Molecular aspects of intestinal calcium absorption

Intestinal Ca2+ absorption is a crucial physiological process for maintaining bone mineralization and Ca2+ homeostasis. It occurs through the transcellular and paracellular pathways. The first route comprises 3steps: the entrance of Ca2+ across the brush border membranes(BBM) of enterocytes through epithelial Ca2+ channels TRPV6, TRPV5, and Cav1.3; Ca2+ movement from the BBM to the basolateral membranes by binding proteins with high Ca2+ affinity(such as CB9k); and Ca2+ extrusion into the blood. Plasma membrane Ca2+ ATPase(PMCA1b) and sodium calcium exchanger(NCX1) are mainly involved in the exit of Ca2+ from enterocytes. A novel molecule, the 4.1R protein, seems to be a partner of PMCA1 b, since both molecules colocalize and interact. The paracellular pathway consists of Ca2+ transport through transmembrane proteins of tight junction structures, such as claudins 2, 12, and 15. There is evidence of crosstalk between the transcellular and paracellular pathways in intestinal Ca2+ transport. When intestinal oxidative stress is triggered, there is a decrease in the expression of several molecules of both pathways that inhibit intestinal Ca2+ absorption. Normalization of redox status in the intestine with drugs such as quercetin, ursodeoxycholic acid, or melatonin return intestinal Ca2+ transport to control values. Calcitriol [1,25(OH)2D3] is the major controlling hormone of intestinal Ca2+ transport. It increases the gene and protein expression of most of the molecules involved in both pathways. PTH, thyroid hormones, estrogens, p ro l a c t i n, g ro w t h h o r m o n e, a n d g l u c o c o r t i c o i d s apparently also regulate Ca2+ transport by direct action, indirect mechanism mediated by the increase of renal 1,25(OH)2D3 production, or both. Different physiological conditions, such as growth, pregnancy, lactation, and aging, adjust intestinal Ca2+ absorption according to Ca2+ demands. Better knowledge of the molecular details of intestinal Ca2+ absorption could lead to the development of nutritional and medical strategies for optimizing the efficiency of intestinal Ca2+ absorption and preventing osteoporosis and other pathologies related to Ca2+ metabolism.     

幽门螺杆菌作用后AGS细胞内钙离子的动态变化研究

背景幽门螺杆菌(Helicobacter pylori,Hp)感染与胃癌发生密切相关,但致病机制尚未清晰。细胞毒素相关蛋白A(CagA)是Ⅰ型Hp的主要毒力因子,在Hp诱导的疾病特别是胃癌的发展中起重要作用。前期的研究发现,Hp作用后人胃腺癌上皮细胞(AGS)的钙离子相关蛋白钙网织蛋白CRT以及钙离子结合蛋白CALNUC磷酸化程度发生改变,提示Hp可能会影响AGS细胞的钙稳态,通过钙离子通路影响细胞的增殖或凋亡。目的研究Hp作用于人胃腺癌上皮细胞后,AGS细胞内钙离子的时序性变化,以及CagA对AGS细胞内钙离子的影响,进一步揭示Hp的致病机制。方法采用钙离子荧光标记示踪法,AGS细胞以Fluo-3/AM荧光指示剂负载,特异性地活体标记AGS内的钙离子,采用激光共聚焦显微镜观察分析Hp26695及其CagA缺失株(Hp26695△CagA)分别作用于AGS细胞1h、2h、3h、4h、5h、6h后,AGS细胞内钙离子的变化情况。结果幽门螺杆菌与AGS细胞相互作用1h,胞内Ca2+部分流失,5h胞内Ca2+大量流失。1～6h内,Hp26695与Hp26695△CagA作用的AGS细胞荧光强度没有显著差异。结论Hp作用会导致AGS细胞内Ca2+剥夺,且与CagA的存在无明显关联性。     

Roles of Na~+/Ca~(2+) exchanger 1 in digestive system physiology and pathophysiology

The Na~+/Ca~(2+) exchanger(NCX) protein family is a part of the cation/Ca~(2+) exchanger superfamily and participates in the regulation of cellular Ca~(2+) homeostasis. NCX1, the most important subtype in the NCX family, is expressed widely in various organs and tissues in mammals and plays an especially important role in the physiological and pathological processes of nerves and the cardiovascular system. In the past few years, the function of NCX1 in the digestive system has received increasing attention; NCX1 not only participates in the healing process of gastric ulcer and gastric mucosal injury but also mediates the development of digestive cancer, acute pancreatitis, and intestinal absorption.This review aims to explore the roles of NCX1 in digestive system physiology and pathophysiology in order to guide clinical treatments.     

(Ca2++Mg2+)-ATPase activity of sickle cell membranes: decreased activation by red blood cell cytoplasmic activator.

Human red blood cells (RBCs) contain a cytoplasmic protein that activates membrane-bound (Ca2+ + Mg2+)-ATPase and the transport of Ca2+. The (Ca2+ + Mg2+)-ATPase of sickle cells showed a less than normal response to this activator. This was true whether the activator was obtained from normal or sickle cells. Activator present in sickle cell hemolysates fully activated the (Ca2+ + Mg2+)-ATPase of normal RBC membranes. These results demonstrate that membranes of sickle cells are defective in their response to the activator. Neither the apparent affinity for calcium nor the apparent affinity for activator was different comparing the (Ca2+ + Mg2+)-ATPase of sickle and normal membranes. Young, mature, and irreversibly sickled cells were separated by density gradient centrifugation, and membranes were prepared from each of these cell populations. No significant differences in ATPase activities were found based on cell age (density). The (Ca2+ + Mg2+)-ATPase of all populations of sickle cells showed a decreased response to the activator. Thus, it appears unlikely that the decreased response of the (Ca2+ + Mg2+)-ATPase of sickle cells is due to membrane damage caused by repeated sickling during the life-span of the cell. Reduced activation of (Ca2+ + Mg2+)-ATPase by the cytoplasmic activator may account for calcium accumulation in sickle cells.     

105例氟骨症患者血清F^—／Ca^＋＋比值的分析

本文报告地方性氟中毒饮水型病区的105例氟骨症患者血清F~-/Ca 比值,其均值为0.028,而天津市正常人血清F~-/Ca~-比值为0.011±0.002。两组人群之间,经统计学处理有显著性差异。疏松型与硬化型患者血清F~-/Ca~-比值之间有显著性差异,硬化型及疏松型分别与混合型相比,无显著性差异。疏松型患者血清F~-/Ca~- 比值随患者X线病理改变的加重而下降;硬化型则上升;而混合型无显著变化。 本文提示,氟骨症患者血清F~-/Ca~-比值可以作为氟对骨质损害的一个参考指标。     

Effect of prolonged omeprazole administration on segmental intestinal Mg~(2+) absorption in male Sprague-Dawley rats

BACKGROUND The exact mechanism of proton pump inhibitors(PPIs)-induced hypomagnesemia(PPIH) is largely unknown. Previous studies proposed that PPIH is a consequence of intestinal Mg~(2+) malabsorption. However, the mechanism of PPIs-suppressed intestinal Mg~(2+) absorption is under debate.AIM To investigate the effect of 12-wk and 24-wk omeprazole injection on the total,transcellular, and paracellular Mg~(2+) absorption in the duodenum, jejunum, ileum,and colon of male Sprague-Dawley rats.METHODS The rats received 20 mg/kg·d subcutaneous omeprazole injection for 12 or 24 wk.Plasma and urinary Mg~(2+), Ca~(2+), and PO_4~(3-)levels were measured. The plasma concentrations of 1α,25-dihydroxyvitamin D3(1α,25(OH)_2D_3), parathyroid hormone(PTH), fibroblast growth factor 23(FGF-23), epidermal growth factor(EGF), and insulin were also observed. The duodenum, jejunum, ileum, and colon of each rat were mounted onto individual modified Using chamber setups to study the rates of total, transcellular, and paracellular Mg~(2+) absorption simultaneously. The expression of transient receptor potential melastatin 6(TRPM6) and cyclin M4(CNNM4) in the entire intestinal tract was also measured.RESULTS Single-dose omeprazole injection significantly increased the intraluminal p H of the stomach, duodenum, and jejunum. Omeprazole injection for 12 and 24 wk induced hypomagnesemia with reduced urinary Mg~(2+) excretion. The plasma Ca~(2+) was normal but the urinary Ca~(2+) excretion was reduced in rats with PPIH. The plasma and urinary PO_4~(3-)levels increased in PPIH rats. The levels of1α,25(OH)_2D_3 and FGF-23 increased, whereas that of plasma EGF decreased in the omeprazole-treated rats. The rates of the total, transcellular, and paracellular Mg~(2+) absorption was significantly lower in the duodenum, jejunum, ileum, and colon of the rats with PPIH than in those of the control rats. The percent suppression of Mg~(2+) absorption in the duodenum, jejunum, ileum, and colon of the rats with PPIH compared with the control rats was 81.86%, 70.59%, 69.45%,and 39.25%, respectively. Compared with the control rats, the rats with PPIH had significantly higher TRPM6 and CNNM4 expression levels throughout the intestinal tract.CONCLUSION Intestinal Mg~(2+) malabsorption was observed throughout the intestinal tract of rats with PPIH. PPIs mainly suppressed small intestinal Mg~(2+) absorption. Omeprazole exerted no effect on the intraluminal acidic pH in the colon. Thus, the lowest percent suppression of total Mg~(2+) absorption was found in the colon. The expression levels of TRPM6 and CNNM4 increased, indicating the presence of a compensatory response to Mg~(2+) malabsorption in rats with PPIH. Therefore, the small intestine is an appropriate segment that should be modulated to counteract PPIH.     

Inter-individual variability in Ca2+ signalling in platelets from healthy volunteers: effects of aspirin and relationship with expression of endomembrane Ca2+-ATPases

Increased Ca²⁺ signal generation may lead to hyperactivity of platelets and contribute to thrombotic complications. Using fura-2-loaded platelets from 51 healthy volunteers, high variability was detected in the Ca²⁺ responses evoked by the receptor agonists, thrombin and collagen, and the inhibitor of sarco/endoplasmic reticulum Ca²⁺-ATPases (SERCA), thapsigargin (Tg). Oral intake of 500 mg aspirin reduced the magnitude of the Ca²⁺ responses, and lowered the intra-individual coefficients of variance of the responses by 50%. However, the corresponding inter-individual variance coefficients were only a little influenced by aspirin intake, pointing to subject-dependent factors in Ca²⁺ handling that are unrelated to thromboxane formation. With each agonist, 6–9% of the subjects had platelets with relatively high Ca²⁺ responses (> mean + SD) both before and after aspirin intake. In 90% (9/10) of these cases the high responsiveness was confirmed in platelets obtained 6–12 months later. The Tg- but not thrombin-induced Ca²⁺ responses correlated inversely with the expression levels of SERCA PL/IM 430 (SERCA-3b) in platelets. After aspirin intake, the Ca²⁺ responses with collagen but not thrombin correlated inversely with SERCA-2b expression. These results suggest that, in the absence of potentiating effects of thromboxane, (i) the amount of PL/IM 430-recognizable SERCA may control the Ca²⁺ signal when SERCA-2b is specifically inhibited (with Tg), and (ii) the expression of SERCA-2b determine the collagen- but not the thrombin-evoked Ca²⁺ signal. Accordingly, limited Ca²⁺-pumping activity by low expression of one of the SERCA isoforms is likely to be one of the factors resulting in increased platelet activity towards collagen or thapsigargin but not thrombin.     

Efonidipine Hydrochloride: A Dual Blocker of L‐ and T‐Type Ca2+ Channels

T‐type Ca²⁺ channels have properties different from those of the L‐type and are involved in cardiac pacemaking and regulation of blood flow, but not in myocardial contraction. Efonidipine is an antihypertensive and antianginal drug with dihydropyridine structure that was recently found to block both L‐ and T‐type Ca²⁺ channels. In isolated myocardial and vascular preparations, efonidipine has potent negative chronotropic and vasodilator effects but only a weak negative inotropic effect. In experimental animals and patients, reduction of blood pressure by the drug was accompanied by no or minimum reflex tachycardia leading to improvement of myocardial oxygen balance and maintenance of cardiac output. Efonidipine increased glomerular filtration rate without increasing intraglomerular pressure. By relaxing both the afferent and efferent arterioles, efonidipine markedly reduced proteinuria. Thus, efonidipine, an L‐ and T‐type dual Ca²⁺ channel blocker, appears to have an ideal profile as an antihypertensive and antianginal drug with organ‐protective effects in the heart and kidney.     

Leptin transiently antagonizes ghrelin and long-lastingly orexin in regulation of Ca2＋ signaling in neuropeptide Y neurons of the arcuate nucleus

AIM： To explore the mechanism for interactions of leptin with ghrelin and orexin in the arcuate nucleus （ARC） activating neuropeptide Y （NPY） neurons during physiological regulation of feeding, METHODS： Single neurons from ARC of adult rats with matured feeding function were isolated. [Ca2＋]i was measured to monitore their activities. The time course of leptin effects on ghrelin-induced versus orexin-induced [Ca2＋]i increases in NPY neurons was studied. RESULTS： Administration of ghrelin or orexin-A at 101~ mol/L increased cytosolic Ca2~ concentration （[Ca2＋]~） in NPY neurons isolated from the ARC of adult rats. Upon administration of leptin at 10^-14-10^-12 mol/L, ghrelin-induced [Ca2＋]i increases were initially （〈 10 min） inhibited but later restored, exhibiting a transient pattern of inhibition. In contrast, orexin-induced [Ca2＋]i increases were inhibited by leptin in a long- lasting manner. Furthermore, a prior administration of leptin inhibited orexin action but not ghrelin action to increase ICa 2＋li, CONCLUSION： Leptin counteracted ghrelin effects transiently and orexin effects long-lastingly in NPY neurons. The transient property with which leptin counteracts ghrelin action in NPY neurons may allow the fasting-associated increase in ghrelin levels to activate NPY neurons in the presence of physiological leptin and to stimulate feeding.     

Possible contribution of the sarcoplasmic reticulum Ca(2+) pump function to electrical and mechanical alternans

We investigated the role of the sarcoplasmic reticulum's (SR) Ca(2+) pump function of the in the mechanism of alternans. We recorded the surface ECG, monophasic action potential (MAP) and left ventricular pressure (LVP) in the canine beating heart. Alternans was induced with an abrupt shortening of the cycle length from 1000 to 350 ms. After the control studies, we administered propranolol or isoproterenol. In the presence of propranolol, we administered milrinone or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). In the presence of isoproterenol, we administered thapsigargin. Isoproterenol and milrinone attenuated both the electrical and mechanical alternans. Thapsigargin, a specific SR Ca(2+) pump inhibitor, and propranolol magnified both types of alternans. DIDS, a Ca(2+)-activated Cl(-) current (I(Cl(Ca))) inhibitor, attenuated the MAP alternans without an affect on the LVP alternans. Thus, the delayed intracellular Ca(2+) cycling caused by the impaired SR Ca(2+) pump function might produce electrical and mechanical alternans. beta-adrenergic stimulation eliminated these alternans. The I(Cl(Ca)) contributed to the appearance of the electrical alternans.     

Local Ca(2+) signaling and EC coupling in heart: Ca(2+) sparks and the regulation of the [Ca(2+)](i) transient

The elementary event of Ca(2+) release in heart is the Ca(2+) spark. It occurs at a low rate during diastole, activated only by the low cytosolic [Ca(2+)](i). Synchronized activation of many sparks is due to the high local [Ca(2+)](i) in the region surrounding the sarcoplasmic reticulum (SR) Ca(2+) release channels and is responsible for the systolic [Ca(2+)](i) transient. The biophysical basis of this calcium signaling is discussed. Attention is placed on the local organization of the ryanodine receptors (SR Ca(2+) release channels, RyRs) and the other proteins that underlie and modulate excitation-contraction (EC) coupling. A brief review of specific elements that regulate SR Ca(2+) release (including SR lumenal Ca(2+) and coupled gating of RyRs) is presented. Finally integrative calcium signaling in heart is presented in the context of normal heart function and heart failure.     

Decrease in cardiotoxicity of contrast media in coronary angiography by addition of calcium ions: A combined experimental and clinical study

The cardiotoxicity of commonly used contrast media such as sodium methylglucamine diatrizoate is related to nonphysiologic concentrations of ions and high osmolality. Following injection there is an excessive decrease in ambient calcium which disturbs the extracellular Na⁺/Ca²⁺ ratio and results in myocardial depression. In animal studies (anesthetized, intact mongrel dogs) this cardiodepressive effect could be fully compensated by adding 22.5 mEq/1 of ionic calcium to the contrast medium. In 16 patients studied for coronary artery disease (CAD) 72 injections of sodium methylglucamine diatrizoate, mainly in the left coronary artery, were thoroughly investigated. There was marked depression of left ventricular pressure (-22%) and (dP/dt)_max (-27%), and marked heart rate slowing (-29%). Added calcium in the concentration cited reduced significantly (p<0.001) the myocardial depression to ?15% and ?13%, respectively, but did not completely compensate it. The heart rate slowing was not altered significantly. It was concluded that the addition of calcium may be helpful when studying high-risk patients and using high injection volumes.     

Preventing all-cause hospitalizations in type 2 diabetes with sodium-glucose cotransporter-2 inhibitors and glucagon-like peptide-1 receptor agonists: A narrative review and proposed clinical approach

Patients with type 2 diabetes (T2D) are at increased risk for hospital admissions, and acute hospitalizations are associated with a worse prognosis. However, outcomes related to all-cause hospital admissions (ACHAs) were often overlooked in trials that demonstrated the cardiovascular and kidney benefits of sodium-glucose cotransporter-2 (SGLT2) inhibitors and glucagon-like peptide-1 receptor agonists (GLP-1RAs). This review includes a contemporary literature summary of emerging data regarding the effects of SGLT2 inhibitors and GLP-1RAs on ACHAs. The role of SGLT2 inhibitors in preventing ACHAs was shown in exploratory investigations of several randomized controlled trials (RCTs) and was further supported by real-world evidence (RWE). However, the association between GLP-1RA use and lower ACHA risk was mainly shown through RWE, with minimal available RCT data. We also discuss the advantages and challenges of studying ACHAs. Finally, we propose an easily memorized (“ABCDE” acronym) clinical approach to evaluating T2D status and treatment in admitted patients, as they transition from hospital to community care. This systematic approach may assist clinicians in recognizing possible pitfalls in T2D management, thereby preventing subsequent hospitalizations and improving patient prognoses. While acute admission can sometimes be perceived as a management failure, it should also be viewed as an opportunity to take action to prevent the next hospitalization.     

Ca2+ -mediated degradation of central nervous system (CNS) proteins: Topographic and species variation

Incubation of homogenates of rat, rabbit, and bovine spinal cord and of bovine brain white and gray matter in the presence of calcium (5 mM) produced an extensive degradation of the neurofilament triplet proteins (NFP; 200 K, 150K, and 69K). The breakdown products of the NFPs were identified by immunoblot. The glial fibrillary acidic protein (GFAP), microtubular proteins (MTP), and myelin proteins were also degraded. The 150 K NFP was more susceptible than the other NFPs. The extent of calcium-mediated degradation was slightly greater with rat spinal cord than the others. Bovine brain white matter had more activity than gray, which had no appreciable degradative activity. The breakdown was prevented by both EGTA and leupeptin but a similar concentration of MgCl₂ (5 mM) had no effect. These results suggest that NFPs are degraded by a Ca²⁺ -activated neutral proteinase in the central nervous system (CNS) of several species. The lesser activity in gray matter suggests that the enzyme is enriched in axons, myelin, and/or oligodendroglial cells.     

N‐(p‐Amylcinnamoyl)anthranilic Acid (ACA): A Phospholipase A2 Inhibitor and TRP Channel Blocker

Phospholipase A₂ enzymes display a superfamily of structurally different enzymes classified in at least nine subfamilies by biochemical and structural properties. N‐(p‐amylcinnamoyl)anthranilic acid commonly referred to as ACA is often used as a broad‐spectrum inhibitor for the characterization of phospholipase A₂–mediated pathways. Compounds like ACA and ACA‐like structures have been described to block the receptor‐induced release of arachidonic acid and subsequent signaling cascades in the pancreas and the cardiovascular system. We showed that ACA directly blocks several transient receptor potential (TRP) channels (TRPC6, TRPM2, TRP and TRPM8). With respect to the published data of ACA in the phospholipase A₂ field, the finding that ACA blocks diacylglycerol‐activated TRP channels is of specific interest as it offers the opportunity to interfere with receptor‐induced calcium‐dependent signaling processes in platelets and vascular smooth muscle cells. Overall, N‐phenylcinnamides, as a new pharmaceutical lead structure, form the first class of synthetic TRP channel blockers and represent a promising start for the development of small organic TRP channel–specific blockers.