Changes in the expression of Na+/K+-ATPase isoenzymes in the left ventricle of diabetic rat hearts: effect of insulin treatment

The Na+/K+-ATPase: A potential therapeutic target in cardiometabolic diseases

Cardiometabolic diseases (CMD) are a direct consequence of modern living and contribute to the development of multisystem diseases such as cardiovascular diseases and diabetes mellitus (DM). CMD has reached epidemic proportions worldwide. A sodium pump (Na⁺/K⁺-ATPase) is found in most eukaryotic cells' membrane and controls many essential cellular functions directly or indirectly. This ion transporter and its isoforms are important in the pathogenesis of some pathological processes, including CMD. The structure and function of Na⁺/K⁺-ATPase, its expression and distribution in tissues, and its interactions with known ligands such as cardiotonic steroids and other suspected endogenous regulators are discussed in this review. In addition, we reviewed recent literature data related to the involvement of Na⁺/K⁺-ATPase activity dysfunction in CMD, focusing on the Na⁺/K⁺-ATPase as a potential therapeutic target in CMD.

Non-Coding RNAs in the Cardiac Action Potential and Their Impact on Arrhythmogenic Cardiac Diseases

Cardiac arrhythmias are prevalent among humans across all age ranges, affecting millions of people worldwide. While cardiac arrhythmias vary widely in their clinical presentation, they possess shared complex electrophysiologic properties at cellular level that have not been fully studied. Over the last decade, our current understanding of the functional roles of non-coding RNAs have progressively increased. microRNAs represent the most studied type of small ncRNAs and it has been demonstrated that miRNAs play essential roles in multiple biological contexts, including normal development and diseases. In this review, we provide a comprehensive analysis of the functional contribution of non-coding RNAs, primarily microRNAs, to the normal configuration of the cardiac action potential, as well as their association to distinct types of arrhythmogenic cardiac diseases.

Effect of a purine derivative containing selenium to improve memory decline and anxiety through modulation of the cholinergic system and Na+/K+-ATPase in an Alzheimer's disease model

Alzheimer's disease (AD) is a worldwide problem, and there are currently no treatments that can stop this disease. To investigate the binding affinity of 6-((4-fluorophenyl) selanyl)-9H-purine (FSP) with acetylcholinesterase (AChE), to verify the effects of FSP in an AD model in mice and to evaluate the toxicological potential of this compound in mice. The binding affinity of FSP with AChE was investigated by molecular docking analyses. The AD model was induced by streptozotocin (STZ) in Swiss mice after FSP treatment (1 mg/kg, intragastrically (i.g.)), 1st-10th day of the experimental protocol. Anxiety was evaluated in an elevated plus maze test, and memory impairment was evaluated in the Y-maze, object recognition and step-down inhibitory avoidance tasks. The cholinergic system was investigated based on by looking at expression and activity of AChE and expression of choline acetyltransferase (ChAT). We evaluated expression and activity of Na⁺/K⁺-ATPase. For toxicological analysis, animals received FSP (300 mg/kg, i.g.) and aspartate aminotransferase, alanine aminotransferase activities were determined in plasma and δ-aminolevulinate dehydratase activity in brain and liver. FSP interacts with residues of the AChE active site. FSP mitigated the induction of anxiety and memory impairment caused by STZ. FSP protected cholinergic system dysfunction and reduction of activity and expression of Na⁺/K⁺-ATPase. FSP did not modify toxicological parameters evaluated and did not cause the death of mice. FSP protected against anxiety, learning and memory impairment with involvement of the cholinergic system and Na⁺/K⁺-ATPase in these actions.

Cardiotonic Steroids Induce Vascular Fibrosis Via Pressure-Independent Mechanism in NaCl-Loaded Diabetic Rats

Endogenous cardiotonic steroid, marinobufagenin (MBG), induces Fli1-dependent tissue fibrosis. We hypothesized that an increase in MBG initiates the development of aortic fibrosis in salt-loaded rats with type 2 diabetes mellitus (DM2) via pressure-independent mechanism. DM2 was induced by a single intraperitoneal administration of 65 mg/kg streptozotocin to neonatal (4-5 days) male Wistar rats. Eight-week-old DM2 rats received water or 1.8% NaCl (DM-NaCl) solution for 4 weeks (n = 16); half of DM-NaCl rats were treated with anti-MBG monoclonal antibody (mAb) (DM-NaCl-AB) during week 4 of salt loading; control intact rats received water (n = 8/group). Blood pressure, MBG, erythrocyte Na/K-ATPase activity, aortic weights, levels of fibrosis markers (Fli1, protein kinase Cδ, transforming growth factor-β1, receptors of the transforming growth factor beta5, fibronectin, collagen-1), and sensitivity of the aortic explants to the vasorelaxant effect of sodium nitroprusside were assessed. No changes in systolic blood pressure were observed while erythrocyte Na/K-ATPase was inhibited by 30%, plasma MBG was doubled, and aortic markers of fibrosis became elevated in DM-NaCl rats versus control. Treatment of DM-NaCl rats with anti-MBG mAb activated Na/K-ATPase, prevented increases in aortic weights, and the levels of fibrosis markers returned to the control levels. The responsiveness of the aortic rings from DM-NaCl rats to the relaxant effect of sodium nitroprusside was reduced (half maximal effective concentration (EC50) = 29 nmol/L) versus control rings (EC50 = 7 nmol/L) and was restored by anti-MBG mAb (EC50 = 9 nmol/L). Our results suggest that in salt-loaded diabetic rats, MBG stimulates aortic collagen synthesis in a pressure-independent fashion and that 2 profibrotic mechanisms, Fli1 dependent and transforming growth factor-β dependent, underlie its effects.

Advanced glycation end-products impair Na⁺/K⁺-ATPase activity in diabetic cardiomyopathy: role of the adenosine monophosphate-activated protein kinase/sirtuin 1 pathway

Decreased Na(+) /K(+) -ATPase activity, and both sirtuin 1 (SIRT1) and adenosine monophosphate-activated protein kinase (AMPK) have been reported to be involved in the development of diabetic cardiomyopathy (DCM). The present study aimed to investigate the advanced glycation end-products (AGE) that impair Na(+) /K(+) -ATPase stability by regulating the AMPK/SIRT1 pathway during progression of DCM. To study type 1 diabetic mellitus (T1DM), a disease model in rats was established by a single intraperitoneal injection of streptozotocin (STZ; 65 mg/kg), and neonatal rat cardiomyocytes were also cultured. Heart function was detected by Doppler, and SIRT1 and AMPK protein expression were detected by immunohistochemistry and western blotting. Na(+) /K(+) -ATPase activity was also monitored. Using in vivo rat models of DCM, we showed that Na(+) /K(+) -ATPase activity decreased when both AMPK and SIRT1 expression were downregulated. In vitro, AGE impaired Na(+) /K(+) -ATPase activity and decreased the AMPK and SIRT1 expression. Sirtuin 1 overexpression increased Na(+) /K(+) -ATPase activity. 5-aminoimidazole-4-carboxamide-3-ribonucleoside (AICAR) upregulated SIRT1 expression and increased Na(+) /K(+) -ATPase activity, which could be partially abolished by splitomicin. Our results suggest that the dysfunction of DCM is related to AGE-induced Na(+) /K(+) -ATPase activity impairment through a mechanism involving the AMPK/SIRT1 pathway.

Effect of 2-Hydroxy-4-methoxy Benzoic Acid Isolated from Hemidesmus indicus on Erythrocyte Membrane Bound Enzymes and Antioxidant Status in Streptozotocin-induced Diabetic Rats

In the present study, the effect of 2-hydroxy-4-methoxy benzoic acid isolated from roots of Hemisdesmus indicus on the erythrocyte membrane bound enzymes and antioxidant status in streptozotocin-induced diabetic rats was investigated. The streptozotocin-induced diabetic rats were treated with 2-hydroxy-4-methoxy benzoic acid (500 μg/kg/day) for 7 weeks by oral intubation and compared with glibenclamide, a standard hypoglycemic agent (100 mg/kg). The erythrocyte membrane was isolated and the activity of Na(+)/K(+)-dependent ATPases, Ca(2+)-ATPases, Mg(2+)-ATPases were determined. Superoxide dismutase, catalase, glutathione peroxidase, glutathione-S-transferase, vitamins C, vitamin E, plasma reduced glutathione and erythrocyte glutathione, reduced glutathione content in the tissues was also assayed. Administration of 2-hydroxy-4-methoxy benzoic acid to diabetic rats significantly (F>0.05 and P<0.001) elevated the activity of total ATPases, Na(+)/k(+) ATPase, Mg(2+) ATPase and Ca(2+) ATPase to near normal level. The activities of catalase, superoxide dismutase and glutathione peroxidase and glutathione-S-transferase in erythrocytes were decreased significantly (F>0.05; P<0.001) in diabetic rats. Diabetic rats treated with 2-hydroxy-4-methoxy benzoic acid showed a significant (F>0.05; <0.001) increase in the enzymic antioxidants in erythrocytes. The elevated levels of vitamin E and low level of vitamin C and glutathione level in plasma and erythrocytes were observed in diabetic rats when compared to control rats and were restored significantly (F>0.05; P<0.001) after the administration of 2-hydroxy-4-methoxy benzoic acid. This study concludes administration of 2-hydroxy-4-methoxy benzoic acid supports the restoration of antioxidant defence, reduces the free radial production, lipid peroxidation and the glycosylation of haemoglobin in diabetic rats.

Insulin induced translocation of Na+/K+ -ATPase is decreased in the heart of streptozotocin diabetic rats

AIM

To investigate the effect of acute insulin administration on the subcellular localization of Na(+)/K(+)-ATPase isoforms in cardiac muscle of healthy and streptozotocin-induced diabetic rats.

METHODS

Membrane fractions were isolated with subcellular fractionation and with cell surface biotinylation technique. Na(+)/K(+)-ATPase subunit isoforms were analysed with ouabain binding assay and Western blotting. Enzyme activity was measured using 3-O-methylfluorescein-phosphatase activity.

RESULTS

In control rat heart muscle alpha1 isoform of Na(+)/K(+) ATPase resides mainly in the plasma membrane fraction, while alpha2 isoform in the intracellular membrane pool. Diabetes decreased the abundance of alpha1 isoform (25 %, P<0.05) in plasma membrane and alpha2 isoform (50%, P<0.01) in the intracellular membrane fraction. When plasma membrane fractions were isolated by discontinuous sucrose gradients, insulin-stimulated translocation of alpha2- but not alpha1-subunits was detected. Alpha1-subunit translocation was only detectable by cell surface biotinylation technique. After insulin administration protein level of alpha2 increased by 3.3-fold, alpha1 by 1.37-fold and beta1 by 1.51-fold (P<0.02) in the plasma membrane of control, and less than 1.92-fold (P<0.02), 1.19-fold (not significant) and 1.34-fold (P<0.02) in diabetes. The insulin-induced translocation was wortmannin sensitive.

CONCLUSION

This study demonstrates that insulin influences the plasma membrane localization of Na(+)/K(+)-ATPase isoforms in the heart. alpha2 isoform translocation is the most vulnerable to the reduced insulin response in diabetes. alpha1 isoform also translocates in response to insulin treatment in healthy rat. Insulin mediates Na(+)/K(+)-ATPase alpha1- and alpha2-subunit translocation to the cardiac muscle plasma membrane via a PI3-kinase-dependent mechanism.

Na+,K+-ATPase is modulated by angiotensin II in diabetic rat kidney--another reason for diabetic nephropathy?

Angiotensin II (ANGII) plays a central role in the enhanced sodium reabsorption in early type 1 diabetes in man and in streptozotocin-induced (STZ) diabetic rats. This study investigates the effect of untreated STZ-diabetes leading to diabetic nephropathy in combination with ANGII treatment, on the abundance and localization of the renal Na(+),K(+)-ATPase (NKA), a major contributor of renal sodium handling. After 7 weeks of STZ-diabetes (i.v. 65 mg kg(-1)) a subgroup of control (C) and diabetic (D7) Wistar rats were treated with ANGII (s.c. minipump 33 microg kg(-1) h(-1) for 24 h; CA and D7A). We measured renal function and mRNA expression, protein level, Serin23 phosphorylation, subcellular distribution, and enzyme activity of NKA alpha-1 subunit in the kidney cortex. Diabetes increased serum creatinine and urea nitrogen levels (C versus D7), as did ANGII (C versus CA, D7 versus D7A). Both diabetes (C versus D7) and ANGII increased NKA alpha-1 protein level and enzyme activity (C versus CA, D7 versus D7A). Furthermore, the combination led to an additive increase (D7 versus D7A, CA versus D7A). NKA alpha-1 Ser23 phosphorylation was higher both in D7 and ANGII-treated rats in the non-cytoskeletal fraction, while no signal was detected in the cytoskeletal fraction. Control kidneys showed NKA alpha-1 immunopositivity on the basolateral membrane of proximal tubular cells, while both D7 and ANGII broadened NKA immunopositivity towards the cytoplasm. Our study demonstrates that diabetes mellitus (DM) increases the mRNA expression, protein level, Ser23 phosphorylation and enzyme activity of renal NKA, which is further elevated by ANGII. Despite an increase in total NKA quantity in diabetic nephropathy, the redistribution to the cystosol suggests the Na(+) pump is no longer functional. ANGII also caused translocation from the basolateral membrane, thus in diabetic states where ANGII level is acutely elevated, the loss of NKA will be exacerbated. This provides another mechanism by which ANGII blockade is likely to be protective.

Effect of maturation on renal Na+/K+-atpase and its susceptibility to nitric oxide-deficient hypertension in rats

1. The present study deals with the effect of maturation on the kinetic properties of renal Na(+)/K(+)-ATPase and its susceptibility to nitric oxide (NO)-deficient hypertension induced by the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). 2. Immature (4-week-old) and adult (12-week-old) male Wistar rats were administered L-NAME (40 mg/kg per day) in their drinking water for 4 weeks. 3. The properties of the ATP- and Na(+)-binding sites of Na(+)/K(+)-ATPase were investigated by activation of the enzyme with increasing concentrations of the energy substrate ATP and/or cofactor Na(+). Unchanged values of K(m) suggest that energy utilization by the enzyme in the kidney of control rats remains unaffected during maturation. Conversely, the decrease in K(Na) values (the concentration of Na(+) necessary to achieve half-maximal reaction velocity) indicates improved affinity for Na(+) in the older group of control rats. 4. Application of L-NAME to all young animals had no significant effect on the functional properties of Na(+)/K(+)-ATPase. 5. In adult animals, the V(max) values remained unchanged after treatment with L-NAME, but the affinities of the ATP- and Na(+)-binding sites were decreased, as indicated by significant increase in K(m) and K(Na) values. 6. Maturation of control rats was accompanied by an increase in the Na(+) affinity of renal Na(+)/K(+)-ATPase without affecting ATP utilization. However, maturation increased the susceptibility of renal Na(+)/K(+)-ATPase to the harmful effects of L-NAME.

Lack of potentiation of bradykinin by angiotensin-(1-7) in a type 2 diabetes model: role of insulin

Considering the growing importance of the interaction between components of kallikrein-kinin and renin-angiotensin systems in physiological and pathological processes, particularly in diabetes mellitus, the aim of the present study was to investigate the interaction between angiotensin-(1-7) (Ang-(1-7)) and bradykinin (BK), important components of these systems in an insulin resistance model of diabetes, and the effect of insulin on it. For this the response of mesenteric arterioles of anesthetized neonatal streptozotocin-induced (n-STZ) diabetic and control rats was evaluated using intravital microscopy. Though capable of potentiating BK in non-diabetic rats, Ang-(1-7) did not potentiate BK in n-STZ rats. Chronic but not acute insulin treatment restored the potentiation. This restorative effect of insulin was abolished by a K+ channel blocker (tetraethylammonium), by nitric oxide synthase inhibitor (N-nitro-L-arginine methyl ester) and by a cyclooxygenase inhibitor (indomethacin). On the other hand, Na(+)-,K(+)-ATPase inhibition (by ouabain) did not abolish the effect of insulin. There was no difference in mRNA and protein expression of B1 and B2 kinin receptor subtypes between n-STZ diabetic and control rats. Insulin treatment did not alter the kinin receptor expression. Our data allow us to conclude that diabetes impaired the interaction between BK and Ang-(1-7) and that insulin restores it. The restoring effect of insulin depends on membrane hyperpolarization, nitric oxide release and cyclooxygenease metabolites but not Na+K+-ATPase. Alteration of kinin receptor expression might not be involved in the restoring effect of insulin on the potentiation of BK by Ang-(1-7).

Cardio protective effect of glucose–insulin infusion on acute digoxin toxicity in rat

We recently observed a case of digoxin and insulin self-poisoning without cardiac repercussion. We raised the hypothesis that insulin may have a cardio-protective effect in case of digoxin toxicity. We have therefore evaluated the effect of glucose-insulin infusion on mortality and ECG abnormalities during acute digoxin toxicity in rats. Before and after a hyperinsulinemia-euglycemia clamp, rats in glucose-insulin-digoxin (GID) group (n=10) received an intravenous infusion of 12ml/h or 2,5ml/h digoxin (0.25mg/ml) respectively until death occured. Animals receiving digoxin or saline solution intravenously served as control (n=10). ECG recording was performed in all animals over the entire period. Serum insulin and digoxin concentrations were measured by ELISA method after digoxin administration. When digoxin was administered after the clamp, all animals in GID group were alive, whereas 80% of animals in the digoxin group were dead (p<0.001) after 30min. The administration of Digoxin provoked rapid death of rats in the digoxin group in 15+/-12min whereas in GID group the survival period was significantly increased to 38+/-3min (p<0.001). Twenty minutes after digoxin administration, P waves disappeared for 78% of animals in digoxin group while they were present in all rats of GID group (p<0.001). Animal death occurred after a digoxin infusion volume of 7.7+/-0.6ml and 3.0+/-2.4ml in GID and digoxin group respectively (p<0.001). Five minutes after digoxin administration, potassium plasmatic level increased significantly in digoxin group as compared to GID group: 7.1+/-2mmol/l versus 4.4+/-0.4mmol/l (p<0.001). When digoxin was infused before the clamp, 40% of animals in GID group were alive after 180min and the other 60% died after 137+/-40min whereas death of rats in the digoxin group occurred within 80+/-10min (p<0.001). The death of animals was preceded by the P waves disappearing. Thirty minutes after digoxin administration, the potassium plasmatic level increased significantly in the digoxin group as compared to the GID group: 6.9+/-0.5mmol/l versus 4.9+/-0.3mmol/l (p<0.001). At the time of death, both volume of digoxin infusion and serum digoxin concentration were increased in GID group as compared to digoxin group: 5.7+/-1.6ml versus 3.3+/-0.4ml (p<0.001) and 10.7+/-8.3mg/l versus 8.5+/-4.6mg/l.

CONCLUSION

Glucose-insulin infusion delayed the abnormalities in cardiac conduction and improved rat survival after acute digoxin toxicity. These results suggest a cardioprotective effect of insulin in case of acute digoxin toxicity.

Down-Regulation of Na+Pump α2Isoform in Isoprenaline-Induced Cardiac Hypertrophy in Rat: Evidence for Increased Receptor Binding Affinity but Reduced Inotropic Potency of Digoxin

Cardiac hypertrophy in rats induces a down-regulation of Na(+),K(+)-ATPase alpha(2) isoform, although its functional consequences are poorly understood. Using a mathematical modeling approach that allows differentiation between effects elicited at the receptor and postreceptor level, we studied uptake, receptor binding kinetics, and positive inotropism of digoxin in single-pass Langendorff-perfused hearts of vehicle- and isoprenaline-pretreated rats (2.4 mg/kg per day over 4 days). Digoxin outflow concentration and left ventricular developed pressure data were measured for three consecutive doses (15, 30, and 45 microg) in the absence and presence of the reverse mode Na(+)/Ca(2+) exchange inhibitor 2-[2-[4-(4-nitrobenzyloxyl-)phenyl]ethyl isothiourea methansulfonate] (KB-R7943) (0.1 microM) in perfusate. In hypertrophied hearts, 1) the amount of alpha(2) receptors was reduced to 52% of control levels; 2) the digoxin binding affinity was increased 12-fold due to a decrease in dissociation rate constants of alpha(1) and alpha(2) receptors, and 3) inotropic responsiveness to digoxin the was attenuated on the stimulus-response level, where the coupling ratio of stimulus to response was reduced to 38% of control values. Only in the lowest dose level (15 microg) was this decrease in inotropic potency counterbalanced by the increase in receptor affinity. The Na(+),K(+)-ATPase isoform shift was not responsible for the diminished inotropic effect of digoxin. Coadministration of KB-R7943 significantly reduced cellular response generation at higher digoxin doses to the same limiting stimulus-response relationship in both the vehicle and isoprenaline group.

Systems analysis of digoxin kinetics and inotropic response in the rat heart: effects of calcium and KB-R7943

To gain more insight into the mechanistic processes controlling the kinetics of inotropic response of digoxin in the perfused whole heart, an integrated kinetic model was developed incorporating digoxin uptake, receptor binding (Na(+)-K(+)-ATPase inhibition), and cellular events linking receptor occupation and response. The model was applied to data obtained in the single-pass Langendorff-perfused rat heart for external [Ca(2+)] of 0.5 and 1.5 mM under control conditions and in the presence of the reverse-mode Na(+)/Ca(2+) exchange inhibitor KB-R7943 (0.1 microM) in perfusate. Outflow concentration and left ventricular developed pressure data measured for three consecutive doses (15, 30, and 45 microg) in each heart were analyzed simultaneously. While disposition kinetics of digoxin was determined by interaction with a heterogeneous receptor population consisting of a high-affinity/low-capacity and a low-affinity/high- capacity binding site, response generation was >80% mediated by binding to the high-affinity receptor. Digoxin sensitivity increased at lower external [Ca(2+)] due to higher stimulus amplification. Coadministration of KB-R7943 significantly reduced the positive inotropic effect of digoxin at higher doses (30 and 45 microg) and led to a saturated and delayed receptor occupancy-response relationship in the cellular effectuation model. The results provide further evidence for the functional heterogeneity of the Na(+)-K(+)-ATPase and suggest that in the presence of KB-R7943 a reduction of the Ca(2+) influx rate via the reverse mode Na(+)/Ca(2+) exchanger might become the limiting factor in digoxin response generation.

Islet transplantation is associated with improvement of renal function among uremic patients with type I diabetes mellitus and kidney transplants

The potential effects of islet transplantation on the renal function of 36 patients with type I diabetes mellitus and kidney transplants were studied with 4 yr of follow-up monitoring. Kidney-islet recipients were divided into two groups, i.e., patients with successful islet transplants (SI-K group) (n = 24, fasting C-peptide levels of >0.5 ng/ml for >1 yr) and patients with unsuccessful islet transplants (UI-K group) (n = 12, fasting C-peptide levels of <0.5 ng/ml). Kidney graft survival rates and function, urinary albumin excretion rates, and sodium handling were compared. Na(+)/K(+)-ATPase activity in protocol kidney biopsies and in red blood cells was cross-sectionally analyzed. The SI-K group demonstrated better kidney graft survival rates (100, 83, and 83% at 1, 4, and 7 yr, respectively) than did the UI-K group (83, 72, and 51% at 1, 4, and 7 yr, respectively; P = 0.02). The SI-K group demonstrated reductions in exogenous insulin requirements and higher C-peptide levels, compared with the UI-K group, whereas GFR values were similar. Microalbuminuria (urinary albumin index) increased significantly in the UI-K group only (UI-K, from 92.0 +/- 64.9 to 183.8 +/- 83.8, P = 0.05; SI-K, from 108.5 +/- 53.6 to 85.0 +/- 39.0, NS). In the SI-K group, but not in the UI-K group, natriuresis decreased at 2 and 4 yr (P < 0.01). The SI-K group demonstrated greater Na(+)/K(+)-ATPase immunoreactivity in renal tubular cells (P = 0.05) and higher activity in red blood cells (P = 0.03), compared with the UI-K group. The Na(+)/K(+)-ATPase activity in red blood cells was positively correlated with circulating C-peptide levels but not with glycated hemoglobin levels. Successful islet transplantation was associated with improvements in kidney graft survival rates and function among uremic patients with type I diabetes mellitus and kidney grafts.

Digoxin uptake, receptor heterogeneity, and inotropic response in the isolated rat heart: a comprehensive kinetic model

The cardiac pharmacokinetics of digitalis glycosides is not well understood. In the present study, a mechanism-based pharmacokinetic/pharmacodynamic model was developed to describe the uptake kinetics, receptor interaction, and positive inotropic effect of digoxin in the single-pass isolated perfused rat heart. Three doses of digoxin (0.1, 0.2, and 0.3 micromol) were administered to the heart (n = 12) as consecutive 1-min infusions followed by 15-min washout periods. Outflow concentration and left ventricular developed pressure were measured and analyzed by the model. The uptake of digoxin by the heart was limited by capillary permeability with a permeation clearance of 2.35 ml/min/g (about one-third of perfusate flow). Binding kinetics was determined by a mixture of two receptor subtypes, a low-affinity/high-capacity binding site (K(D,1) = 20.9 nmol, 89% of total receptors) and a high-affinity/low-capacity binding site (K(D,2) = 1.5 nmol, 11%). The time course of inotropic response was linked to receptor occupation, with higher efficiency of the high-affinity receptor population. The results suggest that, in the rat heart, consecutive inhibition of first the alpha(2)- and then the alpha(1)-isoform of Na(+)/K(+)-ATPase mediates the positive inotropic effect of digoxin with increasing dosage.

Reduced Na-K pump but increased Na-K-2Cl cotransporter in aorta of streptozotocin-induced diabetic rat

The activities of Na-K-ATPase and Na-K-2Cl cotransporter (NKCC1) were studied in the aorta, heart, and skeletal muscle of streptozotocin (STZ)-induced diabetic rats and control rats. In the aortic rings of STZ rats, the Na-K-ATPase-dependent (86)Rb/K uptake was reduced to 60.0 +/- 5.5% of the control value (P < 0.01). However, Na-K-ATPase activity in soleus skeletal muscle fibers of STZ rats and paired control rats was similar, showing that the reduction of Na-K-ATPase activity in aortas of STZ rats is tissue specific. To functionally distinguish the contributions of ouabain-resistant (alpha(1)) and ouabain-sensitive (alpha(2) and alpha(3)) isoforms to the Na-K-ATPase activity in aortic rings, we used either a high (10(-3) M) or a low (10(-5) M) ouabain concentration during (86)Rb/K uptake. We found that the reduction in total Na-K-ATPase activity resulted from a dramatic decrement in ouabain-sensitive mediated (86)Rb/K uptake (26.0 +/- 3.9% of control, P < 0.01). Western blot analysis of membrane fractions from aortas of STZ rats demonstrated a significant reduction in protein levels of alpha(1)- and alpha(2)-catalytic isoforms (alpha(1) = 71.3 +/- 9.8% of control values, P < 0.05; alpha(2) = 44.5 +/- 11.3% of control, P < 0.01). In contrast, aortic rings from the STZ rats demonstrated an increase in NKCC1 activity (172.5 +/- 9.5%, P < 0.01); however, in heart tissue no difference in NKCC1 activity was seen between control and diabetic animals. Transport studies of endothelium-denuded or intact aortic rings demonstrated that the endothelium stimulates both Na-K-ATPase and Na-K-2Cl dependent (86)Rb/K uptake. The endothelium-dependent stimulation of Na-K-ATPase and Na-K-2Cl was not hampered by diabetes. We conclude that abnormal vascular vessel tone and function, reported in STZ-induced diabetic rats, may be related to ion transport abnormalities caused by changes in Na-K-ATPase and Na-K-2Cl activities.

Effects of ouabain on the pressor response to phenylephrine and on the sodium pump activity in diabetic rats

The diabetes mellitus insulin-dependent is usually associated with cardiovascular disorders and with changes in the activity of the Na(+),K(+)-ATPase. The effects of ouabain, a Na(+),K(+)-ATPase inhibitor, on the pressor response of 7-day streptozotocin-induced diabetes were investigated in anesthetized rats and on the vascular reactivity of the perfused rat tail vascular bed. Diabetes was characterized by hyperglycemia (86+/-7.8 vs. 471+/-18.5 mg/dl) without changes in arterial blood pressure. Blood pressure increased after the treatment with 18 microg/kg ouabain in controls but not in diabetic rats; acute hyperglycemia, in non-treated rats, did not change these effects. Control tail vascular beds showed increased maximal response to phenylephrine after treatment with 10 nM ouabain for 1 h; this response was abolished in streptozotocin-treated rats. These rats showed an increased sensitivity to phenylephrine without changing the maximal vasoconstrictor response when compared to control rats. The relaxation induced by acetylcholine was reduced in diabetic rats. The functional activity of the Na(+),K(+)-ATPase was inhibited in vascular beds from diabetic rats, when compared to control rats, and the inhibition of the Na(+),K(+)-ATPase with 10 nM ouabain was not effective in these rats. Results suggested that in 7-day diabetic rats, the increase of arterial blood pressure or the sensitization of the vascular bed produced by ouabain is lost as a consequence of the reduction of the functional activity of the Na(+), K(+)-ATPase probably as a result of insulin lack and a deficient endothelial nitric oxide activity.

Role of mitochondrial dysfunction in calcium signalling alterations in dorsal root ganglion neurons of mice with experimentally-induced diabetes

The role of mitochondrial dysfunction in alterations of calcium signalling in primary sensory neurons has been studied in mice with streptozotocin-induced and genetically predisposed diabetes mellitus before and after additional treatment with insulin infusions. Cytosolic calcium transients triggered by membrane depolarization were measured using a membrane-permeable form of fluorescent indicator indo-1, and their changes after application of mitochondrial uncoupler carbonyl cyanide m-chlorphenylhydrazone were compared in cells of control and diabetic animals. Considerable prolongation of residual elevation of cytosolic calcium after termination of membrane depolarization was observed in diabetic mice, which was expressed mainly in small-sized (nociceptive) neurons. This correlated with the level of hyperglycemia, which was maximal in cells from streptozotocin-treated mice. Insulin partly reversed these changes. Carbonyl cyanide m-chlorophenylhydrazone application to neurons of control mice enlarged the peak of calcium transients and decreased residual calcium elevations, indicating that mitochondria in physiological conditions participate in shaping of these transients by diminishing their peak due to rapid uptake of calcium ions and by prolonging them due to subsequent slow calcium release back into the cytosol. Depression of the calcium accumulating function of mitochondria by carbonyl cyanide m-chlorophenylhydrazone eliminated these changes. The prolonged residual elevation of cytosolic calcium characteristic for neurons of diabetic animals was also eliminated by carbonyl cyanide m-chlorophenylhydrazone, confirming the suggestion that such elevation is determined mainly by mitochondrial dysfunction, the latter being dependent on the level of hyperglycemia. Predominant expression of such changes in small-sized neurons can be explained by the absence in them of effective calcium-buffering by the endoplasmic reticulum. Possible role of the described calcium signalling changes in the origin of neuropathic syndromes is discussed.