Changes in lipoproteins induced by the remnant kidney tissue or binephrectomy in chronic uremic patients treated by hemodialysis

The role of the remmant kidney tissue in uremic patients undergoing hemodialysis treatment has rarely been considered to influence the changes in lipoprotein and lipid metabolism. Twenty hemodialyzed patients with remnant kidneys and 11 anephric patients were studied to examine whether the presence or the absence of remnant kidney leads to qualitative or quantitative changes of the lipids and lipoproteins. Anephric patients showed a significantly higher triglyceride level, 3.66 +/- 0.49 (SEM) mmol/L v 2.34 +/- 0.09 mmol/L in patients with remnant kidneys (P less than .01), higher very-low-density lipoprotein (VLDL) triglycerides, 1.24 +/- 0.30 mmol/L v 0.69 +/- 0.09 (P less than .04), and higher HDL-triglycerides, 1.22 +/- 0.29 mmol/L v 0.66 +/- 0.09 mmol/L (P less than .04). APO-AI was significantly decreased in anephric patients, 95.2 +/- 13.3 mg/dL v 129.7 +/- 6.02 mg/dL in patients with remnant kidneys (P less than .01). APO-B was similar in both groups. All APO-C and APO-E were significantly lower in anephric patients, APO-CI 6.13 +/- 0.87 mg/dL v 8.47 +/- 0.42 mg/dL in patients with remnant kidneys (P less than .01), APO CII 1.00 +/- 0.01 mg/dL v 10.0 +/- 0.01 mg/dL (P less than .0001), APO-CIII 10.12 +/- 1.43 mg/dL v 26.0 +/- 2.86 mg/dL (P less than .0005), and APO-E 8.0 +/- 0.02 mg/dL v 12.0 +/- 0.01 mg/dL (P less than .03). These results point out important differences between women and men. In women binephrectomy promotes a decreased concentration of all APO-C but has no influence on APO-AI concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in plasma lipoproteins in acute malaria

Plasma lipid and lipoprotein concentrations were monitored in 16 patients with acute malaria. Plasma high density lipoprotein (HDL) levels decreased dramatically during the first 3 d after diagnosis to around 0.2 mmol l-1 (reference range 0.8-1.6 mmol l-1). The low HDL levels were related to parasitaemia, and rapidly recovered after successful therapy. Plasma triglyceride concentrations were moderately increased and plasma low density lipoprotein (LDL) cholesterol concentrations decreased during the course of infection. The mechanisms underlying the selective and pronounced decline in HDL cholesterol concentration remain obscure, but the reproducible phenomenon may be useful as an additional diagnostic tool in suspected malaria infection.     

Changes in the distribution and composition of high-density lipoproteins in primary hypothyroidism

The distribution and composition of high-density lipoprotein (HDL) subclasses were investigated in 14 women with severe hypothyroidism who were studied before and during treatment. The plasma concentrations of triglycerides, total cholesterol, HDL cholesterol, and of the apoproteins (apo) A-I, B, and E were increased in the hypothyroid state, while the apo A-II levels did not change significantly. After normalization of the thyroid function tests, the lipid and apoprotein levels were similar to those of normal individuals. Isopycnic ultracentrifugation in the density range 1.020 to 1.210 g/mL showed increases of both cholesterol and apo B in very-low-density lipoprotein (VLDL) and in low-density lipoprotein (LDL). The distribution of the HDL subclasses was modified in the hypothyroid subjects; both the less dense HDL fraction (d 1.063 to 1.100 g/mL; HDL2b), and the denser subclass (d 1.150 to 1.210 g/mL; HDL3b+3c) were increased, while the intermediate density subfraction (d 1.100 to 1.150 g/mL; HDL2a+3a) did not vary significantly. This redistribution of the HDL subfractions was associated with increased concentrations of cholesterol, phospholipid, and apo A-I in HDL2b, and of phospholipid and apo A-I in HDL3b+3c. Treatment of hypothyroidism decreased the concentrations of these fractions, and HDL2a+3a became the major HDL subclass in the euthyroid state. The particle sizes within HDL subfractions, measured by polyacrylamide gradient gel electrophoresis, were identical in the untreated and treated patients. The increased mass of protein and lipid within HDL2b and HDL3b+3c could therefore be attributed to an accumulation of identical-sized particles. The overall lipid and protein composition of the HDL lipoproteins was similar before and during treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in the levels and composition of lipoproteins and apolipoproteins of blood plasma in patients with ischemic heart disease after hemosorption

Prior to and following activated charcoal hemosorption, concentrations of lipids, apolipoproteins AI and B and lipid and protein composition in lipoprotein fractions isolated by ultracentrifugation were determined in the plasma from patients with coronary heart disease. The majority of the patients showed a parallel proportional decrease in plasma atherogenic parameters and all components of very low density lipoproteins and low density lipoproteins, triglycerides in particular. Antiatherogenic parameters, such as high density lipoprotein and apo-AI cholesterol, and all the components in high density lipoprotein subfractions were less reduced. In 19% of the patients, hemosorption failed to affect plasma lipids and apolipoproteins. The findings suggest that, in case of successful hemosorption, apoB-containing lipoproteins are chiefly eliminated as whole complexes from the plasma and that this procedure is most beneficial in hypertriglyceridemia.     

Effects of excess dietary L-cystine on the rat plasma lipoproteins

The effects of excess dietary cystine on the cholesterol and protein contents of rat plasma lipoproteins are described. 5% L-cystine was added to a semisynthetic diet containing 23% casein and 0.05% cholesterol, to the same diet enriched with 1% cholesterol or containing tristearin instead of lard. Rats were fed the diets during 2 months. The addition of cystine led to an increase in the plasma cholesterol level of the rats fed with the basal diet (from 0.92 to 1.56 mg/ml). But it produced a reduction of this level in cholesterol-fed rats (from 1.71 to 1.49 mg/ml). These different changes in the total plasma cholesterol level are explained by the specific effects of cystine on each lipoprotein: whatever the diet, cystine supplementation reduced the chylomicron and VLDL cholesterol contents and increased that of LDL (especially LDL2: density 1.040-1.063) and HDL. This study allowed us to compare 2 conditions which lead to hypercholesterolemia but which have opposite effects on hepatic cholesterogenesis: the supplementation of the same basal diet with 1% cholesterol or 5% cystine. In cholesterol-fed rats, the major part (49%) of plasma cholesterol was found in the chylomicrons and VLDL while the LDL2 cholesterol content was low (0.07 mg/ml plasma). Conversely, cystine-fed rats had a low chylomicron and VLDL plasma content (both enriched in apoprotein E), whereas up to 33% of the plasma cholesterol were carried by LDL2. Thus the production of LDL2 in cholesterol and cystine-fed rats could be related to hepatic cholesterogenesis.     

Changes in plasma lipoproteins accompanying diet therapy in obese diabetics.

Plasma lipoprotein cholesterol and triglyceride levels were measured in 24 obese not-insulin dependent Pima Indian diabetics and 9 obese nondiabetic controls before and after 1-8 months on a 500 calorie diet. The diabetics were divided into 3 groups--severe, recent onset (n = 10), severe long-term (n = 6), and borderline (n = 8). The diet regimen resulted in weight loss and improved glucose tolerance in all of the diabetics, and insulin secretion increased in the 2 groups of severe diabetics. After the period of weight loss, total plasma cholesterol had declined greater than 20%, and LDL cholesterol decreased 25% in all diabetic groups and in the controls. In all diabetic groups, HDL cholesterol did not decline; therefore the ratio of HDL/LDL cholesterol after diet therapy was significantly increased. In the controls HDL cholesterol declined with weight loss, and the distribution of HDL/LDL cholesterol remained constant. Plasma and VLDL triglyceride levels decreased in all groups in those with initial triglyceride levels greater than 150 mg/dl. The results indicate that weight loss in not-insulin dependent diabetics not only improves glucose tolerance, but also lowerss plasma lipids and reverses the dyslipoproteinemia often associated with this disorder. This may influence the risk of arteriosclerotic heart disease in these individuals.     

Oral but not transdermal estrogen replacement therapy changes the composition of plasma lipoproteins

The role of hormone replacement therapy and estrogen replacement therapy (ERT) in cardiovascular disease prevention has not been unambiguously defined yet. The metabolic effects of estrogens may vary depending upon the route of administration. Therefore, we compared the impact of unopposed oral or transdermal ERT on plasma lipids and lipoproteins in 41 hysterectomized women. This was an open-label, randomized, crossover study (with 2 treatments and 2 periods). The 41 hysterectomized women were randomized to receive oral or transdermal 17β-estradiol in the first or second of two 12-week study periods. Plasma lipid and lipoprotein levels were assayed before and after each treatment using standard automated methods. Lipid content of lipoprotein subclasses was assessed by sequential ultracentrifugation. The atherogenic index of plasma (AIP) was calculated as log(triglyceride [TG]/high-density lipoprotein [HDL] cholesterol). The difference between the 2 forms of administration was tested using a linear mixed model. The change from baseline for each of the forms was tested using paired t test. Oral ERT resulted in a significant increase in HDL cholesterol and apolipoprotein A-I levels, whereas it significantly decreased total and low-density lipoprotein (LDL) cholesterol and increased TG concentrations. Transdermal ERT had no such effect. Oral ERT led to a significant TG enrichment of HDL (0.19 ± 0.06 vs 0.27 ± 0.07 mmol/L, P < .001) and LDL particles (0.23 ± 0.08 vs 0.26 ± 0.10 mmol/L, P < .001) compared with baseline, whereas transdermal therapy did not have any effect on lipoprotein subclasses composition. The difference between the 2 treatments was statistically significant for HDL-TG and LDL-TG (0.27 ± 0.07 vs 0.19 ± 0.05 mmol/L, P < .001 and 0.26 ± 0.10 vs 0.22 ± 0.07 mmol/L, P< .001, respectively). The transdermal but not oral ERT significantly reduced the AIP compared with baseline (−0.17 ± 0.26 vs −0.23 ± 0.25, P = .023), making the difference between the therapies statistically significant (−0.23 ± 0.25 vs −0.18 ± 0.22, P = .017). Oral administration of ERT resulted in TG enrichment of LDL and HDL particles. Transdermal ERT did not change the composition of the lipoproteins and produced a significant improvement of AIP. Compared with transdermal ERT, orally administered ERT changes negatively the composition of plasma lipoproteins.     

Changes in plasma high-density lipoproteins after body weight reduction in obese women

We investigated effects of a hypocaloric diet of 5023.2 kJ (1200 kcal) on body weight and plasma lipids in 40 obese female subjects in two groups: (1) 20 obese subjects with normal plasma triglycerides at the onset (means = 148.5 mg/dl), and (2) 20 with hypertriglyceridemia (means = 225.3 mg/dl). The hypocaloric diet was instituted for a mean period of nine months and average body weight loss was 15.6 kg for Gp 1 and 14.0 kg for Gp 2. In Gp 1 there were no significant changes in total plasma cholesterol or triglycerides, but HDL-cholesterol rose significantly from a mean value of 42.5 to 53.6 mg/dl (P less than 0.001). Subjects in Gp 2 showed a significant decrease in plasma triglycerides (from 225.3 to 152.3 mg/dl, P less than 0.001) and an elevation in HDL-cholesterol from 41.2 to 48.2 mg/dl, P less than 0.001. Our results show that losing weight is associated with HDL-cholesterol elevation, independently of variation in plasma triglycerides.     

Changes in heart rate variability in chronic uremic patients during ultrafiltration and hemodialysis

BACKGROUND: The analysis of heart rate variability (HRV) is a useful tool to evaluate cardiac autonomic modulation, which is frequently impaired in chronic uremia. AIMS: The aim of this study was to evaluate HRV in chronic uremics and to separately investigate the acute changes induced by volume depletion and solute removal during a hemodialysis session. METHODS: Fourteen uremic patients (8 males and 6 females, aged 50 +/- 15 years) on maintenance hemodialysis and 14 sex- and age-matched healthy controls were studied. Both groups underwent ambulatory electrocardiogram monitoring to evaluate the HRV time and frequency domain indices. The hemodialysis session was performed by 1 h of high-rate isolated ultrafiltration followed by 3 h of bicarbonate diffusive procedure. RESULTS: In uremic patients, the overall variability in the frequency [low-frequency power (LF): 505 +/- 473, vs. 1,446 +/- 654; high-frequency power (HF): 133 +/- 162 vs. 512 +/- 417; p < 0.001] and time domain indices (standard deviation of normal R-R intervals: 101.9 +/- 33.3 vs. 181.7 +/- 44.1 ms; p < 0.001) was markedly reduced compared to controls, whereas mean heart rate (83 +/- 12.4 vs. 60.9 +/- 8.8 bpm; p < 0.001) and LF/HF ratio (5.8 +/- 3.5 vs. 2.2 +/- 0.8; p < 0.001) were increased. Isolated ultrafiltration produced a marked further decrease in HRV indices, but the subsequent diffusive hemodialysis procedure, with a low ultrafiltration rate, made HRV increase again. CONCLUSIONS: Chronic uremics showed abnormal autonomic modulation with sympathetic-vagal imbalance. The unbalanced hypersympathetic response to body fluid depletion is related to the ultrafiltration rate. Low interdialytic weight gain and a low ultrafiltration rate, associated with adequate hemodialysis, should be the preferable strategy for uremic patients with autonomic dysfunction.     

Fatty acid composition of plasma lipoproteins in control subjects and in patients with malabsorption

The fatty acid composition of cholesterol (cholesteryl) ester, triglyceride, and lecithin has been investigated in whole plasma and individual lipoproteins of healthy control subjects, of patients with malabsorption, and also of patients without malabsorption. The results show a decreased proportion of the essential fatty acid linoleic acid in all three lipid classes in both groups of patients as compared with the healthy controls. This abnormality was more marked in the malabsorbers, especially those with steatorrhoea secondary to intestinal resection. Unequivocal biochemical evidence of essential fatty acid deficiency, as indicated by the appearance of 5, 8, 11 eicosatrienoic acid in plasma lecithin, was observed in two patients, both of whom had undergone major intestinal resections. The results suggest that intestinal resection predisposes to the development of essential fatty acid deficiency.     

Changes in plasma lipoproteins after cardiac rehabilitation in patients not on lipid-lowering drugs

The extent to which a conventional cardiac rehabilitation programme can influence plasma lipoproteins was investigated in a prospective study. The relationship between changes in plasma lipoproteins and baseline characteristics, as well as variables related to the physical training and to dietary habits were assessed in 77 cardiac patients. All patients participated in a physical training programme, including general dietary advice. Patients who received lipid-lowering drugs were excluded from this study. Total plasma cholesterol decreased from 7.1 +/- 1.6 to 6.8 +/- 1.2 mmol l-1 (P less than 0.05), but it remained high in many patients, 61% having a level above 6.5 mmol l-1. The high- and low-density lipoprotein fractions (HDL- and LDL-cholesterol), and the ratio of total cholesterol to HDL-cholesterol, did not change significantly. The change in total plasma cholesterol was greatest (P less than 0.05) in patients who changed their diet in the recommended direction, and was poorly related to the change in maximal workload. It is concluded that a combination of general dietary advice and moderate physical exercise training is followed by a small reduction in total plasma cholesterol levels without changing HDL-cholesterol, and that cardiac rehabilitation should include strict programmes for the reduction of elevated plasma cholesterol.     

Origin and fate of cholesterol in rat plasma lipoproteins in vivo. II. Modelling of cholesterol absorption and its release into plasma lipoproteins

After a single ingestion of a diet containing 14C-cholesterol, cholesterol radioactivity in the stomachal and intestinal contents, in the different organs and in the very low density lipoproteins (VLDL) and chylomicrons was measured at different times during 2 days. Based on the results, a quantitative model of cholesterol absorption and of its release into the VLDL and chylomicrons has been elaborated. This model takes into account the different processes implied in the turnover of intestinal cholesterol and that of the entire organism. It constitutes a coherent whole (satisfactory simulations for the variables studied, suitable mass balances for each compartment and the absence of major contradictions with preexisting quantitative data). Once again the model demonstrates the important part played by the intestine in rat cholesterol system dynamics. It takes into account the existence of two related exogenous and endogenous cholesterol pools from which the cholesterol released by the intestine into the chylomicrons and VLDL originates. The results suggest the existence of an important esterified cholesterol uptake from other plasma lipoproteins by the chylomicrons.     

Effects of simvastatin, bezafibrate and gemfibrozil on the quantity and composition of plasma lipoproteins

Simvastatin, 10-40 mg/d (n = 11), bezafibrate, 600 mg/d (n = 6), and gemfibrozil, 1200 mg/d (n = 5) were administered for 12 weeks after a 4-week placebo period to subjects with initial plasma levels (mg/100 ml. mean +/- SD) of cholesterol (346 +/- 77), and of triglycerides (180 +/- 54). Total LDL-C plasma concentration was lowered 32% by simvastatin and 35% by bezafibrate, but only bezafibrate diminished the triglyceride (41%) and increased HDL-C plasma levels (35%). Plasma lipoprotein fractions obtained by discontinuous gradient ultracentrifugation, namely, VLDL, lighter LDL (LDL-1), heavier LDL (LDL-2) and bulk HDL were chemically analyzed. Simvastatin and bezafibrate significantly diminished the quantity of VLDL and LDL-1 particles, although barely modifying their composition. Neither drug influenced the LDL-2 plasma concentration. Bezafibrate increased the total plasma HDL level little interfering with its chemical composition. Gemfibrozil was the least effective of all drugs but decreased the lipid and protein contents and their ratios in VLDL and LDL-2.     

Changes in the composition and physico-chemical characteristics of serum lipoproteins during ethanol-induced lipaemia in alcoholic subjects.

The effect upon serum lipids and lipoproteins of a large intake of ethanol (180 g in 6 hr) has been studied during the ensuing 24 hr in 21 subjects. These were all men who were habituated to heavy drinking but who were shown to be normolipaemic when maintained on a normal diet with a restricted intake of alcohol. In all the subjects studied, ingestion of this amount of ethanol induced an acute hypertriglyceridaemia which varied in intensity between individuals, with increases ranging from 26% to 377% above the basal value. On the other hand, no significant change occurred in plasma cholesterol. In addition to measurements of triglycerides and cholesterol, the plasma from all the subjects were examined by electrophoresis in agarose and polyacrylamide gels with photometric scanning of the gels. In a subgroup of ten of the subjects, the plasma lipoproteins were separated into very low density (VLDL), low-density (LDL), and high-density (HDL) fractions, which were also analyzed biochemically and by electrophoresis. Following ethanol ingestion, a mean increase of about twofold in triglyceride content, and a decrease in the cholesterol/triglyceride ratio was observed in VLDL, LDL, and HDL. The triglyceride/protein ratio decreased in VLDL and increased in LDL. On electrophoresis of the intact sera following ethanol ingestion visible particulate fat and a band which failed to permeate the gel ('chylomicron-like band') was observed in five out of ten subjects and eight out of these ten subjects showed a prominent prebetalipoprotein (pre-beta) band. This component was found to be present in the lipoprotein fraction of density greater than 1.006 Kg/1. Examination of this fraction from basal specimens revealed a pre-beta band in only two of the ten subjects and the intensity of this band increased in the corresponding fractions from the same subjects after ethanol ingestion. In an additional four subjects no pre-beta was detected in this fraction before ethanol but appeared after ethanol ingestion. No attempt has been made to characterize further the component with these characters but it is suggested that it is probably an intermediate lipoprotein (ILDL), that is, a lipoprotein with characteristics intermediate between VLDL and LDL.     

Anomalies in composition of uremic lipoproteins isolated by gradient ultracentrifugation: relative enrichment of HDL in apolipoprotein C-III at the expense of apolipoprotein A-I

Using a discriminating gradient separation technique combined with careful analysis of lipid and apolipoprotein (apo) composition, serum lipoproteins of 20 chronically uremic patients have been compared with those of 19 normal controls matched for age and sex. In uremic subjects, serum VLDL concentration was markedly increased. These particles were enriched in protein and cholesterol and relatively poor in triglycerides (TG). They contained more frequently apo B-48, and a decreased proportion of apo E. Expressed in percent, total apo C was normal but the ratio of apo C-III2/apo C-III1 was elevated. Uremic IDL, whose serum concentration was markedly increased, were characterized by an increased proportion of protein. Total uremic LDL whose serum concentration was normal, contained less esterified cholesterol (EC) and more TG than normal LDL, their EC/TG ratio being very low. Moreover, the concentration of the mature subfraction of LDL having a mean density of 1.043 g/ml, was markedly decreased in uremic subjects. Taken together, these anomalies indicate a delayed transformation of VLDL into LDL in chronic renal failure. The decreased concentration of total HDL in uremic subjects was more marked in HDL2 (-46%) than HDL3 (-24%). Both uremic HDL2 and HDL3 were relatively enriched in TG, and uremic HDL3 were poorer in EC than normal HDL3.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of chronic and acute changes in plasma composition on vasopressin secretion and cerebrospinal fluid in the rat.

The effects of chronic and acute changes in plasma composition on the osmolality and sodium concentration of cerebrospinal fluid and plasma vasopressin (AVP) concentration have been examined. Chronic elevation of plasma osmolality in three strains of genetically AVP-deficient rats (Brattleboro and New Zealand hypertensive and normotensive Brattleboro) was associated with increased cerebrospinal fluid osmolality by comparison with AVP-replete controls (Long Evans and New Zealand genetically hypertensive and normotensive rats). The linear correlation between plasma and cerebrospinal fluid osmolality did not reflect a similar relationship between plasma and cerebrospinal fluid sodium concentration. Hypertensive animals exhibited a threefold higher plasma AVP concentration in association with significantly elevated cerebrospinal fluid osmolality by comparison with normotensive controls. Although ip hypertonic saline injection elicited parallel increases in plasma and cerebrospinal fluid osmolality and sodium concentration in both hypertensive and normotensive rats, only in the normotensives did this result in an increase in plasma AVP concentration. These results indicate that cerebrospinal fluid is subject to modest chronic and acute changes in osmolality and sodium concentration which may contribute to the osmotic control of AVP secretion. The disturbed control of vasopressin secretion in hypertensive rats may in part be related to the abnormal cerebrospinal fluid composition in these animals.     

Influence of age and sex on composition and lipid fluidity in miniature swine plasma lipoproteins

Age- and sex-related differences were observed in the plasma cholesterol level, the plasma concentrations of certain lipoprotein components, and the HDL lipid phase fluidity in miniature swine from post-weaning (6 weeks) through puberty (6 months), maturity (2-6 years), and old age (10-12 years). Age effects were more dominant in the males, with VLDL protein; LDL protein, triacylglycerol, and phospholipid; and HDL triacylglycerol, phospholipid, cholesterol, and polyunsaturated fatty acids showing statistically significant negative correlations with age. These effects were not observed in females. HDL cholesterol was positively correlated with age in females. Total plasma cholesterol decreased with age in males only, but plasma triacylglycerol was not influenced by age in either sex. Higher concentrations of all lipoprotein lipids were observed in the female minipigs regardless of age. HDL lipids became less fluid with age in the males alone suggesting a physical chemical basis for the lower incidence of heart disease among females. The more fluid HDL circulating in the female may be more capable of mobilizing peripheral tissue cholesterol for catabolism thus protecting her from developing atherosclerotic lesions.     

The composition of plasma lipoproteins and erythrocyte membranes in cholesterol-fed pigs with partial ileal bypass

The composition of plasma lipoproteins and erythrocyte membranes was studied in cholesterol-fed pigs with a partial ileal bypass. Cholesterol feeding caused marked increases in the plasma concentrations of cholesterol and phospholipids. In spite of continuation of cholesterol feeding, PIB reduced plasma concentrations of cholesterol and phospholipids towards basal values. PIB completely counteracted the dietary cholesterol induced alterations in the lipid composition of the apoprotein B containing plasma lipoproteins, but not in the HDL2 fraction. It is suggested that PIB specifically influences the metabolism of the atherogenic, apoprotein B containing lipoproteins. Dietary cholesterol caused significant increases in the ratios of cholesterol:phospholipids and phosphatidylcholine: sphingomyelin in erythrocytes. The high-cholesterol diet also increased the content of linoleic acid in erythrocyte phosphatidylcholine. PIB completely nullified the cholesterol-induced increase in the cholesterol:phospholipid ratio, but not the increase in the phosphatidylcholine:sphingomyelin ratio. The percentage of linoleic acid in erythrocyte phosphatidylcholine was unaffected by PIB. Neither cholesterol feeding nor PIB had an effect on the lipid fluidity of erythrocyte membranes, as measured by fluorescence polarization, using the probe diphenylhexatriene. Possible compensatory mechanisms operating to control homeostasis of lipid fluidity of erythrocyte membranes are discussed.