The alpha(1G)-subunit of a voltage-dependent Ca(2+) channel is localized in rat distal nephron and collecting duct

N-/T-Type vs. L-Type Calcium Channel Blocker in Treating Chronic Kidney Disease: A Systematic Review and Meta-Analysis

Renin-angiotensin system (RAS) inhibitors and calcium channel blockers (CCB) are often used together in chronic kidney disease (CKD). The PubMed, EMBASE, and Cochrane Library databases were searched to identify randomized controlled trials (RCTs) in order to explore better subtypes of CCB for the treatment of CKD. This meta-analysis of 12 RCTs with 967 CKD patients who were treated with RAS inhibitors demonstrated that, when compared with L-type CCB, N-/T-type CCB was superior in reducing urine albumin/protein excretion (SMD, −0.41; 95% CI, −0.64 to −0.18; p < 0.001) and aldosterone, without influencing serum creatinine (WMD, −3.64; 95% CI, −11.63 to 4.35; p = 0.37), glomerular filtration rate (SMD, 0.06; 95% CI, −0.13 to 0.25; p = 0.53), and adverse effects (RR, 0.95; 95% CI, 0.35 to 2.58; p = 0.93). In addition, N-/T-type CCB did not decrease the systolic blood pressure (BP) (WMD, 0.17; 95% CI, −1.05 to 1.39; p = 0.79) or diastolic BP (WMD, 0.64; 95% CI, −0.55 to 1.83; p = 0.29) when compared with L-type CCB. In CKD patients treated with RAS inhibitors, N-/T-type CCB is more effective than L-type CCB in reducing urine albumin/protein excretion without increased serum creatinine, decreased glomerular filtration rate, and increased adverse effects. The additional benefit is independent of BP and may be associated with decreased aldosterone (PROSPERO, CRD42020197560).

Calcium channel blocker in patients with chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) is involved in a progressive deterioration in renal function over the years and is now a global public health problem. Currently, reducing the number of patients progressing to end-stage renal failure is urgently necessary. Hypertension and CKD interact with each other, and good control of blood pressure (BP) can improve CKD patients' prognosis. With the current global trend for more strict BP control, the importance of BP management and the need for medication to achieve this strict goal are increasing. Calcium channel blockers (CCBs), which target voltage-dependent calcium channels, are frequently used in combination with renin-angiotensin-aldosterone system inhibitors for CKD patients because of their strong BP-lowering properties and relatively few adverse side effects. Calcium channels have several subtypes, including L, N, T, P/Q, and R, and three types of CCBs, L-type CCBs, L-/T-type CCBs, and L-/N-type CCBs, that are available. Nowadays, the new functions and effects of the CCBs are being elucidated.

CONCLUSION

We should use different types of CCBs properly depending on their pharmacological effects, such as the strength of antihypertensive effects and the organ protection effects, taking into account the pathophysiology of the patients. In this article, the role and the use of CCBs in CKD patients are reviewed.

Role of hepcidin in oxidative stress and cell death of cultured mouse renal collecting duct cells: protection against iron and sensitization to cadmium

The liver hormone hepcidin regulates systemic iron homeostasis. Hepcidin is also expressed by the kidney, but exclusively in distal nephron segments. Several studies suggest hepcidin protects against kidney damage involving Fe2+ overload. The nephrotoxic non-essential metal ion Cd2+ can displace Fe2+ from cellular biomolecules, causing oxidative stress and cell death. The role of hepcidin in Fe2+ and Cd2+ toxicity was assessed in mouse renal cortical [mCCD(cl.1)] and inner medullary [mIMCD3] collecting duct cell lines. Cells were exposed to equipotent Cd2+ (0.5-5 μmol/l) and/or Fe2+ (50-100 μmol/l) for 4-24 h. Hepcidin (Hamp1) was transiently silenced by RNAi or overexpressed by plasmid transfection. Hepcidin or catalase expression were evaluated by RT-PCR, qPCR, immunoblotting or immunofluorescence microscopy, and cell fate by MTT, apoptosis and necrosis assays. Reactive oxygen species (ROS) were detected using CellROX™ Green and catalase activity by fluorometry. Hepcidin upregulation protected against Fe2+-induced mIMCD3 cell death by increasing catalase activity and reducing ROS, but exacerbated Cd2+-induced catalase dysfunction, increasing ROS and cell death. Opposite effects were observed with Hamp1 siRNA. Similar to Hamp1 silencing, increased intracellular Fe2+ prevented Cd2+ damage, ROS formation and catalase disruption whereas chelation of intracellular Fe2+ with desferrioxamine augmented Cd2+ damage, corresponding to hepcidin upregulation. Comparable effects were observed in mCCD(cl.1) cells, indicating equivalent functions of renal hepcidin in different collecting duct segments. In conclusion, hepcidin likely binds Fe2+, but not Cd2+. Because Fe2+ and Cd2+ compete for functional binding sites in proteins, hepcidin affects their free metal ion pools and differentially impacts downstream processes and cell fate.

Iron transport in the kidney: implications for physiology and cadmium nephrotoxicity

The kidney has recently emerged as an organ with a significant role in systemic iron (Fe) homeostasis. Substantial amounts of Fe are filtered by the kidney, which have to be reabsorbed to prevent Fe deficiency. Accordingly Fe transporters and receptors for protein-bound Fe are expressed in the nephron that may also function as entry pathways for toxic metals, such as cadmium (Cd), by way of "ionic and molecular mimicry". Similarities, but also differences in handling of Cd by these transport routes offer rationales for the propensity of the kidney to develop Cd toxicity. This critical review provides a comprehensive update on Fe transport by the kidney and its relevance for physiology and Cd nephrotoxicity. Based on quantitative considerations, we have also estimated the in vivo relevance of the described transport pathways for physiology and toxicology. Under physiological conditions all segments of the kidney tubules are likely to utilize Fe for cellular Fe requiring processes for metabolic purposes and also to contribute to reabsorption of free and bound forms of Fe into the circulation. But Cd entering tubule cells disrupts metabolic pathways and is unable to exit. Furthermore, our quantitative analyses contest established models linking chronic Cd nephrotoxicity to proximal tubular uptake of metallothionein-bound Cd. Hence, Fe transport by the kidney may be beneficial by preventing losses from the body. But increased uptake of Fe or Cd that cannot exit tubule cells may lead to kidney injury, and Fe deficiency may facilitate renal Cd uptake.

Calcium-permeable ion channels in the kidney

Calcium ions (Ca(2+)) are crucial for a variety of cellular functions. The extracellular and intracellular Ca(2+) concentrations are thus tightly regulated to maintain Ca(2+) homeostasis. The kidney, one of the major organs of the excretory system, regulates Ca(2+) homeostasis by filtration and reabsorption. Approximately 60% of the Ca(2+) in plasma is filtered, and 99% of that is reabsorbed by the kidney tubules. Ca(2+) is also a critical signaling molecule in kidney development, in all kidney cellular functions, and in the emergence of kidney diseases. Recently, studies using genetic and molecular biological approaches have identified several Ca(2+)-permeable ion channel families as important regulators of Ca(2+) homeostasis in kidney. These ion channel families include transient receptor potential channels (TRP), voltage-gated calcium channels, and others. In this review, we provide a brief and systematic summary of the expression, function, and pathological contribution for each of these Ca(2+)-permeable ion channels. Moreover, we discuss their potential as future therapeutic targets.

The Wnt11 Signaling Pathway in Potential Cellular EMT and Osteochondral Differentiation Progression in Nephrolithiasis Formation

The molecular events leading to nephrolithiasis are extremely complex. Previous studies demonstrated that calcium and transforming growth factor-β1 (TGF-β1) may participate in the pathogenesis of stone formation, but the explicit mechanism has not been defined. Using a self-created genetic hypercalciuric stone-forming (GHS) rat model, we observed that the increased level of serous/uric TGF-β1 and elevated intracellular calcium in primary renal tubular epithelial cells (PRECs) was associated with nephrolithiasis progression in vivo. In the setting of high calcium plus high TGF-β1 in vitro, PRECs showed great potential epithelial to mesenchymal transition (EMT) progression and osteochondral differentiation properties, representing the multifarious increased mesenchymal and osteochondral phenotypes (Zeb1, Snail1, Col2A1, OPN, Sox9, Runx2) and decreased epithelial phenotypes (E-cadherin, CK19) bythe detection of mRNAs and corresponding proteins. Moreover, TGF-β-dependent Wnt11 knockdown and L-type Ca²⁺ channel blocker could greatly reverse EMT progression and osteochondral differentiation in PRECs. TGF-β1 alone could effectively promote EMT, but it had no effect on osteochondral differentiation in NRK cells (Rat kidney epithelial cell line). Stimulation with Ca²⁺ alone did not accelerate differentiation of NRK. Co-incubation of extracellular Ca²⁺ and TGF-β1 synergistically promotes EMT and osteochondral differentiation in NRK control cells. Our data supplied a novel view that the pathogenesis of calcium stone development may be associated with synergic effects of TGF-β1 and Ca²⁺, which promote EMT and osteochondral differentiation via Wnt11 and the L-type calcium channel.

L-type calcium channel modulates cystic kidney phenotype

In polycystic kidney disease (PKD), abnormal proliferation and genomic instability of renal epithelia have been associated with cyst formation and kidney enlargement. We recently showed that L-type calcium channel (CaV1.2) is localized to primary cilia of epithelial cells. Previous studies have also shown that low intracellular calcium level was associated with the hyperproliferation phenotype in the epithelial cells. However, the relationship between calcium channel and cystic kidney phenotype is largely unknown. In this study, we generated cells with somatic deficient Pkd1 or Pkd2 to examine ciliary CaV1.2 function via lentiviral knockdown or pharmacological verapamil inhibition. Although inhibition of CaV1.2 expression or function did not change division and growth patterns in wild-type epithelium, it led to hyperproliferation and polyploidy in mutant cells. Lack of CaV1.2 in Pkd mutant cells also decreased the intracellular calcium level. This contributed to a decrease in CaM kinase activity, which played a significant role in regulating Akt and Erk signaling pathways. Consistent with our in vitro results, CaV1.2 knockdown in zebrafish and Pkd1 heterozygous mice facilitated the formation of kidney cysts. Larger cysts were developed faster in Pkd1 heterozygous mice with CaV1.2 knockdown. Overall, our findings emphasized the importance of CaV1.2 expression in kidneys with somatic Pkd mutation. We further suggest that CaV1.2 could serve as a modifier gene to cystic kidney phenotype.

Salt sensitivity of renin secretion, glomerular filtration rate and blood pressure in conscious Sprague-Dawley rats

AIM

We hypothesized that in normal rats in metabolic steady state, (i) the plasma renin concentration (PRC) is log-linearly related to Na(+) intake (NaI), (ii) the concurrent changes in mean arterial pressure (MABP) and glomerular filtration rate (GFR) are negligible and (iii) the function PRC = f(NaI) is altered by β₁-adrenoceptor blockade (metoprolol) and surgical renal denervation (DNX).

METHODS

In catheterized, conscious rats on low-Na(+) diet (0.004% Na(+)), NaI was increased by up to 120-fold, in four 3-day steps, by intravenous saline infusion. MABP was recorded continuously, PRC measured in arterial blood, and GFR estimated by inulin clearance.

RESULTS

Steady states were achieved within 3 days. PRC [mIU L(-1)] was log-linearly related to NaI [mmol kg(-1) day(-1)]: PRC = -9.9 log (NaI) + 22. Set point (22 mIU L(-1) at NaI = 1) and slope (9.9 mIU per decade NaI) were independent of metoprolol administration and DNX. MABP and GFR were markedly salt-sensitive: MABP [mmHg] = 4.9 log (NaI) + 99 (P < 0.01), and GFR [mL min(-1)] = 1.4 log (NaI) + 8.3 (P < 0.01). MABP increased similarly (approx. 10%, P < 0.001) irrespective of pre-treatment. Metoprolol, but not DNX, reduced MABP, HR, and GFR (all P < 0.01). Salt sensitivity of GFR was not observed in DNX rats.

CONCLUSION

Log-linear relations to sodium intake exist not only for PRC, but also for MABP and GFR, which per 10-fold increase in sodium intake rose by 5 mmHg and 1.4 mL min(-1) respectively. Steady-state levels of PRC appear independent of renal nerves. MABP and GFR seem markedly salt sensitive in normal rats.

Functional and pharmacological consequences of the distribution of voltage-gated calcium channels in the renal blood vessels

Calcium channel blockers are widely used to treat hypertension because they inhibit voltage-gated calcium channels that mediate transmembrane calcium influx in, for example, vascular smooth muscle and cardiomyocytes. The calcium channel family consists of several subfamilies, of which the L-type is usually associated with vascular contractility. However, the L-, T- and P-/Q-types of calcium channels are present in the renal vasculature and are differentially involved in controlling vascular contractility, thereby contributing to regulation of kidney function and blood pressure. In the preglomerular vascular bed, all the three channel families are present. However, the T-type channel is the only channel in cortical efferent arterioles which is in contrast to the juxtamedullary efferent arteriole, and that leads to diverse functional effects of L- and T-type channel inhibition. Furthermore, by different mechanisms, T-type channels may contribute to both constriction and dilation of the arterioles. Finally, P-/Q-type channels are involved in the regulation of human intrarenal arterial contractility. The calcium blockers used in the clinic affect not only L-type but also P-/Q- and T-type channels. Therefore, the distinct effect obtained by inhibiting a given subtype or set of channels under experimental settings should be considered when choosing a calcium blocker for treatment. T-type channels seem to be crucial for regulating the GFR and the filtration fraction. Use of blockers is expected to lead to preferential efferent vasodilation, reduction of glomerular pressure and proteinuria. Therefore, renovascular T-type channels might provide novel therapeutic targets, and may have superior renoprotective effects compared to conventional calcium blockers.

Cd²⁺ block and permeation of CaV3.1 (α1G) T-type calcium channels: candidate mechanism for Cd²⁺ influx

Cd²⁺ is an industrial pollutant that can cause cytotoxicity in multiple organs. We examined the effects of extracellular Cd²⁺ on permeation and gating of Ca(v)3.1 (α1G) channels stably transfected in HEK293 cells, by using whole-cell recording. With the use of instantaneous I-V currents (measured after strong depolarization) to isolate the effects on permeation, Cd²⁺ rapidly blocked currents with 2 mM Ca²⁺ in a voltage-dependent manner. The block caused by Cd²⁺ was relieved at more-hyperpolarized potentials, which suggests that Cd²⁺ can permeate through the selectivity filter of the channel into the cytosol. In the absence of other permeant ions (Ca²⁺ and Na⁺ replaced by N-methyl-d-glucamine), Cd²⁺ carried sizable inward currents through Ca(v)3.1 channels (210 ± 20 pA at -60 mV with 2 mM Cd²⁺). Ca(v)3.1 channels have a significant "window current" at that voltage (open probability, ∼1%), which makes them a candidate pathway for Cd²⁺ entry into cells during Cd²⁺ exposure. Incubation with radiolabeled ¹⁰⁹Cd²⁺ confirmed uptake of Cd²⁺ into cells with Ca(v)3.1 channels.

Calcium signaling in renal tubular cells

The kidney handles calcium by filtration and reabsorption. About 60% of the plasma calcium is filterable, and 99% is reabsorbed in the tubule. In the proximal tubule, the reabsorption is passive and paracellular, but in the distal tubule is active and transcellular. Thus, renal tubular cells are exposed to very high concentrations of calcium in both, the extracellular and the intracellular compartments. Extracellular calcium signaling is transmitted by the calcium sensing receptor, located both in the luminal and basolateral sides of tubular cells. This receptor is able to control levels of extracellular calcium and acts in consequence to maintain calcium homeostasis. Furthermore, renal tubular cells possess several calcium channels that regulate some of the cell functions. Among those, voltage gated calcium channels, transient receptor potential channels and N-methyl-D-aspartate receptor channels have been reported to control several functions. Those functions include survival, apoptosis, differentiation, epithelial-mesenchymal transition, and active vitamin D and renin synthesis.

17β-Estradiol induces nongenomic effects in renal intercalated cells through G protein-coupled estrogen receptor 1

Steroid hormones such as 17β-estradiol (E2) are known to modulate ion transporter expression in the kidney through classic intracellular receptors. Steroid hormones are also known to cause rapid nongenomic responses in a variety of nonrenal tissues. However, little is known about renal short-term effects of steroid hormones. Here, we studied the acute actions of E2 on intracellular Ca(2+) signaling in isolated distal convoluted tubules (DCT2), connecting tubules (CNT), and initial cortical collecting ducts (iCCD) by fluo 4 fluorometry. Physiological concentrations of E2 induced transient increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) in a subpopulation of cells. The [Ca(2+)](i) increases required extracellular Ca(2+) and were inhibited by Gd(3+). Strikingly, the classic E2 receptor antagonist ICI 182,780 also increased [Ca(2+)](i), which is inconsistent with the activation of classic E2 receptors. G protein-coupled estrogen receptor 1 (GPER1 or GPR30) was detected in microdissected DCT2/CNT/iCCD by RT-PCR. Stimulation with the specific GPER1 agonist G-1 induced similar [Ca(2+)](i) increases as E2, and in tubules from GPER1 knockout mice, E2, G-1, and ICI 182,780 failed to induce [Ca(2+)](i) elevations. The intercalated cells showed both E2-induced concanamycin-sensitive H(+)-ATPase activity by BCECF fluorometry and the E2-mediated [Ca(2+)](i) increment. We propose that E2 via GPER1 evokes [Ca(2+)](i) transients and increases H(+)-ATPase activity in intercalated cells in mouse DCT2/CNT/iCCD.

A nicardipine-sensitive Ca2+ entry contributes to the hypotonicity-induced increase in [Ca2+]i of principal cells in rat cortical collecting duct

We examined the mechanisms involved in the [Ca(2+)](i) response to the extracellular hypotonicity in the principal cells of freshly isolated rat cortical collecting duct (CCD), using Fura-2/AM fluorescence imaging. Reduction of extracellular osmolality from 305 (control) to 195 mosmol/kgH(2)O (hypotonic) evoked transient increase in [Ca(2+)](i) of principal cells of rat CCDs. The [Ca(2+)](i) increase was markedly attenuated by the removal of extracellular Ca(2+)(.) The application of a P(2) purinoceptor antagonist, suramin failed to inhibit the hypotonicity-induced [Ca(2+)](i) increase. The [Ca(2+)](i) increase in response to extracellular hypotonicity was not influenced by application of Gd(3+) and ruthenium red. On the other hand, a voltage-gated Ca(2+) channel inhibitor, nicardipine, significantly reduced the peak amplitude of [Ca(2+)](i) increase in the principal cells. In order to assess Ca(2+) entry during the hypotonic stimulation, we measured the quenching of Fura-2 fluorescence intensity by Mn(2+). The hypotonic stimulation enhanced quenching of Fura-2 fluorescence by Mn(2+), indicating that a Ca(2+)-permeable pathway was activated by the hypotonicity. The hypotonicity-mediated enhancement of Mn(2+) quenching was significantly inhibited by nicardipine. These results strongly suggested that a nicardipine-sensitive Ca(2+) entry pathway would contribute to the mechanisms underlying the hypotonicity-induced [Ca(2+)](i) elevation of principal cells in rat CCD.

Cilnidipine suppresses podocyte injury and proteinuria in metabolic syndrome rats: possible involvement of N-type calcium channel in podocyte

OBJECTIVES

Clinical studies have indicated the beneficial effect of an L/N-type calcium channel blocker (CCB), cilnidipine, on the progression of proteinuria in hypertensive patients compared with an L-type CCB, amlodipine. In the present study, we examined the effects of cilnidipine and amlodipine on the renal injury in spontaneously hypertensive rat/ND mcr-cp (SHR/ND) and their underlying mechanism.

METHODS AND RESULTS

SHR/ND were treated with vehicle (nU10), cilnidipine [33 mg/kg per day, orally (p.o.); nU11] or amlodipine (20 mg/kg per day, p.o.; nU9) for 20 weeks. SHR/ND developed proteinuria in an age-dependent manner. Cilnidipine suppressed the proteinuria greater than amlodipine did. The immunohistochemical analysis showed that N-type calcium channel and Wilm's tumor factor, a marker of podocyte, were co-expressed. SHR/ND had significantly greater desmin staining, an indicator of podocyte injury, with lower podocin and nephrin expression in the glomeruli than Wistar-Kyoto rat or SHR. Cilnidipine significantly prevented the increase in desmin staining and restored the glomerular podocin and nephrin expression compared with amlodipine. Cilnidipine also prevented the increase in renal angiotensin II content, the expression and membrane translocation of NADPH oxidase subunits and dihydroethidium staining in SHR/ND. In contrast, amlodipine failed to change these renal parameters.

CONCLUSION

These data suggest that cilnidipine suppressed the development of proteinuria greater than amlodipine possibly through inhibiting N-type calcium channel-dependent podocyte injury in SHR/ND.

A voltage-dependent Ca2+ influx pathway regulates the Ca2+-dependent Cl(-) conductance of renal IMCD-3 cells

We have previously shown that the membrane conductance of mIMCD-3 cells at a holding potential of 0 mV is dominated by a Ca2+-dependent Cl(-) current (I(CLCA)). Here we report that I(CLCA) activity is also voltage dependent and that this dependence on voltage is linked to the opening of a novel Al3+-sensitive, voltage-dependent, Ca2+ influx pathway. Using whole-cell patch-clamp recordings at a physiological holding potential (-60 mV), ICLCA was found to be inactive and resting currents were predominantly K+ selective. However, membrane depolarization to 0 mV resulted in a slow, sigmoidal, activation of ICLCA (T(0.5) approximately 500 s), while repolarization in turn resulted in a monoexponential decay in I(CLCA) (T (0.5) approximately 100 s). The activation of I(CLCA) by depolarization was reduced by lowering extracellular Ca2+ and completely inhibited by buffering cytosolic Ca2+ with EGTA, suggesting a role for Ca2+ influx in the activation of I(CLCA). However, raising bulk cytosolic Ca2+ at -60 mV did not produce sustained I(CLCA) activity. Therefore I(CLCA) is dependent on both an increase in intracellular Ca2+ and depolarization to be active. We further show that membrane depolarization is coupled to opening of a Ca2+ influx pathway that displays equal permeability to Ca2+ and Ba2+ ions and that is blocked by extracellular Al3+ and La3+. Furthermore, Al3+ completely and reversibly inhibited depolarization-induced activation of ICLCA, thereby directly linking Ca2+ influx to activation of I(CLCA). We speculate that during sustained membrane depolarization, calcium influx activates ICLCA which functions to modulate NaCl transport across the apical membrane of IMCD cells.

Development of newer calcium channel antagonists: therapeutic potential of efonidipine in preventing electrical remodelling during atrial fibrillation

Calcium channel antagonists are most frequently prescribed for the treatment of hypertension and the majority specifically inhibit the L-type Ca2+ channel. In order to prevent reflex sympathetic over activity caused by L-type calcium channel antagonists (calcium channel blockers [CCBs]), increasing attention has focused on the blockade of the T-type Ca2+ channel. The T-type Ca2+ channel is found in the kidney and can also appear in the ventricle of the heart when in failure. Therefore, the T-type Ca2+ channel is a possible new target for the treatment of nephropathy and heart failure. In clinical trials, the efficacy and safety of T-type CCBs in hypertension and chronic renal disease have been reported. It is well known that the T-type Ca2+ channel is present in the adult atrium and plays a role in the cardiac pacemaker, but recent experimental studies suggest that this current also promotes electrical remodelling of the atrium. Using efonidipine, a dual L- and T-type CCB, it has been demonstrated that atrial electrical remodelling can be diminished in dogs. Furthermore, the T-type Ca2+ channel has recently been found in the pulmonary veins, contributing to the pulmonary vein pacemaker activity and triggered activity. A variety of drugs having T-type CCB effects have been shown to be effective in the management of atrial fibrillation, suggesting that this channel may be a novel therapeutic target.

Ca2+ channel subtypes and pharmacology in the kidney

A large body of evidence has accrued indicating that voltage-gated Ca(2+) channel subtypes, including L-, T-, N-, and P/Q-type, are present within renal vascular and tubular tissues, and the blockade of these Ca(2+) channels produces diverse actions on renal microcirculation. Because nifedipine acts exclusively on L-type Ca(2+) channels, the observation that nifedipine predominantly dilates afferent arterioles implicates intrarenal heterogeneity in the distribution of L-type Ca(2+) channels and suggests that it potentially causes glomerular hypertension. In contrast, recently developed Ca(2+) channel blockers (CCBs), including mibefradil and efonidipine, exert blocking action on L-type and T-type Ca(2+) channels and elicit vasodilation of afferent and efferent arterioles, which suggests the presence of T-type Ca(2+) channels in both arterioles and the distinct impact on intraglomerular pressure. Recently, aldosterone has been established as an aggravating factor in kidney disease, and T-type Ca(2+) channels mediate aldosterone release as well as its effect on renal efferent arteriolar tone. Furthermore, T-type CCBs are reported to exert inhibitory action on inflammatory process and renin secretion. Similarly, N-type Ca(2+) channels are present in nerve terminals, and the inhibition of neurotransmitter release by N-type CCBs (eg, cilnidipine) elicits dilation of afferent and efferent arterioles and reduces glomerular pressure. Collectively, the kidney is endowed with a variety of Ca(2+) channel subtypes, and the inhibition of these channels by their specific CCBs leads to variable impact on renal microcirculation. Furthermore, multifaceted activity of CCBs on T- and N-type Ca(2+) channels may offer additive benefits through nonhemodynamic mechanisms in the progression of chronic kidney disease.

Glucocorticoid impairs growth of kidney outer medulla and accelerates loop of Henle differentiation and urinary concentrating capacity in rat kidney development

In the rat, urinary concentrating ability develops progressively during the third postnatal (P) week and nearly reaches adult level at weaning ( P21) governed by a rise in circulating glucocorticoid. Elevated extracellular osmolality can lead to growth arrest of epithelial cells. We tested the hypothesis that supranormal exposure of rat pups to glucocorticoid before the endogenous surge enhances urinary concentrating ability but inhibits renomedullary cell proliferation. Proliferating-cell nuclear antigen (PCNA)-positive cells shifted from the nephrogenic zone in the first postnatal week to Tamm-Horsfall-positive thick ascending limb (TAL) cells at the corticomedullary junction at P10– 14. Renal PCNA protein abundance was stable in the suckling period and decreased 10-fold after weaning. Renal PCNA protein abundance decreased in response to dexamethasone (DEXA; 100 μg·kg−1·day−1, P8–12). Prolonged administration of DEXA ( P1-P11) reduced selectively the area and thickness of the outer medulla and the number of PCNA-positive cells. DEXA ( P8– 12) increased urinary and papillary osmolality in normohydrated and water-deprived pups and led to osmotic equilibrium between interstitium and urine, whereas apoptotic and GADD153-positive cells increased in the inner medulla. TAL-associated NaCl transporters Na-K-2Cl cotransporter, Na-K-ATPase-α1, Na/H exchanger type 3, and ROMK increased significantly at weaning and in response to DEXA. We conclude that a low level of circulating glucocorticoid is permissive for proliferation of Henle's loop and the outer medulla before weaning. A reduced papillary tonicity is a crucial factor for the reduced capacity to concentrate urine during postnatal kidney development. We speculate that supranormal exposure to glucocorticoid in the suckling period can alter kidney medullary structure and function permanently.

Cyclooxygenase type 2 is increased in obstructed rat and human ureter and contributes to pelvic pressure increase after obstruction

Prostanoids exert physiological effects on ureteral contractility that may lead to pressure changes and pain during obstruction. In the present study, we examined whether (1) obstruction changes the expression of the two cyclooxygenase (COX) isoforms, COX-1 and COX-2 in human and rat ureters and (2) administration of a selective COX-2 inhibitor influences the pelvic pressure change after experimental ureteral obstruction. Rats were subjected to bilateral ureter obstruction. Ureters were removed and dissected into a proximal dilated and distal non-dilated segment. RNA and protein were extracted and analyzed for cyclooxygenase expression by quantitative polymerase chain reaction and Western blotting. Human ureter samples were obtained from patients undergoing radical nephrectomy. Rat and human ureteral samples were processed for immunohistochemistry. COX-1, but not COX-2 mRNA, was readily detected in the normal rat ureter. COX-2 mRNA and protein expression was increased in the proximal dilated ureter compared to distal non-dilated ureter. This increased COX-2 expression was associated with increased urinary prostaglandin E2 (PGE2) excretion after release of obstruction. Immunohistochemistry showed increased COX-2 labeling in surface epithelium and smooth muscle layers in both rat and human obstructed ureters compared to control ureters. Furthermore, contractile PGE2-EP1 and thromboxane TP receptors were expressed in ureteral smooth muscle. Systemic treatment with the COX-2 selective inhibitor parecoxib (5 mg/kg/day) attenuated the pelvic pressure increase during obstruction. In summary, COX-2 expression is significantly increased in the ureteral wall in response to obstruction in the rat and human ureter and COX-2 activity contributes to increased pelvic pressure after obstruction.

Tissue expression and plasma levels of adrenomedullin in renal cancer patients

The peptide AM (adrenomedullin) is stimulated by hypoxia through HIF-1 (hypoxia-inducible factor-1). The majority of human CC-RCCs (clear cell renal cell carcinomas) display mutations in the tumour suppressor protein von Hippel–Lindau, which leads to constitutively elevated HIF-1. We hypothesized that AM is increased in CC-RCC tumours and that AM is a plasma biomarker for CC-RCC. Tumours and non-malignant kidney tissue were obtained from patients that underwent unilateral nephrectomy. Blood samples were drawn at the day of surgery, 3–6 days after surgery and 4–5 weeks after surgery. AM mRNA and peptide expression in tissue and AM plasma concentration were determined. HIF-1α was localized in tissue by immunohistochemistry. AM mRNA was elevated in CC-RCC compared with adjacent renal cortex (6-fold, n=18; P<0.02). There was no difference in AM mRNA between cortex and non-CC-RCC tissue (n=7). AM peptide concentration was elevated in CC-RCC tissue compared with adjacent cortex (4-fold, n=6; P<0.02), whereas there was no difference between cortex and non-CC-RCC tissue (n=5). HIF-1α immunoreactivity was detected in the majority of cell nuclei in 76% of CC-RCC, consistent with constitutive stabilization. In non-CC-RCC, HIF-1α staining was focal. Before surgery there was no difference in plasma AM concentration between tumour types. Nephrectomy increased plasma AM significantly after 3–6 days and a similar pre-surgery level was observed after 4–5 weeks in both groups of tumour patients. We conclude that elevated tissue AM is a distinguishing feature of CC-RCC compared with other kidney tumours. Plasma AM is not suited as a tumour marker for this disease.

Coexpression of Voltage-Dependent Calcium Channels Ca v 1.2, 2.1a, and 2.1b in Vascular Myocytes

Voltage-dependent Ca2+ channels Cav1.2 (L type) and Cav2.1 (P/Q type) are expressed in vascular smooth muscle cells (VSMCs) and are important for the contraction of renal resistance vessels. In the present study we examined whether native renal VSMCs coexpress L-, P-, and Q-type Ca2+ currents. The expression of both Cav2.1a (P-type) and Cav2.1b (Q-type) mRNA was demonstrated by RT-PCR in renal preglomerular vessels from rats and mice. Immunolabeling was performed on A7r5 cells, renal cryosections, and freshly isolated renal VSMCs with anti-Cav1.2 and anti-Cav2.1 antibodies. Conventional and confocal microscopy revealed expression of both channels in all of the smooth muscle cells. Whole-cell patch clamp on single preglomerular VSMCs from mice showed L-, P-, and Q-type currents. Blockade of the L-type currents by calciseptine (20 nmol/L) inhibited 35.6+/-3.9% of the voltage-dependent Ca2+ current, and blocking P-type currents (omega-agatoxin IVA 10 nmol/L) led to 20.2+/-3.0% inhibition, whereas 300 nmol/L of omega agatoxin IVA (blocking P/Q-type) inhibited 45.0+/-7.3%. In rat aortic smooth muscle cells (A7r5), blockade of L-type channels resulted in 28.5+/-6.1% inhibition, simultaneous blockade of L-type and P-type channels inhibited 58.0+/-11.8%, and simultaneous inhibition of L-, P-, and Q-type channels led to blockade (88.7+/-5.6%) of the Ca2+ current. We conclude that aortic and renal preglomerular smooth muscle cells express L-, P-, and Q-type voltage-dependent Ca2+ channels in the rat and mouse.

Ion channel clustering enhances weak electric field detection by neutrophils: apparent roles of SKF96365-sensitive cation channels and myeloperoxidase trafficking in cellular responses

We have tested Galvanovskis and Sandblom's prediction that ion channel clustering enhances weak electric field detection by cells as well as how the elicited signals couple to metabolic alterations. Electric field application was timed to coincide with certain known intracellular chemical oscillators (phase-matched conditions). Polarized, but not spherical, neutrophils labeled with anti-K(v)1.3, FL-DHP, and anti-TRP1, but not anti-T-type Ca(2+) channels, displayed clusters at the lamellipodium. Resonance energy transfer experiments showed that these channel pairs were in close proximity. Dose-field sensitivity studies of channel blockers suggested that K(+) and Ca(2+) channels participate in field detection, as judged by enhanced oscillatory NAD(P)H amplitudes. Further studies suggested that K(+) channel blockers act by reducing the neutrophil's membrane potential. Mibefradil and SKF93635, which block T-type Ca(2+) channels and SOCs, respectively, affected field detection at appropriate doses. Microfluorometry and high-speed imaging of indo-1-labeled neutrophils was used to examine Ca(2+) signaling. Electric fields enhanced Ca(2+) spike amplitude and triggered formation of a second traveling Ca(2+) wave. Mibefradil blocked Ca(2+) spikes and waves. Although 10 microM SKF96365 mimicked mibefradil, 7 microM SKF96365 specifically inhibited electric field-induced Ca(2+) signals, suggesting that one SKF96365-senstive site is influenced by electric fields. Although cells remained morphologically polarized, ion channel clusters at the lamellipodium and electric field sensitivity were inhibited by methyl-beta-cyclodextrin. As a result of phase-matched electric field application in the presence of ion channel clusters, myeloperoxidase (MPO) was found to traffic to the cell surface. As MPO participates in high amplitude metabolic oscillations, this suggests a link between the signaling apparatus and metabolic changes. Furthermore, electric field effects could be blocked by MPO inhibition or removal while certain electric field effects were mimicked by the addition of MPO to untreated cells. Therefore, channel clustering plays an important role in electric field detection and downstream responses of morphologically polarized neutrophils. In addition to providing new mechanistic insights concerning electric field interactions with cells, our work suggests novel methods to remotely manipulate physiological pathways.

Differential Blocking Action of Dihydropyridine Ca2+ Antagonists on a T-Type Ca2+ Channel (α1G) Expressed in Xenopus Oocytes

Recent reports show that efonidipine, a dihydropyridine Ca2+ antagonist, has blocking action on T-type Ca2+ channels, which may produce favorable actions on cardiovascular systems. However, the effects of other dihydropyridine Ca2+ antagonists on T-type Ca2+ channels have not been investigated yet. Therefore, in this study, we examined the effects of dihydropyridine compounds clinically used for treatment of hypertension on a T-type Ca2+ channel subtype, alpha1G, expressed in Xenopus oocytes. These effects were compared with those on T-type Ca2+ channel. Rabbit L-type (alpha1Calpha2/deltabeta1a) or rat T-type (alpha1G) Ca2+ channel was expressed in Xenopus oocytes by injection of cRNA for each subunit. The Ba currents through expressed channels were measured by conventional 2-microelectrode voltage-clamp methods. Twelve DHPs (amlodipine, barnidipine, benidipine, cilnidipine, efonidipine, felodipine, manidipine, nicardipine, nifedipine, nilvadipine, nimodipine, nitrendipine) and mibefradil were tested. Cilnidipine, felodipine, nifedipine, nilvadipine, minodipine, and nitrendipine had little effect on the T-type channel. The blocks by drugs at 10 microM were less than 10% at a holding potential of -100 mV. The remaining 6 drugs had blocking action on the T-type channel comparable to that on the L-type channel. The blocking actions were also comparable to that by mibefradil. These results show that many dihydropyridine Ca2+ antagonists have blocking action on the alpha1G channel subtype. The action of dihydropyridine Ca2+ antagonists in clinical treatment should be evaluated on the basis of subtype selectivity.

Inhibition of cGMP-specific phosphodiesterase type 5 reduces sodium excretion and arterial blood pressure in patients with NaCl retention and ascites

In the present study, we tested the hypothesis that inhibition of renal phosphodiesterase type 5 (PDE5) in patients with liver cirrhosis and ascites increases sodium excretion. The effect of sildenafil citrate was studied in a randomized double-blind. placebo-controlled crossover study. Diuretics were withdrawn, and a fixed sodium diet (100 mmol/day) was given to the patients for 5 days before both study days. After a 60-min basal period, eight patients received either oral sildenafil (50 mg) or placebo. Glomerular filtration rate (GFR) and renal blood flow (RBF) were determined by 99mTc-diethylenetriamine-pentaacetate and (131)I-hippuran clearances. In human nephrectomy specimens, PDE5 mRNA was expressed at similar levels in the cortex (n = 6) and inner medulla (n = 4). Histochemical staining showed PDE5 immunoreactivity in collecting ducts and vascular smooth muscle. At baseline, cirrhotic patients exhibited elevated plasma concentrations of ANP, renin, ANG II, and aldosterone that did not differ on the 2 study days. Basal sodium excretion was similar at the 2 study days (median 17 and 18 mmol, respectively), and patients were in positive sodium balance. Sildenafil increased heart rate, plasma renin activity, plasma ANG II, and aldosterone concentrations significantly after 60 min. Plasma cGMP concentration was increased after 120 and 180 min, and urinary sodium excretion and mean arterial blood pressure were decreased significantly at 120 and 180 min. Plasma ANP concentration, GFR, and RBF did not change after sildenafil. In patients with ascites and cirrhosis, inhibition of PDE5 did not promote natriuresis but led to increased plasma levels of the renin-angiotensin-aldosterone system.

Identification of R(-)-isomer of efonidipine as a selective blocker of T-type Ca2+ channels

Efonidipine, a derivative of dihydropyridine Ca(2+) antagonist, is known to block both L- and T-type Ca(2+) channels. It remains to be clarified, however, whether efonidipine affects other voltage-dependent Ca(2+) channel subtypes such as N-, P/Q- and R-types, and whether the optical isomers of efonidipine have different selectivities in blocking these Ca(2+) channels, including L- and T-types. To address these issues, the effects of efonidipine and its R(-)- and S(+)-isomers on these Ca(2+) channel subtypes were examined electrophysiologically in the expression systems using Xenopus oocytes and baby hamster kidney cells (BHK tk-ts13). Efonidipine, a mixture of R(-)- and S(+)-isomers, exerted blocking actions on L- and T-types, but no effects on N-, P/Q- and R-type Ca(2+) channels. The selective blocking actions on L- and T-type channels were reproduced by the S(+)-efonidipine isomer. By contrast, the R(-)-efonidipine isomer preferentially blocked T-type channels. The blocking actions of efonidipine and its enantiomers were dependent on holding potentials. These findings indicate that the R(-)-isomer of efonidipine is a specific blocker of the T-type Ca(2+) channel.

Testosterone enhances calcium reabsorption by the kidney

The kidney is a target tissue for androgens, but the role of these hormones in the regulation of calcium (Ca2+) reabsorption remains unclear. The present study examines the effects of testosterone on Ca2+ transport by the luminal membranes of proximal and distal nephrons of the rabbit kidney. Tubule suspensions were pre-incubated in the presence or absence of the hormone, and 45C2+ uptake by the luminal membranes was measured using the rapid filtration technique. In the proximal tubules, testosterone did not influence Ca2+ uptake. In the distal tubules, a 5 min incubation with the hormone increased this uptake with a maximal response at 10(-10)M. Ca2+ transport by the distal membranes shows a dual kinetics. Testosterone enhanced the Vmax value of the low affinity component. In an attempt to identify the underlying mechanisms involved in this action, several messenger inhibitors were introduced in the tubule suspension. PD 98059 and U0 126 as well as AG 99 and genistein interfered with the hormone action suggesting the implication of a MEK kinase and a tyrosine kinase. To determine the type of the channels involved in this effect, Ca2+ uptake was measured in the presence of diltiazem, omega-conotoxin MVIIC and mibefradil, i.e. selective inhibitors of the L-type, P/Q type and T-type channels. An inhibition of Ca2+ transport was observed exclusively with mibefradil. These results indicate that testosterone enhances Ca2+ transport by opening a T-type Ca2+ channel in the distal luminal membrane, via MEK kinase and tyrosine kinase dependent mechanisms.

Depolarization-induced calcium influx in rat mesenteric small arterioles is mediated exclusively via mibefradil-sensitive calcium channels

1. In this study, intracellular Ca(2+) was measured as the Fura-2 ratio (R) of fluorescence excited at 340 and 380 nm (F(340)/F(380)) in nonpressurized rat mesenteric small arterioles ( (lumen diameter) 10-25 microm). 2. The response to depolarization using 75 mm KCl was an increase in R from a baseline of 0.96+/-0.01 ([Ca(2+)](i) approximately 74 nm) to 1.04+/-0.01 ( approximately 128 nm) (n=80). The response to 75 mm K(+) was reversibly abolished in Ca(2+)-free physiological saline solution, whereas phentolamine (10 microm) or tetrodotoxin (1 microm) had no effects. LaCl(3) (200 microm) inhibited 61+/-9% of the response. 3. A [K(+)]-response curve indicated that the Ca(2+) response was activated between 15 and 25 mm K(+). The data suggest that the Ca(2+) response was caused by the activation of voltage-dependent Ca(2+) channels. 4. Mibefradil use dependently inhibited the Ca(2+) response to 75 mm K(+) by 29+/-2% (100 nm), 73+/-7% (1 microm) or 89+/-7% (10 microm). Pimozide (500 nm) use dependently inhibited the Ca(2+) response by 85+/-1%. 5. Nifedipine (1 microm) inhibited the Ca(2+) response to 75 mm K(+) by 41+/-12%. The response was not inhibited by calciseptine (500 nm), omega-agatoxin IVA (100 nm), omega-conotoxin MVIIA (500 nm), or SNX-482 (100 nm). 6. Using reverse transcriptase-polymerase chain reaction, it was shown that neither Ca(V)2.1a (P-type) nor Ca(V)2.1b (Q-type) voltage-dependent Ca(2+) channels were expressed in mesenteric arterioles, whereas the Ca(V)3.1 (T-type) channel was expressed. Furthermore, no amplification products were detected when using specific primers for the beta(1b), beta(2), or beta(3) auxiliary subunits of high-voltage-activated Ca(2+) channels. 7. The results suggest that the voltage-dependent Ca(2+) channel activated by sustained depolarization in mesenteric arterioles does not classify as any of the high-voltage-activated channels (L-, P/Q-, N-, or R-type), but is likely to be a T-type channel. The possibility that the sustained Ca(2+) influx observed was the result of a T-type window current is discussed.

Expression of adrenomedullin in hypoxic and ischemic rat kidneys and human kidneys with arterial stenosis

To investigate regional aspects of hypoxic regulation of adrenomedullin (AM) in kidneys, we mapped the distribution of AM in the rat kidney after hypoxia (normobaric hypoxic hypoxia, carbon monoxide, and CoCl2 for 6 h), anemia (hematocrit lowered by bleeding) and after global transient ischemia for 1 h (unilateral renal artery occlusion and reperfusion for 6 and 24 h) and segmental infarct (6 and 24 h). AM expression and localization was determined in normal human kidneys and in kidneys with arterial stenosis. Hypoxia stimulated AM mRNA expression significantly in rat inner medulla (CO 13 times, 8% O2 6 times, and CoCl2 8 times), followed by the outer medulla and cortex. AM mRNA level was significantly elevated in response to anemia and occlusion-reperfusion. Immunoreactive AM was associated with the thin limbs of Henle's loop, distal convoluted tubule, collecting ducts, papilla surface epithelium, and urothelium. AM labeling was prominent in the inner medulla after CO and in the outer medulla after occlusion-reperfusion. The infarct border zone was strongly labeled for AM. In cultured inner medullary collecting duct cells, AM mRNA was significantly increased by hypoxia. AM mRNA was equally distributed in human kidney and AM was localized as in the rat kidney. In human kidneys with artery stenosis, AM mRNA was not significantly enhanced compared with controls, but AM immunoreactivity was observed in tubules, vessels, and glomerular cells. In summary, AM expression was increased in the rat kidney in response to hypoxic and ischemic hypoxia in keeping with oxygen gradients. AM was widely distributed in the human kidney with arterial stenosis. AM may play a significant role to counteract hypoxia in the kidney.

Extracellular ATP-induced calcium signaling in mIMCD-3 cells requires both P2X and P2Y purinoceptors

Kidney tubules are targets for the activation of locally released nucleotides through multiple P2 receptor types. Activation of these P2 receptors modulates cellular Ca(2+) signaling and downstream cellular function. The purpose of this study was to determine whether P2 receptors were present in mIMCD-3 cells, a mouse inner medullary collecting duct cell line, and if so, to examine their link with intracellular Ca(2+) homeostasis. To monitor intracellular Ca(2+) concentration ([Ca(2+)](i)), experiments were conducted using the fluorescent dye fura 2. ATP (0.1-100 microM) produced a dose-dependent increase in [Ca(2+)](i) in a physiological Ca(2+)-containing solution, with an EC(50) of 2.5 microM. The P2-receptor antagonist PPADS reduced the effect of ATP on [Ca(2+)](i), and the P1-receptor agonist adenosine caused only a small increase in [Ca(2+)](i). Preincubation of cells with the phospholipase C antagonist U-73122 blocked the ATP-induced increase in [Ca(2+)](i), indicating P2Y receptors were involved in this process. In a Ca(2+)-free bath solution, thapsigargin and ATP induced intracellular Ca(2+) release from an identical pool. Nucleotides caused an increase in [Ca(2+)](i) in the potency order of UTP = ATP > ATP gamma S > ADP > UDP that is best fitted with the P2Y(2) subtype profile. Although the P2Y agonist UTP induced a similar large transient increase in [Ca(2+)](i) as did ATP, a small but sustained increase in [Ca(2+)](i) occurred only in ATP-stimulated cells, suggesting the role of P2X receptors in Ca(2+) influx. The sustained increase in [Ca(2+)](i) could be blocked by either nonselective cation channel blockers Gd(3+) or P2X antagonists PPADS and PPNDS. Furthermore, when either Gd(3+) or PPNDS was applied to the bath solution before ATP application, the ATP-induced increase in [Ca(2+)](i) was significantly reduced. Both RT-PCR and Western blotting corroborated the presence of P2X(1) and P2Y(2) receptors. These studies demonstrate that mIMCD-3 cells have both P2X and P2Y subtype receptors and that the activation of both P2X and P2Y receptors by extracellular ATP appears to be required to regulate intracellular Ca(2+) signaling.

Identification of Channels Promoting Calcium Spikes and Waves in HT1080 Tumor Cells

Intracellular Ca(2+) signals have been associated with cell polarization and locomotion. As cell motility underlies metastasis, we have sought to better characterize the Ca(2+) signaling events in HT1080 fibrosarcoma cells. We have tested the hypothesis that low voltage-activated (LVA) and nonvoltage-gated (NVG) channels of HT1080 cells participate in dynamic Ca(2+)-signaling events leading to cell migration and invasion. Immunofluorescence microscopy has shown that HT1080 cells express LVA T-type Ca(2+) channels uniformly about the cell periphery, whereas the transient receptor potential-1 (a NVG cation channel) protein appears as punctate spots about a cell's periphery. HT1080 cells exhibit periodic intracellular Ca(2+) spikes. High-speed imaging revealed that the Ca(2+) spikes were composed of a single Ca(2+) wave traveling unidirectionally about the periphery of the cytoplasm in a clockwise fashion (as viewed from basal to apical surfaces). The T-type Ca(2+) channel blocker mibefradil inhibited Ca(2+) spikes and waves on cells and, in parallel, inhibited cell motility and invasion in a dose-dependent manner. Similar changes were noted with the NVG cation channel blockers Gd(3+) and carboxyamido-triazole. The combination of LVA and NVG blockers further reduced Matrigel invasiveness. However, the Ca(2+) channel blockers nicardipine, SKF96365, diltiazem, and verapamil had no effect at appropriate doses. These results indicate that certain LVA and NVG channels regulate HT1080 cell motility. In addition to providing novel information regarding cancer cell motility, we suggest that it may be possible to design drugs that inhibit a key Ca(2+) wave, thereby enhancing the efficacy of emerging therapeutic protocols.

Low endogenous glucocorticoid allows induction of kidney cortical cyclooxygenase-2 during postnatal rat development

In postnatal weeks 2-4, cyclooxygenase-2 (COX-2) is induced in the rat kidney cortex where it is critically involved in final stages of kidney development. We examined whether changes in circulating gluco- or mineralocorticosteroids or in their renal receptors regulate postnatal COX-2 induction. Plasma corticosterone concentration peaked at birth, decreased to low levels at days 3-13, and increased to adult levels from day 22. Aldosterone peaked at birth and then stabilized at adult levels. Gluco- and mineralocorticoid receptor (GR and MR) mRNAs were expressed stably in kidney before, during, and after COX-2 induction. 11 beta-hydroxysteroid dehydrogenase 2 was induced shortly after birth and was widely distributed in the whole collecting duct system in the suckling period and then returned to an adult pattern. Supplementation with corticosterone (20 mg.kg-1.day-1) or GR-specific dexamethasone (1 mg.kg-1.day-1) during low endogenous corticosterone suppressed renal COX-2 mRNA and protein and led to a restricted distribution of COX-2 immunolabeling. The ability of glucocorticoids to affect COX-2 was reflected in colocalization of GR-alpha and COX-2 immunoreactivity and mRNAs in thick ascending limb of Henle's loop. The MR antagonist potassium canrenoate (20 mg.kg-1.day-1) enhanced COX-2 expression from days 5 to 10, but low MR-specific concentrations of DOCA (1 mg.kg-1.day-1) had no effect on COX-2. Renomedullary interstitial cells expressed GR-alpha and COX-2. Dexamethasone suppressed COX-2 in these cells. Thus low plasma concentrations of corticosterone allowed for cortical and medullary COX-2 induction during postnatal kidney development. Increased circulating glucocorticoid in the postnatal period may damage late renal development through inhibition of COX-2.

Rapid Inhibition of Vasoconstriction in Renal Afferent Arterioles by Aldosterone

Aldosterone has been suggested to elicit vessel contraction via a nongenomic mechanism. We tested this proposal in microdissected, perfused rabbit renal afferent arterioles. Aldosterone had no effect on internal diameter in concentrations from 10(-10) to 10(-5) mol/L, but aldosterone abolished the ability of 100 mmol/L KCl to induce vascular contraction. The inhibitory effect of aldosterone was observed from 1 pmol/L. The inhibitory effect was significant after 5 minutes and maximal after 20 minutes and was fully reversible. Actinomycin D (10(-6) mol/L) prolonged the effect of aldosterone. The effect was abolished by the mineralocorticoid receptor antagonist spironolactone (10(-7) mol/L) but not by the glucocorticoid receptor antagonist mifepristone (10(-6) mol/L). The K+-mediated increase of intracellular calcium concentration in afferent arterioles was not affected by aldosterone. Mineralocorticoid receptor was detected by reverse transcription-polymerase chain reaction and immunohistochemistry in rat renal vasculature and rabbit endothelial cells. Inhibition of phosphatidylinositol (PI)-3 kinase with LY 294002 (3x10(-6) mol/L) restored sensitivity to K+ in the presence of aldosterone, and afferent arterioles were immunopositive for PI-3 kinase subunit p110alpha. Inhibition of NO formation by L-NAME (10(-4) mol/L) or inhibition of soluble guanylyl cyclase with 1H-(1,2,4)Oxadiazolo[4,3-a]quinoxaline-1-one restored K+-induced vasoreactivity in the presence of aldosterone. Similar to aldosterone, the NO donor sodium nitroprusside inhibited K+-induced vascular contraction. Geldanamycin (10(-6) mol/L), an inhibitor of heat shock protein 90, abolished aldosterone-induced vasorelaxation. We conclude that aldosterone inhibits depolarization-induced vasoconstriction in renal afferent arterioles by a rapid nongenomic mechanism that is initiated by mineralocorticoid receptor activation and involves PI-3 kinase, protein kinase B, and heat shock protein 90-mediated stimulation of NO generation.

Increased intracellular Ca++ in the macula densa regulates tubuloglomerular feedback

BACKGROUND

Tubuloglomerular feedback is initiated by an increase in NaCl at the macula densa lumen, which in turn increases intracellular Ca++. In the present study, we examined the role of increased intracellular Ca++ in tubuloglomerular feedback and the source of the increased Ca++. We hypothesized that an increase in intracellular Ca++ at the macula densa via the basolateral Na+/Ca++ exchanger, caused by an increase in luminal NaCl, initiates Ca++-mediated Ca++ release from intracellular stores, which is essential for tubuloglomerular feedback.

METHODS

Rabbit afferent arterioles and attached macula densas were simultaneously microperfused in vitro. Tubuloglomerular feedback was induced by increasing macula densa Na+/Cl- from 11/10 mmol/L (low) to 81/80 mmol/L (high) and was measured before and after treatment.

RESULTS

To investigate whether elevations in intracellular Ca++ are required for tubuloglomerular feedback, the calcium ionophore A23187 or the Ca++ chelator BAPTA-AM was added to the macula densa lumen. During the control period, tubuloglomerular feedback decreased afferent arteriole diameter from 18.1 +/- 1.1 microm to 15.3 +/- 0.8 microm. Adding 2 x 10-6 mol/L A23187 to the low NaCl macula densa perfusate induced tubuloglomerular feedback; diameter decreased from 18.0 +/- 1.0 microm to 15.4 +/- 0.9 microm (N = 6; P < 0.01). After adding BAPTA-AM (25 micromol/L) to the macula densa lumen, tubuloglomerular feedback response was completely eliminated. We next studied the source of increased macula densa Ca++ in response to increased NaCl concentration. During the control period, tubuloglomerular feedback decreased afferent arteriole diameter from 18.5 +/- 1.6 microm to 15.3 +/- 1.2 microm (N = 6; P < 0.01). After adding the Na+/Ca++ exchanger inhibitor 2'4'-dichlorobenzamil (10 micromol/L) or KB-R7943 (30 micromol/L) to the bath, the tubuloglomerular feedback response was blocked; however, the afferent arteriole response to angiotensin II or adenosine was not altered. Next, we tested the Ca++-adenosine triphosphatase (ATPase) inhibitor thapsigargin (0.1 micromol/L), which has been reported to inhibit sarcoplasmic reticulum Ca++-ATPase activity and prevent restoration of intracellular Ca++ stores. When thapsigargin was added to the macula densa lumen, it reduced the first tubuloglomerular feedback response by 33% and completely eliminated the second and third tubuloglomerular feedback responses. In the absence of thapsigargin, there was no significant decrease in the tubuloglomerular feedback responses (N = 6). Neither the L-type Ca++ channel blocker nifedipine (25 micromol/L), nor the T-type Ca++ channel blocker pimozide (10 micromol/L), inhibited tubuloglomerular feedback when added to the macula densa lumen.

CONCLUSION

We concluded that (1). increased intracellular Ca++ at the macula densa is required for the tubuloglomerular feedback response; (2). Na+/Ca++ exchange appears to initiate Ca++-mediated Ca++ release from intracellular stores; and (3). luminal L-type or T-type Ca++ channels are not involved in tubuloglomerular feedback.

Stimulation of 11-beta-hydroxysteroid dehydrogenase type 2 in rat colon but not in kidney by low dietary NaCl intake

Data suggest that mineralocorticoid selectivity is differentially regulated in epithelial target tissues. We investigated whether the level of dietary NaCl intake influenced the expression and tissue distribution of 11-beta-hydroxysteroid dehydrogenase type 2 (11betaHSD-2), aldosterone receptor (MR), and glucocorticoid receptor (GR) in rat colon, kidney, and cardiovascular tissue. Rats were fed a diet with 0.01 or 3% NaCl for 10 days. Messenger RNAs were analyzed with ribonuclease protection assay, 11betaHSD-2 protein by Western blot analysis, and localization of GR and 11betaHSD-2 by immunohistochemistry. NaCl restriction elevated plasma renin and aldosterone concentration, whereas corticosterone was unaltered. In distal colon, 11betaHSD-2 mRNA and protein were augmented significantly by low-NaCl intake and immunolabeling was widely distributed in crypt and surface epithelium. The MR mRNA level was decreased, whereas GR mRNA was unaltered in distal colon. MR, GR, and 11betaHSD-2 mRNAs were not changed in kidney cortex and medulla, left cardiac ventricle, and aorta. Immunofluorescence labeling showed that GR and 11betaHSD-2 localization was mutually exclusive in kidney. In colon epithelium, nuclear staining for GR subsided as perinuclear 11betaHSD-2 immunoreactivity increased with NaCl restriction. As a functional correlate of increased 11betaHSD-2 expression in colon, the GR-stimulated sodium-hydrogen exchanger NHE-3 was lowered by NaCl restriction. Inhibition of 11betaHSD-2 activity by carbenoxolone during NaCl restriction stimulated NHE-3 expression in colon. Dexamethasone stimulated NHE-3 both in colon and kidney. These data indicate that mineralocorticoid selectivity is physiologically regulated by NaCl intake at the level of 11betaHSD-2 expression and tissue distribution in the distal colon, but not in the kidney.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Role of T-type calcium channels in myogenic tone of skeletal muscle resistance arteries

T-type calcium channels may be involved in the maintenance of myogenic tone. We tested their role in isolated rat cremaster arterioles obtained after CO(2) anesthesia and decapitation. Total RNA was analyzed by RT-PCR and Southern blotting for calcium channel expression. We observed expression of voltage-operated calcium (Ca(V)) channels Ca(V)3.1 (T-type), Ca(V)3.2 (T-type), and Ca(V)1.2 (L-type) in cremaster arterioles (n = 3 rats). Amplification products were observed only in the presence of reverse transcriptase and cDNA. Concentration-response curves of the relatively specific L-type blocker verapamil and the relatively specific T-type blockers mibefradil and nickel were made on cannulated vessels with either myogenic tone (75 mmHg) or a similar level of constriction induced by 30 mM K(+) at 35 mmHg. Mibefradil and nickel were, respectively, 162-fold and 300-fold more potent in inhibiting myogenic tone compared with K(+)-induced constriction [log(IC(50), M): mibefradil, basal -7.3 +/- 0.2 (n = 9) and K(+) -5.1 +/- 0.1 (n = 5); nickel, basal -4.1 +/- 0.2 (n = 5) and K(+) -1.6 +/- 0.5 (n = 5); means +/- SE]. Verapamil had a 17-fold more potent effect [log(IC(50), M): basal -6.6 +/- 0.1 (n = 5); K(+) -5.4 +/- 0.3 (n = 4); all log(IC(50)) P < 0.05, basal vs. K(+)]. These data suggest that T-type calcium channels are expressed and involved in maintenance of myogenic tone in rat cremaster muscle arterioles.

Tubular and cellular localization of the cardiac L-type calcium channel in rat kidney

BACKGROUND

The mRNAs of several types of calcium channels have been identified in intact rat kidney, and L-type calcium channels cause changes in intracellular calcium in primary cultures of distal tubule cells. The aim of this study was to evaluate the tubular and cellular distribution of the alpha1C subunit of the L-type calcium channel in intact kidney.

METHODS

RT-PCR and Northern blot analysis were used to assess the regional abundance of the mRNA of this channel. Immunocytochemistry combined with confocal microscopy and surface biotinylation were applied to determine the tubular and cellular localization of the protein.

RESULTS

Northern blot and RT-PCR analysis indicated that the mRNA of the alpha1C subunit of the cardiac L-type calcium channel was present in whole rat kidney, kidney tubules and kidney cell lines. Western blot of lysates from whole kidney, kidney tubules or cell lines revealed bands of approximately 190 kD for the alpha1C subunit and approximately 60 kD for the beta3 subunit. Confocal immunohistochemistry indicated that the alpha1C subunit of this channel was co-expressed in cells of the distal tubule that express calbindin-D28K, but not in intercalated cells. The alpha1C subunit was also highly expressed in both outer and inner medullary collecting ducts. Serial confocal microscopic images or surface biotinylation experiments determined that the channel was predominantly on the basolateral membrane but had some distribution on the apical membrane.

CONCLUSIONS

The distribution and cellular localization of the alpha1C subunit of cardiac L-type calcium channel suggest it is probably involved in intracellular and membrane calcium signaling.