Vasoconstrictor responses in thromboxane receptor knockout mice: tubuloglomerular feedback and ureteral obstruction

Concurrent activation of multiple vasoactive signaling pathways in vasoconstriction caused by tubuloglomerular feedback: a quantitative assessment

Tubuloglomerular feedback (TGF) describes the negative relationship between (a) NaCl concentration at the macula densa and (b) glomerular filtration rate or glomerular capillary pressure. TGF-induced vasoconstriction of the afferent arteriole results from the enhanced effect of several vasoconstrictors with an effect size sequence of adenosine = 20-HETE > angiotensin II > thromboxane = superoxide > renal nerves > ATP. TGF-mediated vasoconstriction is limited by the simultaneous release of several vasodilators with an effect size sequence of nitric oxide > carbon monoxide = kinins > adenosine. The sum of the constrictor effects exceeds that of the dilator effects by the magnitude of the TGF response. The validity of the additive model used in this analysis can be tested by determining the effect of combined inhibition of some or all agents contributing to TGF. Multiple independent contributors to TGF are consistent with the variability of TGF and of the factors contributing to TGF resetting.

Thromboxane prostanoid receptors enhance contractions, endothelin-1, and oxidative stress in microvessels from mice with chronic kidney disease

Cardiovascular disease is frequent in chronic kidney disease and has been related to angiotensin II, endothelin-1 (ET-1), thromboxane A2, and reactive oxygen species (ROS). Because activation of thromboxane prostanoid receptors (TP-Rs) can generate ROS, which can generate ET-1, we tested the hypothesis that chronic kidney disease induces cyclooxygenase-2 whose products activate TP-Rs to enhance ET-1 and ROS generation and contractions. Mesenteric resistance arterioles were isolated from C57/BL6 or TP-R+/+ and TP-R-/- mice 3 months after SHAM-operation (SHAM) or surgical reduced renal mass (RRM, n=6/group). Microvascular contractions were studied on a wire myograph. Cellular (ethidium: dihydroethidium) and mitochondrial (mitoSOX) ROS were measured by fluorescence microscopy. Mice with RRM had increased excretion of markers of oxidative stress, thromboxane, and microalbumin; increased plasma ET-1; and increased microvascular expression of p22(phox), cyclooxygenase-2, TP-Rs, preproendothelin and endothelin-A receptors, and increased arteriolar remodeling. They had increased contractions to U-46,619 (118 ± 3 versus 87 ± 6, P<0.05) and ET-1 (108 ± 5 versus 89 ± 4, P<0.05), which were dependent on cellular and mitochondrial ROS, cyclooxygenase-2, and TP-Rs. RRM doubled the ET-1-induced cellular and mitochondrial ROS generation (P<0.05). TP-R-/- mice with RRM lacked these abnormal structural and functional microvascular responses and lacked the increased systemic and the increased microvascular oxidative stress and circulating ET-1. In conclusion, RRM leads to microvascular remodeling and enhanced ET-1-induced cellular and mitochondrial ROS and contractions that are mediated by cyclooxygenase-2 products activating TP-Rs. Thus, TP-Rs can be upstream from enhanced ROS, ET-1, microvascular remodeling, and contractility and may thereby coordinate vascular dysfunction in chronic kidney disease.

Cyclooxygenase metabolites in the kidney

In the mammalian kidney, prostaglandins (PGs) are important mediators of physiologic processes, including modulation of vascular tone and salt and water. PGs arise from enzymatic metabolism of free arachidonic acid (AA), which is cleaved from membrane phospholipids by phospholipase A2 activity. The cyclooxygenase (COX) enzyme system is a major pathway for metabolism of AA in the kidney. COX are the enzymes responsible for the initial conversion of AA to PGG2 and subsequently to PGH2, which serves as the precursor for subsequent metabolism by PG and thromboxane synthases. In addition to high levels of expression of the "constitutive" rate-limiting enzyme responsible for prostanoid production, COX-1, the "inducible" isoform of cyclooxygenase, COX-2, is also constitutively expressed in the kidney and is highly regulated in response to alterations in intravascular volume. PGs and thromboxane A2 exert their biological functions predominantly through activation of specific 7-transmembrane G-protein-coupled receptors. COX metabolites have been shown to exert important physiologic functions in maintenance of renal blood flow, mediation of renin release and regulation of sodium excretion. In addition to physiologic regulation of prostanoid production in the kidney, increases in prostanoid production are also seen in a variety of inflammatory renal injuries, and COX metabolites may serve as mediators of inflammatory injury in renal disease.

Cyclooxygenase 2 inhibition suppresses tubuloglomerular feedback: roles of thromboxane receptors and nitric oxide

Thromboxane (TxA(2)) and nitric oxide (NO) are potent vasoactive autocoids that modulate tubuloglomerular feedback (TGF). Each is produced in the macula densa (MD) by cyclooxygenase-2 (COX-2) and neuronal nitric oxide synthase (nNOS), respectively. Both enzymes are similarly regulated in the MD and their interaction may be an important factor in the regulation of TGF and glomerular filtration rate. We tested the hypothesis that TGF is modified by the balance between MD nNOS-dependent NO and MD COX-2-dependent TxA(2). We measured maximal TGF during perfusion of the loop of Henle (LH) by continuous recording of the proximal tubule stopped flow pressure response to LH perfusion of artificial tubular fluid (ATF) at 0 and 40 nl/min. The response to inhibitors of COX-1 (SC-560), COX-2 [parecoxib (Pxb)], and nNOS (l-NPA) added to the ATF solution was measured in separate nephrons. COX-2 inhibition with Pxb reduced TGF by 46% (ATF + vehicle vs. ATF + Pxb), whereas COX-1 inhibition with SC-560 reduced TGF by only 23%. Pretreatment with intravenous infusion of SQ-29,548, a selective thromboxone/PGH(2) receptor (TPR) antagonist, blocked all of the SC-560 effect on TGF, suggesting that this effect was due to activation of TPR. However, SQ-29,548 only partially diminished the effect of Pxb (-66%). Specific inhibition of nNOS with l-NPA increased TGF, as expected. However, the ability of Pxb to reduce TGF was significantly impaired with comicroperfusion of l-NPA. These data suggest that COX-2 modulates TGF by two proconstrictive actions: generation of TxA(2) acting on TPR and by simultaneous reduction of NO.

Oxidative stress and nitric oxide deficiency in the kidney: a critical link to hypertension?

There is growing evidence that oxidative stress contributes to hypertension. Oxidative stress can precede the development of hypertension. In almost all models of hypertension, there is oxidative stress that, if corrected, lowers BP, whereas creation of oxidative stress in normal animals can cause hypertension. There is overexpression of the p22(phox) and Nox-1 components of NADPH oxidase and reduced expression of extracellular superoxide dismutase (EC-SOD) in the kidneys of ANG II-infused rodents, whereas there is overexpression of p47(phox) and gp91(phox) and reduced expression of intracellular SOD with salt loading. Several mechanisms have been identified that can make oxidative stress self-sustaining. Reactive oxygen species (ROS) can enhance afferent arteriolar tone and reactivity both indirectly via potentiation of tubuloglomerular feedback and directly by microvascular mechanisms that diminish endothelium-derived relaxation factor/nitric oxide responses, generate a cyclooxygenase-2-dependent endothelial-derived contracting factor that activates thromboxane-prostanoid receptors, and enhance vascular smooth muscle cells reactivity. ROS can diminish the efficiency with which the kidney uses O(2) for Na(+) transport and thereby diminish the P(O(2)) within the kidney cortex. This may place a break on further ROS generation yet could further enhance vasculopathy and hypertension. There is a tight relationship between oxidative stress in the kidney and the development and maintenance of hypertension.

TP receptors regulate renal hemodynamics during angiotensin II slow pressor response

We investigated the hypothesis that thromboxane A2(TxA2)-prostaglandin H2receptors (TP-Rs) mediate the hemodynamic responses and increase in reactive oxygen species (ROS) to ANG II (400 ng·kg−1·min−1sc for 14 days) using TP-R knockout (TP −/−) and wild-type (+/+) mice. TP −/− had normal basal mean arterial blood pressure (MAP) and glomerular filtration rate but reduced renal blood flow and increased filtration fraction (FF) and renal vascular resistance (RVR) and markers of ROS (thiobarbituric acid-reactive substances and 8-isoprostane PGF2α) and nitric oxide (NOx). Infusion of ANG II into TP +/+ increased ROS and thromboxane B2(TxB2) and increased RVR and FF. ANG II infusion into TP −/− mice reduced ANG I and increased aldosterone but caused a blunted increase in MAP (TP −/−: +6 ± 2 vs. TP +/+: +15 ± 3 mmHg) and failed to increase FF, ROS, or TxB2but increased NOx and paradoxically decreased RVR (−2.1 ± 1.7 vs. +2.6 ± 0.8 mmHg·ml−1·min−1·g−1). Blockade of AT1receptor of TP −/− mice infused with ANG II reduced MAP (−8 mmHg) and aldosterone but did not change the RVR or ROS. In conclusion, during an ANG II slow pressor response, AT1receptors activate TP-Rs that generate ROS and prostaglandins but inhibit NO. TP-Rs mediate all of the increase in RVR and FF, part of the increase in MAP, but are not implicated in the suppression of ANG I or increase in aldosterone. TP −/− mice have a basal increase in RVR and FF associated with ROS.

Perinatal obstructive nephropathy

Significant advances have been made recently in elucidating the cellular consequences of urinary tract obstruction during renal development. Urinary tract obstruction impairs growth and maturation of the kidney, and can also cause renal maldevelopment. This includes a reduction in the number of nephrons, tubular atrophy, and progressive interstitial fibrosis. Apoptosis (programmed cell death) accounts for much of the loss of tubular epithelial cells. Factors contributing to apoptosis include stretching of cells in dilated tubules, altered renal production of growth factors, and infiltration of the renal interstitium by macrophages. Two major controversies remain regarding the surgical management of congenital obstructive nephropathy: first, which fetuses with bladder outlet obstruction should undergo prenatal intervention, and second, which infants should undergo early pyeloplasty for ureteropelvic junction obstruction? Even after successful surgery for congential obstructive nephropathy, all patients should be followed for hypertension, proteinuria, or renal deterioration.

Deficiency of COX-1 causes natriuresis and enhanced sensitivity to ACE inhibition

BACKGROUND

Prostanoid products of the cyclo-oxygenase (COX) pathway of arachidonic acid metabolism modulate blood pressure (BP) and sodium homeostasis. Conventional non-steroidal anti-inflammatory drugs (NSAIDs), which inhibit both COX isoforms (COX-1 and -2), cause sodium retention, exacerbate hypertension, and interfere with the efficacy of certain anti-hypertensive agents such as angiotensin-converting enzyme (ACE) inhibitors. While a new class of NSAIDs that specifically inhibit COX-2 is now widely used, the relative contribution of the individual COX isoforms to these untoward effects is not clear.

METHODS

To address this question, we studied mice with targeted disruption of the COX-1 (Ptgs1) gene. Blood pressure, renin mRNA expression, and aldosterone were measured while dietary sodium was varied. To study interactions with the renin-angiotensin system, ACE inhibitors were administered and mice with combined deficiency of COX-1 and the angiotensin II subtype 1A (AT1A) receptor were generated.

RESULTS

On a regular diet, BP in COX-1-/- mice was near normal. However, during low salt feeding, BP values were reduced in COX-1-/- compared to +/+ animals, and this reduction in BP was associated with abnormal natriuresis despite appropriate stimulation of renin and aldosterone. Compared to COX-1+/+ mice, the actions of ACE inhibition were markedly accentuated in COX-1-/- mice. Sodium sensitivity and BP lowering also were enhanced in mice with combined deficiency of COX-1 and AT1A receptor.

CONCLUSIONS

The absence of COX-1 is associated with sodium loss and enhanced sensitivity to ACE inhibition, suggesting that COX-1 inhibition does not cause hypertension and abnormal sodium handling associated with NSAID use.

Differences in cortical microcirculation in the kidneys of unilaterally congenital hydronephrotic rats

The surgically induced split hydronephrotic kidney has been generally accepted as a valid model for the assessment of renal microcirculation by means of intravital microscopy. Whereas nearly all previous work on this issue has been done with a transillumination technique, we used an epiillumination model that is suitable for investigation of microvascular perfusion in both normal and hydronephrotic kidneys without surgical manipulation of the ureter. By means of the congenital unilaterally hydronephrotic Tauchi rat, microcirculation of the hydronephrotic and that of the nonhydronephrotic kidney were compared. For that purpose both the hydronephrotic and the nonhydronephrotic kidneys of Tauchi rats were exteriorized on a specially designed microscopy stage. After injection of FITC-dextran and rhodamine 6G, microvascular perfusion was assessed in both kidneys. The new model allowed visualization of arterioles, capillaries, and postcapillary venules in both the hydronephrotic and the nonhydronephrotic kidneys. Glomeruli could only be regularly seen in the hydronephrotic kidney, but also in some normal kidneys. Capillary blood cell velocity was significantly higher in the hydronephrotic kidneys (0.67 +/- 0.03 mm/s) compared to the normal kidney (0.32 +/- 0.05 mm/s; P < 0.05), whereas capillary diameters were smaller (4.2 +/- 0.02 microm vs. 5.7 +/- 0.2 microm; P < 0.05). In addition, the hydronephrotic kidney showed a significantly lower density of perfused microvessels compared to the normal controls. Epiillumination intravital microscopy allows assessment of the cortical microcirculation in both the hydronephrotic and the nonhydronephrotic kidneys without surgical induction of hydronephrosis. The hydronephrotic kidney shows significant microcirculatory differences compared to normal kidneys that should be taken into account when using a hydronephrotic model for pharmacological testing.

G protein-coupled prostanoid receptors and the kidney

Renal cyclooxygenase 1 and 2 activity produces five primary prostanoids: prostaglandin E2, prostaglandin F2alpha, prostaglandin I2, thromboxane A2, and prostaglandin D2. These lipid mediators interact with a family of distinct G protein-coupled prostanoid receptors designated EP, FP, IP, TP, and DP, respectively, which exert important regulatory effects on renal function. The intrarenal distribution of these prostanoid receptors has been mapped, and the consequences of their activation have been partially characterized. FP, TP, and EP1 receptors preferentially couple to an increase in cell calcium. EP2, EP4, DP, and IP receptors stimulate cyclic AMP, whereas the EP3 receptor preferentially couples to Gi, inhibiting cyclic AMP generation. EP1 and EP3 mRNA expression predominates in the collecting duct and thick limb, respectively, where their stimulation reduces NaCl and water absorption, promoting natriuresis and diuresis. The FP receptor is highly expressed in the distal convoluted tubule, where it may have a distinct effect on renal salt transport. Although only low levels of EP2 receptor mRNA are detected in the kidney and its precise intrarenal localization is uncertain, mice with targeted disruption of the EP2 receptor exhibit salt-sensitive hypertension, suggesting that this receptor may also play an important role in salt excretion. In contrast, EP4 receptor mRNA is predominantly expressed in the glomerulus, where it may contribute to the regulation of glomerular hemodynamics and renin release. The IP receptor mRNA is highly expressed near the glomerulus, in the afferent arteriole, where it may also dilate renal arterioles and stimulate renin release. Conversely, TP receptors in the glomerulus may counteract the effects of these dilator prostanoids and increase glomerular resistance. At present there is little evidence for DP receptor expression in the kidney. These receptors act in a concerted fashion as physiological buffers, protecting the kidney from excessive functional changes during periods of physiological stress. Nonsteroidal anti-inflammatory drug (NSAID)-mediated cyclooxygenase inhibition results in the loss of these combined effects, which contributes to their renal effects. Selective prostanoid receptor antagonists may provide new therapeutic approaches for specific disease states.

Considerations for the evaluation of renal function in genetically engineered mice

Transgenic and gene-targeting techniques have opened a new era of physiologic investigation: the field of functional genomics. The nearly exclusive use of the mouse in this discipline has necessitated the development and adaptation of sophisticated techniques for evaluating murine physiology at the cellular, tissue, organ and whole animal levels. Although many of the methodologies for exploring cardiorenal function have been successfully adapted from their use in the rat, there are important limitations and considerations that must be recognized when applying them in the mouse. Investigators have been successful in measuring a wide variety of functional variables at the whole kidney and even single nephron levels. Reviewed here are recent advances in the measurement of blood pressure, renal blood flow, whole kidney electrolyte excretion and clearance rates, single-nephron glomerular filtration rate and transport, and tubuloglomerular feedback.

Eicosanoid regulation of the renal vasculature

Even though it has been recognized that arachidonic acid metabolites, eicosanoids, play an important role in the control of renal blood flow and glomerular filtration, several key observations have been made in the past decade. One major finding was that two distinct cyclooxygenase (COX-1 and COX-2) enzymes exist in the kidney. A renewed interest in the contribution of cyclooxygenase metabolites in tubuloglomerular feedback responses has been sparked by the observation that COX-2 is constitutively expressed in the macula densa area. Arachidonic acid metabolites of the lipoxygenase pathway appear to be significant factors in renal hemodynamic changes that occur during disease states. In particular, 12(S)- hydroxyeicosatetraenoic acid may be important for the full expression of the renal hemodynamic actions in response to angiotensin II. Cytochrome P-450 metabolites have been demonstrated to possess vasoactive properties, act as paracrine modulators, and be a critical component in renal blood flow autoregulatory responses. Last, peroxidation of arachidonic acid metabolites to isoprostanes appears to be involved in renal oxidative stress responses. The recent developments of specific enzymatic inhibitors, stable analogs, and gene-disrupted mice and in antisense technology are enabling investigators to understand the complex interplay by which eicosanoids control renal blood flow.