Effect of indomethacin on force responses and sarcoplasmic reticulum function in skinned skeletal muscle fibers and cytosolic [Ca2+] in myotubes

The MyoRobot: A novel automated biomechatronics system to assess voltage/Ca2+ biosensors and active/passive biomechanics in muscle and biomaterials

We engineered an automated biomechatronics system, MyoRobot, for robust objective and versatile assessment of muscle or polymer materials (bio-)mechanics. It covers multiple levels of muscle biosensor assessment, e.g. membrane voltage or contractile apparatus Ca²⁺ ion responses (force resolution 1µN, 0-10mN for the given sensor; [Ca²⁺] range ~ 100nM-25µM). It replaces previously tedious manual protocols to obtain exhaustive information on active/passive biomechanical properties across various morphological tissue levels. Deciphering mechanisms of muscle weakness requires sophisticated force protocols, dissecting contributions from altered Ca²⁺ homeostasis, electro-chemical, chemico-mechanical biosensors or visco-elastic components. From whole organ to single fibre levels, experimental demands and hardware requirements increase, limiting biomechanics research potential, as reflected by only few commercial biomechatronics systems that can address resolution, experimental versatility and mostly, automation of force recordings. Our MyoRobot combines optical force transducer technology with high precision 3D actuation (e.g. voice coil, 1µm encoder resolution; stepper motors, 4µm feed motion), and customized control software, enabling modular experimentation packages and automated data pre-analysis. In small bundles and single muscle fibres, we demonstrate automated recordings of (i) caffeine-induced-, (ii) electrical field stimulation (EFS)-induced force, (iii) pCa-force, (iv) slack-tests and (v) passive length-tension curves. The system easily reproduces results from manual systems (two times larger stiffness in slow over fast muscle) and provides novel insights into unloaded shortening velocities (declining with increasing slack lengths). The MyoRobot enables automated complex biomechanics assessment in muscle research. Applications also extend to material sciences, exemplarily shown here for spider silk and collagen biopolymers.

hhLIM is a novel F-actin binding protein involved in actin cytoskeleton remodeling

Human heart LIM protein (hhLIM) is a newly cloned protein. In vitro analyses showed that green fluorescent protein (GFP)-tagged hhLIM protein accumulated in the cytoplasm of C2C12 cells and colocalized with F-actin, indicating that hhLIM is an actin-binding protein in C2C12 cells. Overexpression of hhLIM-GFP in C2C12 cells significantly stabilized actin filaments and delayed depolymerization of the actin cytoskeleton induced by cytochalasin B treatment. Expression of hhLIM-GFP in C2C12 cells also induced significant changes in the organization of the actin cytoskeleton, specifically, fewer and thicker actin bundles than in control cells, suggesting that hhLIM functions as an actin-bundling protein. This hypothesis was confirmed using low-speed co-sedimentation assays and direct observation of F-actin bundles that formed in vitro in the presence of hhLIM. hhLIM has two LIM domains. To identify the essential regions and sites for association, a series of truncated mutants was constructed which showed that LIM domain 2 has the same activity as full-length hhLIM. To further characterize the binding sites, the LIM domain was functionally destructed by replacing cysteine with serine in domain 2, and results showed that the second LIM domain plays a central role in bundling of F-actin. Taken together, these data identify hhLIM as an actin-binding protein that increases actin cytoskeleton stability by promoting bundling of actin filaments.

Postnatal adrenalectomy impairs urinary concentrating ability by increased COX-2 and leads to renal medullary injury

We hypothesized that aldosterone promotes development of the renal medulla in the postnatal period and that cyclooxygenase-2 (COX-2) activity contributes to renal dysfunction after impaired aldosterone signaling. To test these hypotheses, rat pups underwent either sham operation or adrenalectomy at postnatal day 10. Adrenalectomized rats were divided into no steroid substitution (ADX), corticosterone replacement (ADX-C), and corticosterone and DOCA substitution (ADX-CD) groups that received subcutaneous pellets with steroids. Without replacement, pups failed to thrive and exhibited impaired urinary-concentrating ability. The renal medulla was significantly smaller, and the medullary interstitial osmolality was lower in the ADX group, whereas COX-2 and PGE2 tissue levels were significantly elevated compared with levels shown in sham animals. Substitution with DOCA and corticosterone corrected these changes, whereas corticosterone replacement alone improved survival but not weight gain and urinary-concentrating ability. Administration of a COX-2 inhibitor to ADX rats (parecoxib, 5 mg.kg(-1).day(-1), days 17-20) increased weight gain, urinary-concentrating ability, and papillary osmolality. After fluid deprivation, parecoxib attenuated weight loss and the increase in plasma Na+ concentration and osmolality. It is concluded that mineralocorticoid is required for normal postnatal development of the renal medulla. COX-2 contributes to impaired urine-concentrating ability, NaCl loss, and extracellular volume depletion in postnatal mineralocorticoid deficiency.

Effect of tumor necrosis factor α on electrically induced calcium transients elicited in C2C12 skeletal myotubes

Diseases involving chronic inflammation can lead to prolonged exposure of skeletal muscle to inflammatory cytokines such as tumor necrosis factor alpha (TNFalpha), which may contribute to the skeletal muscle weakness seen in these conditions. In this study we examined the effect of a prolonged exposure to TNFalpha on intracellular Ca(2+) transients elicited in skeletal C(2)C(12) myotubes. A 48-h exposure to TNFalpha (10 ng/mL) significantly reduced the peaks, time to peak, and rate of Ca(2+) decay of electrically induced Ca(2+) transients elicited in C(2)C(12) skeletal myotubes. TNFalpha exposure had no significant effect on the resting Ca(2+) levels. The results of this study indicate that prolonged exposure to TNFalpha decreases sarcoplasmic reticulum Ca(2+) release in cultured skeletal muscle cells. This altered Ca(2+) release could contribute to the muscle weakness found in conditions involving chronic inflammation.

Role of renal cortical cyclooxygenase-2 expression in hyperfiltration in rats with high-protein intake

Renal cortical cyclooxygenase-2 (COX-2) is restricted to the macula densa and adjacent cortical thick ascending limbs (MD/cTALH). Renal cortical COX-2 increases in response to diabetes and renal ablation, both of which are characterized by hyperfiltration and reduced NaCl delivery to the MD due to increased proximal NaCl reabsorption. High-protein intake also induces hyperfiltration and decreases NaCl delivery to the MD due to increased NaCl reabsorption proximally. We investigated whether high protein induces cortical COX-2 and whether cortical COX-2 contributes to high protein-induced hyperfiltration and increased intrarenal renin biosynthesis. Cortical COX-2 increased after protein loading but decreased after protein restriction. COX-2 inhibition attenuated high protein-induced hyperfiltration but had no effect on high protein-induced intrarenal renin elevation. Therefore, induction of cortical COX-2 contributed to high protein-induced hyperfiltration but not intrarenal renin elevation. In the kidney cortex, neuronal nitric oxide synthase (nNOS) is also localized to the MD, and interactions between intrarenal nNOS and COX-2 systems have been proposed. Cortical COX-2 elevation seen in salt restriction was blocked by nNOS inhibiton. Cortical nNOS expression also increased after protein loading, and inhibition of nNOS activity completely reversed high protein-induced cortical COX-2 elevation and hyperfiltration. These results indicate that NO is a mediator of high protein-induced cortical COX-2 elevation and suggest that both intrarenal nNOS and COX-2 systems appear to regulate afferent arteriolar tone and subsequent hyperfiltration seen in high-protein intake.

The effect of taurine depletion on the contractile properties and fatigue in fast-twitch skeletal muscle of the mouse

Taurine increases force production in skeletal muscle, and taurine levels may fall during exercise. The contractile properties and fatigability of extensor digitorum longus (EDL) muscles depleted of taurine by guanodinoethane sulfonate (GES) treatment were investigated. GES treatment decreased muscle taurine levels to <40% of controls. Peak twitch force levels were 23% of controls in GES treated EDL muscles (p < 0.05), but maximal specific force was unaffected. The force-frequency relationship was examined and significantly less force was produced by the GES treated muscles compared to controls at stimulation frequencies from 50 to 100 Hz (p < 0.05). GES treated EDL muscles exhibited significantly slower rates of fatigue than controls (p < 0.05). In skinned fibres, 20 mM GES had a small but significant effect on force production, indicating that GES may have some minor taurine-like effects. In this study, a fall in taurine levels decreased force output, and increased the endurance of EDL skeletal muscles.

Neonatal sympathectomy reduces NADPH oxidase activity and vascular resistance in spontaneously hypertensive rat kidneys

Neonatal sympathectomy reduces arterial pressure in spontaneously hypertensive rats (SHR). In SHR transplanted with a kidney from sympathectomized SHR, arterial pressure was lower and less Na+ sensitive than in SHR transplanted with a kidney from hydralazine-treated SHR. This study was performed to identify underlying renal mechanisms. Tests for differential renal mRNA expression of nine a priori selected genes revealed robust differences for renal medullary expression of the NADPH oxidase subunit p47phox. Therefore, we investigated the effects of neonatal sympathectomy on renal mRNA expression of NADPH oxidase subunits, NADPH oxidase activity, and renal function. In 10-wk-old sympathectomized SHR fed a 0.6% NaCl diet, medullary p47phox and gp91phox expression was 40% less than in hydralazine-treated SHR. Also, after a 1.8% NaCl diet, medullary p47phox mRNA expression was lower in sympathectomized than in hydralazine-treated SHR. We found lower cortical (-30%, P<0.01) and medullary (-30%, P<0.05) NADPH oxidase activities in sympathectomized than in hydralazine-treated or untreated SHR. Glomerular filtration rate, renal blood flow, medullary blood flow, and fractional Na+ excretion in kidney grafts from sympathectomized and hydralazine-treated donors (n=8 per group) were similar at baseline and in response to a 20-mmHg rise in renal perfusion pressure. Renal vascular resistance was lower in kidneys from sympathectomized than hydralazine-treated donors (25+/-2 vs. 32+/-4 mmHg.min.ml-1, P<0.05). The results indicate that the sympathetic nervous system contributes to the level of renal NADPH oxidase activity and to perinatal programming of alterations in renal vascular function that lead to elevated renal vascular resistance in SHR.

Interactions between 11beta-hydroxysteroid dehydrogenase and COX-2 in kidney

The syndrome of apparent mineralocorticoid excess (SAME) is an autosomal recessive form of salt-sensitive hypertension caused by deficiency of the kidney type 2 11beta-hydroxysteroid dehydrogenase (11betaHSD2). In this disorder, cortisol is not inactivated by 11betaHSD2, occupies mineralocorticoid receptors (MRs), and causes excessive sodium retention and hypertension. In renal medulla, prostaglandins derived from cyclooxygenase-2 (COX-2) stimulate sodium and water excretion, and renal medullary COX-2 expression increases after mineralocorticoid administration. We investigated whether medullary COX-2 also increases in rats with 11betaHSD2 inhibition and examined its possible role in the development of hypertension. 11betaHSD2 inhibition increased medullary and decreased cortical COX-2 expression in adult rats and induced high blood pressure in high-salt-treated rats. COX-2 inhibition had no effect on blood pressure in control animals but further increased blood pressure in high-salt-treated rats with 11betaHSD2 inhibition. COX-1 inhibition had no effect on blood pressure in either control or experimental animals. 11betaHSD2 inhibition also led to medullary COX-2 increase and cortical COX-2 decrease in weaning rats, primarily through activation of MRs. In the suckling rats, medullary COX-2 expression was very low, consistent with a urinary concentrating defect. 11betaHSD2 inhibition had no effect on either cortical or medullary COX-2 expression in the suckling rats, consistent with low levels of circulating corticosterone in these animals. These data indicate that COX-2 plays a modulating role in the development of hypertension due to 11betaHSD2 deficiency and that 11betaHSD2 regulates renal COX-2 expression by preventing glucocorticoid access to MRs during postnatal development.

Cross talk between the intrarenal dopaminergic and cyclooxygenase-2 systems

In mammalian kidney, dopamine produced in the proximal tubule (PT) acts as an autocrine/paracrine natriuretic hormone that inhibits salt and fluid reabsorption in the PT. In high-salt-treated animals, PT dopamine activity increases and inhibits reabsorption, leading to increased salt and fluid delivery to the macula densa (MD) and subsequent natriuresis and diuresis. Regulated cyclooxygenase-2 (COX-2) in the MD represents another intrinsic system mediating renal salt and water homeostasis. Renal cortical COX-2 is inversely related to salt intake, and decreased extracellular NaCl stimulates COX-2 expression in cultured MD/cortical thick ascending limb cells. The current study investigated interactions between renal dopamine and cortical COX-2 systems. In rats fed a control diet, the dopamine precursor l-dihydroxyphenylalanine (l-DOPA) or the DA1 receptor agonist SKF-81297 suppressed cortical COX-2 expression. High salt suppressed cortical COX-2 expression, which was attenuated by inhibition of dopamine production with benserazide or the DA1 receptor antagonist, SCH-23390. In contrast, l-DOPA or the dopamine-metabolizing enzyme inhibitor entacapone suppressed low-salt-induced cortical COX-2 expression. Inhibition of PT reabsorption with the carbonic anhydrase inhibitor acetazolamide suppressed cortical COX-2 expression. In contrast, treatment with distally acting diuretics led to elevation of cortical COX-2. These results indicate that dopamine modulates renal cortical COX-2 expression by modifying PT reabsorption.

The renin-angiotensin system in kidney development: role of COX-2 and adrenal steroids

Recent data from studies in rodents with targeted gene disruption and pharmacological antagonists have shown that the renin-angiotensin-aldosterone system (RAAS) and cyclooxygenase type-2 (COX-2) are necessary for late stages of kidney development. The present review summarizes data on the developmental changes of RAAS and COX-2 and the pathways by which they are activated; their possible interplay and the mechanisms by which they affect kidney development. Intrarenal and circulating renin and angiotensin II (ANG II) are stimulated at birth in most mammals. In rats, renin and ANG II stay significantly elevated in the suckling period while aldosterone stabilizes at an adult level. COX-2 is stimulated in thick ascending limb of Henle's loop in the suckling period at a time when urine concentrating ability is not developed. Data suggest that this induction is mediated by combined low plasma glucocorticoid concentration and by a low NaCl intake. Studies with selective inhibitors of COX-2 and COX-2 null mice show that COX-2 activity stimulates renin secretion from JG-cells during postnatal kidney development and that lack of COX-2 activity leads to pathological change in cortical architecture and eventually to renal failure. In the postnatal period, ANG II initiates and maintains pelvic and ureteric contractions necessary for urine flow. Lack of ANG II in the neonatal period is thought to cause injury by a chronic increase of renal pelvic pressure. Aldosterone is crucial for survival and growth in the neonatal period through its effects on sodium reabsorption and the intrarenal sensitivity to aldosterone is increased in the postnatal period. Final maturation of the kidney occurs through an intimate interplay between a low dietary sodium intake and a non-responsive HPA-axis which stimulates cortical COX-2 activity. COX-2 supports increased activity of the RAAS and may contribute to a low concentrating ability.

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.