Cholesterol depletion affects the Ca2+ influx but not the Ca2+ pump in human erythrocytes

Infrared spectroscopic studies on the phosphatidylserine bilayer interacting with calcium ion: effect of cholesterol

Fourier transform infrared (IR) spectroscopic studies of phosphatidylserine/cholesterol/Ca2+ complexes are reported using the synthetic phosphatidylserines (PS) 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine (POPS), and 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS). IR spectra reveal that cholesterol does not significantly alter the binding nature of Ca2+ to PS molecules; Ca2+ binds to the phosphate ester group of PS in the presence of cholesterol up to 50 mol% as in the case of pure PS bilayers. However, the IR data indicate that the presence of cholesterol induces disorder of the acyl chain packing, increases the degree of immobilization of the interfacial and polar regions, and increases the degree of dehydration of the PS/Ca2+ complexes.

Calcium homeostasis of human erythrocytes and its pathophysiological implications

In human red cells, Ca is mainly bound to the inner side of the plasma membrane. A smaller part may be present within intracellular Ca storing vesicles, while only a few percent of total red cell Ca is in ionized form. In some hemolytic anemias (sickle cell anemia, beta-thalassemia), an increased number of endocytotic vesicles storing Ca is probably responsible for the elevation of total red cell Ca content. Red cell Ca inward transport, which is partially susceptible to inhibition by Ca entry blockers, has been reported to be enhanced by physiological shear stress and enrichment in membrane cholesterol, as well as in some hemolytic anemias. Normal intracellular ionized Ca levels have been assessed in several diseases where elevated Ca inward transport rates or decreased Ca efflux through the Ca pump (hemolytic anemias, cystic fibrosis, essential hypertension) had been observed previously. Thus, red cell Ca homeostasis is apparently capable of keeping ionized Ca levels within the physiological range of 20-60 nM under most pathological conditions investigated so far. Conceptually, changes in red cell Ca homeostasis (or also in other red cell membrane parameters) may be of pathophysiological importance in two respects: 1) A disturbance may be directly responsible for some of the symptoms associated with a disease. This is the case in sickle cell anemia, where red cell dehydration is possibly caused by transient elevations of intracellular ionized calcium, which may activate K efflux through the Ca-activated K channel. The presence of dehydrated red cells will, in turn, lead to microvascular occlusion, a pathophysiologically important phenomenon in sickle cell anemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte membrane Ca2+ ATPase: reactivities of human A, AS, and S erythrocytes with steroid hormones

In a preliminary study we reported a significant enhancement of Ca2+ ATPase activity in sickle cell membranes in the presence of progesterone and testosterone. In this work the reactivity of various classes of steroid hormones with the membranes of hemoglobin variants was investigated. A consistent universal stimulation of Ca2+ ATPase activity in sickle cell membranes by the different classes of steroid hormones does not appear to correlate with any major structural differences of the hormones or the presence of reactive functional groups. The universal interaction of the hormones with sickle cell membranes probably enhances Ca2+ efflux through the Ca2+ ATPase without directly affecting the characteristics of the pump.

Dietary cholesterol influences cardiac beta-adrenergic receptor adenylate cyclase activity in the marmoset monkey by changes in membrane cholesterol status

The activity of the beta-adrenergic receptor/adenylate cyclase system of the marmoset monkey heart was investigated following dietary cholesterol supplementation (0.5%). After 22 weeks, plasma cholesterol levels in the cholesterol group were more than twice that of the control group. In the cholesterol-fed group, the affinity for ICYP binding to cardiac membranes was elevated more than 2-fold, while the receptor number was decreased by 31%. Isoproterenol, norepinephrine and sodium fluoride stimulated adenylate cyclase activity was significantly higher in the cholesterol-fed group although the fold stimulation over basal levels was not affected. The most prominent change in the cardiac membrane lipids was an increase in the cholesterol to phospholipid ratio in marmoset monkeys fed cholesterol. These results indicate that in the marmoset, membrane cholesterol is an important factor in determining various properties of the cardiac beta-adrenergic receptor particularly receptor affinity which may impact on the response of the beta-adrenergic receptor/adenylate cyclase system of the heart to catecholamines. This result is in agreement with dietary fatty acid supplements designed to increase cardiac membrane cholesterol in this animal species (McMurchie, E.J. et al. (1988) Biochim. Biophys. Acta 937, 347-358). Elevated membrane cholesterol enhances beta-adrenergic receptor affinity and certain aspects of adenylate cyclase activity. This is a likely mechanism whereby atherogenic diets could promote cardiac arrhythmia in non-human primates and indeed in man.