Update on the role of endothelial cells in trauma.

PURPOSE: This review gives an overview of physiological processes, mainly regarding vascular endothelial cells and their important role in hemostasis, information processing, and communication during trauma. An insight is given into molecules and cells involved in the first innate immune response through to the behavior of endothelial cells in developing trauma. The goal of this review is to show the overlap of crucial factors related to the endothelium and the development of trauma. METHODS: A systemic literature search was performed using Google scholar and PubMed. RESULTS: The results of the literature search showed that the endothelium, especially the vascular endothelium, is involved in various cellular and subcellular pathways of activation, suppression, and transfer of information. A variety of molecules and cells are orchestrated, subsequently the endothelium gets in contact with a traumatizing event. CONCLUSION: The endothelium is one of the first barriers that comes into contact with exo- and endogenous trauma-related signals and is a pivotal point in activating subsequent pathways and cascades by transfer of information.

Site of action of moxonidine in the rat nephron.

Moxonidine is a centrally acting antihypertensive agent which has been found to exert its blood pressure lowering effect by interaction with (alpha2-adrenoceptors and imidazoline receptors of the I(1)-type. These receptors have also been demonstrated to be present in the rat kidney. In the present study, clearance and micropuncture techniques were applied to anaesthetized rats to localize the site of action of moxonidine within the nephron. The clearance data show that moxonidine (0.25 mg/kg i.v., followed by a continuous i.v. infusion of 0.25 mg/h) induced a marked increase in urine flow and urinary excretion of sodium, chloride and potassium. The changes in urine flow and urinary solute excretion were accompanied by an enhanced glomerular filtration rate. The micropuncture experiments revealed that moxonidine significantly increased glomerular filtration rate of superficial nephrons, and significantly inhibited fractional reabsorption of fluid, sodium, potassium and chloride by similar amounts (by 9.0%-9.8%) in superficial proximal tubules. Regarding fluid and sodium reabsorption, the proximal effect of moxonidine was continuously weakened by a compensatory increase of reabsorption in the loop of Henle and the subsequent distal nephron segments. The inhibitory effect of moxonidine on fractional proximal potassium reabsorption was completely compensated in the loop of Henle, but the drug induced a net secretion of potassium into the segments lying beyond the early distal tubule, probably as a consequence of the increased tubule fluid and sodium load delivered to them. The experiments have identified the proximal tubule as the principal nephron site where the diuretic action of moxonidine arises. The proximal effect may be related to the increased glomerular filtration rate and to a direct inhibitory interaction of moxonidine with the proximal Na+/H+ exchanger.

Involvement of protein phosphatases in differential regulation of renal proximal tubular PAH and sodium-dependent dicarboxylate transport.

It has been demonstrated that the basolateral organic allion (PAH) transporter and the sodium-dependent dicarboxylate transporter of rabbit renal proximal tubules are regulated differentially. A variety of protein kinases has been shown to be involved in the regulation of organic anion transport while dicarboxylate uptake, to which the first is coupled functionally, is not influenced by these kinases. This study was undertaken to elucidate whether respective transporter activities are modulated differentially by protein phosphatases as well. The experiments were performed on isolated S, segments of proximal tubules microdissected from rabbit kidneys without the use of enzymatic agents. 3H-PAH was used as marker substance of the PAH transporter, 14C-glutarate as a marker of the sodium dicarboxylate cotransporter. 30 s tubular uptake measurements were performed. Vanadate (10(-3) M), a selective inhibitor of tyrosine phosphatase, did not reduce PAH uptake significantly, while inhibitors of the serine threonine phosphatases 1 and 2A, okadaic acid and calyculin A (10(-6) M, each) induced a significant decrease of 30 s PAH uptake (by 32.3% +/- 7.9% and 25.6% +/- 6.4%) but not a change in dicarboxylatc transport. These findings indicate that, in addition to a variety of protein kinases, serine threonine phosphatases have a role in the regulation of renal basolateral PAH transport. There is no effect of these phosphatases on basolateral 30s gutaltarate transport. Thus, additional evidence for differential regulation of short-time activiity of the transporters for PAH and dicarboxylates is provided.

Potassium-sparing renal effects of trimethoprim and structural analogues.

BACKGROUND/AIMS: The antibiotic antagonists of folic acid trimethoprim, tetroxoprim, pyrimethamine and their antineoplastic analogue methotrexate have structural characteristics in common with the potassium-sparing diuretic triamterene. They may, therefore, share with triamterne potassium-sparing renal effects. METHODS: Clearance studies were performed on anesthetized male Sprague-Dawley rats, and the drug effects on glomerular filtration rate and on urinary excretion of sodium, chloride, and potassium were studied. RESULTS: Trimethoprim and tetroxoprim, injected intravenously at doses ranging from 0.3 to 30 mg/kg, induced dose-dependent natriuretic and antikaliuretic renal effects, whereas pyrimethamine at doses ranging from 1 to 10 mg/kg and methotrexate at doses ranging from 10 to 50 mg/kg did not affect urinary sodium and potassium excretion. An antikaliuretic effect was also observed after application of the structurally related antiprotozoal compound pentamidine at doses ranging from 3 to 10 mg/kg. In contrast to trimethoprim and tetroxoprim, the antikaliuresis was accompanied by a marked decrease of urinary sodium and chloride excretion at all of the doses tested. At 10 mg/kg, pentamidine induced a pronounced fall of the glomerular filtration rate (by 43.5%). CONCLUSIONS: Trimethoprim and tetroxoprim share with potassium-sparing diuretics natriuretic and antikaliuretic renal effects which may be caused by similar mechanisms in the distal nephron, whereas pyrimthamine and methotrexate do not. A depression of renal hemodynamics is an important factor involved in the antikaliuretic effect of pentamidine.

Effect of nebivolol, a novel beta 1-selective adrenoceptor antagonist with vasodilating properties, on kidney function.

Nebivolol (CAS 99200-09-6) is a novel beta 1-selective adrenoceptor antagonist which possesses vasodilating properties and lowers systemic vascular resistance in dogs and humans, presumably by a nitric oxide-related mechanism. In the present study, clearance techniques were applied to anaesthetized male Sprague Dawley rats, and the effects of nebivolol on renal hemodynamics (glomerular filtration rate and renal plasma flow), on urinary excretion of sodium, chloride and potassium, on renal NO-excretion, and on plasma renin activity were studied. Nebivolol doses ranging from 0.1 to 2 mg/kg i.v. were tested. The results revealed that nebivolol dose-dependently increased glomerular filtration rate, urine flow and urinary excretion of sodium and chloride. Potassium excretion was only inconsistently increased and showed no dose dependency. At a dose of 1 mg/kg, the drug also significantly increased renal plasma flow measured as 3H-PAH (p-aminohippurate) clearance. The effect of nebivolol on the glomerular filtration rate could be abolished by L-NMMA (N6-monomethyl-L-arginine) (1 mg/kg), a non-selective inhibitor of NO-synthase and by iminoethyllysine (1 mg/kg), a relatively selective inhibitor of the inducible NO-synthase, but not by 7-nitroindazole (1 mg/kg), a relatively selective inhibitor of the neuronal NO-synthase isoform. The saluretic effect of nebivolol was diminished by all of the three NO-synthase inhibitors, but could not be completely reversed. At a dose of 2 mg/kg, nebivolol increased renal NO-excretion by 70.7%. This effect could be completely abolished by L-NMMA (1 mg/kg). Plasma renin activity was lowered by nebivolol (2 mg/kg) from 14.6 +/- 1.49 to 6.5 +/- 1.66 ng angiotensin I/ml/h (p < 0.01). The results demonstrate that, in anaesthetized rats, nebivolol exerts significant renal vasodilating effects and increases urinary excretion of fluid and solutes. The actions of nebivolol on renal hemodynamics are assumed to be mediated by a stimulation of the NO-synthase, probably the inducible isoform. Since none of the NO-synthase inhibitors could completely abolish the saluretic effect of nebivolol, an additional mechanism, not related to NO, may be involved in the tubular action of this drug.

Regulation of the renal basolateral transport system for organic anions by calcium channel blockers.

Calcium channel blockers have been reported to exert multiple effects on renal tubular function. Therefore, the effects of the calcium channel blockers nifedipine and verapamil on the cellular uptake of tritiated para -aminohippuric acid (PAH) into microdissected non-perfused rabbit kidney S2 proximal tubule segments were investigated to study a possible influence of calcium channel blockers on renal PAH transport. Since the tubules were entirely collapsed, the accumulated radioactivity overwhelmingly reflects the transport across the basolateral membrane. Tubular PAH accumulation was increased by 1 and 10 microm verapamil, and by 1 microm nifedipine, but it was unaffected by lower or higher concentrations of these calcium channel blockers. The increase in PAH accumulation caused by 1 microm nifedipine or 10 microm verapamil was independent from changes in intracellular Ca(2+) and inhibited by staurosporine (1 and 10 n m) a potent inhibitor of protein kinase C (PKC). Our results indicate that the calcium channel blockers nifedipine and verapamil increase the transport of organic anions across the basolateral membrane of proximal tubules, probably by an activation of PKC. Furthermore, the increased tubular PAH transport may disturb the estimation of the renal PAH-clearance.

Effects of inorganic divalent cations on the renal basolateral transport system for organic anions.

Recent work demonstrated that the heavy metal ion Cd2+ increases the transport of p-aminohippuric acid (PAH) across the basolateral membrane of microdissected non-perfused rabbit kidney S2 proximal tubule segments. Usually, such ions induce damage of various renal transport systems, therefore the effects of divalent metal ions Zn2+, Co2+ and Ni2+ on this transporter were investigated. Addition of Ni2+ or Zn2+ to the bathing solution leads to a significant reduction of basolateral PAH transport, with IC50 values of 2 x 10(-5) and 10(-6) M, respectively, whereas Co2+ failed to inhibit PAH accumulation. Simultaneous incubation with thrombin (10(-9)M), which is known to increase [Ca2+]i, abolished the effects of the divalent ions. Our results indicate that Ni2+ and Zn2+ reduce cellular PAH uptake. Because Ni2+ and Zn2+ are calcium channel blockers, these effects are probably due to a reduction of [Ca2+]i by an interaction of these metals with binding sites in the calcium channel, whereas Co2+ does not affect these binding sites. This finding is supported by the fact that thrombin abolished the cation effects.

Evidence for differential regulation of renal proximal tubular p-aminohippurate and sodium-dependent dicarboxylate transport.

In renal proximal tubules, the basolateral organic anion [p-aminohippurate (PAH)] transporter is functionally coupled to the sodium-dependent dicarboxylate transporter. This study was undertaken to elucidate whether protein kinases differentially modulate the activities of these transporters. In isolated S(2) segments of proximal tubules microdissected from rabbit kidneys, we investigated whether the transporters are regulated by tyrosine kinases, phosphatidylinositol 3-kinase (PI3K), and mitogen-activated protein kinase (MAPK). The tubules were collapsed; hence, tubular uptake of the marker substances [(3)H]PAH and [(14)C]glutarate reflects transport across the basolateral cell membrane. Genistein, a selective inhibitor of tyrosine kinase, diminished PAH uptake at 10(-7) M by 15.6 +/- 11.7% and at 10(-6) M by 25.6 +/- 9.1%. An inactive analog of genistein, diadzein, was without effect even at a concentration 100-fold higher than the lowest concentration of genistein, which produced significant reduction of PAH uptake. At 10(-7) M, wortmannin, a selective inhibitor of PI3K, reduced PAH uptake by 24.1 +/- 11.3% and, at 10(-6) M, it reduced it by 32.9 +/- 11.8%. The selective inhibitor of MAPK, PD98059, diminished PAH uptake at 5 x 10(-5) M by 23.2 +/- 6.8% and at 10(-4) M by 18.3 +/- 5.2%. Glutarate uptake was not reduced by any of these protein kinase inhibitors. Insulin had no effect on PAH uptake. These findings indicate that, in addition to protein kinase A, protein kinase C and calcium/calmodulin-dependent protein kinase II (former studies from this laboratory), as well as tyrosine kinases, PI3K, and MAPK, modulate renal basolateral PAH transport, whereas none of these protein kinases affects basolateral glutarate transport. Thus, the results provide evidence for differential regulation of basolateral transporters for PAH and dicarboxylates.

Effects of moxonidine and clonidine on potassium excretion in Sprague-Dawley rats.

Recently we demonstrated that clonidine and moxonidine exert specific action on fractional fluid and Na+ excretion in anaesthetised Sprague-Dawley rats. Classically, most of the diuretics used induce an increased K+ excretion, at least in part due to Na+ load in the distal tubule and exchange of Na+ by K+. Therefore, we studied the effects of moxonidine and clonidine on K+ excretion in anaesthetised Sprague-Dawley rats. Moxonidine (0.25 and 0.5 mg kg-1 body wt. i.v.) increased transiently K+ (1.0 +/- 0.3 -1.9 +/- 0.4 and 0.9 +/- 0.2 -2.9 +0.7 micromol min-1 100 g body wt.) and Na+ (1.4 +/- 1.0 -6. 9 +/- 3.1 and 0.8 +/- 0.36 -6.6 +/- 1.5 micromol min-1100 g body wt.) excretion. Clonidine (0.25 mg kg-1) caused a more pronounced increase in K+ (1.0 +/- 0.1 -2.7 +/- 0.4 micromol min-1 100 g body wt.) and Na+ (0.6 +/- 0.3 -9.5 +/- 0.4 micromol min-1 100 g body wt.) excretion, whereas the higher dose of 0.5 mg kg-1 body wt. had less effect as compared to moxonidine (K+: 0.8 +/- 0.1 -1.7 +/- 0.2 micromol min-1 100 g body wt., Na+: 0.3 +/- 0.1 -3.4 +/- 1.0 micromol min-1 100 g body wt.). The increased electrolyte excretion returned (similar to moxonidine) to baseline levels within 20 min after injection of the drugs. Antagonists such as idazoxan and yohimbine did not change K+ and Na+ excretion by their own. Both, the non-selective imidazoline/alpha2-adrenoceptor antagonist idazoxan and the pure alpha2-adrenoceptor antagonist yohimbine attenuated the moxonidin-induced effects on K+ and Na+ excretion. This could be also observed with clonidine and simultaneous injection of these two antagonists. Our results demonstrate that moxonidine and clonidine also increase renal K+ excretion in this animal model. K+and Na+ excretion show a parallel behaviour, indicating that the increased K+ excretion is mainly due to Na+ load in the tubular system.

Role of the calcium/calmodulin-dependent protein kinase II in the regulation of the renal basolateral PAH and dicarboxylate transporters.

The aim of the present study was to investigate whether the activities of the renal basolateral organic anion transporter (PAH transporter) and the sodium-dependent dicarboxylate transporter are modulated by the calcium/calmodulin-dependent multifunctional protein kinase II (CaM kinase II). The studies were performed on isolated S2 segments of proximal tubules microdissected from rabbit kidneys without the use of enzymatic agents. 3H-PAH was used as marker substance of the anion transporter, and 14C-glutarate as a marker of the sodium/dicarboxylate cotransporter. Because the tubules were not perfused, and hence were collapsed, the tubular uptake of the marker substances reflects transport across the basolateral cell membrane. To obtain uptake rates most closely related to initial transport rates, 30 s tubular uptake measurements were performed. The results show that a selective inhibitor of CaM kinase II, KN93, inhibited tubular PAH uptake. The smallest effective dose was 10(-7) M. An inactive analogue of KN93, KN92, was without effect, even at the high concentration of 10(-5) M. In contrast to PAH transport, tubular 14C-glutarate uptake was not affected by KN93 (10(-5) M). PAH transport was also inhibited after elevation of intracellular Ca2+ by the Ca(2+)-ionophore A 23187 and by the polycationic antibiotic neomycin, but not by the intracellular Ca2+ modulators thapsigargin and ryanodine. The effect of the Ca(2+)-ionophore could be abolished by KN93, but not by Rp-cAMPs, an inhibitor of protein kinase A, indicating that this event was mediated by CaM kinase II, but not by PKA. The results provide the first evidence that, in addition to the protein kinases A and C (previous studies from this lab), CaM kinase II has a role in the regulation of the renal basolateral PAH transporter, whereas the renal basolateral dicarboxylate transporter does not depend on CaM kinase II activity.

Effects of extracellular cadmium on renal basolateral organic anion transport.

Heavy metal ions are well-known nephrotoxic agents. Usually, they induce a damage of various renal transport systems. Cd2+, however, has been shown to enhance renal basolateral organic cation transport. Therefore, the effects of Cd2+ on the basolateral p-aminohippuric acid (PAH) uptake of microdissected nonperfused rabbit kidney S2 proximal tubule segments were investigated. Incubation with Cd2+ induced a bell-shaped concentration response curve with a 2-fold increase of the PAH transport at a Cd2+ concentration of 10(-6) M. Since Cd2+ has been described to activate protein kinase C (PKC), the effects of the PKC inhibitor staurosporine were also tested. Staurosporine (10(-8) M) could, however, not reduce the effects of Cd2+. Cd2+ may exert its effects by an as yet unknown mechanism that is different from the PKC-activating mechanism of phorbol esters. A stimulation of the tricarboxylic acid cycle in kidney cells by Cd2+ may, however, increase the delivery of alpha-ketoglutarate. Indeed, incubation of proximal tubules with alpha-ketoglutarate increased PAH transport across the basolateral membrane significantly. Thus, the observed effects of Cd2+ may be due to an enhanced transport of p-aminohippuric acid by stimulation of exchange of PAH with alpha-ketoglutarate. The modulation of renal organic anion transport may be another aspect of Cd2+ nephrotoxicity.

Effect of the novel T-selective calcium channel antagonist mibefradil on kidney function in comparison with amlodipine.

In the present study, the renal effects of mibefradil (CAS 116666-63-8), a novel calcium channel antagonist which more selectively blocks T-type than L-type calcium channels, was tested by applying clearance techniques to anaesthetized rats. The effects of mibefradil on kidney function and on arterial blood pressure were compared with those of the long acting dihydropyridine-type calcium antagonist amlodipine (CAS 88150-42-9). The results show that, within a dosage range of 0.1 to 1.0 mg/kg i.v., mibefradil induced a dose-dependent decrease of arterial blood pressure. Kidney function was not significantly affected at a dose of 0.1 mg/kg. By increasing the dose to 0.3 mg/kg, mibefradil induced a significant increase in urine flow, renal sodium, chloride and potassium excretion. Also fractional sodium and chloride excretions were significantly enhanced at this dose. The diuretic and saluretic effects of mibefradil were accompanied by a significant increase in the glomerular filtration rate. At the highest dose of 1 mg/kg used, the blood pressure lowering effect of mibefradil was most pronounced and glomerular filtration rate rose only slightly and not significantly. At this dose, the enhancement of urine flow and urinary electrolyte excretion was smaller than at the dose of 0.3 mg/kg. The actions of mibefradil were qualitatively similar to those of the dihydropyridine derivate amlodipine which at a dose of 0.3 mg/kg produced nearly identical renal effects to mibefradil, but exerted stronger antihypertensive effects. This study demonstrates that mibefradil shares with amlidopine the property to induce, at appropriate doses, diuretic and saluretic effects with a concomitant increase in glomerular filtration rate.

Basolateral transport of glutarate in proximal S2 segments of rabbit kidney: kinetics of the uptake process and effect of activators of protein kinase A and C.

The kinetics of tubular glutarate uptake, the coupling of glutarate to p-aminohippurate (PAH) transport and the effect of activators of protein kinase A and C on glutarate uptake were studied using isolated S2 segments of proximal tubules microdissected from rabbit kidneys without the use of enzymatic agents. Because the tubules were not perfused, and hence were collapsed, the tubular uptake of [14C]glutarate reflects transport across the basolateral cell membrane. To obtain uptake rates most closely related to initial transport rates, 30 s glutarate uptake measurements were performed. In a first set of experiments it could be shown that preloading proximal S2 segments with glutarate (10(-3 )M) stimulated [3H]PAH uptake indicating that glutarate may be a substrate of the PAH /dicarboxylate exchanger. The kinetic data revealed a Km value of 0. 62 mM and a Vmax value of 84.1 pmol nl-1min-1 for tubular [14C]glutarate uptake across the basolateral cell membrane. In contrast to basolateral PAH transport (previous studies from this laboratory), tubular 30 s [14C]glutarate uptake was not affected by either the phorbol ester phorbol 12-myristate 13-acetate (PMA, 10(-7 )M), an activator of protein kinase C, or by the membrane-permeant analogues of cAMP, dibutyryl cyclic AMP (db-cAMP, 10(-4 )M) and 8-bromoadenosine 3',5'-cyclic monophosphate (Br-cAMP, 10(-4 )M). The results indicate that the protein kinases A and C have no function in the regulation of the renal basolateral dicarboxylate transporter. This finding agrees well with the structural feature of the recently cloned rabbit renal dicarboxylate transporter which does not contain any putative phosphorylation sites for protein kinase C or cAMP-dependent kinase.

Effect of activation of protein kinase A and of protein kinase C on the kinetics of the renal basolateral PAH transporter.

The aim of the present study was to examine the effect of activation of the protein kinase A (PKA) and protein kinase C (PKC) pathways on 3H-p-aminohippurate (PAH) uptake of isolated S2 segments of proximal tubules, microdissected from rabbit kidneys without the use of enzymatic agents. Because the tubules were not perfused, and hence were collapsed, the tubular uptake of 3H-PAH reflects transport across the basolateral membrane. The phorbol ester phorbol 12-myristate 13-acetate (PMA) (10(-7) M), an activator of PKC, significantly increased tubular 3H-PAH uptake with steady state conditions (by 115%), whereas dibutyryl cyclic adenosine monophosphate (db-cAMP) (10(-4) M) and forskolin (10(-4) M) significantly inhibited it (by 42% and 52%, respectively). Kinetic data, which were based on 15 sec PAH uptake measurements, revealed that PMA, after a 10 min incubation period, significantly enhanced Km and Vmax of the PAH transporter (Km from 174 +/- 22 to 447 +/- 91 microM, Vmax from 2.76 +/- 0.24 to 16.67 +/- 1.85 pmol nL-1 min-1), whereas db-cAMP significantly decreased Vmax (from 2.76 +/- 0.24 to 1.82 +/- 0.19 pmol nL-1 min-1). The Km value was also numerically lowered by dibutyryl-cAMP (from 174 +/- 22 to 139 +/- 21 microM), but this change did not reach statistical significance. The data provide evidence that short time activation of the PKC pathway 1) enhances the effectiveness of PAH transport into proximal S2 segments across the basolateral cell membrane, 2) increases the maximum transport rate of the PAH transporter and 3) decreases its affinity for PAH. Activation of the cAMP/PKA pathway induces the opposite effects.

Effects of moxonidine and clonidine on renal function and blood pressure in anesthetized rats.

Using classical clearance techniques, the renal effects of moxonidine and clonidine were studied in the rat. Moxonidine (0.25 and 0.5 mg/kg body weight i.v.) increased transiently fractional fluid and Na+ excretion. Similarly, clonidine in the same i.v. doses caused a sustained increase in fractional fluid excretion, but only a short lasting increase in fractional Na+ excretion that was similar to the effect of moxonidine. Water diuresis experiments showed that the agonists mostly exert their actions within the proximal tubule. Antagonists such as idazoxan and yohimbine did not change fractional fluid excretion and Na+ excretion by their own. Both, the nonselective imidazoline/alpha 2-adrenoceptor antagonist idazoxan and the pure alpha 2-adrenoceptor antagonist yohimbine attenuated the moxonidine induced effects on fractional fluid excretion and Na+ excretion. The clonidine induced increase in fractional fluid and Na(+)-excretion was inhibited by yohimbine and desmopressin only. The effects on systemic blood pressure after moxonidine or clonidine application were similar. The initial blood pressure increases after moxonidine application, however, was less than with clonidine, whereas the hypotensive potency was more pronounced. Similar to the effects on kidney function, idazoxan and yohimbine had no influence on systemic blood pressure by themselves. Immediately after moxonidine application, a short lasting increase in renal plasma flow and glomerular filtration rate could be observed. Our results demonstrate, that moxonidine exerts renal effects, which are different from those of clonidine. At present time, renal effects mediated by imidazoline receptors and alpha 2-adrenoceptors cannot be clearly distinguished.

Renal and blood pressure effects of moxonidine and clonidine in spontaneously hypertensive rats.

Recently we could demonstrate that the imidazoline receptor agonist moxonidine exerts specific renal effects in Sprague Dawley rats [Hohage et al. 1997]. Interestingly, the effects of this compound are attenuated in one kidney-one clip hypertensive rats [Li et al. 1994]. In this study, we therefore investigated the effects of moxonidine as compared to clonidine in genetically determined spontaneously hypertensive rats. Moxonidine in a concentration of 0.5 mg/kg b.w.i.v. induced a significant and long-lasting increase of both urine flow from 11.9 +/- 2.1 microliters/min x 100 g b.w. to 50.3 +/- 12.5 microliters/min x 100 g b.w. and of Na(+)-excretion from 2.2 +/- 0.5 mumol/min x 100 g b.w. to 8.4 +/- 1.9 mumol/min x 100 g b.w. In contrast to moxonidine, the effects of clonidine (0.5 mg/kg b.w.i.v.) on urine flow and Na(+)-excretion were negligible. The antagonists idazoxan, effaroxan and rauwolscine abolished the effects of moxonidine on urine flow and Na(+)-excretion, whereas 4-aminopyridine, phenformine and 1,2,3,4-tetrahydro-9-aminoacridine, which have been described to interact with imidazoline binding sites, had no effect. Addition of the antagonists idazoxan, effaroxan and rauwolscine attenuated the initial blood pressure increase immediately after intravenous application, whereas 4-aminopyridine, phenformine and 1,2,3,4-tetrahydro-9-aminoacridine had no influence on this side-effect. Our results provide further evidence that imidazoline receptor agonists such as moxonidine exhibit renal effects, different from the modulation in urine flow and Na(+)-excretion following renal alpha 2 adrenoceptor stimulation. An upregulation of imidazoline receptors in hypertension may contribute to the effects observed.