Insulin effects on human red blood cells

Purine nucleotides and their metabolites in erythrocytes of streptozotocin diabetic rats

OBJECTIVES

In the present study it was tried to obtain a complete overview of purine nucleotide metabolism in erythrocytes of streptozotocin (STZ) induced diabetes mellitus rats.

METHODS

Erythrocyte levels of the main nucleotides (ATP, ADP, AMP, GTP, GDP, GMP, IMP, NAD+, NADP+), nucleosides (Ado, Guo, Ino) and the base Hyp were measured using the HPLC method. The parameters that can be deduced from their concentrations: TAN, TGN and AEC, GEC expressed by the ratio of high/low energy nucleoside phosphates were calculated. The effects of streptozotocin-induced diabetes on the concentration and metabolism of rat erythrocyte purine and pyridine nucleotides and the activity of Na+, K+-ATPase as well as Ca2+-ATPase were investigated.

RESULTS

Increased dephosphorylation of adenine nucleotides (found as the increased concentration of Ado and Hyp and the decrease in AEC value) and the decrease in ATP and TAN and the changes in the concentrations of NAD+ and NADP+ suggest serious purine and pyridine metabolism disruptions in diabetic erythrocytes and decrease in ATPases activity.

CONCLUSION

The observations suggest that purine nucleotide degradation is markedly accelerated in erythrocytes of STZ diabetic rats.

Na pump and plasma membrane structure in L-cell fibroblasts expressing rat liver fatty acid binding protein

Although the intracellular fatty acid binding proteins have been investigated for nearly two decades and purified proteins are now available, little is known regarding the function of these proteins in intact cells. Therefore, L-cell fibroblasts transfected with cDNA encoding for rat liver fatty acid binding protein (L-FABP) were examined as to whether L-FABP expression in intact cells modifies plasma membrane enzyme activities, fluidity, and lipids. Plasma membrane Na/K-ATPase activity was 65.9 +/- 18.7 and 38.6 +/- 22.8 (P less than 0.001) nmol/mg protein x min for control and high-expression transfected cells, respectively. Consistent with this observation, [3H] ouabain binding to whole cells was significantly decreased from 3.7 +/- 0.3 to 2.0 +/- 0.8 pmol ouabain bound/mg cell protein in control and high-expression cells, respectively, whereas the cell's affinity for ouabain was not significantly altered. Unexpectedly, Western blot analysis indicated that transfected cells had higher levels of Na+, K(+)-ATPase protein; in contrast, the activities of 5'-nucleotidase and Mg-ATPase were unaltered. The effects of L-FABP expression on plasma membrane Na/K-ATPase function appeared to be mediated through alterations in plasma membrane lipids and/or structure. The plasma membrane cholesterol/phospholipid ratio decreased and the bulk plasma membrane fluidity increased in the high-expression cells. In conclusion, plasma membrane Na/K-ATPase activity in L cells may be regulated in part through expression of cytosolic L-FABP.

Changes in polyphosphoinositides and phosphatidic acid of erythrocyte membranes in diabetes

We studied metabolic pool size of polyphosphoinositides and phosphatidate of erythrocyte membranes from normal and diabetic subjects using 32P for 20-h incubation, a sufficiently long period to reach isotopic equilibrium between monoesterphosphate bond and gamma-phosphate of ATP. Phosphatidylinositol 4-monophosphate (PtdIns4P), phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and phosphatidate were the phospholipids labelled. Metabolic pools of individual phospholipids were estimated, based on their proportionate and absolute radioactivity. A significant decline in radioactivity of phosphatidate and PtdIns(4,5)P2 was seen in erythrocytes from the diabetic subjects, indicating suppression of the metabolically labile pool of these two phospholipids. There was no significant change in PtdIns4P radioactivity between the groups. The direct effect of insulin on phosphorylation of polyphosphoinositides and phosphatidate was also evaluated by a short incubation period of erythrocyte membranes with [gamma-32P]-ATP. Added insulin increased the incorporation of 32P into phosphatidate in a dose-dependent manner that reached a steady state at 2 nM. We conclude that the metabolically labile pool size of phosphatidate is decreased and that of polyphosphoinositides is altered in erythrocyte membranes from diabetic patients.

Insulin-dependent release of 5'-nucleotidase and alkaline phosphatase from liver plasma membranes

Insulin treatment of isolated liver plasma membranes induced the release of 5'-nucleotidase and alkaline phosphatase. This effect was maximal at physiological hormone concentrations, being 36% and 17% for 5'-nucleotidase and alkaline phosphatase respectively, and was fully mimicked by the phosphatidylinositol specific phospholipase C (PI-PLC), thus confirming the presence of a glycosylphosphatidylinositol anchoring-system for these exofacial enzymatic proteins. The complete inhibition of insulin dependent enzyme release by neomycin is strongly supportive of an involvement of membrane-located PI-PLC activity. In addition, the insulin-like effect on enzyme release induced by the GTP non-hydrolysable analog, GTP-gamma-S, and its sensitivity to the pertussis toxin are in favour of a mediatory role exerted by the G proteins system, in the transduction of some actions of insulin.

The effects of glucose, insulin and metformin on the order parameters of isolated red cell membranes. An electron paramagnetic resonance spectroscopic study

Human red blood cell (RBC) membranes (RBC ghosts) were treated with glucose, insulin and metformin. The order parameters of RBC membranes were determined by 5- and 16-doxyl-stearic acid spin labels. Metabolic effects were excluded using an isolated system of RBC membranes. The membranes were incubated with glucose in physiological (5 mM), renal threshold (10 mM) and manifested diabetic (20 mM) concentrations for limited times. High concentrations of glucose (10, 20, 100 mM) increase the order parameters of RBC membranes significantly. Insulin by itself has a similar effect which is, however, not strictly concentration-dependent. By contrast, metformin at therapeutic concentrations (0.5 and 5.0 microM) decreases the order parameters. At 50 microM concentration the metformin effect is expressed less and recurs at 100 microM concentration. The effects are significant with 5-doxyl-stearic acid, but are not significant with the 16-doxyl derivative. When RBC membranes are co-incubated with 20 mM glucose and metformin at 0.5 and 5.0 microM concentrations the order parameters as determined by 5-doxyl-stearic acid remain normal (= control values). Higher concentrations of metformin (50 and 100 microM) cause an overshoot to very low order parameters. Insulin at 10, 100 and 200 mU/L does not influence significantly the effects of metformin. Addition of physiological amounts of bovine serum albumin does not abolish the effects of metformin. Metformin, at therapeutic concentrations (0.5 and 5.0 microM), maintains the normal fluidity at the polar interface of isolated RBC membranes by counterbalancing non-enzymatic glycosylation with 20 mM glucose in vitro.

Erythrocytes from diabetics with neuropathy have fewer sodium pumps

Erythrocytes from diabetics with neuropathy had fewer binding sites for ouabain and, therefore, fewer sodium pumps than did erythrocytes from diabetics without neuropathy or from age-matched controls (P less than 0.001). Diabetics with neuropathy excreted in their urine significantly less C-peptide immunoreactivity (CPR) than those without neuropathy, which is a sign of lower endogenous insulin secretion levels. Significant positive correlations existed between the number of pumps and tibial motor nerve conduction velocity (MNCV) and urinary CPR in diabetics, and between urinary CPR and MNCV. This suggests that there is close correlation between the fall in the number of pumps in diabetics and diabetic neuropathy and insulin deficiency.

Insulin sensitivity of rat muscle sodium pump

The aim of the present study was to examine the possible physiological responsiveness of the sodium pump to insulin in rat muscle, an effect that has never been convincingly demonstrated. The insulin stimulation of the sodium pump was estimated by two well-established parameters: ouabain binding to pieces of soleus muscle, and Na/K-ATPase activity of purified plasma membranes. For both parameters the dose dependence of the insulin effect on the sodium pump shows the characteristic bell-shaped stimulation pattern, with a maximum in the physiological hormone concentration range. This result has not been observed in previous studies where insulin concentrations two to three orders of magnitude higher were used. It can be concluded that an effect of insulin on the regulation of the Na pump in muscle might well be operating in vivo.

In vitro insulin action on erythrocyte glucose metabolism in normal and diabetic rats

Alloxan-induced diabetes in rats significantly impaired the capacity of the erythrocytes to metabolise glucose in vitro to either lactic acid or CO2. Both these metabolic activities were initially insensitive to insulin in normal as well as in diabetic animals; but became responsive when these cells were subjected to insulin and glucose 'starvation' for 1 h through incubation in their absence. This action of insulin in starved cells showed concentration dependence and required preincubation with the hormone prior to addition of glucose.