Proximal nephron Na+/H+ exchange is regulated by alpha 1A- and alpha 1B-adrenergic receptor subtypes

Landscape of GPCR expression along the mouse nephron

Kidney transport and other renal functions are regulated by multiple G protein-coupled receptors (GPCRs) expressed along the renal tubule. The rapid, recent appearance of comprehensive unbiased gene expression data in the various renal tubule segments, chiefly RNA sequencing and protein mass spectrometry data, has provided a means of identifying patterns of GPCR expression along the renal tubule. To allow for comprehensive mapping, we first curated a comprehensive list of GPCRs in the genomes of mice, rats, and humans (https://hpcwebapps.cit.nih.gov/ESBL/Database/GPCRs/) using multiple online data sources. We used this list to mine segment-specific and cell type-specific expression data from RNA-sequencing studies in microdissected mouse tubule segments to identify GPCRs that are selectively expressed in discrete tubule segments. Comparisons of these mapped mouse GPCRs with other omics datasets as well as functional data from isolated perfused tubule and micropuncture studies confirmed patterns of expression for well-known receptors and identified poorly studied GPCRs that are likely to play roles in the regulation of renal tubule function. Thus, we provide data resources for GPCR expression across the renal tubule, highlighting both well-known GPCRs and understudied receptors to provide guidance for future studies.

Zebrafish as a Model System for Investigating the Compensatory Regulation of Ionic Balance during Metabolic Acidosis

Zebrafish (Danio rerio) have become an important model for integrative physiological research. Zebrafish inhabit a hypo-osmotic environment; to maintain ionic and acid-base homeostasis, they must actively take up ions and secrete acid to the water. The gills in the adult and the skin at larval stage are the primary sites of ionic regulation in zebrafish. The uptake of ions in zebrafish is mediated by specific ion transporting cells termed ionocytes. Similarly, in mammals, ion reabsorption and acid excretion occur in specific cell types in the terminal region of the renal tubules (distal convoluted tubule and collecting duct). Previous studies have suggested that functional regulation of several ion transporters/channels in the zebrafish ionocytes resembles that in the mammalian renal cells. Additionally, several mechanisms involved in regulating the epithelial ion transport during metabolic acidosis are found to be similar between zebrafish and mammals. In this article, we systemically review the similarities and differences in ionic regulation between zebrafish and mammals during metabolic acidosis. We summarize the available information on the regulation of epithelial ion transporters during acidosis, with a focus on epithelial Na⁺, Cl- and Ca2+ transporters in zebrafish ionocytes and mammalian renal cells. We also discuss the neuroendocrine responses to acid exposure, and their potential role in ionic compensation. Finally, we identify several knowledge gaps that would benefit from further study.

Antidiuretic and antinatriuretic response to high salt load in normotensive Wistar-Kyoto rats: Role of alpha-1A-adrenoreceptors

Altered renal adrenergic responses have been recognized as pathophysiological responses to high salt intake. This study aims to investigate the influence of 6 weeks of high salt diet on α_1A -adrenoceptor regulation of renal tubular antinatriuretic and antidiuretic response in normal Wistar Kyoto rats. To achieve the above objective, antinatriuretic and antidiuretic response to phenylephrine was measured in the absence and presence of 5-methylurapidil (5-MeU) using the inulin clearance method. Systemic mean arterial blood pressure and renal haemodynamics were also measured simultaneously. Six weeks of high salt intake in Wistar-Kyoto (WKY) rats did not bring any significant increase in mean arterial blood pressure. WKY rat on high salt diet (WKYHNa) showed an exaggerated increase in absolute and fractional sodium excretion. There was a significant involvement of α_1A -adrenoceptor in carrying out renal tubular antinatriuretic and antidiuretic response in Wistar Kyoto rats on normal sodium diet (WKYNNa). However, α_1A -adrenoceptor played a minimal role in handling the tubular reabsorptive response in WKY rats on high salt diet.

Role of the paraventricular nucleus in the reflex diuresis to pulmonary lymphatic obstruction in rabbits

The changes in urine flow and renal sympathetic nerve activity (RSNA) due to pulmonary lymphatic obstruction (PLO) were examined in anesthetized, artificially ventilated New Zealand white rabbits. PLO was produced by pressurizing an isolated pouch created in the right external jugular vein at the points of entry of the right lymphatic ducts. During this maneuver, urine flow increased from 8.5 ± 0.3 mL/10 min to 12 ± 0.5 mL/10 min (P < 0.0001) and RSNA increased from 24.0 ± 4 to 40.0 ± 5 μV·s (P < 0.0001). Bilateral lesioning of the paraventricular nucleus (PVN) of the hypothalamus or cervical vagotomy abolished these responses. PLO increased c-fos gene expression in the PVN. The increase in urine flow due to PLO was attenuated by muscimol and abolished by kynurenic acid microinjections into the PVN. The results show that (i) neurons in the PVN are an important relay site in the reflex arc, which is activated by PLO; and (ii) this activation is regulated by glutamatergic and partly by GABAergic input to the PVN.

Autonomic regulation of kidney function

The kidneys play a central role in cardiovascular homeostasis by ensuring a balance between the fluid taken in and that lost and excreted during everyday activities. This ensures stability of extracellular fluid volume and maintenance of normal levels of blood pressure. Renal fluid handling is controlled via neural and humoral influences, with the former determining a rapid dynamic response to changing intake of sodium whereas the latter cause a slower longer-term modulation of sodium and water handling. Activity in the renal sympathetic nerves arises from an integration of information from the high and low pressure cardiovascular baroreceptors, the somatosensory and visceral systems as well as the higher cortical centers. Each sensory system provides varying input to the autonomic centers of the hypothalamic and medullary areas of the brain at a level appropriate to the activity being performed. In pathophysiological states, such as hypertension, heart failure and chronic renal disease, there may be an inappropriate sympathoexcitation causing sodium retention which exacerbates the disease process. The contribution of the renal sympathetic nerves to these cardiovascular diseases is beginning to be appreciated with the demonstration that renal denervation of resistant hypertensive patients results in a long-term normalization of blood pressure.

Oxidative stress and alpha1-adrenoceptor-mediated stimulation of the Cl-/HCO3- exchanger in immortalized SHR proximal tubular epithelial cells

BACKGROUND AND PURPOSE

This study evaluated the signalling coupled to the alpha1-adrenoceptor-induced stimulation of the Cl-/HCO3- exchanger in hypertension.

EXPERIMENTAL APPROACH

The Na+ -independent HCO3- transport system activity was assayed as the initial rate of pHi recovery after an alkaline load (CO2/HCO3 removal) in immortalized renal proximal tubular epithelial cells from spontaneously hypertensive rat (SHR) and their normotensive control (Wistar Kyoto rat; WKY).

KEY RESULTS

Noradrenaline increased Cl-/HCO3- exchanger activity with EC50 values of 0.6 and 5.3 microM in SHR and WKY cells, respectively. These effects were abolished by prazosin, but not by yohimbine. Phenylephrine increased Cl-/HCO3- exchanger activity in SHR and WKY cells (EC50 of 2.6 and 4.9 microM, respectively). Phenylephrine-mediated increase in Cl-/HCO3- exchanger activity in WKY and SHR cells was inhibited by protein kinase C (PKC), MAPK/ERK kinase (MEK) and p38 mitogen-activated protein kinase (p38 MAPK) inhibitors. The expression of alpha1A- and alpha1B-adrenoceptors was identical in WKY and SHR cells. SHR cells generated more H2O2 than WKY cells. In SHR cells, the NADPH oxidase inhibitor apocynin reduced their increased ability to generate H2O2 and abolished their hypersensitivity to phenylephrine, but failed to affect basal Cl-/HCO3- exchanger activity. H2O2-dependent stimulation of Cl-/HCO3- exchange activity was significantly higher in SHR than in WKY cells.

CONCLUSIONS AND IMPLICATIONS

Differences between WKY and SHR cells on their sensitivity to alpha1-adrenoceptor stimulation did not correlate with the abundance of alpha1A- and alpha1B-adrenoceptors and may be related to the increased generation of H2O2, which may amplify the response downstream of alpha1-adrenoceptor activation.

Arrestins and spinophilin competitively regulate Na+,K+-ATPase trafficking through association with a large cytoplasmic loop of the Na+,K+-ATPase

The activity and trafficking of the Na(+),K(+)-ATPase are regulated by several hormones, including dopamine, vasopressin, and adrenergic hormones through the action of G-protein-coupled receptors (GPCRs). Arrestins, GPCR kinases (GRKs), 14-3-3 proteins, and spinophilin interact with GPCRs and modulate the duration and magnitude of receptor signaling. We have found that arrestin 2 and 3, GRK 2 and 3, 14-3-3 epsilon, and spinophilin directly associate with the Na(+),K(+)-ATPase and that the associations with arrestins, GRKs, or 14-3-3 epsilon are blocked in the presence of spinophilin. In COS cells that overexpressed arrestin, the Na(+),K(+)-ATPase was redistributed to intracellular compartments. This effect was not seen in mock-transfected cells or in cells expressing spinophilin. Furthermore, expression of spinophilin appeared to slow, whereas overexpression of beta-arrestins accelerated internalization of the Na(+),K(+)-ATPase endocytosis. We also find that GRKs phosphorylate the Na(+),K(+)-ATPase in vitro on its large cytoplasmic loop. Taken together, it appears that association with arrestins, GRKs, 14-3-3 epsilon, and spinophilin may be important modulators of Na(+),K(+)-ATPase trafficking.

Modulation of succinate transport in Hep G2 cell line by PKC

The cellular uptake of the tricarboxylic acid cycle (TCA) intermediates is very important for cellular metabolism. However, the transport pathways for these intermediates in liver cells are not well characterized. We have examined the transport of succinate and citrate in the human hepatoma cell line Hep G2 and found that it exhibited a higher rate of succinate compared to citrate transport, which was sodium dependent. Comparison of the transport properties of Hep G2 to that of human retinal pigment epithelial (HRPE) cells transfected with human sodium dicarboxylate transporters, hNaDC-1, hNaDC-3, and hNaCT indicated that Hep G2 cells express a combination of hNaDC-3 and hNaCT. Short period activation of protein kinase C (PKC) by phorbol 12-myristate, 13-acetate (PMA) and alpha-adrenergic receptor agonist, phenylephrine (PE), downregulated sodium-dependent succinate transport presumably via hNaDC-3. The inhibition by PMA was partially prevented by cytochalasin D, suggesting that PKC reduces the hNaDC-3 activity, at least in part, by increased endocytosis. In contrast, activation of PKA by both forskolin and epidermal growth factor (EGF) had no effect on succinate transport. Our results suggest that Hep G2 cells provide a useful model for studies of di- and tricarboxylate regulation of human liver.

Phenol injury-induced hypertension stimulates proximal tubule Na+/H+ exchanger activity

Injection of 50 microl 10% phenol into rat renal cortex activates renal sympathetic nerve activity which provokes acute hypertension that persists for weeks. We have previously shown with membrane fractionation that phenol injury caused a redistribution of the main proximal tubule (PT) apical transporter NHE3 (Na+/H+ exchanger isoform 3) to low density membranes enriched in apical microvilli. The aim of this study was to determine whether phenol injury increases PT apical Na+/H+ exchanger (NHE) activity. NHE activity was measured in vivo as the initial rate of change in intracellular pH (dpH(i)/dt) during luminal Na+ removal in PT preloaded with the pH-sensitive fluorescence dye BCECF. Injection of 50 microl 10% phenol increased blood pressure from 113 +/- 5.2 to 130 +/- 4.6 mmHg without changing glomerular filtration rate or urine output. NHE activity increased 2.6-fold by 70 min after phenol injury. The increase of NHE activity was accompanied with an increase of tubular reabsorption. Total NHE activity/NHE3 protein in cortical brush-border membrane (BBM) vesicles, measured by acridine orange quench and immunoblot, respectively, was unchanged by phenol injury. In conclusion, acute phenol injury provokes coincident increases in PT apical NHE activity, redistribution of NHE3 into low density apical membranes, and hypertension. The increase in NHE activity may contribute to the lack of pressure-diuresis and the maintenance of chronic hypertension in this model.

Acute regulation of OAT3-mediated estrone sulfate transport in isolated rabbit renal proximal tubules

We investigated the regulation of organic anion transport driven by the organic anion transporter 3 (OAT3), a multispecific OAT localized at the basolateral membrane of the renal proximal tubule. PMA, a PKC activator, inhibited uptake of estrone sulfate (ES), a prototypic substrate for OAT3, in a dose- and time-dependent manner. This inhibition was reduced by 100 nM bisindoylmaleimide I (BIM), a specific PKC inhibitor. The alpha(1)-adrenergic receptor agonist phenylephrine also inhibited ES uptake, and this effect was reduced by BIM. These results suggest that PKC activation downregulates OAT3-mediated organic anion transport. In contrast, epidermal growth factor (EGF) increased ES uptake following activation of MAPK. Exposure to PGE(2) or dibutyryl (db)-cAMP also enhanced ES uptake. Stimulation produced by PGE(2) and db-cAMP was prevented by the PKA inhibitor H-89, indicating that this stimulation required PKA activation. In addition, inhibition of cyclooxygenase 1 (COX1) (but not COX2) inhibited ES uptake. Furthermore, the stimulatory effect of EGF was eliminated by inhibition of either COX1 or PKA. These data suggest that EGF stimulates ES uptake by a process in which MAPK activation results in increased PGE(2) production that, in turn, activates PKA and subsequently stimulates ES uptake. Interestingly, EGF did not induce upregulation immediately following phenylephrine-induced downregulation; and phenylephrine did not induce downregulation immediately after EGF-induced upregulation. These data are the first to show the regulatory response of organic anion transport driven by OAT3 in intact renal proximal tubules.

Modulation of the Na(+)-H(+) antiport activity by adrenaline on erythrocytes from normal and obese individuals

The effect of adrenaline on normal and obese human Na(+)-H(+) antiport (NHE 1) erythrocyte activity has been studied. Adrenaline increased both intracellular pH (pHi) and Na(+) influx in erythrocyte suspensions. This effect of adrenaline was inhibited by amiloride or EIPA, indicating that adrenaline stimulated NHE 1. Phorbol myristicate ester (PMA), a protein kinase C (PKC) stimulator, increased the activity of NHE 1 whereas calphostin C, a PKC inhibitor, partially inhibited NHE 1 activation induced by adrenaline. The effect of adrenaline to NHE 1 was counteracted by prazocin and by propranolol as well indicating the involvement of both alpha and beta 2 adrenergic receptors. The effect of adrenaline on erythrocyte NHE 1 activity was significantly more profound in obese compared to normal subjects. These data indicate that adrenaline induces an increase of pHi and Na(+) uptake of human erythrocytes through stimulation of NHE 1 activity. The significantly more profound stimulation of NHE 1 activity by adrenaline in obese as compared to normal subjects is discussed.

Responses of proximal tubule sodium transporters to acute injury-induced hypertension

Renal injury-induced by phenol injection activates renal sympathetic afferent pathways, increases norepinephrine release from the posterior hypothalamus, activates renal efferent pathways, and provokes a rapid and persistent hypertension. This study aimed to determine whether phenol injury provoked a redistribution of proximal Na(+) transporters from internal stores to the apical cell surface mediated by sympathetic activation, a response that could contribute to generation or maintenance of hypertension. Anesthetized rats were cannulated for arterial blood pressure tracing and saline infusion and then 50 microl 10% phenol or saline was injected into one renal cortex (n = 7 each). Fifty minutes after injection, kidneys were removed and renal cortex membranes from injected kidneys were fractionated on sorbitol gradients and pooled into three windows (WI-WIII) that contained enriched apical brush border (WI); mixed apical, intermicrovillar cleft and dense apical tubules (WII); and intracellular membranes (WIII). Na(+) transporter distributions were determined by immunoblot and expressed as percentage of total in gradient. Acute phenol injury increased blood pressure 20-30 mmHg and led to redistribution of Na(+)/H(+) exchanger type 3 (NHE3) out of WIII (from 22.79 +/- 4.75 to 10.79 +/- 2.01% of total) to WI (13.07 +/- 1.97 to 27.15 +/- 4.08%), Na(+)-P(i) cotransporter 2 out of WII (68.72 +/- 1.95 to 59.76 +/- 2.21%) into WI (9.5 +/- 1.62 to 18.7 +/- 1.45%), and a similar realignment of dipeptidyl-peptidase IV immunoreactivity and alkaline phosphatase activity to WI. Renal denervation before phenol injection prevented the NHE3 redistribution. By confocal microscopy, NHE3 localized to the brush border after phenol injection. The results indicate that phenol injury provokes redistribution of Na(+) transporters from intermicrovillar cleft/intracellular membrane pools to apical membranes associated with sympathetic nervous system activation, which may contribute to phenol injury-induced hypertension.

Role of the alpha1D-adrenergic receptor in the development of salt-induced hypertension

In an attempt to elucidate whether there is a specific alpha1-adrenergic receptor (alpha1-AR) subtype involved in the genesis or maintenance of hypertension, the alpha1D-AR subtype was evaluated in a model of salt-induced hypertension. The alpha1D-AR-deficient (alpha1D-/-) and control (alpha1D+/+) mice (n=8 to 14 in each group) were submitted to subtotal nephrectomy and given 1% saline as drinking water for 35 days. Blood pressure (BP) was monitored by tail-cuff readings and confirmed at the end point by direct intraarterial BP recording. The alpha1D-/- mice had a significantly (P=0.0004) attenuated increase in BP response in this protocol (baseline 94.6+/-2.8 versus end point 107.4+/-4.5 mm Hg) compared with that of their wild-type counterparts (alpha1D+/+), from a baseline 97.4+/-2.9 to an end point 139.4+/-4.5 mm Hg. Seven of 15 alpha1D+/+ mice died with edema, probably owing to renal failure, whereas 14 of 15 alpha1D-/- mice were maintained for 35 days. Body weight, renal remnant weight, and residual renal function were similar in the 2 groups, whereas the values of plasma catecholamines (epinephrine, norepinephrine, and dopamine) were higher in alpha1D+/+ than in the alpha1D-/- mice. These data suggest that alpha1D-AR plays an important role in developing a high BP in response to dietary salt-loading, and that agents having selective alpha1D-AR antagonism could have significant therapeutic potential in the treatment of hypertension.

Protecting the myocardium from ischemic injury: a critical role for alpha(1)-adrenoreceptors?

Ischemic preconditioning (IPC) refers to the ability of short periods of ischemia to make the myocardium more resistant to a subsequent ischemic insult. It is the most powerful form of endogenous protection against myocardial infarction and has been demonstrated in all species evaluated to date. However, the cellular mechanisms that drive IPC remain poorly understood. This hypothesis describes an important role for alpha(1)-adrenoreceptors in mediating IPC and discusses the underlying mechanisms by which this is likely achieved. alpha(1)-Adrenoreceptors are present in the myocardium of all mammalian species, and several lines of evidence suggest that they play an important role in mediating IPC. During periods of myocardial hypoxia/ischemia, cardiomyocytes have to rely solely on anaerobic glycolysis for energy production; for this, the cells have to depend on increased glucose entry inside the cell as well as increased glycolysis. Stimulation of alpha(1)-adrenoreceptors increases glucose transport inside the cardiomyocytes by translocating glucose transporter (GLUT)-1 and GLUT-4 from the cytoplasm to the plasma membrane, enhances glycogenolysis by activating phosphorylase kinase, increases the rate of glycolysis by activating the enzyme phosphofructokinase, reduces intracellular acidity produced during excessive glycolysis by activating the Na(+)/H(+) exchanger, and inhibits apoptosis by increasing the levels of the antiapoptotic protein Bcl-2. Myocardial ischemia produces an increase in the expression of alpha(1)-adrenoreceptors in cardiomyocytes, as well as increases the levels of its agonist norepinephrine by several fold. During ischemic states, upregulation of alpha(1)-adrenoreceptors and increase in norepinephrine release could be a powerful adaptive mechanism that drives IPC. An understanding into the role of alpha(1)-adrenoreceptors in mediating IPC could not only point to newer treatments for limiting myocardial damage during myocardial infarction or heart surgery, but could also help in avoiding the use of alpha(1)-antagonists in patients with ischemic heart disease.

alpha(1)-Adrenergic receptors activate NHE1 and NHE3 through distinct signaling pathways in epithelial cells

The Na+/H+ exchanger (NHE) regulates intracellular pH, cell volume, Na+ absorption and H+ secretion in epithelial cells of the renal proximal tubule (PT). alpha(1)-Adrenergic receptors (ARs) increase NHE activity in PT cells. The purpose of this study was to determine the mechanism of alpha(1)-AR activation of NHE isoforms expressed in PT cells. Northern and Western blotting demonstrate transcripts and protein expression of NHE1 and NHE3 in PT cells. An anti-NHE1 antibody predominately labels protein expressed at basal and lateral membranes. In contrast, NHE3 protein is expressed exclusively at the apical membrane. To determine NHE isoforms regulated by alpha(1)-ARs, antisense oligodeoxynucleotides (AS-ODNs) specific for NHE1 and NHE3 isoforms were introduced into cells with streptolysin O permeabilization. Cells incubated with AS-ODNs a total of three times exhibited a reduction in protein expression of ~85%. Na uptake and changes in intracellular pH (pH(i)) were used as measures of NHE activity in PT cells. alpha(1)-AR stimulation increased Na uptake from 8.5 to 13.8 nmol. min(-1). mg protein(-1). AS-ODNs to NHE3 significantly reduced alpha(1)-AR stimulated Na uptake and increases in pH(i); no effect was observed in sense-ODN-treated cells. Inhibition of NHE1 but not NHE3 expression abolishes amiloride-suppressible NHE activity. alpha(1)-AR stimulation of NHE1 is inhibited by the protein kinase C (PKC) inhibitor calphostin C whereas NHE3 activity is abolished by the mitogen-activated protein kinase (MAPK) inhibitor PD-98059. In PT cells transfected with MAPK kinase MEKK1(COOH), a truncated version of MEKK1 that activates MAPK, NHE3 but not NHE1 activity is stimulated. We conclude that alpha(1)-ARs activate distinct signaling pathways to regulate specific NHE isoforms localized on opposite membranes in polarized renal epithelial cells. alpha(1)-AR activation of NHE1 is regulated by PKC whereas NHE3 is controlled by MAPK and serves to separately regulate pH(i), Na absorption, and proton excretion in PT cells.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

PKC regulation of organic anion secretion in perfused S2 segments of rabbit proximal tubules

To examine the role of protein kinase C (PKC) in organic anion (OA) secretion, we used epifluorescence microscopy to study steady-state transepithelial secretion of 1 microM fluorescein (FL) by isolated perfused S2 segments of rabbit renal proximal tubules. Addition of 100 nM phorbol 12-myristate 13-acetate (PMA), a known PKC activator, to the bathing medium decreased steady-state secretion of FL by approximately 30% after 25 min. This inhibition was irreversible and, indeed, increased to approximately 40% at 25 min following removal of PMA [10 microM 1,2-dioctanoyl-sn-glycerol (DOG) produced a comparable inhibition]. The inhibition produced by PMA was blocked when 100 nM of either staurosporine (ST) or bisindolylmaleimide I (BIM), both known PKC inhibitors, was added to the bath for a 20-min preexposure followed by the addition of PMA. ST or BIM alone had no significant effect on FL secretion, suggesting that the basal FL secretion rate was not under influence of PKC. Addition of 1 microM of either the peptide hormone bradykinin (BK) or the alpha(1)-receptor agonist phenylephrine (PE), both of which stimulate PKC via a ligand-receptor-PKC coupling reaction, to the bath also inhibited FL secretion by approximately 22 and approximately 27%, respectively. However, the inhibition was completely reversible after removal of BK or PE. Pretreatment of tubules with 100 nM BIM eliminated the inhibition of FL secretion produced by exposure to PE. We conclude that PKC negatively regulates the net secretion of OAs in rabbit renal proximal tubules. The data indicate that BK or catecholamines can play a physiological role in regulating OA secretion via PKC activation.

Vascular alpha 1D-adrenoceptors: are they related to hypertension?

Heterogeneity of vascular alpha 1-adrenoceptor subtypes has been revealed by pharmacological and molecular biology studies (i.e., alpha 1A-, alpha 1B-, and alpha 1D-adrenoceptors). The alpha 1D-adrenoceptor subtype is predominantly involved in the contraction of a variety of vessels and its role in the control of blood pressure has been suggested, a phenomenon probably related to aging. Recent advances in the use of young pre-hypertensive rats and adult spontaneously hypertensive rats with one kidney and Grollman-type renal hypertension suggest vascular alpha 1D-adrenoceptor involvement in the increased blood pressure. The possible role of alpha 1D-adrenoceptors in the genesis/maintenance of hypertension is discussed in this review.

Inhibition of initial transport rate of basolateral organic anion carrier in renal PT by BK and phenylephrine

The effect of ligands for phospholipase C-coupled receptors and of protein kinase C (PKC) stimulation with phorbol ester [phorbol 12-myristate 13-acetate (PMA)] or 1,2-dioctanoyl-sn-glycerol on the activity of the basolateral organic anion transporter (OAT) in S2 segments of single, nonperfused rabbit proximal tubules (PT) was measured with the use of fluorescein and epifluorescence microscopy. The initial uptake rate (25 s, OAT activity) was measured in real time by using conditions similar to those found in vivo. Stimulation of PKC with PMA or 1,2-dioctanoyl-sn-glycerol led to an inhibition of OAT activity, which could be prevented by 10(-7) mol/l of the PKC-specific inhibitor bisindolylmaleimide. The alpha(1)-receptor agonist phenylephrine as well as the peptide hormone bradykinin induced a reversible decrease of OAT activity, which was prevented by bisindolylmaleimide. The observed effect was not due to a decrease in the concentration of the counterion alpha-ketoglutarate or to impaired alpha-ketoglutarate recycling, because it was unchanged in the continuous presence of alpha-ketoglutarate or methyl succinate. We conclude that physiological stimuli can inhibit the activity of OAT in rabbit PT via PKC. The effect is not mediated by alterations in counterion availability but by a direct action on the OAT.

Alpha1- and alpha2-adrenoceptor control of sodium transport reverses in developing hypertension

Alpha-Adrenergic receptor (AR) activation enhances sodium retention in certain forms of hypertension. The objective of the present study was to understand the role of alpha-ARs in regulating sodium transport by distal tubules (DT). DT cells were isolated from kidneys of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at 6 weeks, when hypertension is developing, or at 12 weeks, when hypertension is established. The alpha1-AR agonist phenylephrine increased 22Na uptake by 50% into DT cells of 6-week SHR; no effect was observed with WKY cells. The alpha2-AR agonist B-HT 933 increased uptake by only 10%. At 12 weeks, the pattern of alpha-AR regulation was reversed: alpha1-AR-induced sodium uptake was only 15%, whereas alpha2-AR activation increased sodium uptake by 35% in SHR and WKY cells. alpha1-AR-induced sodium uptake in 6-week SHR cells was abolished by prazosin; alpha2-AR-stimulated sodium uptake was blocked by yohimbine in 12-week SHR and WKY. Competitive binding studies were performed with [3H]prazosin and alpha1A-, alpha1B-, and alpha1D-selective antagonists with DT cell membranes from 6- and 12-week SHR and WKY. alpha2-AR subtypes were determined with [3H]rauwolscine and alpha2A- and alpha2B-selective antagonists. Expression of alpha1B-ARs was increased 4-fold in DT cells during the developing phase of hypertension in SHR. No change was detected in alpha2-AR expression. DT cells transiently increase [Ca2+]i in response to alpha1-AR agonists from 6-week but not 12-week SHR. Conversely, alpha2-AR agonists increase [Ca2+]i at 12 weeks. In summary, during developing hypertension, alpha1-ARs increase sodium uptake and [Ca2+]i in SHR cells. Expression of alpha1B-ARs is selectively upregulated during developing hypertension. In established hypertension (and normotension), alpha2-ARs regulate sodium transport and [Ca2+]i in DT cells. We conclude that a molecular switch of alpha1-AR and alpha2-AR signaling occurs in DT cells during the development of hypertension.

Neural control of renal function

The renal nerves are the communication link between the central nervous system and the kidney. In response to multiple peripheral and central inputs, efferent renal sympathetic nerve activity is altered so as to convey information to the major structural and functional components of the kidney, the vessels, glomeruli, and tubules, each of which is innervated. At the level of each of these individual components, information transfer occurs via interaction of the neurotransmitter released at the sympathetic nerve terminal-neuroeffector junction with specific postjunctional receptors coupled to defined intracellular signaling and effector systems. In response to normal physiological stimuli, changes in efferent renal sympathetic nerve activity contribute importantly to homeostatic regulation of renal blood flow, glomerular filtration rate, renal tubular epithelial cell solute and water transport, and hormonal release. Afferent input from sensory receptors located in the kidney participates in this reflex control system via renorenal reflexes that enable total renal function to be self-regulated and balanced between the two kidneys. In pathophysiological conditions, abnormal regulation of efferent renal sympathetic nerve activity contributes significantly to the associated abnormalities of renal function which, in turn, are of importance in the pathogenesis of the disease.