Effect of cholesterol and dipalmitoyl phosphatidylcholine enrichment on the kinetics of Na-Li exchange of human erythrocytes

Alterations in heart and kidney membrane phospholipids in hypertension as observed by 31P nuclear magnetic resonance

Abnormalities of phospholipids in hypertension have previously been described in human erythrocyte, platelet, and plasma lipoproteins. Since the heart and kidney are adversely affected by hypertension, we investigated possible alterations in their membrane phospholipids, which could play a role in the derangement of intracellular ion balance widely observed in hypertension. The phospholipid compositions of heart and kidney from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were determined by using 31P nuclear magnetic resonance (NMR) spectroscopy. Absolute contents of all phospholipids in hypertensive hearts and kidneys were significantly higher than in normotensive hearts and kidneys. Expressed as a fraction of total phospholipid, cardiolipin (CL) and phosphatidylethanolamine plasmalogen (PEp) were significantly increased in SHR hearts compared to WKY hearts (CL and PEp were 7.95+/-0.22% and 13.16+/-0.35% in SHR vs. 7.01+/-0.20% and 11.19+/-0.42% in WKY rats, P< or =0.05), but phosphatidylethanolamine (PE) and phosphatidylcholine (PC) were significantly decreased in SHR (PE and PC were 22.46+/-0.37% and 44.81+/-0.43% in SHR vs. 24.02+/-0.44% and 46.01+/-0.50% in WKY rats, P< or =0.05). In the phospholipids extracted from rat kidneys, the percentage of PE was significantly higher for SHR than for WKY rats (20.37+/-0.60% vs. 18.43+/-0.37%, P< or =0.05), while PEp and phosphatidylserine (PS) were significantly lower for SHR (PEp and PS were 10.22+/-0.36% and 8.42+/-0.28% in SHRs vs. 11.29+/-0.36% and 9.71+/-0.40% in WKY rats, P< or =0.05). The above alterations in phospholipid composition might contribute to the higher oxygen consumption in the hypertensive heart and abnormal intracellular ion concentrations and ion transport in the heart and the kidney in hypertension.

Correlations of Na+-Li+ exchange activity with Na+ and Li+ binding and phospholipid composition in erythrocyte membranes of white hypertensive and normotensive individuals: a nuclear magnetic resonance investigation

Enhanced Na+-Li+ exchange activity has been reported in red blood cells (RBCs) of white patients with essential hypertension compared with RBCs of normotensive individuals. To understand the factors responsible for this finding, we applied novel and conventional spectroscopic and kinetic methods to blood samples from 10 hypertensive and 10 normotensive individuals. We measured the kinetic parameters (V std, V max, and K m) for RBC Na+-Li+ exchange by atomic absorption spectrophotometry and used 23Na and 7Li nuclear magnetic resonance relaxation methods to measure Na+ and Li+ binding to RBC membranes as well as 31P nuclear magnetic resonance spectroscopy to measure membrane phospholipid compositions. We found significant differences between the two groups for the affinity of Na+ for the RBC membrane (0.202 +/- 0.054 mmol/L-1 for hypertensive patients versus 0.296 +/- 0.071 mmol/L-1 for normotensive subjects, P<.005). The kinetic parameters of RBC Na+-Li+ exchange (V std, V max, and K m) were 0.32 +/- 0.09 and 0.66 +/- 0.17 mmol Li+/L cell.h and 160 +/- 62 mmol/L, respectively, for hypertensive patients versus 0.21 +/- 0.06 and 0.32 +/- 0.14 mmol Li+/L cell.h and 86 +/- 69 mmol/L for normotensive subjects (P<.05). The fractions of phosphatidylserine and phosphatidylethanolamine were 0.153 +/- 0.009 and 0.294 +/- 0.016 for hypertensive patients versus 0.138 +/- 0.013 and 0.325 +/- 0.018 for normotensive subjects (P<.05). The Na+ binding constants were negatively correlated with the Km values for both the hypertensive (r=-.61, P=.01) and normotensive (r=-.43, P=.04) groups. Changes in lipid-protein interactions in the RBC membranes of hypertensive patients appear to be responsible for weaker Na+ binding to the membrane and for the faster rates of RBC Na+-Li+ exchange.

Effects of membrane incorporation of short-chain phospholipids on sodium pump function in human erthrocytes

Erythrocyte membrane incorporation of exogenous short-chain diacyl phosphatidylcholines (PC) has been quantified by gas chromatography of fatty acid methyl esters of extracted membrane lipids after incubation of cells with sonicated aqueous suspensions of PC. The PCs studied included didecanoyl PC (C10-PC), dilauroyl PC (C12-PC), dimyristoyl PC (C14-PC) and dipalmitoyl PC (C16-PC). PC incorporation of 10-15 mol% was achieved by incubation at 37 degrees C for 0.5-24 h. Control cells incubated in saline alone showed a progressive reduction in endogenous polyunsaturated acyl chain content. Incubation with C10-PC and C16-PC was associated with reductions in membrane cholesterol. Experiments were performed with mixtures of PC and cholesterol in order to minimise this effect. Short-chain PC incorporation was associated with increases in intracellular Na+ and reduced intracellular K+ concentrations. Sodium pump activity was measured as the ouabain-sensitive rate of 86Rb+ influx and was significantly reduced by all PCs tested; mean reductions were 13-30%. These results confirm that the sodium pump in situ is sensitive to lipid acyl chain composition.

The effect of lithium therapy upon the composition of the human erythrocyte membrane

The membrane phospholipids and membrane proteins from the erythrocytes of manic depressive patients undergoing lithium therapy and of normal controls were examined by high performance TLC and gel electrophoresis, respectively. The phospholipid composition of the erythrocyte membranes of the normal controls was close to that reported previously. The lipid composition of the erythrocyte membranes of the patients was very similar but differed from the normal controls in that the proportion of sphingomyelin and cholesterol appeared to be lower, whereas phosphatidylserine appeared to be higher in the patients by amounts comparable with the sums of the 95% confidence limits on the measurements. No major differences in the protein composition of the erythrocyte membranes were detected.

Human lymphocyte sodium-hydrogen exchange. The influences of lipids, membrane fluidity, and insulin

The relation between serum lipids, membrane fluidity, insulin, and the activity of the sodium-hydrogen exchanger was investigated in human lymphocytes from 83 subjects. Subjects had a wide range of serum lipids and no concurrent disease. Lymphocyte membrane anisotropy (inversely related to membrane fluidity) was measured with a fluorescence polarization method. Sodium-hydrogen exchange maximal proton efflux rate, affinity for external sodium, and resting pH were determined with the intracellular pH-sensitive fluorochrome 2',5'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. Sodium-hydrogen exchange maximal proton efflux rate was negatively correlated with the age of the subject (p = 0.03). Membrane anisotropy correlated with serum triglyceride (p = 0.04). Multiple regression analysis demonstrated that the maximal proton efflux rate in human lymphocytes was significantly related to age (p = 0.005), systolic blood pressure (p = 0.04), membrane anisotropy (p = 0.03), and serum cholesterol (p = 0.03). Incubation of lymphocytes with insulin failed to affect sodium-hydrogen exchange kinetics, intracellular buffering power, or resting intracellular pH. These results suggest that membrane-bound transport proteins may be influenced by serum lipids and the fluidity of the lipid membrane in which they are bound, but they are unlikely to be affected by insulin.

Relations of sodium-lithium countertransport kinetics to plasma and red cell membrane phospholipids in hyperlipidemia

As compared to 7 normolipidemic donors, the maximal velocity of sodium-lithium countertransport was accelerated by nearly 70% in 10 patients with elevated levels of triglyceride-rich lipoproteins and tended to be stimulated also in 5 patients with hypercholesterolemia. No significant differences were observed between normolipidemia and both hyperlipidemic groups for the apparent affinities of the transport system for intracellular sodium and extracellular lithium. Strong positive relations of the maximal activity of sodium-lithium countertransport to the percentages of red cell membrane phosphatidylcholine (r = 0.85, 2P < 0.001), the phosphatidylcholine/sphingomyelin (r = 0.82, 2P < 0.001) and the phosphatidylcholine/phosphatidylethanolamine ratio (r = 0.81, 2P < 0.001) were seen in all donors. A negative correlation was found to membrane sphingomyelin (r = -0.72, 2P < 0.001). Also plasma phosphatidylcholine and sphingomyelin exhibited positive and negative associations, respectively, to the maximal activity of sodium-lithium countertransport (r = 0.66, 2P < 0.01 and r = -0.78, 2P < 0.001). Among several plasma lipoprotein parameters investigated, total triglycerides or VLDL cholesterol levels showed independent relations to both the plasma and the membrane phosphatidylcholine/sphingomyelin ratio as well as to the maximal velocity of sodium-lithium countertransport. The results indicate that an increase in red cell membrane phosphatidylcholine and a concomitant fall in sphingomyelin are closely associated with the acceleration of sodium-lithium countertransport in hyperlipidemia.

Relationship between myoinositol influx and lipids in diabetic neuropathy

Diabetic neuropathy is associated with abnormalities in lipid metabolism and has been postulated to be associated with abnormal myoinositol metabolism. Leucocyte myoinositol influx was measured using a triple isotope method in long-standing type 1 (insulin-dependent) diabetic patients with and without diabetic neuropathy and in a group of matched controls. No differences in fasting lipid, glucose concentrations or glycated haemoglobin were found in the diabetic groups. Myoinositol influx was significantly but negatively correlated with the serum very low density lipoprotein (VLDL) cholesterol concentration in patients with and without neuropathy but not in the controls. The VLDL cholesterol concentration also correlated negatively with the transporter Km (Rs = -0.87, P < 0.005, neuropathic group only) and Vmax (Rs = -0.93, P < 0.001, neuropathic group; Rs = -0.59, P < 0.05, non-neuropathic group). Myoinositol influx correlated with the glycated haemoglobin only in the diabetic patients without neuropathy. The lipid relationships in diabetic subjects were independent of glucose control, which suggests that myoinositol influx mechanisms represent another transporter affected by intracellular lipid metabolism. The control of VLDL metabolism by fibrates could offer a method for reducing the progression of diabetic neuropathy.