326-F333, 2000.--Endothelin-1 (ET-1) inhibits transport in various nephron segments, and the thick ascending limb of the loop of Henle (TALH) expresses ET-1 receptors. In many tissues, activation of [ET.sub.B] receptors stimulates release of NO, and we recently reported that endogenous NO inhibits TALH chloride flux ([J.sub.Cl]). However, the relationship between ET-1 and NO in the control of nephron transport has not been extensively studied. We hypothesized that ET-1 decreases NaCl transport by cortical TALHs through activation of [ET.sub.B] receptors and release of NO. Exogenous ET-1 (1 nM) decreased [J.sub.Cl] from 118.3 [+ or -] 15.0 to 62.7 [+ or -] 13.6 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1] (48.3 [+ ro -] 8.2% reduction), whereas removal of ET-1 increased [J.sub.Cl] in a separate group of tubules from 87.6 [+ or -] 10.7 to 115.2 [+ ro -] 10.3 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1] (34.5 [+ or -] 6.2% increase). To determine whether NO mediates the inhibitory effects of ET-1 on [J.sub.Cl], we examined the effect of inhibiting of NO synthase (NOS) with [N.sup.G]-nitro-L-arginine methyl ester (L,-NAME) on ET-1-induced changes in [J.sub.Cl]. L-NAME (5 mM) completely prevented the ET-1-induced reduction in [J.sub.Cl], whereas D-NAME did not. L-NAME alone had no effect on [J.sub.Cl]. These data suggest that the effects of ET-1 are mediated by NO. Blockade of [ET.sub.B] receptors with BQ-788 prevented the inhibitory effects of 1 nM ET-1. Activation of [ET.sub.B] receptors with sarafotoxin S6c mimicked the inhibitory effect of ET-1 on [J.sub.Cl] (from 120.7 [+ or -] 12.6 to 75.4 [+ or -] 13.3 pmol [multiplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]). In contrast, [ET.sub.A] receptor antagonism with BQ-610 did not prevent ET-1-mediated inhibition of TALH [J.sub.Cl] (from 96.5 [+ or -] 10.4 to 69.5 [+ or -] 8.6 pmol [muliplied by] [mm.sup.-1] [multiplied by] [min.sup.-1]). Endothelin increased intracellular calcium from 96.9 [+ or -] 14.0 to 191.4 [+ or -] 11.9 nM, an increase of 110.8 [+ or -] 26.1%. We conclude that exogenous endothelin indirectly decreases TALH [J.sub.Cl] by activating [ET.sub.B] receptors, increasing intracellular calcium concentration, and stimulating NO release. These data suggest that endothelin acts as a physiological regulator of TALH NO synthesis, thus inhibiting chloride transport and contributing to the natriuretic effects of ET-1 observed in vivo. kidney; nitric oxide; tubular transport

Examines the stimulation of alpha[sub 2]-adrenergic receptors inhibiting transport in nephron segments. Description of clonidine; Decrease of NaCl absorption by cortical thick ascending limb of the loop of Henle; Activation of nitric oxide synthase.

Examines the hypothesis that endothelin-1 (ET-1) decreases NaCl transport by cortical thick ascending limb of the loop of Henle through activation of ET[sub B] receptors and release of nitric oxide (NO). Preparation of isolated nephron segments; Net chloride flux; Intracellular calcium concentration.

The management of sodium intake is clinically important in many disease states including heart failure, kidney disease, and hypertension. Tenapanor is an inhibitor of the sodium-proton (Na(+)/H(+)) exchanger NHE3, which plays a prominent role in sodium handling in the gastrointestinal tract and kidney. When administered orally to rats, tenapanor acted exclusively in the gastrointestinal tract to inhibit sodium uptake. We showed that the systemic availability of tenapanor was negligible through plasma pharmacokinetic studies, as well as autoradiography and mass balance studies performed with (14)C-tenapanor. In humans, tenapanor reduced urinary sodium excretion by 20 to 50 mmol/day and led to an increase of similar magnitude in stool sodium. In salt-fed nephrectomized rats exhibiting hypervolemia, cardiac hypertrophy, and arterial stiffening, tenapanor reduced extracellular fluid volume, left ventricular hypertrophy, albuminuria, and blood pressure in a dose-dependent fashion. We observed these effects whether tenapanor was administered prophylactically or after disease was established. In addition, the combination of tenapanor and the blood pressure medication enalapril improved cardiac diastolic dysfunction and arterial pulse wave velocity relative to enalapril monotherapy in this animal model. Tenapanor prevented increases in glomerular area and urinary KIM-1, a marker of renal injury. The results suggest that therapeutic alteration of sodium transport in the gastrointestinal tract instead of the kidney--the target of current drugs--could lead to improved sodium management in renal disease.