Alterations in phospholipid-dependent (Na+ +K+)-ATPase activity due to lipid fluidity. Effects of cholesterol and Mg2+

Content and activity of the testis-specific isoform of angiotensin-converting enzyme are reduced in frozen-thawed bull spermatozoa

Sperm cryopreservation and thawing reduces fertility and alters the content and function of various sperm proteins. Previously, we reported that a testes-specific isoform of angiotensin-converting enzyme (tACE) was required for capacitation of bovine spermatozoa. The aim of the present study was to determine effects of sperm cryopreservation and thawing on the content, activity and localisation of tACE in bovine spermatozoa. Relative median fluorescence intensity (flow cytometry) was greater (P<0.01), tACE content (110 kDa protein) in sperm proteins was higher (P<0.01) and there was greater tACE enzyme activity (mean (±s.e.m.) 0.16±0.01 vs 0.06±0.02UmL-1; P<0.01) in fresh versus frozen-thawed spermatozoa (n=6 bulls). In fresh spermatozoa, tACE was immunolocalised in the acrosomal and principal piece regions of the sperm head and tail respectively. However, in frozen-thawed spermatozoa, there were four patterns of localisation: most frozen-thawed spermatozoa (64%) had fluorescence in the acrosomal ridge, whereas in 17% and 9% of spermatozoa the signal was limited to the post-acrosomal region and the equatorial segment respectively; in the remainder (10%), there was no signal. We conclude that cryopreservation and thawing decrease the content and activity of tACE and cause it to be translocated to other parts of the sperm head.

Molecular Dynamics of POPC Phospholipid Bilayers through the Gel to Fluid Phase Transition: An Incoherent Quasi-Elastic Neutron Scattering Study

The microscopic dynamics for the gel and liquid-crystalline phase of highly aligned D2O-hydrated bilayers of 1-palmitoyl-oleoyl-sn-glycero-phosphocholine (POPC) were investigated in the temperature range from 248 to 273 K by using incoherent quasi-elastic neutrons scattering (QENS). We develop a model for describing the molecular motions of the liquid phase occurring in the 0.3 to 350 ps time range. Accordingly, the complex dynamics of hydrogen are described in terms of simple dynamical processes involving different parts of the phospholipid chain. The analysis of the data evidences the existence of three different motions: the fast motion of hydrogen vibrating around the carbon atoms, the intermediate motion of carbon atoms in the acyl chains, and the slower translational motion of the entire phospholipid molecule. The influence of the temperature on these dynamical processes is investigated. In particular, by going from gel to liquid-crystalline phase, we reveal an increase of the segmental motion mainly affecting the terminal part of the acyl chains and a change of the diffusional dynamics from a localized rattling-like motion to a confined diffusion.

Membrane Lipid Microenvironment Modulates Thermodynamic Properties of the Na+-K+-ATPase in Branchial and Intestinal Epithelia in Euryhaline Fish In vivo

We have analyzed the effects of different native membrane lipid composition on the thermodynamic properties of the Na⁺-K⁺-ATPase in different epithelia from the gilthead seabream Sparus aurata. Thermodynamic parameters of activation for the Na⁺-K⁺-ATPase, as well as contents of lipid classes and fatty acids from polar lipids were determined for gill epithelia and enterocytes isolated from pyloric caeca, anterior intestine and posterior intestine. Arrhenius analyses of control animals revealed differences in thermal discontinuity values (Td) and activation energies determined at both sides of Td between intestinal and gill epithelia. Eyring plots disclosed important differences in enthalpy of activation (ΔH^‡) and entropy of activation (ΔS^‡) between enterocytes and branchial cells. Induction of n-3 LCPUFA deficiency dramatically altered membrane lipid composition in enterocytes, being the most dramatic changes the increase in 18:1n-9 (oleic acid) and the reduction of n-3 LCPUFA (mainly DHA, docosahexaenoic acid). Strikingly, branchial cells were much more resistant to diet-induced lipid alterations than enterocytes, indicating the existence of potent lipostatic mechanisms preserving membrane lipid matrix in gill epithelia. Paralleling lipid alterations, values of Ea₁, ΔH^‡ and ΔS^‡ for the Na⁺-K⁺-ATPase were all increased, while Td values vanished, in LCPUFA deficient enterocytes. In turn, Differences in thermodynamic parameters were highly correlated with specific changes in fatty acids, but not with individual lipid classes including cholesterol in vivo. Thus, Td was positively related to 18:1n-9 and negatively to DHA. Td, Ea₁ and ΔH^‡ were exponentially related to DHA/18:1n-9 ratio. The exponential nature of these relationships highlights the strong impact of subtle changes in the contents of oleic acid and DHA in setting the thermodynamic properties of epithelial Na⁺-K⁺-ATPase in vivo. The effects are consistent with physical effects on the lipid membrane surrounding the enzyme as well as with direct interactions with the Na⁺-K⁺-ATPase.

Na, K-ATPase: Ubiquitous Multifunctional Transmembrane Protein and its Relevance to Various Pathophysiological Conditions

The Na(+), K(+)-ATPase (NKA) is an ubiquitous enzyme consisting of α, β and γ subunits, and is responsible for the creation and maintenance of the Na(+) and K(+) gradients across the cell membrane by transporting 3 Na(+) out and 2 K(+) into the cell. Sodium pump regulation is tissue as well as isoform specific. Intracellular messengers differentially regulate the activity of the individual NKA isozymes. Regulation of specific NKA isozymes gives cells the ability to precisely coordinate NKA activity to their physiological requirements. It is the only known receptor for the cardiac glycosides used to treat congestive heart failure and cardiac arrhythmias. Endogenous ligands structurally similar to cardiac glycosides may act as natural regulators of the sodium pump in heart and other tissues. Identification of naturally occurring regulators of  NKA could initiate the discovery of new hormone-like control systems involved in the etiology of selected disease processes, hence the importance of understanding the relation of the sodium pump and its ligands to disease. Diabetes has a marked effect on the metabolism of a variety of tissues and because the NKA is critical for the membrane potential and many transports, a change in its activity in diabetes would have profound consequence in these tissues. NKA is also involved in hypertension, salt balance, cardiovascular and renal disorders, sperm capacitation, cell volume regulation, apoptosis, rheumatoid arthritis, sepsis, neurological disorders, lung edema clearance and preeclampsia. NKA activity and expression in the collecting duct of kidney are modulated physiologically by hormones like aldosterone, vasopressin, and insulin. NKA enzyme activity and subunit levels are reduced in carcinoma, NKA-β levels were highly reduced in an invasive form of human renal clear cell carcinoma, androgen-dependent prostate cancer, in early stages of urothelial cancer, as well as in poorly differentiated, highly motile carcinoma cell lines obtained from various tissues suggesting a functional link between reduced NKA-β expression and cancer progression. It could be a target for the development of anticancer drugs as it serves as a signal transducer, it is a player in cell adhesion and its aberrant expression and activity are implicated in the development and progression of different cancers.

KEYWORDS

Na(+), K(+)-ATPase (NKA); Cardiotonic steroids (CTS); Diabetes; Hypertension; Cardiovascular and renal disorders; Signal transducer; Anticancer drugs.

Effect of lupeol and lupeol linoleate on lipemic – hepatocellular aberrations in rats fed a high cholesterol diet

Cholesterol feeding has been often used to study the etiology of hypercholesterolaemia-related metabolic disturbances. The aim of the present study is to investigate the effects of a pentacyclic triterpene, lupeol, and its ester derivative on hepatic abnormalities associated with hypercholesterolemic rats. Hypercholesterolaemia was induced in male Wistar rats by feeding them with a high cholesterol diet (HCD) containing normal rat chow supplemented with 4% cholesterol and 1% cholic acid, for 30 days. Lupeol and lupeol linoleate were supplemented (50 mg/kg body wt/day) during the last 15 days. Increased hepatic lipid profile along with abnormalities in lipid-metabolizing enzyme activities were seen in hypercholesterolemic rats. An apparent increase in the expression of Acyl-CoA cholesterol acyltransferase mRNA was seen in HCD fed rats. The activities of hepatic marker enzymes, which serve as indices of cellular injury, were altered in HCD fed rats. Treatment with triterpenes significantly modulated the abnormalities induced by hypercholesterolaemia. Also, an increased (P >0.001) faecal excretion of cholesterol and bile acids were observed in lupeol and lupeol linoleate group when compared with HCD fed group. Therefore, it can be concluded that triterpenes treatment afforded substantial protection against the anomalies, which are manifested during the early stage of hypercholesterolemic atherogenesis.

Temperature-activity relationship for the intestinal Na+-K+-ATPase of Sparus aurata. A role for the phospholipid microenvironment?

The temperature dependence for Na(+)-K(+)-ATPase has been examined in the proximal-distal axis of the intestine of gilthead seabream (Sparus aurata), i.e. pyloric caeca (PC), anterior intestine (AI) and posterior intestine (PI). Data derived from the Arrhenius plots showed differences in terms of temperature discontinuity points ( Td) (13.29 degrees C, 16.39 degrees C and 17.48 degrees C for PC, AI and PI, respectively) and activation energy ratios (Ea(2)/Ea(1)) obtained at both sides of Td (2.38, 1.98 and 1.78, for PC, AI and PI, respectively). The analyses of polar lipids showed differences in the levels of certain fatty acids among intestinal regions. The content of each fatty acid and different fatty acid ratios were correlated with the corresponding Td and Ea(2)/Ea(1) values. Regression analyses revealed the existence of strong negative correlations between docosahexaenoic acid (22:6n-3, DHA) or the DHA/monoenes ratio and Td. No obvious relationships were observed for other polyunsaturated fatty acids (PUFA) nor saturated fatty acids. The results obtained in the present study indicate that the heterogeneous values of Td displayed by the Na(+)-K(+)-ATPase along the intestinal tract could be related to a modulatory role of certain fatty acid within the lipid microenvironment of the enzyme.

Effects of eicosapentaenoic acid on blood pressure, cell membrane fatty acids, and intracellular sodium concentration in essential hypertension

This study was designed to clarify the effects of orally administered eicosapentaenoic acid (EPA) on blood pressure, intracellular sodium content, and cell membrane fatty acid composition in patients with essential hypertension. After a 4-week run-in period, a study group of 17 male patients was assigned to an 8-week treatment with EPA (2.7 g/day) or placebo in a randomized, double-blind fashion with a crossover at week 4. Systolic blood pressure (SBP) was lower after treatment with EPA than after treatment with placebo (152.9+/-17.3 vs. 162.6+/-20.6 mmHg; p<0.01), while diastolic blood pressure was not statistically different. Compared with the placebo treatment, EPA supplementation resulted in a decrease in intraerythrocyte sodium content (R-Na; 11.17+/-0.63 vs. 10.44+/-1.28 nmol/l cells; p<0.05) accompanied by an increase (p<0.001) in erythrocyte membrane EPA content. The increase in membrane EPA content was related to the decrease in SBP (r=-0.52, p<0.05) and the decrease in R-Na (r=-0.57, p<0.02) during EPA treatment. The decrease in R-Na correlated positively with the decrease in SBP (r=0.54, p<0.05), and correlated negatively with the change in Na+-K+ ATPase activity (r= -0.59, p<0.02). However, the change in Na+-K+ ATPase activity did not directly correlate with the change in membrane EPA content. In conclusion, oral EPA supplementation increased membrane EPA content and reduced SBP in patients with essential hypertension. Based on the association between the increase in membrane EPA content and the decrease in intracellular sodium concentration, EPA may lower blood pressure by altering the activities of the membrane sodium transport systems.

In vivo and in vitro influence of etretinate on erythrocyte membrane fluidity

The molecular mechanisms underlying the action of synthetic retinoids have been studied intensively, but they are not fully understood yet. It is well known that retinoids exert their effects on gene expression via the retinoic acid receptor. Some observations suggest that the main aromatic retinoid etretinate (Tigason) exerts its therapeutic effect in psoriasis also through an action on the cell membrane. In this paper, we present the results of previously unreleased experiments (when Tigason was still in use) concerning the in vivo and in vitro influence of etretinate on erythrocyte membrane fluidity in psoriatic patients. Erythrocytes from healthy subjects and topically treated psoriatics were chosen as control groups. Membrane fluidity was measured by the electron paramagnetic resonance (EPR) spin-labelling technique. Erythrocytes from psoriatic patients had lower membrane fluidity, a lower antioxidant activity and a greater susceptibility to peroxidation than those from healthy subjects. After treatment with etretinate, a significant increase in erythrocyte membrane fluidity and in antioxidant activity as well as a decrease in lipid peroxidation were observed in erythrocytes from patients. Local therapy of psoriatic lesions had no influence on the improvement in membrane fluidity and antioxidant activity of erythrocytes. Incubation of erythrocytes from healthy controls and topically treated psoriatics with etretinate in vitro confirmed its fluidizing effect on erythrocyte membranes. These data may indicate that two mechanisms lead to an increase in erythrocyte membrane fluidity in psoriatic patients treated with Tigason: the first one, indirect, by improvement of the antioxidant defence system and cell protection against lipid peroxidation, and the second one, by a direct fluidizing effect of etretinate on the erythrocyte membrane.

24,25-(OH)(2)D(3) regulates cartilage and bone via autocrine and endocrine mechanisms

The purpose of this paper is to summarize recent advances in our understanding of the physiological role of 24(R),25(OH)(2)D(3) in bone and cartilage and its mechanism of action. With the identification of a target cell, the growth plate resting zone (RC) chondrocyte, we have been able to use cell biology methodology to investigate specific functions of 24(R),25(OH)(2)D(3) and to determine how 24(R),25(OH)(2)D(3) elicits its effects. These studies indicate that there are specific membrane-associated signal transduction pathways that mediate both rapid, nongenomic and genomic responses of RC cells to 24(R),25(OH)(2)D(3). 24(R),25(OH)(2)D(3) binds RC chondrocyte membranes with high specificity, resulting in an increase in protein kinase C (PKC) activity. The effect is stereospecific; 24R,25(OH)(2)D(3), but not 24S,25-(OH)(2)D(3), causes the increase, indicating a receptor-mediated response. Phospholipase D-2 (PLD2) activity is increased, resulting in increased production of diacylglycerol (DAG), which in turn activates PKC. 24(R),25(OH)(2)D(3) does not cause translocation of PKC to the plasma membrane, but activates existing PKCalpha. There is a rapid decrease in Ca(2+) efflux, and influx is stimulated. 24(R),25(OH)(2)D(3) also reduces arachidonic acid release by decreasing phospholipase A(2) (PLA(2)) activity, thereby decreasing available substrate for prostaglandin production via the action of cyclooxygenase-1. PGE(2) that is produced acts on the EP1 and EP2 receptors expressed by RC cells to downregulate PKC via protein kinase A, but the reduction in PGE(2) decreases this negative feedback mechanism. Both pathways converge on MAP kinase, leading to new gene expression. One consequence of this is production of new matrix vesicles containing PKCalpha and PKCzeta and an increase in PKC activity. The chondrocytes also produce 24(R),25(OH)(2)D(3), and the secreted metabolite acts directly on the matrix vesicle membrane. Only PKCzeta is directly affected by 24(R),25(OH)(2)D(3) in the matrix vesicles, and activity of this isoform is inhibited. This effect may be involved in the control of matrix maturation and turnover. 24(R),25(OH)(2)D(3) causes RC cells to mature along the endochondral developmental pathway, where they become responsive to 1alpha,25(OH)(2)D(3) and lose responsiveness to 24(R),25(OH)(2)D(3), a characteristic of more mature growth zone (GC) chondrocytes. 1alpha,25(OH)(2)D(3) elicits its effects on GC through different signal transduction pathways than those used by 24(R),25(OH)(2)D(3). These studies indicate that 24(R),25(OH)(2)D(3) plays an important role in endochondral ossification by regulating less mature chondrocytes and promoting their maturation in the endochondral lineage.

The nutritional significance of sterol metabolic constraints in the generalist grasshopper Schistocerca americana

Sterols are essential nutrients for grasshoppers, as well as all other insects, but metabolic constraints can limit which phytosterols support normal growth and development. In the current study, the generalist grasshopper Schistocerca americana was used to address two questions related to grasshopper sterol nutrition: (1) how does sterol quantity influence growth and survival, and (2) how do mixtures of suitable and unsuitable sterols at different concentrations influence growth and survival? Results from the first experiment indicated that this grasshopper species had a minimum sterol requirement of 0.05% dry weight; as sterol quantity increased above this concentration, however, survival and performance were not enhanced. Results from the second experiment revealed two novel aspects of sterol nutrition in grasshoppers: (1) when suitable sterols were limiting, most individuals could not use unsuitable sterols to meet the minimum sterol requirement (i.e. no sparing occurred), and (2) above a certain threshold, unsuitable sterols were deleterious even when suitable sterols were present at a concentration that alone permits normal growth and development. We discuss these physiological findings in terms of how sterol metabolic constraints in grasshoppers might influence foraging.

Two different modes of inhibition of the rat brain Na+, K(+)-ATPase by triterpene glycosides, psolusosides A and B from the holothurian Psolus fabricii

Effects of two triterpene glycosides, isolated from the holothurian Psolus fabricii, on rat brain Na+, K(+)-ATPase (Na, K-pump; EC 3.6.1.3) were investigated. Psolusosides A and B (PsA and PsB) inhibited rat brain Na+, K(+)-ATPase with I50 values of 1 x 10(-4) M and 3 x 10(-4) M, respectively. PsA significantly stimulated [3H]ATP binding to Na+, K(+)-ATPase, weakly increased [3H]ouabain binding to the enzyme, and inhibited K(+)-phosphatase activity to a smaller degree than the total reaction of ATP hydrolysis. In contrast, PsB decreased [3H]ATP binding to Na+, K(+)-ATPase, and had no effect on [3H]ouabain binding to the enzyme. K(+)-Phosphatase activity was inhibited by PsB in parallel with Na+, K(+)-ATPase activity. The fluorescence intensity of tryptophanyl residues of Na+, K(+)-ATPase was increased by PsA and decreased by PsB in a dose-dependent manner. The excimer formation of pyrene, a hydrophobic fluorescent probe, was decreased by PsA only. The different characteristics of inhibition mode for these substances were explained by peculiarities of their chemical structures and distinctive affinity to membrane cholesterol.

1,25(OH)2D3 regulates protein kinase C activity through two phospholipid-dependent pathways involving phospholipase A2 and phospholipase C in growth zone chondrocytes

We have previously shown that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a major role in growth zone chondrocyte (GC) differentiation and that this effect is mediated by protein kinase C (PKC). The aim of the present study was to identify the signal transduction pathway used by 1,25(OH)2D3 to stimulate PKC activation. Confluent, fourth passage GC cells from costochondral cartilage were used to evaluate the mechanism of PKC activation. Treatment of GC cultures with 1,25(OH)2D3 elicited a dose-dependent increase in both inositol-1,4,5-trisphosphate and diacylglycerol (DAG) production, suggesting a role for phospholipase C and potentially for phospholipase D. Addition of dioctanoylglycerol to plasma membranes isolated from GCs increased PKC activity. Neither pertussis toxin nor choleratoxin had an inhibitory effect on PKC activity in control or 1,25(OH)2D3-treated GCs, indicating that neither Gi nor Gs proteins were involved. Phospholipase A2 inhibitors, quinacrine, OEPC (selective for secretory phospholipase A2), and AACOCF3 (selective for cytosolic phospholipase A2), and the cyclooxygenase inhibitor indomethacin decreased PKC activity, while the phospholipase A2 activators melittin and mastoparan increased PKC activity in GC cultures. Arachidonic acid and prostaglandin E2, two downstream products of phospholipase A2 action, also increased PKC activity. These results indicate that 1,25(OH)2D3-dependent stimulation of PKC activity is regulated by two distinct phospholipase-dependent mechanisms: production of DAG, primarily via phospholipase C and production of arachidonic acid via phospholipase A2.

Reduced platelet serotonin content and release in familial hypercholesterolaemia

Plasma and platelet serotonin (5-HT) concentrations, and resting and collagen-induced 5-HT release in platelet-rich plasma were studied in normal and familial hypercholesterolaemic (FH) subjects. Platelet 5-HT concentrations were significantly reduced (-37%, P < 0.01) in FH patients whilst mean plasma concentrations, although increased, were not significantly different from those in normal subjects. Platelet 5-HT correlated negatively with plasma cholesterol when the data for normal subjects and FH, patients were combined (r = -0.48, P = 0.005). It also correlated negatively with low-density lipoprotein (LDL) (FH data, r = -0.59, P = 0.03; normal and FH data, r = -0.49, P = 0.004) but positively with high-density lipoprotein (HDL) (FH r = 0.79, P = 0.001; normal and FH, r = 0.37, P = 0.03). Collagen (5-160 micrograms/ml) stimulated platelet 5-HT release occurred in a concentration-dependent manner. In FH patients stimulated 5-HT release was reduced (10 micrograms/ml collagen, -40%, P < 0.05) and accompanied by increased collagen EC50 values (P < 0.02). Resting 5-HT release was increased substantially in FH patients but not significantly. Our data provide evidence for a relationship between circulating cholesterol and platelet serotonergic mechanisms. It is proposed that abnormalities relating to platelet-plasma 5-HT dynamics, perhaps due to enhanced platelet activity or decreased platelet uptake, may contribute to the cardiovascular complications in FH.

Increased hepatic Na,K-ATPase activity during hepatic regeneration is associated with induction of the beta1-subunit and expression on the bile canalicular domain

Cellular and molecular mechanisms regulating the activity of the sodium pump or Na,K-ATPase during proliferation of hepatocytes following 70% liver resection have not been defined. Na,K-ATPase may be regulated by synthesis of its alpha- and beta-subunits, by sorting to either the sinusoidal or apical plasma membrane domains, or by increasing membrane lipid fluidity. This study investigated the time course of changes during hepatic regeneration for Na, K-ATPase activity, lipid composition and fluidity, and protein content of liver plasma membrane subfractions. As early as 4 h after hepatic resection, Na,K-ATPase activity was increased selectively in the bile canalicular fraction. It reached a new steady state at 12 h and remained elevated for 2 days. Although hepatic regeneration was associated with a reduced cholesterol/phospholipid molar ratio and increased fluidity, measured with two different probes, these changes in lipid metabolism were in the sinusoidal membrane domain. The Na,K-ATPase beta1-subunit, but not the alpha1-subunit, was increased selectively at the bile canalicular surface as shown by immunoblotting of liver plasma membrane subfractions and the morphological demonstration at both the light and electron microscopic levels. Furthermore, cycloheximide blocked the rise in beta1-subunit mRNA levels. Since the time course for beta1-subunit accumulation was similar to that for activation of Na,K-ATPase activity, this change implicated the beta1-subunit in activating sodium pump activity.

Effect of 17 beta-estradiol on chondrocyte membrane fluidity and phospholipid metabolism is membrane-specific, sex-specific, and cell maturation-dependent

In this study we examined the hypothesis that 17 beta-estradiol exerts both rapid and direct, nongenomic effects of cells in the endochondral pathway. To do this, we used a cell culture model in which chondrocytes at two distinct stages of cell maturation are isolated from the costochondral cartilage of male and female rats, and examined the short-term effect of 17 alpha- and 17 beta-estradiol on [14C]arachidonic acid turnover in the cell layer and phospholipase A2 specific activity in plasma membranes and extracellular matrix vesicles isolated from similarly prepared cultures. In addition, the effect of 17 alpha- and 17 beta-estradiol on plasma membrane and matrix vesicle membrane fluidity was assessed. The effect of hormone on arachidonic acid turnover was rapid, time- and concentration-dependent, stereo-specific, and cell maturation-specific. Only resting zone cells from female rats were affected, and only 17 beta-estradiol elicited a response. Similarly, only female rat resting zone chondrocytes exhibited a change in phospholipase A2 activity after a 24 h exposure to hormone, causing an increase in enzyme activity in the matrix vesicles, but not plasma membranes. When isolated membranes were incubated directly with hormone, membrane fluidity was decreased in both plasma membranes and matrix vesicles isolated from female rat resting zone chondrocyte cultures. This nongenomic effect was dose-dependent and stereo-specific and differentially expressed in the two membrane fractions with respect to time course and magnitude of response. These results support the hypothesis that 17 beta-estradiol has a rapid action on chondrocyte membrane lipid metabolism and suggest that specific membrane components, characteristic of a particular sex and state of cell maturation, are involved in the nongenomic effects of this sex hormone on isolated matrix vesicles and plasma membranes.

Alteration in liver plasma membrane phospholipids and protein kinase activities during the development of chick embryo

The changes in phospholipid compositions, membrane fluidity and protein kinase A, protein kinase C, tyrosine and casein kinase activities in chick embryo liver plasma membranes during development have been investigated. The percentage participation of sphingomyelin increased while that of phosphatidylserine decreased during chick embryo development. The alterations in membrane sphingomyelin accompanied an increase of steady-state fluorescence anisotropy (rs) of membrane bilayer. Regression analysis indicated positive linear correlations between the percentage participation of sphingomyelin in total membrane phospholipids and (i) protein kinase C (r = 0.903); (ii) casein kinase (r = 0.936); (iii) protein kinase A (r = 0.850); (iv) tyrosine kinase (r = 0.960) activities. We suggest that sphingomyelin might be an specific activator for all types of protein kinase activities investigation.

Alterations in sodium metabolism as an etiological model for hypertension

An adequate matching for race, sex, stage of the menstrual cycle, family history of hypertension, and the amount of sodium and other electrolytes in the diet should be a prerequisite for valid conclusions when interpreting the erythrocyte concentration and fluxes of sodium in essential hypertensive patients in comparison with normal subjects. Alterations in intracellular sodium concentration and transmembrane sodium transport systems as causes of essential hypertension are postulated. This review article describes how this abnormal sodium and calcium metabolism translates into increased systemic vascular resistance through altered vasoactive responses and/or vasculature structural changes.

Role of rat liver plasma membrane phospholipids in regulation of protein kinase activities

The role of rat liver plasma membrane phospholipids in the regulation of protein kinase A, protein kinase C and tyrosine kinase activities was investigated. Plasma membrane composition was modified by phospholipase A2, phospholipase C and phospholipase D treatment and subsequent incorporation of various phospholipids. Phospholipase A2 deactivated the three types of protein kinases, while phospholipase C and D affected the enzymes in a different manner. Phosphatidylcholine and sphingomyelin were found to be the most effective activators of protein kinase A and tyrosine kinase. Incorporation of sphingomyelin and phosphatidylserine into partially delipidated plasma membranes resulted in a significant stimulation of protein kinase C activity. Since sphingomyelin appeared to be a specific effector of the three types of protein kinases under investigation, one might suggest that its role in cellular signaling could be manifested via regulation of protein kinase C as well as via modulation of protein kinase A and tyrosine kinase activities.

Pravastatin has no direct effect on transmembrane cationic transport systems in human erythrocytes and platelets

In vitro incubation of human erythrocytes and platelets with the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor pravastatin in the concentration range 1 nM to 10 microM did not affect the activity of the Na(+)-K(+)-pump, Na(+)-K(+)-cotransport or Na(+)-Li(+)-countertransport, or the ground membrane leak for Na+ and K+. The data indicate that pravastatin has no direct effect on transmembrane cationic transport systems in red blood cells or platelets.

Temperature dependence of embryonic cardiac gap junction conductance and channel kinetics

We have investigated the effects of temperature on the conductance and voltage-dependent kinetics of cardiac gap junction channels between pairs of seven-day embryonic chick ventricle myocytes over the range of 14-26 degrees C. Records of junctional conductance (Gj) and steady-state unit junctional channel activity were made using the whole-cell double patch-clamp technique while the bath temperature was steadily changed at a rate of about 4 degrees C/min. The decrease in Gj upon cooling was biphasic with a distinct break at 21 degrees C. In 12 cell pairs, Q10 was 2.2 from 26 to 21 degrees C, while between 21 and 14 degrees C it was 6.5. The mean Gj at 22 degrees C (Gj22) was 3.0 +/- 2.1 nS, ranging in different preparations from 0.24 to 6.4 nS. At room temperature, embryonic cardiac gap junctions contain channels with conductance states near 240, 200, 160, 120, 80 and 40 pS. In the present study, we demonstrate that cooling decreases the frequency of channel openings at all conductance levels, and at temperatures below 20 degrees C shifts the prevalence of openings from higher to lower conductance states: all 240 pS openings disappear below 20 degrees C; 200 pS openings are suppressed at 17 degrees C; below 16 degrees C 160 and 120 pS events disappear and only 80 and 40 pS states are seen. Temperature also affected the voltage-dependent kinetics of the channels. Application of a 6 sec, 80 mV voltage step across the junction (Vj80) caused a biexponential decay in junctional conductance. Decay was faster at lower temperatures, whereas the rate of recovery of Gj after returning to Vj0 was slowed. Cooling reduced the fast decay time constant, increased both recovery time constants, and decreased the magnitude of Gj decay, thus leaving a 10-16% larger residual conductance (Gss/Ginit, +/- 80 mV Vj) at 18 than at 22 degrees C. From these results we propose that embryonic chick cardiac gap junctions contain at least two classes of channels with different conductances and temperature sensitivities.

Effect of ethanol administration and withdrawal on serotonin receptor subtypes and receptor-mediated phosphoinositide hydrolysis in rat brain

The effect of short-term (15 days) and long-term (60 days) ethanol treatment and withdrawal on agonist-stimulated phosphoinositide (Pl) hydrolysis, serotonin receptor subtypes (5HT1A and 5HT2), and alpha 1-adrenergic receptors were studied in rat cerebral cortex. Short-term ethanol treatment had no significant effect on serotonin (5HT), norepinephrine (NE), and calcium ionophore (A23187)-stimulated [3H]-inositol-1-phosphate ([3H]-IP1) formation and 5-HT2 receptors as measured by 125I-lysergic acid diethylamide (125I-LSD) binding, in rat cerebral cortex. However, 15 days of ethanol treatment, followed by 24 hr of withdrawal resulted in a decrease in Bmax of 125I-LSD binding without significant change in KD, as well as a decrease in 5HT-stimulated [3H]-IP1 formation in rat cerebral cortex. 5HT1A and alpha 1-adrenergic receptors were determined by using [3H]-8-hydroxy-2-(di-N-propylamino)tetralin and [3H]-prazosin as radioligand, respectively. We also observed that long-term ethanol treatment had no significant effect on Bmax and KD of 5HT2, 5HT1A, and alpha 1-adrenergic receptors, as well as NE and A23187-stimulated [3H]-IP1 formation, but significantly decreased the 5HT-stimulated [3H]-IP1 formation in rat cerebral cortex. It is possible that a decrease in 5HT-induced PI turnover after long-term ethanol exposure may be due to a decrease in coupling of 5HT2 receptors to G protein or PLC enzyme, whereas the decrease in 5HT-induced PI turnover after withdrawal may be due to a decrease in functional 5HT2 receptor number.

Phospholipid N-methylation in diabetic erythrocytes: effects on membrane Na+, K+ ATPase activity

Phospholipid methylation was quantified in non-diabetic and streptozotocin diabetic rat erythrocytes. While the total mass of methylated lipids remained the same in both groups, the relative abundance of individual methylated lipid species differed significantly in diabetic erythrocytes. Moreover, incubation of erythrocytes membranes with S-adenosyl methionine, a substrate for methyl transferases, not only increased membrane lipid methylation but also decreased Na+, K+ ATPase activity significantly. These results suggest that phospholipid methylation may cause the observed depression of erythrocyte Na+, K+ ATPase activity in diabetes and could contribute to the altered rheology of erythrocytes in diabetes.

Loss and recovery of activities of alpha+ and alpha isozymes of (Na(+) + K+)-ATPase in cortical focal ischemia: GM1 ganglioside protects plasma membrane structure and function

Alterations in cellular membrane structure and the subsequent failure of its function after CNS ischemia were monitored by analyzing changes in the plasma membrane marker enzyme (Na(+) + K(+)-ATPase. The levels of two isozymes of (Na(+) + K(+)-ATPase, alpha+ and alpha, which have distinct cellular and anatomical distributions, were studied to determine if differential cellular damage occurs in primary and peri-ischemic injury areas. The efficacy of monosialoganglioside (GM1) treatment was assessed, since this glycosphingolipid has been shown to reduce ischemic injury by protecting cell membrane structure/function. Using a rat model of cortical focal ischemia, levels of both ATPase isozyme activities were assayed in total membrane fractions from primary ischemic tissue (parietal cortex) and three peri-ischemic tissue areas (frontal, occipital, and temporal cortex) at 1, 3, 5, 7, and 14 days after ischemia. No significant loss of either isozyme's activity occurred in any tissue area at 1 day after ischemia. At 5 days, in the primary ischemic area, both isozyme activity levels decreased by 70-75%. The alpha+ enzyme activity loss persisted up to 14 days, while a 17% recovery in alpha activity occurred. In the three peri-ischemic tissue areas, enzyme activity losses ranged from 42%-59% at 3 days after ischemia. A complete restoration of both isozyme activities was seen at 14 days. After three days of GM1 ganglioside treatment there was no loss of total (Na*+) + K(+)-ATPase activity in the three peri-ischemic areas, and a significantly reduced loss in the primary infarct tissue. An autoradiographic analysis of brain coronal sections using 3H-ouabain supports the enzymatic data and GM1 effects. Reductions in 3H-ouabain binding in all cortical layers at 3 days after ischemia were visualized. GM1 treatment significantly reduced these 3H-ouabain binding losses. In summary, time-dependent quantitative changes in activity levels of ATPase isozymes (alpha+ and alpha) reflect the different degree of membrane damage that occurs in primary vs. peri-ischemic tissues (e.g., irreversible vs. reversible membrane damage), and that ischemia affects cell membranes of all neural elements in a largely similar fashion. GM1 ganglioside was found to reduce plasma membrane damage in all CNS cell types.

Phospholipid-dependence of rat liver plasma membrane protein kinase activities--a new approach

The influence of the phospholipid composition and fluidity on protein kinase A and protein kinase C activities in rat liver plasma membranes was studied. We observed that enrichment of membranes with phosphatidylglycerol, phosphatidylserine, phosphatidylethanolamine and dioleoylphosphatidylcholine caused activation of both protein kinases. Phosphatidylglycerol was found to be most effective activator. The enrichment of plasma membranes with dipalmitoylphosphatidylcholine and sphingomyelin led to decrease in protein kinase A and C activities. The stimulatory effect of phosphatidylglycerol was confirmed in plasma membranes pretreated with exogenous phospholipases A2, C and D, and subsequently enriched with phosphatidylglycerol. We suggest that besides the specific presence of definite phospholipids protein kinases A and C require a more fluid membrane lipid bilayer to display an optimal activity.

Influence of phospholipid environment on the phosphatidylethanolamine: ceramide-phosphorylethanolamine transferase activity in rat liver plasma membranes

1. Plasma membranes were treated with phospholipase A2, phospholipase C or phospholipase D. The phosphatidylethanolamine:ceramide-phosphorylethanolamine transferase was deactivated by phospholipase C treatment, whereas phospholipase A2 and phospholipase D did not affect the enzyme. 2. Incorporation of phosphatidylethanolamine and phosphatidylglycerol into partially delipidated plasma membranes resulted in significant stimulation of the transferase, whereas inclusion of sphingomyelin and phosphatidylserine suppressed the enzyme activity. Our results suggest that phosphatidylserine is a regulator of sphingomyelin level in membranes. 3. The activity of phosphatidylethanolamine:ceramide-phosphorylethanolamine transferase was not influenced by the fluidity of its lipid environment.

S-adenosyl-L-methionine reverses the cholestatic effect of ethinylestradiol in rat hepatocytes by increasing its catabolism

In this study, ethinylestradiol inhibited the uptake of taurocholate by cultured rat hepatocytes, increasing the Km while leaving the Vmax unchanged. S-Adenosyl-L-methionine (SAMe) had no effect on taurocholate uptake or release, but was able to reverse the competitive inhibition induced by ethinylestradiol. S-Adenosyl-L-homocysteine did not reverse this inhibition, which suggests that the methyl group of SAMe affects its activity. Several possible mechanisms for the action of SAMe were investigated. The methylation of cell membrane phospholipids was eliminated as a possible mechanism. The presence of SAMe greatly increased the catabolism of ethinylestradiol by hepatocytes and reduced its covalent binding to hepatocyte macromolecules. In culture supernatants, both highly polar (conjugated) and non-conjugated metabolites could be detected. Moreover, most of the metabolites were methylated. This suggests that SAMe may revert the effects of ethinylestradiol of taurocholate uptake by increasing its catabolic rate by hepatocytes.

Role of phosphatidylethanol in membranes. Effects on membrane fluidity, tolerance to ethanol, and activity of membrane-bound enzymes

We investigated the effect of phosphatidylethanol (PEt) on fluidity and membrane tolerance to the fluidization induced by ethanol as well as on the activity of two membrane-bound enzymes, Na+/K+ ATPase and 5'-nucleotidase. PEt was synthesized from 1,2-dimyristoylphosphatidylcholine and phosphatidylcholine from bovine brain and studies were performed to determine the optimal experimental conditions for the insertion of PEt in natural bilayers. The effects of PEt, evaluated by differential scanning calorimetry or fluorescence polarization techniques, were studied in model membranes made of synthetic phospholipids or made of total lipids extracted from rat brain crude mitochondrial fraction (P2 fraction) and from natural membranes (P2 fraction). The presence of PEt increased the fluidity of artificial as well of natural membranes, but tolerance to the addition of ethanol, displayed by dimyristoylphosphatidylcholine vesicles and by natural membranes containing PEt, was lacking in vesicles made of dimyristoylphosphatidylethanolamine and in artificial bilayers reconstituted from total P2 lipid extracts, suggesting an involvement of PC on PEt-induced ethanol resistance. Na+/K+ ATPase activity was enhanced by the addition of small amounts of ethanol (up to 50 mM) and progressively inhibited at higher concentrations, while 5'-nucleotidase was not affected up to 400 mM ethanol. The presence of PEt in the bilayer exerted the opposite effects on the two enzymes, reducing the Na+/K+ ATPase activation induced by ethanol and enhancing 5'-nucleotidase activity. The mechanisms of the PEt-induced modifications are discussed.

Effects of lovastatin treatment on red blood cell and platelet cation transport

Hypercholesterolemia frequently accompanies hypertension, and it has been suggested that by affecting membrane lipid composition, hypercholesterolemia may cause or accentuate abnormalities in several red blood cell transports associated with hypertension. Such an effect might obfuscate the relation of membrane markers to hypertension and decrease their usefulness in genetic studies of the heritable basis of hypertension. To determine if changing plasma lipids affects membrane transport, we studied the effects of the cholesterol-lowering agent lovastatin on red blood cell lithium-sodium countertransport and sodium-potassium-chloride cotransport, red blood cell sodium and water content, and platelet amiloride-sensitive volume responsiveness to cytoplasmic acidification, an indirect measure of sodium-proton exchange that has been proposed as a new membrane marker for hypertension. In a 24-week, placebo-controlled, double-blinded, randomized trial, lovastatin significantly lowered total and low density lipoprotein cholesterol and raised high density lipoprotein cholesterol. Red blood cell lithium-sodium countertransport and sodium-potassium-chloride cotransport were not significantly altered. Red blood cell sodium content decreased significantly in the lovastatin-treated group, probably as a result of an increase in red blood cell sodium-potassium pump activity. Platelet amiloride-sensitive responses to cytoplasmic acidification were significantly depressed by lovastatin treatment, suggesting that lowering plasma cholesterol may suppress platelet sodium-proton exchange. It has been hypothesized that the hyperlipidemias frequently observed in essential hypertensive patients may alter membrane lipid composition and affect membrane cation transport activities.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of receptors for prostaglandin E1/prostacyclin and insulin in human erythrocytes and platelets

Prostaglandin E1/I2 and insulin receptors of human erythrocyte and platelet are capable of modulating each other's activity. This modulation of the receptor activity and number in one system by a second receptor system in human platelet and erythrocyte seems to be beneficial. Insulin increases the PGE1 binding to platelets and thereby enhances the platelet antiaggregatory action of prostaglandin by increasing cyclic AMP levels. Similarly, PGE1 increases insulin binding to human erythrocyte, and thereby reduces the optimum concentration of insulin for a maximal reduction in membrane microviscosity. During ischemia the reduced response of platelets to the inhibitory effect of PGE1 or PGI2 relates to the impaired PGE1/I2 receptor activity. Treatment of these platelets with insulin at physiological concentrations can normalise the PGE1/I2 receptor activity. This review focuses on the relationship between the two receptor systems in human blood cells.

Membrane transport of ions in hypertension

A variety of disturbances in transmembrane monovalent and divalent cation fluxes has been described in blood cells from hypertensive patients. Other membrane properties, such as fluidity and calcium binding, are also altered. It is now abundantly clear that some of the inconsistencies in this field are due to poor matching of patients and controls. However, even when careful matching is carried out, differences in membrane functions are still seen. It is suggested that these are due to a disturbance in the physicochemical properties of the cell membrane, related to changes in cell membrane phospholipid fluidity. This change could maintain peripheral resistance either by directly or indirectly increasing tone or by predisposing to resistance vessel hypertrophy. Recent evidence emphasizes the role of the latter rather than the former in experimental hypertension. It is postulated that overactivity of the phosphoinositide second messenger system as a result of alteration in all membrane properties predisposes genetically susceptible individuals to resistance-vessel hypertrophy and hypertension.

Effects of the novel antidepressant S-adenosyl-methionine on alpha 1- and beta-adrenoceptors in rat brain

alpha 1- and beta-adrenoceptors were studied ex vivo in the brains of rats receiving repeated daily treatment with the standard antidepressant imipramine or the atypical antidepressant S-adenosyl-L-methionine (SAM), which has minimal effects on monoamine reuptake or turnover. Consistent with past studies, a decrease in the density of beta receptors at three weeks and an increase in the affinity of alpha 1 receptors for the agonist phenylephrine at one week of treatment was observed with imipramine. By comparison, an increase in the density of beta receptors and a decrease in the affinity of alpha 1 receptors for phenylephrine was observed at one week of treatment with SAM. These changes were no longer apparent at three weeks of treatment. The results suggest that treatment with SAM does lead to changes in adrenergic neurotransmission, but that down regulation of beta receptors or increased agonist affinity of alpha 1 receptors may not be necessary for the production of antidepressant effects.

Changes of electron spin resonance membrane fluidity in hexadecane-induced hyperproliferative epidermis

To study some of the biochemical and physical states of membranes associated with hyperproliferation, the effect of topical hexadecane on membrane fluidity in guinea pig epidermis was investigated by electron spin resonance using a 5-doxylstearic acid spin labeling agent. Guinea pig epidermal cells were separated into three regions of keratinocytes by Percoll density gradient centrifugation. Membrane fluidity and Na+, K+-ATPase activity were higher in hyperproliferating epidermal cells than in control. The free cholesterol content and the molar ratio of free cholesterol to phospholipid were found to decrease significantly. Also elevated levels of palmitic acid, stearic acid and omega-3 unsaturated fatty acid derived from phospholipid were observed. Normal differentiation of epidermis was found to be accompanied by a decrease in membrane fluidity, whereas a relatively high membrane fluidity was maintained in the hexadecane-induced hyperproliferation.

The influence of the lipid bilayer phase state on the p-aminohippurate (PAH) transport and the activity of the alkaline phosphatase in brush-border membrane vesicles from normal and mutant rats

The kinetic parameters of p-aminohippurate transport and activity of the alkaline phosphatase were studied using brush-border membrane vesicles isolated from the kidney cortex of normal and mutant (strain of Campbell) rats. p-Aminohippurate (PAH) transport of both normal and mutant animals was carried out by the mechanism of facilitated diffusion. The apparent Michaelis constant at 36 degrees C was equal to 7 mM, the maximal rate of PAH transport was 15 nmol/min per mg protein and the constant of inhibition by probenecid was 0.5 mM for normal rats and, respectively, 29 mM, 62 nmol/min per mg protein and 1.4 mM for mutant rats. The Arrhenius plot for the PAH transport and activity of the alkaline phosphatase showed the breakpoints at 28-30 degrees C for normal rats and at 36-38 degrees C for the Campbell strain rats. The thermotropic phase transitions detected by the EPR method with 5-doxylstearate as a probe were recorded at 21-30 degrees C and 30-35 degrees C for normal and mutant rats, respectively. Therefore, characteristic features of the PAH carrier and alkaline phosphatase activity in normal and Campbell strain rats are determined by the difference in the phase state of their membrane lipid bilayers. We suppose that mutation in the Campbell strain gives rise to a membrane pleiotropic effect which enables us to understand the mechanism of genetic control of the lipid structure and membrane fluidity.

Changes in the phospholipid composition and phospholipid asymmetry of ram sperm plasma membranes after cryopreservation

The changes in the phospholipid composition of spermatozoa plasma membranes after freezing were determined by thin-layer chromatography. The results showed an augmentation of the diphosphatidylglycerol and a diminution of phosphatidylglycerol, phosphatidylserine, and phosphatidylethanolamine in sperm plasma membranes after freezing. In intact sperm cells we observed an elevation of the sphingomyelin and phosphatidylinositol levels and a diminution of the phosphatidylethanolamine and diphosphatidylglycerol levels. The effect of freezing on the phospholipid distribution between the inner and outer monolayers of the plasma membrane was also studied using exogenous phospholipases and trinitrobenzene sulfonate. The most important change we observed after freezing, was the translocation of diphosphatidylglycerol from the inner to the outer monolayer of the plasma membrane.

Inhibition of rat brain Na+-K+-ATPase by triterpene glycosides from holothurians

The effect of triterpene glycosides from holothurians on Na+-K+-ATPase of rat brain was investigated. The marine glycosides are irreversible inhibitors of the enzyme with an average I50 value of 10(-4) M. ATP had a low protective effect against inhibition. The inhibitory effect was increased by preincubation with MgCl2. There was alteration of the activation curve of Na+-K+-ATPase by NaCl and KCl in the presence of glycosides. Triterpene glycosides inhibited the K+-phosphatase activity, but to a smaller degree than the ATPase activity. Na+-K+-ATPase of pig kidney was less sensitive to the marine triterpene glycosides than the brain enzyme. The marine glycosides did not alter the specific binding of [3H]-ouabain to the Na+-K+-ATPase.

Physicochemical characteristics of interaction of toxic triterpene glycosides from holothurians with rat brain Na+-K+-ATPase

High-angle X-ray diffraction spectra showed that triterpene glycosides form crystalline complexes with membrane cholesterol. Electron microscopy demonstrated a decreased vesicle size, of the membrane preparation from rat brain which is enriched in Na+-K+-ATPase, by the triterpene glycosides. The Arrhenius plot was linear in the presence of triterpene glycosides. The half-width of the phosphatidylcholine N-methyl proton line in proton NMR spectra was not altered in the presence of marine glycosides. The excimer formation of pyrene, a hydrophobic fluorescent probe, was significantly decreased by triterpene glycosides. The increase of tryptophanyl residue fluorescence demonstrated a change of the Na+-K+-ATPase conformation after treatment with cytotoxic glycosides.

Changes in parameters of lipoprotein metabolism during rat hepatic development

Maintenance of whole body cholesterol homeostasis is determined in part by the liver. Thus, changes in expression of hepatic parameters important in the regulation of cholesterol metabolism may play key roles in determining how homeostasis is maintained. The expression of hepatic lipoprotein uptake systems was studied during development using as a ligand very-low density lipoproteins rich in apolipoprotein E that had been obtained from hypercholesterolemic adult rats. These lipoproteins can serve as ligands for cell surface receptors recognizing apolipoproteins B and/or E. Uptake was lowest in freshly isolated fetal rat hepatocytes, increased substantially in hepatocytes from neonates and was intermediate in those from adults. Binding of these lipoproteins to liver membranes prepared from fetal, neonatal, suckling, weaned and adult rats was lowest in fetal preparations, while those from suckling, weaned and adult livers behaved similarly. Numbers of binding sites in neonatal liver membranes were similar to those in adult, but showed a different affinity. On the basis of this data, the ability of hepatocytes to recognize and remove apolipoprotein B/E-containing lipoproteins from the plasma appears to be a function of the differential expression or regulation of lipoprotein-uptake systems during development.

Luminescence spectroscopic approaches in studying cell surface dynamics

The major elements of membranes, such as proteins, lipids and polysaccharides, are in dynamic interaction with each other (Albertset al.1983). Protein diffusion in the lipid matrix of the membrane, the lipid diffusion and dynamic domain formation below and above their transition temperature from gel to fluid state, have many functional implications. This type of behaviour of membranes is often summarized in one frequently used word membrane fluidity (coined by Shinitzky & Henkart, 1979). The dynamic behaviour of the cell membrane includes rotational, translational and segmental movements of membrane elements (or their domain-like associations) in the plane of, and perpendicular to the membrane. The ever changing proximity relationships form a dynamic pattern of lipids, proteins and saccharide moieties and are usually described as ‘cell-surface dynamics’ (Damjanovichet al.1981). The knowledge about the above defined behaviour originates from experiments performed mostly on cytoplasmic membranes of eukaryotic cells. Nevertheless numerous data are available also on the mitochondrial and nuclear membranes, as well as endo (sarco-)plasmic reticulum (Martonosi, 1982; Slater, 1981; Siekevitz, 1981).

Temperature-dependencies of various catalytic activities of membrane-bound Na+/K+-ATPase from ox brain, ox kidney and shark rectal gland and of C12E8-solubilized shark Na+/K+-ATPase

The temperature dependence of ouabain-sensitive ATPase and phosphatase activities of membrane fragments containing the Na+/K+-ATPase were investigated in tissue from ox kidney, ox brain and from shark rectal glands. The shark enzyme was also tested in solubilized form. Arrhenius plots of the Na+/K+-ATPase activity seem to be linear up to about 20 degrees C, and non-linear above this temperature. The Arrhenius plots of mammalian enzyme (ox brain and kidney) were steeper, especially at temperatures below 20-30 degrees C, than that of shark enzyme. The Na+-ATPase activity showed a weaker temperature-dependence than the Na+/K+-ATPase activity. The phosphatase reactions measured, K+-stimulated, Na+/K+-stimulated and Na+/K+/ATP-stimulated, also showed a weaker temperature-dependence than the overall Na+/K+-ATPase activity. Among the phosphatase reactions, the largest change in slope of the Arrhenius plot was observed with the Na+/K+/ATP)-stimulated phosphatase reaction. The Arrhenius plots of the partial reactions were all non-linear. Solubilization of shark enzyme in C12E8 did not change the curvature of Arrhenius plots of the Na+/K+-ATPase activity or the K+-phosphatase activity. Since solubilization involves a disruption of the membrane and an 80% delipidation, the observed curvature of the Arrhenius plot can not be attributed to a property of the membrane as such.

Red cell membrane (Na+ +K+)-ATPase in diabetes mellitus

The red cell membrane (Na+ +K+)-ATPase activity is significantly elevated in diabetes mellitus. The osmotic fragility of diabetic red cells is also increased. In vivo insulin treatment restores the enzyme activity and the osmotic fragility to the normal level. In vitro insulin treatment of diabetic red cells was found to inhibit the further increase in its activity, but it failed to restore the activity to the normal level as in vivo. In diabetes increased Km of (Na+ +K+) ATPase for ATP was observed but Vmax remained the same. Arrhenius plot of this enzyme was also altered in diabetes.

Increased ischemia-reperfusion injury to the heart associated with short-term, diet-induced hypercholesterolemia in rabbits

The effects of increased dietary cholesterol content on coronary vascular hemodynamics and endothelial cell transport function were assessed in isolated rabbit hearts during 3.5 hours of reperfusion after 30 minutes of global, no-flow ischemia. In control hearts from rabbits fed normal chow, perfusion pressure, left ventricular end-diastolic pressure, maximum +dP/dt, and the rate of intravascular clearance of radiolabelled albumin remained constant during 5 hours of continuous perfusion, while the mean transit time of radiolabelled albumin increased 1.6 X baseline. In ischemic hearts from rabbits fed normal chow, perfusion pressure increased 59% during reperfusion while left ventricular end-diastolic pressure and maximum +dP/dt returned toward control levels. The rate of intravascular clearance of radiolabelled albumin decreased 36%, and the mean transit time of albumin increased approximately 3 X baseline. Ischemia-reperfusion injury to the cardiac vasculature and musculature was markedly increased in hearts of rabbits fed chow supplemented with 2% cholesterol for 2-3 weeks compared to rabbits fed the same diet for a longer duration (5-16 weeks) or rabbits fed normal chow. Prior to ischemia, permeation of the coronary vasculature by albumin was increased twofold in rabbits fed cholesterol for 2-3 weeks while myocyte contractile function was normal relative to chow-fed controls or the group fed cholesterol for 5-16 weeks. These effects of acute cholesterol feeding precede occlusive atherosclerotic coronary artery disease and occur at plasma cholesterol concentrations one third of those in rabbits fed cholesterol for the longer duration.(ABSTRACT TRUNCATED AT 250 WORDS)