Ablation of miRNA-22 protects against obesity-induced adipocyte senescence and ameliorates metabolic disorders in middle-aged mice

High-fat diet (HFD) promotes obesity-related metabolic complications by activating cellular senescence in white adipose tissue (WAT). Growing evidence supports the importance of microRNA-22 (miR-22) in metabolic disorders and cellular senescence. Recently, we showed that miR-22 deletion attenuates obesity-related metabolic abnormalities. However, whether miR-22 mediates HFD-induced cellular senescence of WAT remains unknown. Here, we uncovered that obese mice displayed increased pri-miR-22 levels and cellular senescence in WAT. However, miR-22 ablation protected mice against HFD-induced WAT senescence. In addition, in vitro studies showed that miR-22 deletion prevented preadipocyte senescence in response to Doxorubicin (Doxo). Loss-of-function studies in vitro and in vivo revealed that miR-22 increases H2ax mRNA and γH2ax levels in preadipocytes and WAT without inducing DNA damage. Intriguingly, miR-22 ablation prevented HFD-induced increase in γH2ax levels and DNA damage in WAT. Similarly, miR-22 deletion prevented Doxo-induced increase in γH2ax levels in preadipocytes. Adipose miR-22 levels were enhanced in middle-aged mice fed a HFD than those found in young mice. Furthermore, miR-22 deletion attenuated fat mass gain and glucose imbalance induced by HFD in middle-aged mice. Overall, our findings indicate that miR-22 is a key regulator of obesity-induced WAT senescence and metabolic disorders in middle-aged mice.

miRNA-22 is involved in the aortic reactivity in physiological conditions and mediates obesity-induced perivascular adipose tissue dysfunction

AIMS

Blood vessels are surrounded by perivascular adipose tissue (PVAT), which plays an important role in vascular tonus regulation due to its anticontractile effect; however, this effect is impaired in obesity. We previously demonstrated that miRNA-22 is involved in obesity-related metabolic disorders. However, the impact of miRNA-22 on vascular reactivity and PVAT function is unknown.

AIM

To investigate the role of miRNA-22 on vascular reactivity and its impact on obesity-induced PVAT dysfunction.

MAIN METHODS

Wild-type and miRNA-22 knockout (KO) mice were fed a control or a high-fat (HF) diet. To characterize the vascular response, concentration-responses curves to noradrenaline were performed in PVAT- or PVAT+ thoracic aortic rings in absence and presence of L-NAME. Expression of adipogenic and thermogenic markers and NOS isoforms were evaluated by western blotting or qPCR.

KEY FINDINGS

HF diet and miRNA-22 deletion reduced noradrenaline-induced contraction in PVAT- aortic rings. Additionally, miRNA-22 deletion increased noradrenaline-induced contraction in PVAT+ aortic rings without affecting its sensitivity; however, this effect was not observed in miRNA-22 KO mice fed a HF diet. Interestingly, miRNA-22 deletion reduced the contraction of aortic rings to noradrenaline via a NOS-dependent mechanism. Moreover, HF diet abolished the NOS-mediated anticontractile effect of PVAT, which was attenuated by miRNA-22 deletion. Mechanistically, we found that PVAT from miRNA-22 KO mice fed a HF diet presented increased protein expression of nNOS.

SIGNIFICANCE

These results suggest that miRNA-22 is important for aorta reactivity under physiological circumstances and its deletion attenuates the loss of the NOS-mediated anticontractile effect of PVAT in obesity.

Abstract P314: Loss Of Set7 Prevents Isoproterenol-induced Heart Dysfunction

Recent studies have revealed the influence of histone-modifying enzymes in cardiac remodeling and dysfunction. The Set7 methyltransferase regulates the expression of several genes through methylation of histones and modulates the activity of non-histone proteins. However, the role of Set7 in heart dysfunction remains unknown. To answer this question, wild type (WT) and Set7 knockout (KO) male mice were injected with isoproterenol (iso) or saline (s) subcutaneously for 14 days. The WTiso mice displayed decreased Set7 activity in the heart compared to WTs mice (Table 1). Both WTiso and KOiso mice exhibited increased heart weight to tibia length ratio (HW/TL) and cardiomyocyte area. However, KOiso mice had higher HW/TL and cardiomyocyte area compared to WTiso mice. Sirius Red staining revealed that both WT and KO mice injected with iso had increased myocardial fibrosis compared to their controls. Nonetheless, loss of Set7 attenuated iso-induced myocardial fibrosis. Echocardiogram showed that WTiso mice had lower ejection fraction (EF) and fractional shortening (FS), and higher E/A ratio compared to WTs mice. Conversely, KOiso mice did not show alteration on these parameters compared to their controls. RNA sequencing analysis revealed that biological processes related to oxidant detoxication, cellular respiration, and anti-inflammatory response were enriched in the heart of KOiso mice compared to WTiso mice. On the other hand, biological processes related to cell aging, interferon production, and immune response were downregulated in the heart of KOiso mice compared to WTiso mice. Collectively, our data suggest that Set7 deletion prevents iso-induced heart dysfunction.

miRNA-22 deletion limits white adipose expansion and activates brown fat to attenuate high-fat diet-induced fat mass accumulation

BACKGROUND

Obesity, characterized by excessive expansion of white adipose tissue (WAT), is associated with numerous metabolic complications. Conversely, brown adipose tissue (BAT) and beige fat are thermogenic tissues that protect mice against obesity and related metabolic disorders. We recently reported that deletion of miR-22 enhances energy expenditure and attenuates WAT expansion in response to a high-fat diet (HFD). However, the molecular mechanisms involved in these effects mediated by miR-22 loss are unclear.

METHODS AND RESULTS

Here, we show that miR-22 expression is induced during white, beige, and brown adipocyte differentiation in vitro. Deletion of miR-22 reduced white adipocyte differentiation in vitro. Loss of miR-22 prevented HFD-induced expression of adipogenic/lipogenic markers and adipocyte hypertrophy in murine WAT. In addition, deletion of miR-22 protected mice against HFD-induced mitochondrial dysfunction in WAT and BAT. Loss of miR-22 induced WAT browning. Gain- and loss-of-function studies revealed that miR-22 did not affect brown adipogenesis in vitro. Interestingly, miR-22 KO mice fed a HFD displayed increased expression of genes involved in thermogenesis and adrenergic signaling in BAT when compared to WT mice fed the same diet.

CONCLUSIONS

Collectively, our findings suggest that loss of miR-22 attenuates fat accumulation in response to a HFD by reducing white adipocyte differentiation and increasing BAT activity, reinforcing miR-22 as a potential therapeutic target for obesity-related disorders.

Ubiquitin proteasome system (UPS) activation in the cardiac hypertrophy of hyperthyroidism

Ubiquitin proteasome system (UPS) is the main proteolytic pathway in eukaryotic cells. Changes in proteasome expression and activity have been associated to cardiovascular diseases as cardiac hypertrophy. Considering that cardiac hypertrophy is commonly associated to hyperthyroidism condition, the present study aimed to investigate the contribution of UPS in cardiac hypertrophy induced by thyroid hormones. Hyperthyroidism was induced in male Wistar rats by intraperitoneal injections of triiodothyronine (T3; 7  μg/100 g of body weight) for 7 days and confirmed by raised levels of total T3 and decreased levels of total T4. In addition, systolic blood pressure and heart rate were significantly increased in hyperthyroid group. Cardiac hypertrophy was confirmed in hyperthyroid group by increased heart weight/tibia length ratio and by increased α-MHC/β-MHC relative expression. Both catalytic (20SPT) and regulatory subunits (19SPT) of the constitutive proteasome were upregulated in hyperthyroid hearts. In addition, the transcripts that encode immunoproteasome subunits were also elevated. Furthermore, ATP-dependent chymotrypsin-like activity (26SPT) was significantly increased in hyperthyroid group. Despite the upregulation and activation of UPS in hyperthyroid hearts, the content of polyubiquitinated proteins was unaltered in relation to control. Together, these results evidence the activation of cardiac proteasome by thyroid hormones, which possibly contribute to the maintenance of protein quality control and regulation of cardiac hypertrophy in response to thyroid hormones.

MyD88 activation in cardiomyocytes contributes to the heart immune response to acute Trypanosoma cruzi infection with no effect on local parasite control

Cardiomyopathy is the most serious consequence of Chagas disease, a neglected human disorder caused by Trypanosoma cruzi infection. Because T. cruzi parasites invade cardiomyocytes, we sought to investigate whether these cells recognize the parasite in vivo by receptors signaling through the MyD88 adaptor, which mediates the activation pathway of most Toll-like receptors (TLRs) and IL-1/IL-18 receptors, and influence the development of acute cardiac pathology. First, we showed that HL-1 cardiac muscle cell line expresses MyD88 gene and protein at resting state and after T. cruzi infection. To evaluate the role in vivo of MyD88 expression in cardiomyocytes, we generated Mer+MyD88flox+/+ mice in which tamoxifen treatment is expected to eliminate the MyD88 gene exclusively in cardiomyocytes. This Cre-loxP model was validated by both PCR and western blot analysis; tamoxifen treatment of Mer+MyD88flox+/+ mice resulted in decreased MyD88 gene and protein expression in the heart, but not in the spleen, while had no effect on littermates. The elimination of MyD88 in cardiomyocytes determined a lower increase in CCL5, IFNγ and TNFα gene transcription during acute infection by T. cruzi parasites of the Y strain, but it did not significantly modify heart leukocyte infiltration and parasitism. Together, our results show that cardiomyocytes can sense T. cruzi infection through MyD88-mediated molecular pathways and contribute to the local immune response to the parasite. The strong pro-inflammatory response of heart-recruited leukocytes may overshadow the effects of MyD88 deficiency in cardiomyocytes on the local leukocyte recruitment and T. cruzi control during acute infection.

Lactate up-regulates the expression of lactate oxidation complex-related genes in left ventricular cardiac tissue of rats

Background

Besides its role as a fuel source in intermediary metabolism, lactate has been considered a signaling molecule modulating lactate-sensitive genes involved in the regulation of skeletal muscle metabolism. Even though the flux of lactate is significantly high in the heart, its role on regulation of cardiac genes regulating lactate oxidation has not been clarified yet. We tested the hypothesis that lactate would increase cardiac levels of reactive oxygen species and up-regulate the expression of genes related to lactate oxidation complex.

Methods/Principal Findings

Isolated hearts from male adult Wistar rats were perfused with control, lactate or acetate (20mM) added Krebs-Henseleit solution during 120 min in modified Langendorff apparatus. Reactive oxygen species (O₂^●-/H₂O₂) levels, and NADH and NADPH oxidase activities (in enriched microsomal or plasmatic membranes, respectively) were evaluated by fluorimetry while SOD and catalase activities were evaluated by spectrophotometry. mRNA levels of lactate oxidation complex and energetic enzymes MCT1, MCT4, HK, LDH, PDH, CS, PGC1α and COXIV were quantified by real time RT-PCR. Mitochondrial DNA levels were also evaluated. Hemodynamic parameters were acquired during the experiment. The key findings of this work were that lactate elevated cardiac NADH oxidase activity but not NADPH activity. This response was associated with increased cardiac O₂^●-/H₂O₂ levels and up-regulation of MCT1, MCT4, LDH and PGC1α with no changes in HK, PDH, CS, COXIV mRNA levels and mitochondrial DNA levels. Lactate increased NRF-2 nuclear expression and SOD activity probably as counter-regulatory responses to increased O₂^●-/H₂O₂.

Conclusions

Our results provide evidence for lactate-induced up-regulation of lactate oxidation complex associated with increased NADH oxidase activity and cardiac O₂^●-/H₂O₂ driving to an anti-oxidant response. These results unveil lactate as an important signaling molecule regulating components of the lactate oxidation complex in cardiac muscle.

Acute actions of thyroid hormone on blood vessel biochemistry and physiology

PURPOSE OF REVIEW

Description of the progress about the vascular effects promoted by thyroid hormones.

RECENT FINDINGS

Over the past few years, a number of studies have shown that in addition to genomic effects on blood vessels, thyroid hormones exert extranuclear nongenomic effects on vascular smooth muscle cells and endothelium. These nongenomic effects occur rapidly and do not involve thyroid hormone response elements-mediated transcriptional events. In this context, the genomic and nongenomic events promoted by thyroid hormones act in concert to control the vascular hemodynamic and regulate the cardiovascular function.

SUMMARY

Considering the antiatherogenic property of thyroid hormones and the rapid effects produced by this molecule as a vasodilator, including that in the coronary bed, a better understanding of the molecular mechanisms involved in its action may contribute to the development of drugs that can be clinically used to increase the known benefits promoted by thyroid hormones in cardiovascular physiology.

Expression of circadian clock and melatonin receptors within cultured rat cardiomyocytes

Melatonin, the pineal gland hormone, provides entrainment of many circadian rhythms to the ambient light/dark cycle. Recently, cardiovascular studies have demonstrated melatonin interactions with many physiological processes and diseases, such as hypertension and cardiopathologies. Although membrane melatonin receptors (MT1, MT2) and the transcriptional factor RORα have been reported to be expressed in the heart, there is no evidence of the cell-type expressing receptors as well as the possible role of melatonin on the expression of the circadian clock of cardiomyocytes, which play an important role in cardiac metabolism and function. Therefore, the aim of this study was to evaluate the mRNA and protein expressions of MT1, MT2, and RORα and to determine whether melatonin directly influences expression of circadian clocks within cultured rat cardiomyocytes. Adult rat cardiomyocyte cultures were created, and the cells were stimulated with 1 nM melatonin or vehicle. Gene expressions were assayed by real-time polymerase chain reaction (PCR). The mRNA and protein expressions of membrane melatonin receptors and RORα were established within adult rat cardiomyocytes. Two hours of melatonin stimulation did not alter the expression pattern of the analyzed genes. However, given at the proper time, melatonin kept Rev-erbα expression chronically high, specifically 12 h after melatonin treatment, avoiding the rhythmic decline of Rev-erbα mRNA. The blockage of MT1 and MT2 by luzindole did not alter the observed melatonin-induced expression of Rev-erbα mRNA, suggesting the nonparticipation of MT1 and MT2 on the melatonin effect within cardiomyocytes. It is possible to speculate that melatonin, in adult rat cardiomyocytes, may play an important role in the light signal transduction to peripheral organs, such as the heart, modulating its intrinsic rhythmicity.

Increased blood pressure in the offspring of diabetic mothers

Studies were conducted in rats to determine the effect of maternal diabetes and the consequent hyperglycemia on cardiovascular function in the offspring. Diabetes was induced in pregnant Wistar rats through streptozotocin injection (50 mg/kg). Cardiovascular parameters were measured in 2-mo-old offspring animals of diabetic (OD, n=12) and control rats (OC, n=8). Arterial pressure (AP), heart rate (HR), baroreflex sensitivity, and vascular responsiveness to phenylephrine (PH) and sodium nitroprusside (SN) were measured. Angiotensin-converting enzyme (ACE) activity in heart, kidney, and lung was determined. OD rats exhibited increases in systolic AP (138+/-8 vs. 119+/-6 mmHg, OD vs. OC), with no change in HR (342+/-21 vs. 364+/-39 beats per minute (bpm), OD vs. OC). The reflex tachycardia elicited by SN was reduced in OD rats, as indicated by the slope of the linear regression (-2.2+/-0.4 vs. -3.6+/-0.8 bpm/mmHg, OD vs. OC). Vascular responsiveness to PH was increased 63% in OD rats compared with OC. OD rats showed increases in ACE activity in heart, kidney, and lung (1.13+/-0.24, 3.04+/-0.86, 40.8+/-8.9 vs. 0.73+/-0.19, 1.7+/-0.45, 28.1+/-6 nmol His-Leu.min-1 mg protein-1, OD vs. OC). Results suggest that diabetes during pregnancy affects cardiovascular function in offspring, seen as hypertension, baroreflex dysfunction, and activation of tissue renin-angiotensin system.

Glucocorticoid regulation of angiotensin-converting enzyme in primary culture of adult cardiac fibroblasts

The effect of dexamethasone on angiotensin-converting enzyme (ACE) in primary culture of adult cardiac fibroblasts was analyzed in this study. ACE is central to cardiac remodeling in conditions such as myocardial infarction (MI). Some studies indicate that glucocorticoids are often increased post-MI, whereas other studies suggest that glucocorticoids stimulate ACE activity in various cell types. Most cardiac cells are fibroblasts, which have an important function in cardiac remodeling. Therefore, we studied the effects of glucocorticoids on ACE activity and mRNA levels in primary cultures of adult rat cardiac fibroblasts. Steady-state ACE activity was very low, but it increased sixfold with dexamethasone (1 microM for 48 h) treatment. ACE activation occurred within 12 h and peaked at 96 h, after treatment. RNase-protection assays revealed an associated threefold increase (P < 0.05) in ACE mRNA. Dexamethasone's stimulatory effect was abolished by an RNA synthesis inhibitor (actinomycin D, 5 microg/ml) but was potentiated by a protein synthesis inhibitor (cycloheximide, 5 microg/ml). The glucocorticoid-mediated response appears to be specific, because mineralocorticoid treatment did not alter ACE activity. These findings indicate that both transcriptional and posttranscriptional mechanisms are involved in glucocorticoid regulation of ACE expression in rat cardiac fibroblasts.

Stimulatory effect of dexamethasone on angiotensin-converting enzyme in neonatal rat cardiac myocytes

Angiotensin-converting enzyme (ACE) plays a central role in cardiac remodeling associated with pathological conditions such as myocardial infarction. The existence of different cell types in the heart expressing components of the renin-angiotensin system makes it difficult to evaluate their relative role under physiological and pathological conditions. Since myocytes are the predominant cellular constituent of the heart by mass, in the present study we studied the effects of glucocorticoids on ACE activity using well-defined cultures of neonatal rat cardiac myocytes. Under steady-state conditions, ACE activity was present at very low levels, but after dexamethasone treatment ACE activity increased significantly (100 nmol/l after 24 h) in a time-dependent fashion. These results demonstrate the influence of dexamethasone on ACE activity in rat cardiac myocytes. This is consistent with the idea that ACE activation occurs under stress conditions, such as myocardial infarction, in which glucocorticoid levels may increase approximately 50-fold.

Luminal arginine vasopressin stimulates Na(+)-H+ exchange and H(+)-ATPase in cortical distal tubule via V1 receptor

Bicarbonate reabsorption was evaluated by stationary microperfusion of in vivo early distal (ED) and late distal (LD) segments of rat kidney. Intratubular pH was recorded by double-barreled H ion-exchange resin/reference (1 M KCl) microelectrodes for the determination of HCO3- reabsorption. In the presence of luminal arginine vasopressin (AVP, 10(-9) M), a significant increase in HCO3- reabsorption was observed both in ED (from 0.931 +/- 0.061 to 2.12 +/- 0.171 nmol.cm-2.s-1] and LD segments [from 0.542 +/- 0.086 to 1.67 +/- 0.111 nmol.cm-2.s-1]. The addition of the V1-receptor antagonist [(d (CH2)5, Tyr (Et)2) arginine vasopressin] (10(-5) M) to luminal perfusion blocked luminal AVP mediated stimulation in ED and LD segments. 5-(N, N-hexamethylene) amiloride (10(-4) M) added to luminal perfusion inhibited luminal AVP-mediated stimulation in ED (by 63.7%) and LD (by 34.1%) segments. The addition of Bafilomycin A1 (2 x 10(-7) M) to the luminal perfusion did not affect luminal AVP-mediated stimulation in ED segments, but reduced it (by 31.7%) in LD segments. Our results indicate that luminal AVP acts to stimulate the Na(+)-H+ exchange in ED and LD segments via activation of V1 receptors, as well as the vacuolar H(+)-ATPase in LD segments.

Mechanisms and regulation of H+ transport in distal tubule epithelial cells

The mechanism of acidification in the cortical distal tubule of mammalian kidney was analysed by "in vivo" microperfusion and using MDCK cells in culture, by electrophysiological and by cell pH microfluorescence techniques. An electrogenic effect of the vacuolar H(+)-ATPase, which has been localized to the intercalated cells of the cortical distal tubule (connecting segment and initial collecting duct) was only observed after blocking Cl- channels by NPPB. In MDCK cells, the recovery of cell pH after an acid pulse in Na(+)-free medium was also depressed by NPPB, indicating that Cl- ions have an important role in the function of H+ ATPase. The regulation by hormonal agents of distal H+ transport due to Na+/H+ exchange and to vacuolar H+ ATPase, was also studied by microperfusion and cell pH techniques. Angiotensin and vasopressin at picomolar concentrations stimulated both transport mechanisms in late distal tubule, and only Na+/H+ exchange in the early segment. In MDCK cells, cell pH recovery in the presence of Na+ was stimulated by picomolar concentrations of angiotensin and vasopressin, and inhibited by micromolar levels, both effects being reverted by micromolar ANP. Studies with specific antagonists suggest that the luminal effect of angiotensin is mediated by AT1 receptors, and of vasopressin by V1 receptors. There is evidence that cell Ca2+ may have an important regulatory role in the action of these hormones.

Interactions of ANP and ANG II in tubular nephron acidification

In order to examine the effects and the interaction of angiotensin II (ANG II, 1 pM) and atrial natriuretic peptide (ANP, 1 microM) on the kinetics of bicarbonate reabsorption in the rat middle proximal tubule, we performed in vivo experiments using a stopped-flow microperfusion technique with the determination of lumen pH by Sb microelectrodes. These studies confirmed that ANG II added to the luminal or peritubular capillary perfusion fluid stimulates proximal bicarbonate reabsorption and showed that ANP alone does not affect this process, but impairs the stimulation caused by ANG II. We also studied the effects and the interaction of these hormones in cortical distal nephron acidification. Bicarbonate reabsorption was evaluated by the acidification kinetic technique in early (ED) and late (LD) distal tubules in rats during in vivo stopped-flow microperfusion experiments. The intratubular pH was measured with a double-barreled microelectrode with H(+)-sensitive resin. The results indicate that ANG II acted by stimulating Na+/H+ exchange in ED (81%) and LD (54%) segments via activation of AT1 receptors, as well as vacuolar H(+)-ATPase in LD segments (33%). ANP did not affect bicarbonate reabsorption in either segment and, as opposed to what was seen in the proximal tubule, did not impair the stimulation caused by ANG II. To investigate the mechanism of action of these hormones in more detail, we studied cell pH dependence on ANG II and ANP in MDCK cells using the fluorescent probe BCECF. We showed that the velocity of cell pH recovery was almost abolished in the absence of Na+, indicating that it is dependent on Na+/H+ exchange. ANP (1 microM) alone had no effect on this recovery but reversed both the acceleration of H+ extrusion at low ANG II levels (1 pM and 1 nM), and inhibition of H+ extrusion at higher ANG II levels (100 nM). To obtain more information on the mechanism of interaction of these hormones, we also studied their effects on the regulation of intracellular free calcium concentration, [Ca2+]i, monitored with the fluorescent probe Fura-2 in MDCK cells in suspension. The data indicate that the addition of increasing concentrations of ANG II (1 pM to 1 microM) to the cell suspension led to a progressive increase in [Ca2+]i to 2-3 times the basal level. In contrast, the addition of ANP (1 microM) to the cell suspension led to a very rapid 60% decrease in [Ca2+]i and reduced the increase elicited by ANG II, thus modulating the effect of ANG II on [Ca2+]i. These results may indicate a role of [Ca2+]i in the regulation of the H+ extrusion process mediated by Na+/H+ exchange and stimulated/impaired by ANG II. The data are compatible with stimulation of Na+/H+ exchange by increases of [Ca2+]i in the lower range, and inhibition at high [Ca2+]i levels.

Effect of luminal angiotensin II and ANP on early and late cortical distal tubule HCO3- reabsorption

Bicarbonate reabsorption was evaluated by stationary microperfusion "in vivo" early distal (ED) and late distal (LD) segments of at kidney. Intratubular pH was recorded by double-barreled of H+ exchange resin/reference (1 M KCl) microelectrodes for the determination of HCO3- reabsorption. In the presence of angiotensin II (ANG II) (10(-12) M), a significant increase in HCO3- reabsorption was observed both in ED (from 0.930 +/- 0.060 to 2.64 +/- 0.210 nmol.cm-2.s-1 in luminally perfused tubules and from 0.850 +/- 0.040 to 2.03 +/- 0.210 nmol.cm-2.s-1 during capillary perfusion) and LD segments from 0.310 +/- 0.130 to 2.16 +/- 0.151 nmol.cm-2.s-1 during luminal perfusion and from 0.530 +/- 0.031 to 2.16 +/- 0.211 nmol.cm-2.s-1 with capillary perfusion). The addition of the AT1-receptor antagonist losartan (10(-6) M) to luminal perfusion blocked luminal ANG II-mediated stimulation in ED and LD segments. 5-(N,N-hexamethylene)amiloride (10(-4) M) added to luminal perfusion inhibited luminal ANG II-mediated stimulation in ED (by 81%) and LD (by 54%) segments. The addition of bafilomycin A1 (2 x 10(-7) M) to luminal perfusion does not affect luminal ANG II-mediated stimulation in ED segments but reduces it in LD segments (by 33%). During the addition of atrial natriuretic peptide (ANP) (10(-6) M) or ANG II plus ANP in both segments, no significant differences in HCO3- reabsorption were observed. Our results indicate that luminal ANG II acts to stimulate Na+/H+ exchange in ED and LD segments via activation of AT1 receptors, as well as the vacuolar H(+)-adenosinetriphosphatase in LD segments. ANP does not affect HCO3- reabsorption in either ED or LD segments and does not impair the stimulation caused by ANG II.