The effect of in vivo glucose administration on human erythrocyte Ca2+-ATPase activity and on enzyme responsiveness in vitro to thyroid hormone and calmodulin

The influence of different glucose tolerance on QTc interval: a population-based study

BACKGROUND

Corrected QT (QTc) interval has been reported to be associated with type 2 diabetes. This study aimed to explore the relationship between different glucose tolerance and QTc intervals among middle-aged and older Chinese individuals.

METHODS

We conducted a cross-sectional analysis that included 9898 subjects (3194 men and 6704 women) in a Chinese population. Glucose tolerance was studied during the oral glucose tolerance test (OGTT). Insulin, blood pressure, hemoglobin A1c (HbA1c), serum lipids, hepatic transaminases and waist-to-hip ratio were assessed. The QTc interval was derived from ECG recordings, and the subjects were stratified based on different glucose tolerance.

RESULTS

QTc interval levels were increased significantly in the subjects with abnormal glucose metabolism compared with the normal glucose regulation group. Multiple regression analyses showed that the QTc interval was significantly associated with fasting plasma glucose, 2-h OGTT plasma glucose and HbA1c. The odds ratio of prolonged QTc was 1.396 for impaired glucose regulation (IFG)/impaired fasting glucose (IGT) (95% CI 0.126-1.730), and 1.342 for type 2 diabetes (95% CI 0.142-1.577) after all potential confounders were adjusted.

CONCLUSIONS

Impaired glucose tolerance (IGR) and diabetes are associated with prolonged QTc intervals among middle-aged and older Chinese individuals. Abnormal glucose regulation can be used to monitor the QTc interval in the population.

Alterations in erythrocyte membrane transporter expression levels in type 2 diabetic patients

Type 2 diabetes mellitus (T2DM) is one of the most common multifactorial diseases and several membrane transporters are involved in its development, complications and treatment. We have recently developed a flow-cytometry assay panel for the quantitative determination of red cell membrane protein levels with potential relevance in diseases. Here we report a detailed phenotypic analysis of a medium scale, clinically based study on the expression of T2DM-related membrane proteins, the GLUT1, GLUT3, MCT1, URAT1, ABCA1, ABCG2 and the PMCA4 transporters in erythrocytes. By comparing age-matched control subjects and three groups of T2DM patients (recently diagnosed, successfully managed, and patients with disease-related complications), we found significant differences in the membrane expression levels of the transporters in these groups. This is a first detailed analysis of T2DM related alterations in erythrocyte membrane transporter protein levels, and the results suggest significant changes in some of the transporter expression levels in various patient groups. By performing a further, more detailed analysis of the clinical and molecular biology parameters, these data may serve as a basis of establishing new, personalized diagnostic markers helping the prevention and treatment of type 2 diabetes.

The Effect of Glucose Variability on QTc Duration and Dispersion in Patients with Type 2 Diabetes Mellitus

Objective:

Glycemic variability (GV) is a new term with the episodes of hyper and hypoglycemia in diabetic patients. Both prolonged QT interval and QTd are potential risk factors for malignant ventricular arrhythmias affecting the mortality of different groups of patients including diabetes mellitus. In this study, we aimed to evaluate if the glucose variability increasing the QTc interval and QTc dispersion in type 2 diabetes mellitus.

Methods:

We included 275 consecutive patients with type 2 diabetes. We quantified the GV with standard deviation (SD) and coefficient of variation (CV) from 7 point glucose measures. We investigated the relationship of GV parameters with QT parameters.

Results:

The prevalence of prolonged QTc duration was 21%, no patients have prolonged QTc dispersion (> 80 ms). SD of the patients with prolonged QTc duration was significantly higher than the others (45.14 ±24.45 vs. 37.78 ±9.03 p<0.05). There was also a significant relationship between SD and QTc dispersion (r: 0.164; p: 0.007). There were no relationship between the QT parameters and microvascular diabetic complications. SD and HbA1c levels were significantly higher on the patients having peripheral neuropathy (p<0.005).

Conclusion:

The result of this study demonstratess that increased glycemic variability is associated with prolonged QTc duration and QTc dispersion. It is important to focus on targeting optimal glycemic control with GV as an additional goal point along with the traditional following parameters such as fasting-postprandial blood glucose and HbA1c.

Prevalence and risk factors for prolonged QT interval and QT dispersion in patients with type 2 diabetes

AIMS

Prolonged QT interval is associated with cardiac arrhythmias and sudden death. The present study determined the prevalence of prolonged QT interval and QT dispersion and defined their clinical and metabolic predictors in patients with type 2 diabetes.

METHODS

Cross-sectional study included 501 patients with type 2 diabetes. A standard 12-lead electrocardiogram was recorded. QT corrected for heart rate (QTc) >440 ms and QT dispersion (QTd) >80 ms were considered abnormally prolonged. QTc ≥ 500 ms was considered a high-risk QTc prolongation. Demographic, clinical and laboratory data were collected. Independent risk factors for prolonged QTc and QTd were assessed using logistic regression analysis.

RESULTS

Prevalence of QTc > 440 ms and QTd > 80 ms were 44.1 and 3.6 %, respectively. Prevalence of high-risk QTc (≥500 ms) was 2 % only. Independent risk factors for QTc prolongation >440 ms were mean blood glucose (β = 2.192, p < 0.001), treatment with sulphonylurea (β = 5.198, p = 0.027), female gender (β = 8.844, p < 0.001), and coronary heart disease (β = 8.636, p = 0.001). Independent risk factors for QTc ≥ 500 ms were coronary heart disease (β = 4.134, p < 0.001) and mean blood glucose level (β = 1.735, p < 0.001). The independent risk factor for prolonged QTd was only coronary heart disease (β = 5.354, p < 0.001).

CONCLUSIONS

Although the prevalence of prolonged QTc > 440 ms is significant, the prevalence of high-risk QTc (≥500 ms) and QTd > 80 ms is very low in patients with type 2 diabetes. Hyperglycaemia and coronary heart disease are strong predictors of high-risk QTc.

Effect of (-)epicatechin in modulating calicum-atpase activity in normal and diabetic human erythrocytes

Aqueous extract of the wood ofPterocarpus marsupium, commonly known as 'bijasar', is used in Indian Ayurveda system of medicine for treatment of diabetes. The active anti-diabetic principle in its aqueous extract has been found to be (-)epicatechin, a flavonoid. The present work was undertaken to study the effect of (-)epicatechin on erythrocyte membrane Ca(++)-ATPase from type 2 diabetic patients. The activity of erythrocyte Ca(++)-ATPase was significantly lower in type 2 diabetics.In vitro insulin treatment of erythrocyte ghosts, resulted in the increase of Ca(++)-ATPase activity in diabetic patients. Treatment with (-)epicatechin (1mM) resulted in an increase in the activity of erythrocyte Ca(++)-ATPase in both normal individuals and type 2 diabetic patients. The insulin like effect of (-)epicatechin on erythrocyte membrane Ca(++)-ATPase in type 2 diabetics is an interesting finding. Further work is needed to elucidate the mechanism of action of (-)epicatechin on modulation of erythrocyte membrane bound enzymes.

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.

Acute hyperglycaemia disturbs cardiac repolarization in Type 1 diabetes

AIMS

Patients with Type 1 diabetes have an increased risk of cardiovascular mortality. Notably, a prolonged heart rate adjusted QT interval (QTc) is a predictor of sudden cardiovascular death. Therefore, the objectives of this study were to investigate whether acute hyperglycaemia affects the QTc duration and the QTc dispersion in patients with Type 1 diabetes and in healthy volunteers.

METHODS

Acute hyperglycaemia (15 mmol/l) for 120 min was induced in 35 males (22 men with Type 1 diabetes and 13 age-matched non-diabetic volunteers). All participants were non-smokers without any diabetic complications. Electrocardiogram recordings were performed at normoglycaemia and at 0, 60 and 120 min of hyperglycaemia.

RESULTS

Compared with normoglycaemia, acute hyperglycaemia increased the QTc interval in both patients with Type 1 diabetes (390 +/- 6 vs. 415 +/- 5 ms, P < 0.001) and in healthy volunteers (378 +/- 5 vs. 412 +/- 8 ms, P < 0.01). During hyperglycaemia, the QTc dispersion was prolonged in healthy volunteers (36 +/- 4 ms vs. 54 +/- 7 ms, P < 0.05) but not in patients with Type 1 diabetes (45 +/- 3 ms at baseline vs. 48 +/- 5 ms, NS).

CONCLUSIONS

Acute hyperglycaemia alters myocardial ventricular repolarization in patients with Type 1 diabetes and in healthy volunteers and might consequently be an additional risk factor for cardiovascular events.

Normal Ca2+ extrusion by the Ca2+ pump of intact red blood cells exposed to high glucose concentrations

The ATPase activity of the plasma membrane Ca2+ pump (PMCA) has been reported to be inhibited by exposure of red blood cell (RBC) PMCA preparations to high glucose concentrations. It has been claimed that this effect could have potential pathophysiological relevance in diabetes. To ascertain whether high glucose levels also affect PMCA transport function in intact RBCs, Ca2+ extrusion by the Ca2+-saturated pump [PMCA maximal velocity (V(max))] was measured in human and rat RBCs exposed to high glucose in vivo or in vitro. Preincubation of normal human RBCs in 30-100 mM glucose for up to 6 h had no effect on PMCA V(max). The mean V(max) of RBCs from 15 diabetic subjects of 12.9 +/- 0.7 mmol. 340 g Hb(-1). h(-1) was not significantly different from that of controls (14.3 +/- 0.5 mmol. 340 g Hb(-1). h(-1)). Similarly, the PMCA V(max) of RBCs from 11 streptozotocin-diabetic rats was not affected by plasma glucose levels more than three times normal for 6-8 wk. Thus exposure to high glucose concentrations does not affect the ability of intact RBCs to extrude Ca2+.

Effects of streptozotocin-induced diabetes and pentylenetetrazol-induced seizure on brain cortex (Ca2+)ATPase activity in rats

The aim of the present study was to obtain information about the effects of pentylenetetrazol-induced status epilepticus (SE) and streptozotocin-induced diabetes on brain cortex Ca(2+)ATPase activity. Treatment with pentylenetetrazol (PTZ) and streptozotocin (STZ) to rats resulted in significant decrease in brain cortex Ca(2+)ATPase activity as compared with controls. However, PTZ-treated diabetic rats had a slight but non-significant decrease in enzyme activity. Treatment with PTZ caused a more pronounced effect in inhibiting enzyme activity than that of treatment with STZ. Our results concluded that reduced brain cortex Ca(2+)ATPase activity following PTZ and STZ treatments to rats, may be an initial biochemical lesion which triggers a sequence of events which may culminate in cell death.

Effect of glucose and deoxyglucose on the redistribution of calcium in ehrlich ascites tumour and Zajdela hepatoma cells and its consequences for mitochondrial energetics. Further arguments for the role of Ca(2+) in the mechanism of the crabtree effect

The distribution of Ca(2+) in intact cells was monitored with fluorescent probes: fura-2 for cytosolic [Ca(2+)] and rhod-2 for mitochondrial [Ca(2+)]. It was found that in neoplastic cells, such as Ehrlich ascites tumour and Zajdela hepatoma, but not in non-malignant cells, such as fibroblasts, glucose and deoxyglucose elicited release of Ca(2+) from endoplasmic reticulum stores and an increase in Ca(2+) concentration in the cytosol. Parallel to this, a decrease in the rate of Ca(2+) extrusion from the cell and an enhanced uptake of Ca(2+) by mitochondria were observed. The increase in mitochondrial [Ca(2+)] was accompanied by an increase in the mitochondrial membrane potential and the reduction state of nicotinamide nucleotides. F(1)F(o)-ATPase in submitochondrial particles of Zajdela hepatoma was strongly inhibited in the presence of micromolar Ca(2+) concentrations, whereas this activity in submitochondrial particles from rat liver appeared to be less sensitive to Ca(2+). Indications of glycosylation of Ehrlich ascites tumour cell proteins were also obtained. These data strengthen the proposal [Bogucka, K., Teplova, V.V., Wojtczak, L. and Evtodienko, Y. V. (1995) Biochim. Biophys. Acta 1228, 261-266] that the Crabtree effect is produced by mobilization of cell calcium, which is subsequently taken up by mitochondria and inhibits F(1)F(o)-ATP synthase.

Analysis of phosphorylation and mutation of tyrosine residues of calmodulin on its activation of the erythrocyte Ca(2+)-transporting ATPase

The role played by the phosphorylation sites of calmodulin on its ability to activate the human erythrocyte Ca(2+)-transporting ATPase (Ca(2+)-ATPase) was evaluated. Phosphorylation of mammalian calmodulin on serine/threonine residues by casein kinase II decreased its affinity for Ca(2+)-ATPase by twofold. In contrast, tyrosine phosphorylation of mammalian calmodulin by the insulin-receptor kinase did not significantly alter calmodulin-stimulated Ca(2+)-ATPase activity. Two variant calmodulins, each containing only one tyrosine residue (the second Tyr is replaced by Phe) were also examined: [F138]calmodulin, a mutant containing tyrosine at position 99, and wheat germ calmodulin which has tyrosine at position 139. The concentrations of [F138]calmodulin and wheat germ calmodulin required for half-maximal activation of Ca(2+)-ATPase were tenfold and fourfold higher, respectively, than mammalian calmodulin. Phosphorylation at Tyr99 of [F138]calmodulin shifted its affinity for Ca(2+)-ATPase towards that of mammalian calmodulin. However, phosphorylation at Tyr139 of wheat germ calmodulin had essentially no effect on its interaction with Ca(2+)-ATPase. Thus, all of the observed effects of both phosphorylation and substitution of residues of calmodulin are on its affinity for Ca(2+)-ATPase, not on Vmax. The effects are dependent on the site of phosphate incorporation. Replacement of tyrosine with phenylalanine has a larger effect than phosphorylation of tyrosine, suggesting that the observed functional alterations reflect a secondary conformational change in the C-terminal half of calmodulin, the region that is important in its activation of Ca(2+)-ATPase.

Effect of pH upon Ca(2+)-ATPase activity of rat pancreatic islets: its possible contribution to the inhibitory effect of different insulin secretagogues

This work was undertaken in an attempt to elucidate the possible mechanism by which insulin secretagogues produce a fast and transient drop in the Ca(2+)-ATPase activity of the pancreatic islet membrane. For this purpose, the enzyme activity was measured in either homogenates or partially purified membranes of islets previously incubated under different experimental conditions. Ca(2+)-ATPase activity measured in homogenates of islets preincubated with 8 mM glucose decreased significantly compared to control islets incubated with 2.8 mM glucose. The inhibition was also observed when the enzyme activity was measured in homogenates of islets preincubated with 2.8 mM glucose plus 20 mM propionic acid as well as with glucose 2.8 mM in a buffer equilibrated with a gas mixture of O2 and either 12% or 30% CO2. Ca(2+)-ATPase activity decreased significantly in partially purified islet membranes preincubated for 3 min with glucose (2 and 8 mM), 15 mM KCl and 2 mM tolbutamide. These substances did not affect the Ca(2+)-ATPase activity when added directly to the enzyme assay medium. The enzyme activity also decreased when measured in membranes preincubated at pH 6.5. The addition of 1 mM ATP to the preincubation medium protected the Ca(2+)-ATPase activity from the inhibition induced by glucose, KCl and tolbutamide as well as from the one produced by acidic pH in the medium. On account of these results, we suggest that insulin secretagogues, as well as either acidification of B-cell cytosol or islet membrane incubation medium, produce changes at the islet membrane level which promote a decrease in the Ca(2+)-ATPase activity. A shift of the E1-E2 equilibrium of the phosphoenzyme towards E1 may account for such decreased activity. Changes in Ca(2+)-ATPase activity could either favour the decrease or the increase in the cytosolic concentration of Ca2+ in B-cells. Therefore, negative and positive modulation of its activity might allow Ca(2+)-ATPase to play a role in the switch-on and -off mechanism for intracellular Ca2+ signal regulation of B-cell secretion of insulin.

Glucose-induced alterations of cytosolic free calcium in cultured rat tail artery vascular smooth muscle cells

We have previously suggested that hyperglycemia per se may contribute to diabetic hypertensive and vascular disease by altering cellular ion content. To more directly investigate the potential role of glucose in this process, we measured cytosolic free calcium in primary cultures of vascular smooth muscle cells isolated from Sprague-Dawley rat tail artery before and after incubation with 5 (basal), 10, 15, and 20 mM glucose. Glucose significantly elevated cytosolic free calcium in a dose- and time-dependent manner, from 110.0 +/- 5.4 to 124.5 +/- 9.0, 192.7 +/- 20.4, and 228.4 +/- 21.9 nM at 5, 10, 15, and 20 mM glucose concentrations, respectively. This glucose-induced cytosolic free calcium elevation was also specific, no change being observed after incubation with equivalent concentrations of L-glucose or mannitol. This glucose effect was also dependent on extracellular calcium and pH, since these calcium changes were inhibited in an acidotic or a calcium-free medium, or by the competitive calcium antagonist lanthanum. We conclude that ambient glucose concentrations within clinically observed limits may alter cellular calcium ion homeostasis in vascular smooth muscle cells. We suggest that these cellular ionic effects of hyperglycemia may underlie the predisposition to hypertension and vascular diseases among diabetic subjects and/or those with impaired glucose tolerance.

Human red cell membrane fluidity and calcium pump activity in normolipidaemic type II diabetic subjects

Red cell membrane cholesterol, 1,6-diphenyl-1,3,5-hexatriene (DPH) and 1-[(4-trimethylammonium)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) anisotropies and basal and calmodulin-stimulated calcium pump activities were compared in 16 normolipidaemic Type 2 (non-insulin-dependent) diabetic patients and 20 normolipidaemic control subjects using the Mann-Whitney U-test. Serum cholesterol, membrane cholesterol, and membrane DPH and TMA-DPH anisotropies were similar in the two groups but both basal and calmodulin-stimulated calcium pump activities were reduced in the diabetic group: basal activity (median (inter-quartile range), mumol mg-1 h-1) 1.66 (1.18-1.97) vs 2.09 (1.90-2.50), p < 0.005 and calmodulin-stimulated activity 4.19 (3.07-5.48) vs 5.53 (4.70-6.88), p < 0.006. Although there were no correlations between glycaemic control and membrane anisotropy and between glycaemic control and calcium pump activity, the reduction in calcium pump activity is most likely due to a direct effect of diabetes on the calcium pump protein itself.

Direct effect of insulin on 45calcium uptake in human erythrocytes

OBJECTIVE

High blood pressure is prevalent in obesity and non-insulin dependent diabetes mellitus; both conditions, with insulin resistance and essential hypertension, have been associated with increasing intra-erythrocytic levels of calcium ions. We tested the hypothesis of whether insulin itself might be responsible for the abnormal red cell cytosolic free calcium.

DESIGN

The ionic effects of insulin were studied on the kinetics of 45calcium uptake in vitro in normal human erythrocytes.

SETTING

The study was performed in the outpatient clinic of a central hospital.

SUBJECTS

Sixteen healthy, normotensive individuals with normal body mass index were recruited for the study.

MAIN OUTCOME MEASURES

Blood from eight individuals was used for time-dependent studies of 45calcium uptake in erythrocytes and blood from another eight individuals was used for dose-dependent studies of insulin effect.

RESULTS

The rate of 45calcium influx in red blood cells has two components, a fast component (0-10 min), which measures the initial rate of 45calcium influx, and a slow component (10-60 min) probably reflecting a relatively large backflux of calcium (calcium efflux), which accordingly determines an apparent low rate of 45calcium influx between 10-60 min. The uptake was linear with time between 10-120 min regardless of insulin being present or not. Insulin at a concentration of 120 mU L-1 significantly decreased the 45calcium uptake in a time-dependent fashion between 10-120 min. The uptake was 508 (+/- 59) at 60 min in the presence of insulin vs. a control value of 529 (+/- 59) pmol mL red blood cells-1 (P < 0.001). The corresponding figures at 120 min were 742 (+/- 109) and 767 (+/- 127), respectively (P = 0.02). Inconsistent results were obtained on 45calcium uptake at 60 min by varying insulin concentrations from 40-640 mU L-1 and a dual effect of insulin on 45calcium uptake could not be excluded, one at a fairly low concentration of insulin (40-120 mU L-1) and another at a high concentration (160-640 mU L-1).

CONCLUSION

The data indicate a direct role of insulin in the transport process of calcium into normal human erythrocytes.

19F nuclear magnetic resonance studies of free calcium in heart cells

19F nuclear magnetic resonance is used in conjunction with 5,5'-difluoro-1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (5FBapta), a fluorinated calcium chelator, to report steady-state intracellular free calcium levels ([Ca2+]i) in populations of resting, quiescent, isolated adult heart cells. 31P nuclear magnetic resonance shows that 5FBapta-loaded cells maintain normal intracellular high-energy phosphates, pH, and free Mg2+. The intracellular free calcium concentration of well perfused, isolated heart cells is 61 +/- 5 nM, measured with 5FBapta, which has a dissociation constant (Kd) for calcium chelation of 500 nM. A similar value is obtained with Quin-MF, another fluorinated calcium chelator with Kd and maximum calcium sensitivity at 80 nM. We find that the steady-state level of intracellular free calcium is increased by decreased extra-cellular sodium concentration, omission of extracellular magnesium, decreased extracellular pH, hyperglycemia, and upon treatment with lead acetate. Further, extracellular ATP caused a large transient increase in [Ca2+]i. Thus, while heart cells maintain a very low level of intracellular free Ca2+, acute alterations in extracellular environment can cause derangement of calcium homeostasis, resulting in measurable increases in [Ca2+]i.

The erythrocyte calcium pump is inhibited by non-enzymic glycation: studies in situ and with the purified enzyme

In a previous paper we demonstrated that incubation of either intact erythrocytes or erythrocytes membranes with glucose decreases the activity of the membrane Ca(2+)-ATPase [González Flecha, Bermúdez, Cédola, Gagliardino and Rossi (1990) Diabetes 39, 707-711]. The aim of the present work was to obtain information about the mechanism of this inhibition. For this purpose, experiments were carried out with purified Ca(2+)-ATPase, inside-out vesicles and membranes from human erythrocytes. Incubation of the purified Ca(2+)-ATPase with glucose led to a decay in the enzyme activity of up to 50% of the control activity under the conditions used. The decrease in ATPase activity was concomitant with labelling by [6-3H]glucose of the purified Ca2+ pump; the kinetic properties of both processes were almost identical, suggesting that inhibition is a consequence of the incorporation of glucose into the Ca(2+)-ATPase molecule. In inside-out vesicles, glucose also promoted inhibition of Ca(2+)-ATPase activity as well as of active Ca2+ transport. Arabinose, xylose, mannose, ribose, fructose and glucose 6-phosphate (but not mannitol) were also able to inactive the ATPase. The activation energy for both the decrease in ATPase activity by glucose and the labelling of the pump with [6-3H]glucose was about 65 kJ/mol. Furthermore, inorganic phosphate enhanced the inactivation of the Ca(2+)-ATPase by glucose. This evidence strongly suggests that inhibition is a non-enzymically catalysed process. Inactivation of the Ca(2+)-ATPase by glucose was enhanced by reductive alkylation with sodium borohydride. Aminoguanidine, an inhibitor of the formation of the advanced end products of glycosylation, did not prevent the deleterious effect of glucose on the enzyme activity. Therefore it is concluded that inactivation of the Ca2+ pump is a consequence of the glycation of this protein.

Acute cellular actions of thyroid hormone and myocardial function

The mechanisms of actions of thyroid hormone in various tissues are largely viewed as cell nucleus-mediated. However, several actions of this hormone are definitively extranuclear, and these include effects on the activities of Ca(2+)-adenosine triphosphatases (ATPases) of myocardial sarcolemma and, apparently, sarcoplasmic reticulum in animal models. Both effects would serve to reduce cytoplasmic (sarcoplasmic) [Ca2+]. Sarcoplasmic reticulum uptake of Ca2+ from sarcoplasm is mediated by Ca(2+)-ATPase and is deficient in end-stage heart failure; thyroid hormone can enhance sarcoplasmic reticulum Ca(2+)-ATPase activity acutely via an extranuclear mechanism or indirectly via the myosin-associated Ca(2+)-ATPase gene. Such actions would serve to improve myocardial relaxation, thus improvement in diastolic dysfunction, and may be cardioprotective if excessive levels of sarcoplasmic [Ca2+] develop during reperfusion of previously ischemic tissue. Action of thyroid hormone on sarcolemmal Ca(2+)-ATPase activity will enhance Ca2+ efflux, and a recently described effect of the hormone on myocardial Na+ inactivation current may serve to increase or reduce sarcoplasmic [Ca2+], depending upon the vector of Na+/Ca2+ exchange. This article reviews acute effects of thyroid hormone on the heart that are extranuclear in mechanism.

Cellular ionic effects of insulin in normal human erythrocytes: a nuclear magnetic resonance study

Elevated erythrocyte cytosolic free calcium, and suppressed free magnesium and pH values are associated with the hyperinsulinaemia and insulin resistance of hypertension, obesity, and Type 2 (non-insulin-dependent) diabetes mellitus. To determine the role of insulin in this process, we utilized 19F- and 31P-nuclear magnetic resonance spectroscopy to study the cellular ionic effects of insulin in vitro on normal human erythrocytes. Insulin elevated cytosolic free calcium levels in a dose- and time-dependent manner. The effect began at 10 microU/ml, peaked at 200 microU/ml, and continued at both the 500 microU/ml and 1000 microU/ml doses. At 200 microU/ml, free calcium levels rose from 24.6 +/- 2.5 nmol/l to a peak value at 120 min of 66.4 +/- 11 nmol/l (p < 0.05 vs basal), levels remaining elevated throughout the incubation (45.7 +/- 5.6 nmol/l at 60 min, and 47.9 +/- 9.1 nmol/l at 180 min, p < 0.05 vs basal, respectively). Similarly, insulin also increased intracellular free magnesium at all time points (basal: 177 +/- 11 mumol/l; 60 min: 209 +/- 19 mumol/l; 120 min: 206 +/- 22 mumol/l; and 180 min: 202 +/- 12 mumol/l; p < 0.05 vs basal at all times). No insulin-induced changes in pH were observed. We conclude: (i) that insulin in physiological concentrations may participate in regulating divalent cations in the mature human erythrocyte, (ii) that insulin per se cannot account for the previously described cellular ionic lesions of hypertension and diabetes, and (iii) that future clinical studies of cell ion metabolism should be conducted in the fasting state, be controlled for ambient circulating insulin levels, or both.

Hexose-specific inhibition in vitro of human red cell Ca(2+)-ATPase activity

In a concentration-dependent manner (5.5-27.5 mmol/l), D-glucose incubated in vitro with human erythrocyte membranes at 37 degrees C for 1 h inhibited membrane Ca(2+)-ATPase activity by up to 75%. The IC50 was 11 mmol/l. L-Glucose was ineffective, as were 3-O-methylglucose, 2-deoxyglucose, sorbitol and myo-inositol. In contrast, D-fructose decreased Ca(2+)-ATPase activity nearly as effectively as D-glucose and mannose and galactose at 11 mmol/l were less than 50% as effective as D-glucose. Tunicamycin (12 pmol/l), but not 10 mmol/l aminoguanidine, progressively antagonized in vitro the D-glucose effect on the enzyme. Erythrocyte membrane Ca(2+)-ATPase activity may be regulated by glycosylation, rather than nonenzymatic glycation.

Modifications in platelet membrane transport functions in insulin-dependent diabetes mellitus and in gestational diabetes

The pathogenesis of plasma membrane alterations present in diabetes mellitus is unclear. To add new insights to the question, platelet membrane properties were evaluated in 16 women presenting impaired glucose tolerance at the 28-29th week of gestation (GDM) and in 8 women with insulin-dependent diabetes mellitus (IDDM). 15 healthy pregnant women (HPW) and 21 healthy non-pregnant (HNPW) women were the control group for GDM and IDDM, respectively. Pregnancy (HPW vs. HNPW) provoked an increase in Ca(2+)-ATPase activity and a decrease in membrane fluidity; in contrast, Na+/K(+)-ATPase, intracellular free Ca2+ concentrations, membrane cholesterol and phospholipid content did not vary. Both GDM and IDDM showed lower Na+/K(+)-ATPase activity and higher Ca2+ concentration, compared to HPW and HNPW, respectively, whereas Ca(2+)-ATPase activity was higher only in IDDM; furthermore, membrane fluidity was lower in GDM and higher in IDDM. Finally, GDM showed higher membrane cholesterol content. Both GDM and IDDM showed a very good metabolic control so that variations reported cannot be due to hyperglycemia; it is tempting to suggest that membrane variations are present before the clinical metabolic alteration. Furthermore, both GDM and IDDM were on insulin therapy, therefore: (i) insulin may be the pathogenetic factor of higher intracellular free Ca2+ concentrations and lower Na+/K(+)-ATPase activity since they both varied accordingly in GDM and IDDM, but not of (ii) changes in Ca(2+)-ATPase, membrane fluidity and cholesterol content which did not vary accordingly in GDM and IDDM.

Glucose induces lipid peroxidation and inactivation of membrane-associated ion-transport enzymes in human erythrocytes in vivo and in vitro

Erythrocytes of diabetic subjects (non-insulin dependent) were found to have eight- to ten-fold higher levels of endogenously formed thiobarbituric acid reactive malonyldialdehyde (MDA), thirteen-fold higher levels of phospholipid-MDA adduct, 15-20% reduced Na(+)-K(+)-ATPase activity with unchanged Ca+2-ATPase activity, as compared with the erythrocytes from normal healthy individuals. Incubation of normal erythrocytes with elevated concentrations (15-35 mM) of glucose, similar to that present in diabetic plasma, led to the increased lipid peroxidation, phospholipid-MDA adduct formation, reduction of Na(+)-K(+)-ATPase (25-50%) and Ca+2-ATPase (50%) activities. 2-doxy-glucose was 80% as effective as glucose in the lipid peroxidation and lipid adduct formation. However, other sugars, such as fructose, galactose, mannose, fucose, glucosamine and 3-O-methylmannoside, and sucrose, tested at a concentration of 35 mM, resulted in reduced (20-30%) lipid peroxidation without the formation of lipid-MDA adduct. Kinetic studies show that reductions in Na(+)-K(+)-ATPase and Ca+2-ATPase activities precede the lipid peroxidation as the enzyme inactivation occur within 30 min of incubation of erythrocytes with high concentration (15-35 mM) of glucose, while lipid peroxidation product, MDA appears at 4 hr and lipid-MDA adducts at 8 hr. The lipoxygenase pathway inhibitors, 5,8,11-eicosatriynoic acid and Baicalein (5,6,7-trihydroxyflavone), reduced the glucose-induced lipid peroxidation by 30% and MDA-lipid adduct formation by 26%. Indomethacin, a cyclooxygenase pathway inhibitor, had no discernible effect on the lipid peroxidation in erythrocytes. However, the inhibitors of lipid peroxidation, 3-phenylpyrazolidone, metyrapone, and the inhibitors of lipoxygenase pathways did not ablate the glucose-induced reduction of Na(+)-K(+)-ATPase and Ca+2-ATPase activities in erythrocytes. Erythrocytes produce 15-HETE (15-hydroxy-eicosatetraenoic acid), which is augmented by glucose. These results suggest that the formation of lipoxygenase metabolites potentiate the glucose-induced lipid peroxidation and that the inactivation of Na(+)-K(+)-ATPase and Ca+2-ATPase occurs as a result of non-covalent interaction of glucose with these enzymes.

Glucose inhibits the high-affinity (Ca2+ + Mg2+)-ATPase in the plasma membrane of a glucose-responsive insulinoma

(Ca2+ + Mg2+)-ATPase enzyme activity of a purified plasma membrane preparation from a glucose responsive rat insulinoma, was characterized as Ca2(+)-dependent dephosphorylation of [gamma-32P]ATP. A high-affinity enzyme with a Km(ATP) ranging from 20 to 30 microM and a Km(Ca2+) of 1 microM was identified. Glucose inhibited this high-affinity enzyme in a dose-dependent manner, with no significant inhibition at a concentration between 0 and 5 mM, 50% inhibition at 13.3 mM and 94.5% inhibition at 30 mM. The inhibitory effect of glucose was immediate and rapidly reversible. The effect was stereospecific for the alpha-anomer. These findings support the concept that glucose acts directly at the beta-cell plasma membrane and is involved in the maintenance of elevated intracellular free calcium concentrations associated with insulin release by directly or indirectly inhibiting energy-dependent calcium efflux. Glyceraldehyde (20 mM) increased enzyme activity 3-fold, while other metabolic fuels had no effect. This suggests that inhibition of the enzyme is not an obligatory requirement for insulin release. Calmodulin stimulated the enzyme activity in calmodulin-depleted but not in undepleted membranes. Trifluoperazine (30-100 microM) inhibited (Ca2+ + Mg2+)-ATPase in a dose-dependent manner (14-61% activity) and the activity was also inhibited by vanadate (0.1-1.0 mM) and NaCl (150 mM).

Role of cellular calcium metabolism in abnormal glucose metabolism and diabetic hypertension

The prevalence of hypertension in patients with non-insulin-dependent diabetes mellitus (NIDDM) is considerably higher than in the non-diabetic population. Insulin resistance may contribute to this increased prevalence. Abnormal cellular calcium (Ca2+) homeostasis may link insulin resistance and high blood pressure in patients with NIDDM. Observations of abnormal cellular Ca2+ homeostasis in animal models of NIDDM and obesity as well as in diabetic patients are consistent with this hypothesis. Abnormalities in cellular Ca2+ homeostasis are also found in hypertensive animals and humans. Alterations in cell membrane phospholipid content and distribution may be the primary cause of abnormal plasma membrane Ca2+ fluxes in patients with NIDDM and hypertension.

Thyroid hormone regulation of membrane Ca2(+)-ATPase activity

The Ca2(+)-ATPase of plasma membranes from a variety of tissues is subject to stimulation in vitro, and apparently in vivo, by physiological concentrations of iodothyronines regarded as biologically active in other bioassay systems. This calmodulin-dependent action of thyroid hormone is nongenomic, that is, directly on the cell membrane and independent of the cell nucleus. In the case of human erythrocyte Ca2(+)-ATPase, this assay of thyroid hormone bioactivity is attractive as an in vitro, readily-studied model of hormone action in a human cell. Enzyme activity is paralleled, as expected, by changes in calcium pump activity. Thyroid hormone action in this system is subject to modulation by glucose and by a variety of compounds which, like iodothyronines, are hydrophobic. The mechanism of thyroid hormone action on membrane Ca2(+)-ATPase involves, at least in part, membrane lipids, including components of the phosphatidylinositol cycle. The physiologic role of thyroid hormone action on cell membrane Ca2(+)-ATPase is speculative. In plasma membranes of nonexcitable and excitable tissues, ambient thyroid hormone may set basal activity of Ca2(+)-ATPase or magnitude of the enzymatic response to calmodulin Ca2+.

The effect of oral glucose on the leucocyte sodium pump in normal and obese subjects

The effect of oral glucose (40 g/m2 body surface area) on the leucocyte 22Na efflux rate constants (ERC) was studied in 13 normal weight and 10 obese subjects. The ouabain-sensitive 22Na ERC was higher in leucocytes isolated from fasting obese subjects (median [range] for obese 2.77 [2.33-3.11] vs normals 1.91 [1.57-2.77] h-1, P less than 0.001). There was no difference in the ouabain-resistant 22Na ERC. Oral glucose raised the ouabain-sensitive 22Na ERC after 2 h in normal subjects (1.91 [1.57-2.77] to 2.41 [2.11-3.02] h-1, P less than 0.001). The ouabain-resistant 22Na ERC fell from 0.71 [0.32-1.10] to 0.46 [0.35-0.68] h-1, P less than 0.008. Conversely, in obese subjects, the ouabain-sensitive ERC fell (2.77 [2.33-3.11] to 2.59 [2.11-2.92] h-1, P less than 0.06). There was no significant change in ouabain-resistant 22Na ERC 2 h after oral glucose. The fasting leucocyte 22Na ouabain-sensitive ERC correlated with fasting plasma insulin levels and insulin resistance (rs = 0.48, P less than 0.01 for both). The change in this ERC with oral glucose correlated with the incremental insulin response over 2 h (rs = -0.53, P less than 0.006) and to the insulin resistance (rs = -0.56, P less than 0.003). The failure of oral glucose to stimulate the leucocyte sodium pump in obesity could partially account for the defect in dietary thermogenesis in obesity. This defect in stimulation of the sodium pump is related to insulin resistance.