Pathways through which glucose induces a rise in [Ca2+]i of polymorphonuclear leukocytes of rats

A Bittersweet Response to Infection in Diabetes; Targeting Neutrophils to Modify Inflammation and Improve Host Immunity

Chronic and recurrent infections occur commonly in both type 1 and type 2 diabetes (T1D, T2D) and increase patient morbidity and mortality. Neutrophils are professional phagocytes of the innate immune system that are critical in pathogen handling. Neutrophil responses to infection are dysregulated in diabetes, predominantly mediated by persistent hyperglycaemia; the chief biochemical abnormality in T1D and T2D. Therapeutically enhancing host immunity in diabetes to improve infection resolution is an expanding area of research. Individuals with diabetes are also at an increased risk of severe coronavirus disease 2019 (COVID-19), highlighting the need for re-invigorated and urgent focus on this field. The aim of this review is to explore the breadth of previous literature investigating neutrophil function in both T1D and T2D, in order to understand the complex neutrophil phenotype present in this disease and also to focus on the development of new therapies to improve aberrant neutrophil function in diabetes. Existing literature illustrates a dual neutrophil dysfunction in diabetes. Key pathogen handling mechanisms of neutrophil recruitment, chemotaxis, phagocytosis and intracellular reactive oxygen species (ROS) production are decreased in diabetes, weakening the immune response to infection. However, pro-inflammatory neutrophil pathways, mainly neutrophil extracellular trap (NET) formation, extracellular ROS generation and pro-inflammatory cytokine generation, are significantly upregulated, causing damage to the host and perpetuating inflammation. Reducing these proinflammatory outputs therapeutically is emerging as a credible strategy to improve infection resolution in diabetes, and also more recently COVID-19. Future research needs to drive forward the exploration of novel treatments to improve infection resolution in T1D and T2D to improve patient morbidity and mortality.

Dance Training Improves Cytokine Secretion and Viability of Neutrophils in Diabetic Patients

Background

Evidence suggests that exercise improves neutrophil function. The decreased functional longevity of neutrophils and their increased clearance from infectious sites contribute to the increased susceptibility to infection and severity of infection observed in patients with diabetes.

Objective

Herein, we investigated the effects of a dance program on neutrophil number, function, and death in type 2 diabetes mellitus (T2DM) patients and healthy volunteers.

Methods

Ten patients with T2DM and twelve healthy individuals participated in a moderate-intensity dance training program for 4 months. The plasma levels of leptin, free fatty acids (FFAs), tumour necrosis factor-α (TNF-α), C-reactive protein (CRP), interleukin-1β (IL-1β), and interleukin-1 receptor antagonist (IL-1ra); neutrophil counts; extent of DNA fragmentation; cell membrane integrity; and production of TNF-α, interleukin-8 (IL-8), interleukin-6 (IL-6), and IL-1β in neutrophils were measured before and after training.

Results

Training reduced plasma levels of TNF-α (1.9-fold in controls and 2.2-fold in patients with T2DM) and CRP (1.4-fold in controls and 3.4-fold in patients with T2DM). IL-1ra levels were higher in the control group (2.2-fold) after training. After training, neutrophil DNA fragmentation was decreased in patients with T2DM (90%), while the number of neutrophils increased (70% in controls and 1.1-fold in patients with T2DM).

Conclusion

Dance training is a nonpharmacological strategy to reduce inflammation and improve neutrophil clearance in patients with T2DM.

The role of cytokines in the functional activity of phagocytes in blood and colostrum of diabetic mothers

Immune response changes induced by diabetes are a risk factor for infections during pregnancy and may modify the development of the newborn's immune system. The present study analyzed colostrum and maternal and cord blood of diabetic women to determine (1) the levels of the cytokines IFN-γ and TGF-β and (2) phagocytic activity after incubation with cytokines. Methods. Colostrum and maternal and cord blood samples were classified into normoglycemic (N = 20) and diabetic (N = 19) groups. Cytokine levels, superoxide release, rate of phagocytosis, bactericidal activity, and intracellular Ca²⁺ release by phagocytes were analyzed in the samples. Irrespective of glycemic status, IFN-γ and TGF-β levels were not changed in colostrum and maternal and cord blood. In maternal blood and colostrum, superoxide release by cytokine-stimulated phagocytes was similar between the groups. Compared to spontaneous release, superoxide release was stimulated by IFN-γ and TGF-β in normoglycemic and diabetic groups. In the diabetic group, cord blood phagocytes incubated with IFN-γ exhibited higher phagocytic activity in response to EPEC, and maternal blood exhibited lower microbicidal activity. These data suggest that diabetes interferes in maternal immunological parameters and that IFN-γ and TGF-β modulate the functional activity of phagocytes in the colostrum, maternal blood, and cord blood of pregnant diabetic women.

Preserved function of the plasma membrane calcium pump of red blood cells from diabetic subjects with high levels of glycated haemoglobin

The activity of the plasma membrane Ca(2+)-pump decreases steeply throughout the 120 days lifespan of normal human red blood cells. Experiments with isolated membrane preparations showed that glycation of a lysine residue near the catalytic site of the pump ATPase had a powerful inhibitory effect. This prompted the question of whether glycation is the mechanism of age-related decline in pump activity in vivo. It is important to investigate this mechanism because the Ca(2+) pump is a major regulator of Ca(2+) homeostasis in all cells. Its impaired activity in diabetic patients, continuously exposed to high glycation rates, may thus contribute to varied tissue pathology in this disease. We measured Ca(2+)-pump activity as a function of red cell age in red cells from diabetics continuously exposed to high glucose concentrations, as documented by their high mean levels of glycated haemoglobin. The distribution of Ca(2+)-pump activities was indistinguishable from that in non-diabetics, and the pattern of activity decline with cell age in the diabetics' red cells was identical to that observed in red cells from non-diabetics. These results indicate that in intact cells the Ca(2+) pump is protected from glycation-induced inactivation.

Mechanisms and significance of cell volume regulation

Survival of human and animal cells requires avoidance of excessive alterations of cell volume. The osmolarity amassed by cellular accumulation of organic substances must be compensated by lowering cytosolic ion concentrations. The Na+/K+ ATPase extrudes Na+ in exchange for K+, which can permeate the cell membrane through K+ channels. K+ exit generates a cell-negative potential difference across the cell membrane, driving the exit of anions such as Cl-. The low cytosolic Cl- concentrations counterbalance the excess cellular osmolarity by organic substances. Cell volume regulation following cell swelling involves releasing ions through activation of K+ channels and/or anion channels, KCl-cotransport, or parallel activation of K+/H+ exchange and Cl-/HCO3- exchange. Cell volume regulation following cell shrinkage involves accumulation of ions through activation of Na+,K+,2Cl- cotransport, Na+/H+ exchange in parallel to Cl-/HCO3- exchange, or Na+ channels. The Na+ taken up is extruded by the Na+/K+ ATPase in exchange for K+. Shrunken cells further accumulate organic osmolytes such as sorbitol and glycerophosphorylcholine, and monomeric amino acids by altered metabolism and myoinositol (inositol), betaine, taurine, and amino acids by Na+ coupled transport. They release osmolytes during cell swelling. Challenges of cell volume homeostasis include transport, hormones, transmitters, and drugs. Moreover, alterations of cell volume participate in the machinery regulating cell proliferation and apoptotic cell death. Deranged cell volume regulation significantly contributes to the pathophysiology of several disorders such as liver insufficiency, diabetic ketoacidosis, hypercatabolism, fibrosing disease, sickle cell anemia, and infection.

Neutrophils and monocytes as potentially important sources of proinflammatory cytokines in diabetes

Neutrophils and monocytes play a central role in host defence. The invading leucocytes are capable of synthesizing and releasing a variety of proinflammatory mediators including cytokines. Given the importance of cytokines in the progression of chronic and acute inflammatory processes, we aimed to ascertain whether the release of interleukin (IL)-8, IL-1beta, tumour necrosis factor (TNF)-alpha and IL-1ra of neutrophils and monocytes was modified in diabetes. To this end, we measured the release of cytokines in suspensions of cell culture in basal and lipopolysaccharide (LPS)-stimulated conditions. In basal conditions, neutrophils of diabetics release 1.6, 3.2, 1.9 and 1.9-fold higher amounts of IL-8, IL-1beta, TNF-alpha and IL-1ra, respectively, than do healthy controls. Under our experimental conditions, this effect was more evident for neutrophils than for monocytes. Incremental cytokine production was also found to occur when neutrophils were stimulated with LPS. IL-8, IL-1beta and TNF-alpha increased, respectively, by 4.0, 1.7 and 2.8-fold. Although the effect was more marked for neutrophils, monocytes showed a tendency for increased cytokine production. The discovery of this increase in cytokines released by the neutrophils of diabetics contributes towards a clearer understanding of other deficiencies described for neutrophils in diabetes, such as the migration of neutrophils to inflammatory sites, phagocytes, release of lytic proteases, production of reactive oxygen species and apoptosis. The excessive production of cytokines may lead to inappropriate activation and tissue injury and even to increased susceptibility to invasive microorganisms. Thus, the increased responsiveness of neutrophils of diabetics demonstrated in this study may be considered part of the scenario of diabetes physiopathology.

High Glucose Up-Regulates ENaC and SGK1 Expression in HCD-Cells

BACKGROUND/AIM

Diabetic nephropathy is associated with progressive renal damage, leading to impaired function and end-stage renal failure. Secondary hypertension stems from a deranged ability of cells within the kidney to resolve and appropriately regulate sodium resorption in response to hyperglycaemia. However, the mechanisms by which glucose alters sodium re-uptake have not been fully characterised.

METHODS

Here we present RT-PCR, western blot and immunocytochemistry data confirming mRNA and protein expression of the serum and glucocorticoid inducible kinase (SGK1) and the alpha conducting subunit of the epithelial sodium channel (ENaC) in a model in vitro system of the human cortical collecting duct (HCD). We examined changes in expression of these elements in response to glucose challenge, designed to mimic hyperglycaemia associated with type 2 diabetes mellitus. Changes in Na+ concentration were assessed using single-cell microfluorimetry.

RESULTS

Incubation with glucose, the Ca2+-ionophore ionomycin and the cytokine TGF-beta1 were all found to evoke significant and time-dependent increases in both SGK1 and alphaENaC protein expression. These molecular changes were correlated to an increase in Na+-uptake at the single-cell level.

CONCLUSION

Together these data offer a potential explanation for glucose-evoked Na+-resorption and a potential contributory role of SGK1 and ENaCs in development of secondary hypertension, commonly linked to diabetic nephropathy.

Amlodipine reverses the elevation in [Ca2+]i and the impairment of phagocytosis in PMNLs of NIDDM patients

BACKGROUND

Patients with diabetes mellitus display an elevation in the basal levels of [Ca2+]i of polymorphonuclear leukocytes (PMNLs) and impaired phagocytosis. These derangements are due to the hyperglycemia of diabetes. Calcium channel blockers reverse these abnormalities both in in vitro studies and in diabetic rats. These observations suggest that calcium channel blockers may be useful in the treatment of patients with uncontrolled diabetes. The present study examined this issue.

METHODS

A total of 32 normal subjects and 36 patients with uncontrolled noninsulin-dependent diabetes mellitus (NIDDM) were studied with and without treatment with amlodipine both in a cross-sectional and longitudinal design approach.

RESULTS

In addition to the elevation in basal levels of [Ca2+]i and the impaired phagocytosis, there was also down-regulation of the mRNA of Fc gamma RIII receptors in the PMNLs of the diabetic patients. Treatment of the patients with a small dose of amlodipine (5 mg/day) corrected these abnormalities despite persistent hypoglycemia. This beneficial effect of nifedipine was noted as long as the therapy with the drug was maintained.

CONCLUSION

The results show that the elevation in [Ca2+]i of the PMNLs is associated with down-regulation of the mRNA of their Fc gamma RIII receptors, which is at least, in part, responsible for the impaired phagocytosis. These derangements in the metabolism and function of the PMNLs are most likely responsible for the increased susceptibility of the diabetic patients to infection. Calcium channel blockers may be a beneficial adjunct therapy in patients with uncontrolled diabetes.

Effect of ATP on Ca2+ uptake in the presence of high glucose in renal proximal tubule cells

1. Calcium regulation has been reported to be associated with the development of diabetic nephropathy. Thus, changes in Ca2+ uptake induced by ATP, an important regulator of Ca2+ uptake, in the diabetic condition and related signal pathways were examined in primary cultures of rabbit renal proximal tubule cells (PTC). 2. Under low (5 mmol/L) glucose conditions, 10-4 mol/L ATP inhibited Ca2+ uptake early on (< 30 min), whereas Ca2+ uptake was stimulated at later time points (> 2 h). However, under high (25 mmol/L) glucose conditions, ATP stimulated both the early and late uptake of Ca2+. 3. The adenylate cyclase inhibitor SQ 22536, the protein kinase (PK) A inhibitor PKI amide 14-22, Rp-cAMP, staurosporine, bisindolylmaleimide I and H-7 (PKC inhibitors) blocked the change in ATP effect on Ca2+ uptake in the presence of 25 mmol/L glucose. However, none one of these drugs blocked the effect of ATP on Ca2+ uptake in the presence of 5 mmol/L. 4. At 25 mmol/L, glucose increased cAMP content and PKC activity, whereas ATP had no effect on either parameter. 5. In conclusion, high glucose levels alter ATP-induced Ca2+ uptake via cAMP and PKC pathways in the PTC.

Admission hyperglycemia as a prognostic indicator in trauma

OBJECTIVE

The purpose of this study was to assess the utility of two levels of hyperglycemia as predictors for mortality and infectious morbidity in traumatically injured patients.

METHODS

All patients >or= 17 years old presenting to a Level I trauma center as a "trauma alert" or a "trauma code" from January 1, 2000, through December 31, 2000, were reviewed. Hypoglycemic patients (glucose concentration < 70 mg/dL) were excluded (n = 4). Patients were considered hyperglycemic with an admission glucose concentration > 200 mg/dL (moderate hyperglycemia) or an admission glucose concentration in the upper quartile for the group (mild hyperglycemia [glucose concentration > 135 mg/dL]).

RESULTS

Seven hundred thirty-eight patients were included in the study. Hyperglycemia was associated with increased mortality among both patients with moderate hyperglycemia (34.1% vs. 3.7%, p < 0.01) and those with mild hyperglycemia (15.5% vs. 2%, p < 0.01) compared with corresponding normoglycemic groups. Hyperglycemia proved to be an independent predictor of mortality and of hospital and intensive care unit length of stay after multiple logistic regression while controlling for age, Injury Severity Score, Revised Trauma Score, and gender. Infectious complications, including pneumonia (9.4% vs. 2%, p = 0.001), urinary tract infections (6.6% vs. 1.4%, p = 0.001), wound infections (4.9% vs. 0.6%, p = 0.039), and bacteremia (5% vs. 1.1%, p = 0.004), were significantly increased in patients with elevated glucose concentrations. Hyperglycemia is an independent predictor of increased infectious morbidity controlling for age, gender, and Injury Severity Score in multiple logistic regression models.

CONCLUSION

Hyperglycemia independently predicts increased intensive care unit and hospital length of stay and mortality in the trauma population. It is associated with increased infectious morbidity. These associations hold true for mild hyperglycemia (glucose concentration > 135 mg/dL) and moderate hyperglycemia (glucose concentration > 200 mg/dL).

Inhibition of interferon-gamma expression by osmotic shrinkage of peripheral blood lymphocytes

A hypertonic environment, as it prevails in renal medulla or in hyperosmolar states such as hyperglycemia of diabetes mellitus, has been shown to impair the immune response, thus facilitating the development of infection. The present experiments were performed to test whether hypertonicity influences activation of T lymphocytes. To this end, peripheral blood lymphocytes (PBL) of cytomegalovirus (CMV)-positive donors were stimulated by human leukocyte antigen (HLA)-A2-restricted CMV epitope NLVPMVATV to produce interferon (IFN)-gamma at varying extracellular osmolarity. As a result, increasing extracellular osmolarity during exposure to the CMV antigen indeed decreased IFN-gamma formation. Addition of NaCl was more effective than urea. A 50% inhibition was observed at 350 mosM by addition of NaCl. The combined application of the Ca(2+) ionophore ionomycin (1 microg/ml) and the phorbol ester phorbol 12-myristate 13-acetate (PMA; 5 microg/ml) stimulated IFN-gamma production, an effect again reversed by hyperosmolarity. Moreover, hyperosmolarity abrogated the stimulating effect of ionomycin (1 microg/ml) and PMA (5 microg/ml) on the transcription factors activator protein (AP)-1, nuclear factor of activated T cells (NFAT), and NF-kappaB but not Sp1. In conclusion, osmotic cell shrinkage blunts the stimulatory action of antigen exposure on IFN-gamma production, an effect explained at least partially by suppression of transcription factor activation.

Impairment of type III group B Streptococcus-stimulated superoxide production and opsonophagocytosis by neutrophils in diabetes

The effect of hyperglycemia upon susceptibility to bacterial infection in diabetes mellitus is incompletely elucidated. The present experiments assessed the effect of hyperglycemia upon neutrophil-mediated phagocytosis of type III group B Streptococcus (GBS). Type III GBS was chosen for study because the incidence of invasive GBS disease is substantially increased in type 2 diabetic compared with nondiabetic subjects. The hypothesis tested was that severe hyperglycemia would alter neutrophil metabolism by diverting NADPH from superoxide production into the aldose reductase-dependent polyol pathway that converts glucose into sorbitol and thus would impair opsonophagocytosis (OP) of type III GBS. Neutrophils from 10 adults with type 2 diabetes had no intrinsic phagocytic defect under baseline glycemic conditions. After equilibration in 60 or 120 mM glucose or in 60 mM choline chloride, OP activity was reduced significantly (P < or = 0.03). Neutrophil superoxide production correlated with glucose concentration and also was significantly reduced during hyperglycemia (P < 0.05). Addition of III GBS capsular polysaccharide-specific IgG in a sufficient concentration supported efficient OP, even during hyperglycemia. Alrestatin, an aldose reductase inhibitor, increased superoxide production and significantly improved OP of type III GBS (P = 0.03). Thus, diversion of NADPH into the polyol pathway is one mechanism by which OP of GBS III is impaired during hyperglycemia, and this effect is mitigated when levels of capsular polysaccharide-specific IgG are sufficient.

Urea signaling to ERK phosphorylation in renal medullary cells requires extracellular calcium but not calcium entry

The renal cell line mIMCD3 exhibits markedly upregulated phosphorylation of the extracellular signal-regulated kinase (ERK) 1 and 2 in response to urea treatment (200 mM for 5 min). Previous data have suggested the involvement of a classical protein kinase C (cPKC)-dependent pathway in downstream events related to urea signaling. We now show that urea-inducible ERK activation requires extracellular calcium; unexpectedly, it occurs independently of activation of cPKC isoforms. Pharmacological inhibitors of known intracellular calcium release pathways and extracellular calcium entry pathways fail to inhibit ERK activation by urea. Fura 2 ratiometry was used to assess the effect of urea treatment on intracellular calcium mobilization. In single-cell analyses using subconfluent monolayers and in population-wide analyses using both confluent monolayers and cells in suspension, urea failed to increase intracellular calcium concentration. Taken together, these data indicate that urea-inducible ERK activation requires calcium action but not calcium entry. Although direct evidence is lacking, one possible explanation could include involvement of a calcium-dependent extracellular moiety of a cell surface-associated protein.

Deranged transcriptional regulation of cell-volume-sensitive kinase hSGK in diabetic nephropathy

Transforming growth factor beta (TGF-beta) has been shown to participate in the pathophysiology of diabetic complications. As shown most recently, TGF-beta stimulates the expression of a distinct serine/threonine kinase (hSGK) which had previously been cloned as an early gene transcriptionally regulated by cell volume alterations. The present study was performed to elucidate transcription and function of hSGK in diabetic nephropathy. As shown by Northern blotting, an increase of extracellular glucose concentration increased hSGK mRNA levels in cultured cells, an effect qualitatively mimicked by osmotic cell shrinkage or treatment with TGF-beta (2 microgram/liter), phorbol 12,13-didecanoate (1 microM), or the Ca(2+) ionophore ionomycin (1 microM) and blunted by high concentrations of nifedipine (10 and 100 microM). In situ hybridization revealed that hSGK transcription was markedly enhanced in diabetic nephropathy, with particularly high expression in mesangial cells, interstitial cells, and cells in thick ascending limbs of Henle's loop and distal tubules. According to voltage clamp and tracer flux studies in Xenopus oocytes expressing the renal epithelial Na(+) channel ENaC or the mouse thick ascending limb Na(+),K(+),2Cl(-) cotransporter BSC-1, coexpression with hSGK stimulated ENaC and BSC-1 11-fold and 6-fold, respectively, effects reversed by kinase inhibitors staurosporine (1 microM) and chelerythrine (1 microM) and not elicited by inactive hSGK. In conclusion, excessive extracellular glucose concentrations enhance hSGK transcription, which in turn stimulates renal tubular Na(+) transport. These observations disclose an additional element in the pathophysiology of diabetic nephropathy.

Rotavirus infection induces an increase in intracellular calcium concentration in human intestinal epithelial cells: role in microvillar actin alteration

Rotaviruses, which infect mature enterocytes of the small intestine, are recognized as the most important cause of viral gastroenteritis in young children. We have previously reported that rotavirus infection induces microvillar F-actin disassembly in human intestinal epithelial Caco-2 cells (N. Jourdan, J. P. Brunet, C. Sapin, A. Blais, J. Cotte-Laffitte, F. Forestier, A. M. Quero, G. Trugnan, and A. L. Servin, J. Virol. 72:7228-7236, 1998). In this study, to determine the mechanism responsible for rotavirus-induced F-actin alteration, we investigated the effect of infection on intracellular calcium concentration ([Ca(2+)](i)) in Caco-2 cells, since Ca(2+) is known to be a determinant factor for actin cytoskeleton regulation. As measured by quin2 fluorescence, viral replication induced a progressive increase in [Ca(2+)](i) from 7 h postinfection, which was shown to be necessary and sufficient for microvillar F-actin disassembly. During the first hours of infection, the increase in [Ca(2+)](i) was related only to an increase in Ca(2+) permeability of plasmalemma. At a late stage of infection, [Ca(2+)](i) elevation was due to both extracellular Ca(2+) influx and Ca(2+) release from the intracellular organelles, mainly the endoplasmic reticulum (ER). We noted that at this time the [Ca(2+)](i) increase was partially related to a phospholipase C (PLC)-dependent mechanism, which probably explains the Ca(2+) release from the ER. We also demonstrated for the first time that viral proteins or peptides, released into culture supernatants of rotavirus-infected Caco-2 cells, induced a transient increase in [Ca(2+)](i) of uninfected Caco-2 cells, by a PLC-dependent efflux of Ca(2+) from the ER and by extracellular Ca(2+) influx. These supernatants induced a Ca(2+)-dependent microvillar F-actin alteration in uninfected Caco-2 cells, thus participating in rotavirus pathogenesis.

Effect of diabetes on calcium/calmodulin dependent protein kinase-II from rat brain

Changes in the protein levels and activity of Ca2+/Calmodulin dependent protein kinase II (CaM kinase II) level were studied in cytosolic and particulate fractions from cerebral hemisphere, cerebellum, brain stem, thalamus and hypothalamus regions of rat brain after 4 and 12 weeks of induction of diabetes. Streptozotocin induced diabetes, resulted in pronounced increase of CaM kinase II activity as determined by the kinase activity assay. The total amount of enzyme protein (alpha-subunit specific) also showed increase as revealed by western blotting. Parallel studies were also made in age matched control rats and insulin treated diabetic rats. The increase in CaM kinase II activity was more pronounced in the 12 weeks diabetic group. Insulin treatment of diabetic rats, resulted in recovery of enzyme activity near to control values from majority of the brain regions studied. The expression of alpha-subunit specific CaM kinase II correlates with the enzyme activity in the diabetic rat brain.

Effect of thyroparathyroidectomy on urinary acidification in diabetic rats

In previous studies we have shown stimulation of renal acid excretion in the proximal tubules of rats with diabetes of short duration, with no important alterations in glomerular hemodynamics; on the other hand, in thyroparathyroidectomized rats (TPTX model), a significant decrease in renal acid excretion, glomerular filtration rate (GFR) and renal plasma flow (RPF) was detected. Since important changes in the parathyroid hormone-vitamin D-Ca axis are observed in the diabetic state, the present study was undertaken to investigate the renal repercussions of thyroparathyroidectomy in rats previously made diabetic by streptozotocin (45 mg/kg). Four to 6 days after the induction of diabetes (DM), a group of rats were thyroparathyroidectomized (DM + TPTX). Renal functional parameters were evaluated by measuring the inulin and sodium para-aminohippurate clearance on the tenth day. The decrease in the GFR and RPF observed in TPTX was not reversed by diabetes since the same alterations were observed in DM + TPTX. Net acid (NA) excretion was unchanged in DM (6.19 +/- 0.54), decreased in TPTX (3.76 +/- 0.25) and returned to normal levels in DM + TPTX (5.54 +/- 0.72) when compared to the control group (6.34 +/- 0.14 mumol min-1 kg-1). The results suggest that PTH plays an important vasodilator role regarding glomerular hemodynamics, since in its absence the impairment in GFR and RPF was not reversed by the diabetic state. However, with respect to acid excretion, the presence of diabetes was able to overcome the negative stimulus represented by TPTX.

Mechanisms through which high glucose concentration raises [Ca2+]i in renal proximal tubular cells

BACKGROUND

The basal levels of cytosolic calcium ([Ca2+]i) of renal proximal tubular cells of rats with streptozotocin-induced diabetes are elevated. It is possible that this phenomenon is mediated by the hyperglycemia, which may cause both increased calcium influx into and/or decreased calcium efflux out of these cells.

METHODS

We examined whether high glucose concentration in vitro causes acute rise in [Ca2+]i of freshly isolated renal proximal tubular cells and explored the pathways that are involved in such an event.

RESULTS

There were dose and time dependent increments in [Ca2+]i of renal proximal tubular cells exposed to high concentrations of glucose. A similar effect was observed with equimolar concentrations of mannitol or choline chloride but not urea. A substantial part of the rise in [Ca2+]i was inhibited when the media contained verapamil, nifedipine, amlodipine or ryanodine and when the cells were placed in a calcium free media. Inhibitors of G protein(s) (GDPbetaS or pertussis toxin), inhibitors of cAMP-protein kinase A pathway (RpcAMP or H-89), inhibitors of protein kinase C (staurosporine or calphostin) and inhibitor of Na+-H+ exchanger (HOE 694) blocked the rise in a dose dependent manner. High glucose concentration also caused a decrease in ATP content of these cells and a reduction in the Vmax of their Ca2+ATPase.

CONCLUSIONS

The results are consistent with the formulation that the osmotic activity (cell shrinkage) of the high glucose concentration may activate a stretch receptor with subsequent stimulation of various cellular pathways including G protein(s), cAMP-protein kinase A and phospholipase C systems and calcium channels. Activation of these cellular pathways permits both calcium influx into renal tubular cells and mobilization of calcium from their intracellular stores. Further, a decrease in calcium efflux secondary to the reduction in the Vmax of Ca2+ ATPase may occur. It is possible that the rise in [Ca2+]i is critical for the stimulation of the events that lead to restoration of cell volume to normal.

High glucose concentration causes a rise in [Ca2+]i of cardiac myocytes

Diabetes mellitus is associated with an elevation in the basal levels of cytosolic calcium ([Ca2+]i) of cardiac myocytes. This may be due in part to a glucose-induced elevation in [Ca2+]i. The present study examined this issue and explored the cellular pathways responsible for such a phenomenon. A total of 30 mM glucose, mannitol or choline chloride, but not urea, induced a time- and dose-dependent rise in the [Ca2+]i of cardiac myocytes. G protein inhibition by GDP beta S or pertussis toxin produced significant inhibition (> or = 80%) in the rise in [Ca2+]i. Incubation of cardiac myocytes in a calcium free medium or in media containing verapamil, nifedipine or amlodipine almost completely abolished the rise in [CA2+], while ryanodine produced only small reduction (10%) in the glucose-induced rise in [Ca2+]i. Rp-cAMP or H-89, inhibitors of the cAMP-protein kinase A pathway, produced a modest decrease in the rise in [Ca2+]i, while staurosporine (an inhibitor of PKC) and HOE 694 (an inhibitor of the Na(+)-H+ exchanger) had no effect on the rise in [Ca2+]i. The results indicate that the osmotic activity of glucose (cell shrinkage) activates G protein(s), most likely through a stretch receptor, which in turn stimulates calcium channels inhibitable by verapamil, nifedipine and amlodipine, thus permitting a calcium influx into the cardiac myocytes. The increased calcium entry may stimulate a calcium release from intracellular stores by a calcium-induced calcium release process. Thus, in cardiac myocytes direct activation of calcium channels, and to a small extent activation of the cAMP-protein kinase A, and calcium-induced calcium release mediate the high glucose-induced acute rise in their [Ca2+]i.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Effect of osmolarity on LDL binding and internalization in hepatocytes

The present study has been performed to elucidate a possible role of cell volume in low-density lipoprotein (LDL) binding and internalization (LDL(b+i)). As shown previously, increase of extracellular osmolarity (OSMe) and K+ depletion, both known to shrink cells, interfere with the formation of clathrin-coated pits and thus with LDL(b+i). On the other hand, alterations of cell volume have been shown to modify lysosomal pH, which is a determinant of LDL(b+i). LDL(b+i) have been estimated from heparin-releasable (binding) or heparin-insensitive (internalization) uptake of 125I-labeled LDL. OSMe was modified by alterations of extracellular concentrations of ions, glucose, urea, or raffinose. When OSMe was altered by varying NaCl concentrations, LDL(b+i) decreased (by 0.5 +/- 0.1%/mM) with increasing OSMe and LDL(b+i) increased (by 1.2 +/- 0.1%/mM) with decreasing OSMe, an effect mainly due to altered affinity; the estimated dissociation constant amounted to 20.6, 48.6, and 131.6 micro/ml at 219, 293, and 435 mosM, respectively. A 25% increase of OSMe increased cytosolic (by 0.46 +/- 0.03) and decreased lysosomal (by 0.14 +/- 0.02) pH. Conversely, a 25% decrease of OSMe decreased cytosolic (by 0.28 +/- 0.02) and increased lysosomal (by 0.17 +/- 0.02) pH. Partial replacement of extracellular Na+ with K+ had little effect on LDL(b+i), although it swelled hepatocytes and increased lysosomal and cytosolic pH. Hypertonic glucose, urea, or raffinose did not exert similar effects despite a shrinking effect of hypertonic raffinose. Monensin, which completely dissipates lysosomal acidity, virtually abolished LDL(b+i). In conclusion, the observations reveal a significant effect of ionic strength on LDL(b+i). The effect is, however, not likely to be mediated by alterations of cell volume or alterations of lysosomal pH.

Elevated cytosolic calcium and impaired proliferation of B lymphocytes in type II diabetes mellitus

Patients with diabetes mellitus have increased susceptibility to infection attributable, at least in part, to defective function of polymorphonuclear leukocytes (PMNLs) and B cells. Certain data suggest that cytosolic calcium ([Ca2+]i) is elevated in various cells in diabetes, and high [Ca2+]i adversely affects cell function. Indeed, the [Ca2+]i of PMNLs of diabetic patients is elevated, and phagocytosis of the PMNLs is impaired. The current study examines whether the [Ca2+]i of B cells is also elevated in diabetes and whether this derangement impairs B cell function. We studied 32 patients with non-insulin-dependent diabetes mellitus (NIDDM) and eight normal subjects. All patients had hyperglycemia (11.6 +/- 0.80 mmol/L) and elevated HbA1c (13.2% +/- 0.99%). The basal levels of [Ca2+]i of the B cells (113 +/- 3.3 nmol/L) were significantly (P < 0.01) higher than the values in normal subjects (85 +/- 1.7 nmol/L). There was a direct and significant correlation (r = 0.88; P < 0.01) between the [Ca2+]i of B cells and the blood levels of glucose. Proliferation of B cells in response to Staphylococcus aureus Cowan I (SAC) was significantly impaired in these patients (7.3 +/- 0.48 x 10(3) cpm v 12.5 +/- 0.61 x 10(3) cpm in normal subjects). Normalization of blood glucose with the hypoglycemic agents, glyburide, was associated with the return of both [Ca2+]i of B cells and their proliferation in response to SAC to normal. The results show that hyperglycemia in type II diabetes mellitus is associated with a significant increase in [Ca2+]i of B cells and with a decrease in their proliferation in response to mitogen. These derangements are reversed after the correction of the hyperglycemia. The data of the current study and those previously reported in PMNLs provide for a new pathogenetic process underlying the dysfunction of these cells in diabetes mellitus.

Elevation of [Ca2+]i of renal proximal tubular cells and down-regulation of mRNA of PTH-PTHrP, V1a and AT1 receptors in kidney of diabetic rats

An elevation in intracellular calcium ([Ca2+]i) in rats with chronic renal failure and elevated blood levels of PTH is associated with down-regulation of the mRNA of many proteins. Similarly, in phosphate depleted animals that have normal renal function and low blood levels of PTH, [Ca2+]i is elevated and the mRNA of PTH-PTHrP receptor is down-regulated. The effect of elevation in [Ca2+]i on molecular machinery of many proteins may represent a generalized phenomenon. Diabetes mellitus may also be associated with a rise in [Ca2+]i and therefore down-regulation of the mRNA of proteins may also occur. The present study examined the effect of streptozotocin-induced diabetes mellitus in rats on the [Ca2+]i of the renal proximal tubular cells and on their mRNAs of the PTH-PTHrP, V1a and AT1 receptors. The basal levels of [Ca2+]i of these cells increased significantly (P < 0.01) after one day of diabetes and remained elevated thereafter. There was a significant (r = 0.67, P < 0.01) direct correlation between the [Ca2+]i of the cells and blood levels of glucose up to 350 mg/dl, and the value of [Ca2+]i plateaued with higher concentrations of glucose. Three days of amlodipine therapy prevented and reversed the elevated levels of [Ca2+]i despite marked hyperglycemia. The mRNA of all three receptors in the kidney were down-regulated and this defect was prevented by amlodipine which normalized the [Ca2+]i of the cells. The results show that: (1) the hyperglycemia of IDDM in rats causes a significant elevation in the basal levels of [Ca2+]i of the renal proximal tubular cells and down-regulation of their mRNA of PTH-PTHrP, V1a and AT1 receptors; (2) normalization of the [Ca2+]i of these cells by treatment of the diabetic rats with amlodipine prevented the elevation of [Ca2+]i and the down-regulation of the mRNA of these receptors; (3) these effects occurred in the presence of normal renal function and normal blood of PTH and phosphorus.