Hepatic sinusoidal endothelial fenestrae express plasma membrane Ca++pump and Ca++Mg++‐ATPase

Abstract: Background/Aim: In general, intracytoplasmic free calcium ions (Ca⁺⁺) are maintained at a very low concentration in mammalian tissue by extruding Ca⁺⁺ against a high concentration of extracellular Ca⁺⁺, mainly through the activity of the plasma membrane Ca⁺⁺pump‐ATPase. The aim of the present study was to demonstrate by electron cytochemical and immunogold methods the ultrastructural localization of two different types of plasma membrane Ca⁺⁺‐ATPase, i.e. Ca⁺⁺Mg⁺⁺‐ATPase and Ca⁺⁺pump‐ATPase in the hepatic sinusoidal endothelium. Methods: Liver tissues and the isolated hepatic sinusoidal endothelial cell (SEC)s were subjected to the following procedures. The ultrastructural localizations of Ca⁺⁺Mg⁺⁺‐ATPase were examined by an electron cytochemical method. The ultrastructural localization of Ca⁺⁺pump‐ATPase was identified by an electron immunogold method. Results: The cytochemical reaction of Ca⁺⁺Mg⁺⁺‐ATPase was found to be localized on the outer sites of the plasma membrane of sinusoidal endothelial fenestrae (SEF). The immunogold particles indicating the presence of Ca⁺⁺ pump‐ATPase were identified on the inner sites (cytoplasmic) of the invaginated plasma membrane of SEF. Conclusions: Both Ca⁺⁺Mg⁺⁺‐ATPase and Ca⁺⁺pump‐ATPase demonstrated on the SEF plasma membrane may be involved in the regulation of intracytoplasmic Ca⁺⁺ concentration.     

Hepatic stellate cells express Ca2+ pump-ATPase and Ca2+-Mg2+-ATPase in plasma membrane of caveolae

Intracytoplasmic free calcium ions (Ca²⁺) are maintained at a very low concentration in mammalian tissue by the extrusion of Ca²⁺ across a steep extracellular Ca²⁺ gradient, mainly through the activity of plasma membrane Ca²⁺ pump-ATPase. The present study aimed to identify, by electron cytochemical and electron immunogold methods, the ultrastructural localizations of two types of plasma membrane Ca²⁺-ATPase; Ca²⁺-Mg²⁺-ATPase and Ca²⁺ pump-ATPase, in hepatic stellate cells. Liver tissues and isolated hepatic stellate cells (HSCs) were studied. The ultrastructural localization of Ca²⁺-Mg²⁺-ATPase activity was examined by the electron cytochemical method of Ando. The localization of Ca²⁺ pump-ATPase was identified by immunofluorescence. The ultrastructural localization of Ca²⁺ pump-ATPase was identified by the electron immunogold method. The cytochemical reaction products of Ca²⁺-Mg²⁺-ATPase activity were localized on the outer (cavity) side of the plasma membrane of caveolae. Immunofluorescence of Ca²⁺ pump-ATPase was seen as small dots along the cell edge in HSCs. Immunogold particles indicating the presence of Ca²⁺ pump-ATPase were identified on the inner (cytoplasmic) side of the plasma membrane of caveolae. We localized Ca²⁺ pump-ATPase on the inner side of the plasma membrane caveolae and Ca²⁺-Mg²⁺-ATPase on the outer leaflet of the caveolar plasma membrane in stellate cells, suggesting that Ca²⁺ pump-ATPase may play a key role in the Ca²⁺ reflux. Received: March 7, 2000 / Accepted: July 7, 2000     

Endothelial nitric oxide synthase and caveolin-1 are co-localized in sinusoidal endothelial fenestrae.

BACKGROUND/AIMS: Nitric oxide is synthesized in diverse mammalian tissues by a family of calmodulin-dependent nitric oxide synthases (NOS). Caveolin, the principal structural protein in caveolae, interacts with endothelial NOS leading to enzyme inhibition in a reversible process modulated by Ca++-calmodulin. The aim of the present study was to clarify the ultrastructural localization of eNOS and caveolin-1 in hepatic sinusoidal endothelium by an electron immunogold method. METHODS: Male Wistar rats were used. Liver tissues and hepatic sinusoidal endothelial cells isolated from rat livers by collagenase infusion were studied. For immunohistochemistry, liver specimens were reacted with anti-eNOS or anti-caveolin-1 antibody. The ultrastructural localization of eNOS or caveolin-1 was identified by electron microscopy using an immunogold post-embedding method. RESULTS: Immunohistochemical studies using liver tissues localized endothelial NOS in hepatic sinusoidal lining cells, portal veins and hepatic arteries; and caveolin-1 in sinusoidal lining cells, bile canaliculi, portal vein and hepatic arteries. Immunogold particles indicating the presence of eNOS and caveolin-1 were demonstrated on the plasma membrane of sinusoidal endothelial fenestrae in liver tissue and also in isolated sinusoidal endothelial cells. CONCLUSION: Endothelial NOS and caveolin are co-localized on sinusoidal endothelial fenestrae, suggesting that interaction of the two may modulate cellular regulation of NO synthesis.     

Endothelial nitric oxide synthase and caveolin‐1 are co‐localized in sinusoidal endothelial fenestrae

Abstract: Background/Aims: Nitric oxide is synthesized in diverse mammalian tissues by a family of calmodulin‐dependent nitric oxide synthases (NOS). Caveolin, the principal structural protein in caveolae, interacts with endothelial NOS leading to enzyme inhibition in a reversible process modulated by Ca⁺⁺‐calmodulin. The aim of the present study was to clarify the ultrastructural localization of eNOS and caveolin‐1 in hepatic sinusoidal endothelium by an electron immunogold method. Methods: Male Wistar rats were used. Liver tissues and hepatic sinusoidal endothelial cells isolated from rat livers by collagenase infusion were studied. For immunohistochemistry, liver specimens were reacted with anti‐eNOS or anti‐caveolin‐1 antibody. The ultrastructural localization of eNOS or caveolin‐1 was identified by electron microscopy using an immunogold post‐embedding method. Results: Immunohistochemical studies using liver tissues localized endothelial NOS in hepatic sinusoidal lining cells, portal veins and hepatic arteries; and caveolin‐1 in sinusoidal lining cells, bile canaliculi, portal vein and hepatic arteries. Immunogold particles indicating the presence of eNOS and caveolin‐1 were demonstrated on the plasma membrane of sinusoidal endothelial fenestrae in liver tissue and also in isolated sinusoidal endothelial cells. Conclusion: Endothelial NOS and caveolin are co‐localized on sinusoidal endothelial fenestrae, suggesting that interaction of the two may modulate cellular regulation of NO synthesis.     

Regulatory mechanisms of hepatic microcirculation

Hepatic microvasculature receives blood from two types of afferent vessels: the terminal portal venule (TPVn) and the terminal hepatic arteriole (THAo). The TPVns directly connect with the capillary bed in the liver parenchyma, which is referred to as sinusoids. Hepatic arterial blood pours into the hepatic sinusoids not only indirectly via the anastomosis between the THAo and the portal venule (PVn), but also directly through the THAo or the capillaries derived from the arterial capillary network around the bile duct. From a regulatory point of view, the hepatic arterial system is considered to be supplementary, but hepatic arterial flow is essential for supplying oxygen to sinusoidal blood flow as well as to the bile ducts, portal venules and nerves in the portal tract. The main regulators of hepatic sinusoidal blood flow are present in the portal venous system. By intravital and scanning electron microscopy, it is evident that a potent vasoconstrictor endothelin (ET)-1 causes a contraction of the SEF via the ET_B receptors, as well as a significant contraction of the PVn and TPVn, resulting in an increase in sinusoidal and pre-sinusoidal microvascular resistance. This phenomenon implies that the TPVn, particularly the transitional part to the sinusoid, would provide an essential regulatory site for hepatic sinusoidal blood flow as an inlet sphincter-like function. The endothelial cell linings along the hepatic sinusoids are characterized by the presence of a large number of sieve plate-like pores, 100 nm in diameter, i.e. the sinusoidal endothelial fenestrae (SEF). The SEF are dynamic structures, forming the racemose invaginations of the endothelial plasma membrane across the endothelium, and regulating not only the permeability of hepatic sinusoids, but also the sinusoidal blood flow by the Ca++ -actomyosin-mediated contraction and dilatation of the SEF. Our recent immunoelectron microscopic and Western blot studies have revealed that caveolin-1, i.e. the principal structural protein of caveolae, and endothelial nitric oxide synthase (eNOS) co-exist in the plasma membrane of the SEF, implying that the SEF may correspond to a permanent (stationary) type of fused and interconnected caveolae, thus contributing to the local control of hepatic sinusoidal blood flow by the regulation of NO synthesis.     

Local regulators of hepatic sinusoidal microcirculation: recent advances

This article reviews our recent studies on the local regulation of hepatic microcirculation with special reference to the inlet sphincter-like structures, the roles of sinusoidal endothelial cells and the mechanism of dynamic changes in the sinusoidal endothelial fenestrae (SEF) as well as in the terminal portal venules and the terminal hepatic arterioles induced by the potent vasoconstrictor endothelin (ET)-1. There are two types of sphincter-like structures at the entering sites of hepatic sinusoids. One is located at the junction between the terminal portal venule and the sinusoid, and is characterized by the large endothelial cells surrounded with Ito cells (hepatic stellate cells: HSCs). The other is located at the junction between the terminal hepatic arteriole and the sinusoid, and corresponds to the precapillary sphincter since our enzymohistochemical demonstration of arterial capillaries in close association with the sinusoids combined with intravital microscopy has revealed that the terminal hepatic arteriole directly terminates in the sinusoid. It is essential for the local control of hepatic sinusoidal blood flow that the dynamic contracting and relaxing changes not only in these inlet sphincter-like structures but also in the SEF correspond with those of the HSCs, both of which are mediated by the sinusoidal endothelium-derived vasoconstrictor endothelins (ETs) and vasodilator nitric oxide (NO). The contractility of the SEF and HSCs depends on the intracellular Ca++-calmodulin-actomyosin system.     

Pharmacological and immunohistochemical characterization of calmodulin-stimulated (Ca(2+)+Mg(2+))-ATPase in cultured porcine aortic endothelial cells

Plasma membrane (Ca(2+)+Mg(2+))-ATPase and Ca(2+) transport activities, best characterized in human erythrocytes, are stimulated by calmodulin and thought to play a crucial role in the termination of cellular Ca(2+) signaling in all cells. In plasma membranes isolated from cultured porcine aortic endothelial cells, the (Ca(2+)+Mg(2+))-ATPase was not readily measured. This is in part because of an overabundance of nonspecific Ca(2+)- and/or Mg(2+)-activated ecto-5'-nucleotide phosphohydrolases. Moreover, addition of exogenous calmodulin (10(-9) to 10(-6) mol/L) produced no measurable stimulation of ATPase activities, suggesting a permanently activated state or, alternatively, a complete lack thereof. To establish and verify the presence of a calmodulin-regulated (Ca(2+)+Mg(2+))-ATPase activity in these endothelial cells, immunohistochemical localization using a monoclonal mouse anti-(Ca(2+)+Mg(2+))-ATPase antibody (clone 5F10) was applied to intact pig aorta endothelium, cultured endothelial monolayers, and isolated endothelial plasma membrane fractions. This approach clearly demonstrated Ca(2+) pump immunoreactivity in each of these preparations. To confirm functional calmodulin stimulation of the (Ca(2+)+Mg(2+))-ATPase, 10(-5) mol/L calmidazolium (R24571) was added to the isolated plasma membrane preparation, which lowered the (Ca(2+)+Mg(2+))-ATPase activity from 143.0 to 78.15 nmol P(i)/mg protein x min(-1). This calmidazolium-reduced activity could then be stimulated 113.1+/-0.8% in a concentration-dependent manner by the addition of exogenous calmodulin (10(-7) to 2 x 10(-6) mol/L) with an EC(50) of 3.45+/-0.04 x 10(-7) mol/L (n=4). This represents a competitive lowering of the apparent calmodulin affinity by approximately 100 compared with other unopposed calmodulin-stimulated processes. Together, these findings support evidence for the presence of a calmodulin-stimulated plasma membrane (Ca(2+)+Mg(2+))-ATPase activity in cultured porcine aortic endothelial cells.     

Biochemical separation of Na+,K+-ATPase from a "purified" light density, "canalicular"-enriched plasma membrane fraction from rat liver

Cytochemical studies suggest that Na+,K+-ATPase is localized to sinusoidal and lateral portions of the hepatocyte plasma membrane whereas Mg++-ATPase and alkaline phosphatase are luminal or canalicular membrane markers. To validate further these cytochemical findings, we have isolated from the nuclear pellet of rat liver homogenates a liver plasma membrane (LPM) fraction enriched in all three enzyme markers, as previously described (Biochimica et Biophysica Acta 1975; 401:59-52). Following tight Dounce homogenization, the vesiculated membrane preparation was further separated on a multiple-step discontinuous sucrose density gradient (d 1.12 to 1.22). Na+,K+-ATPase activity "dissociated" from Mg++-ATPase activity, sedimenting at densities of 1.14 and greater. Further studies were carried out in two-step discontinuous sucrose gradients (1.13 and d greater than 1.13), and a light density fraction (d 1.13) was further characterized in calcium-free media (since addition of calcium increased contamination with intracellular membranes). Electron microscopy demonstrated a homogeneous vesicular membrane population in contrast to the heavy density fraction (d greater than 1.13) which contained membrane sheets and junction complexes as well as vesicles. The light density fraction was highly enriched n Mg++-ATPase (42.1 x homogenate specific activity) and alkaline phosphatase (64.6 x homogenate), 3 to 4 times their activities in the original LPM. In contrast, Na+,K+-ATPase activity in the light density fraction, diminished 16-fold from values in the original unfractionated LPM. All but 15% of total Na+,K+-ATPase activity in the original LPM could be accounted for in unwashed preparations. Neither cholesterol/phospholipid ratios nor an analysis of peptides on sodium dodecyl sulfate gel electrophoresis demonstrated differences in the composition of the light vs. heavy density subfractions, although there were relative increases in several peptide bands in the light density subfraction. These studies provide further supporting biochemical evidence for the concept that Na+,K+-ATPase resides on different membrane domains than does Mg++-ATPase and alkaline phosphatase and further characterizes a vesiculated membrane preparation highly enriched in putative "canalicular" enzyme markers.     

Vascular endothelial growth factor increases fenestral permeability in hepatic sinusoidal endothelial cells

Abstract: Vascular endothelial growth factor (VEGF) is an important regulator of vasculogenesis and vascular permeability. Hepatic sinusoidal endothelial cells (SECs) possess sieve‐like pores that form an anastomosing labyrinth structure by the deeply invaginated plasma membrane. Caveolin is the principal structural protein in caveolae. In this study, we examined the role of VEGF on the fenestration and permeability of SECs and the relation with caveolin‐1. SECs isolated from rat livers by collagenase infusion method were cultured for 24 h with (10 or 100 ng/ml) or without VEGF. The cells were then examined by transmission and scanning electron microscopy (EM). The expression of caveolin was investigated by confocal immunofluorescence, immunogold EM, and Western blot. Endocytosis and intracellular traffic was studied using horseradish peroxidase (HRP) reaction as a marker of fluid phase transport in SECs. Both transmission and scanning EM showed an increased number of sinusoidal endothelial fenestrae (SEF) in SECs cultured with VEGF. By confocal immunofluorescence, SECs cultured with VEGF displayed prominent caveolin‐1‐positive aggregates in the cytoplasm, especially surrounding the nucleus region. Immunogold EM depicted increased caveolin‐1 reactivity on vesicles and vacuoles of VEGF‐treated SECs compared with VEGF‐nontreated cells. However, there was no change in the level of caveolin‐1 protein expression on Western blot. After HRP injection, an increase of electron‐dense tracer filled the SEF in cells treated with VEGF. Our results suggested that VEGF induced fenestration in SECs, accompanied by an increased number of caveolae‐like vesicles. Increased caveolin‐1 might be associated with vesicle formation but not with fenestration. Increased fenestration may augment hepatic sinusoidal permeability and trans‐endothelial transport.     

Myosin-mediated Ca++-regulation of actomyosin-adenosinetriphosphatase from porcine aorta.

The nature of the Ca++-sensitive regulatory system for contraction of vascular smooth muscle is considered in detail. Smooth muscle actomyosin prepared from the medial layer of porcine aorta is analyzed chemically and its ATPase (adenosinetriphosphatase, EC 3.6.1.14) activities are investigated. The Mg++-ATPase of this vascular actomyosin is sensitive to the concentration of calcium in the range from 0.1 mM to 10 nM. The calcium sensitivity is maintained in the presence of excess pure actin from skeletal muscle and is abolished in the presence of pure skeletal myosin. It is concluded that the regulatory properties of this natural actomyosin from smooth muscle are in the myosin portion of the protein complex and are not bound to actin-tropomyosin as in skeletal muscle.     

The endothelin-1 receptor-mediated pathway is not involved in the endothelin-1-induced defenestration of liver sinusoidal endothelial cells.

BACKGROUND/AIMS: We previously reported that endothelin (ET)-1 may be involved in the contraction of hepatic sinusoidal endothelial fenestrae (SEF). Rho has emerged as an important regulator of the actin cytoskeleton and consequently cell morphology. To clarify the role of ET receptors [endothelin A receptor (ETAR) and endothelin B receptor (ETBR)] in ET-1-induced defenestration, we studied the size of hepatic SEF under various experimental conditions. METHODS: Liver sinusoidal endothelial cells (LSECs) isolated from rat livers by collagenase perfusion were cultured and divided into four groups: control, ET-1 (10(-6) -10(-10) M)-treated, ET-1+selective ETAR antagonist (BQ610)-treated and ET-1+ETBR antagonist (BQ788)-treated groups. SEF morphology was observed by scanning electron microscopy. Protein expressions of ETAR and ETBR, Rho A and phosphorylated myosin light-chain kinase were analyzed by Western blotting. F-actin stress fiber formation was observed by confocal microscopy. Active Rho was measured by Ren's modification. Intracellular free Ca2+ concentration ([Ca2+]i) was measured by fluorescence digital imaging using fura-2 AM by Aqua cosmos. RESULTS: ET-1 induced a reduction in the number and size of SEF. ETAR antagonist pretreatment inhibited defenestration induced by low ET-1 concentrations (10(-8) -10(-10) M), whereas ETBR antagonist pretreatment did not block defenestration at low to high ET-1 concentrations (10(-6) -10(-10) M). F-actin stress fibers, Rho A levels and phosphorylated myosin light-chain kinase levels remained the same in various treatments. Active Rho was not detected in control and various treatments. ET-1 did not increase [Ca2+]i. Western blot showed prominent ETBR but scarce ETAR protein expression in LSECs. CONCLUSIONS: The present findings demonstrated that ETBR- and ETAR-induced contractile mechanisms are not involved in ET-1-induced defenestration, and that Rho is also not activated. Therefore, ET-1 induces hepatic defenestration by mechanisms other than receptor-mediated contraction.     

Calmodulin stimulation of plasmalemmal Ca2+-pump of canine aortic smooth muscle

Plasma-membrane-enriched fractions of canine aortic smooth muscle possess an ATP-supported Ca2+ accumulation which has an absolute requirement for Mg2+ and a high affinity for Ca2+ (Km approximately 0.5 microM). The rate of ATP-supported Ca2+ transport is not affected by several calmodulin antagonists, but is stimulated by exogenously added calmodulin. The maximal effect of calmodulin on the rate of ATP-dependent Ca2+ transport (at 5.0 microM Ca2+) occurs at 10 micrograms/ml calmodulin and represents an approximately 3-fold stimulation. This calmodulin stimulation of Ca2+ transport does not require pretreatment of the membranes by EGTA and is an intrinsic property of the plasma membranes. A high-affinity Ca2+-ATPase (Km for Ca2+ approximately 0.5 microM) is also present in the aortic smooth muscle plasma membrane. This high-affinity Ca2+-ATPase does not require Mg2+ for catalytic activity, but is in fact inhibited by increasing Mg2+ concentrations. Calmodulin at concentrations effective for the stimulation of the ATP-dependent Ca2+ transport has no effect on the high-affinity Ca2+-ATPase activity or on the basal ATPase activity stimulated by 5 mM Mg2+ or Ca2+. Our results indicate that isolated plasma membranes of canine aortic smooth muscle contain no endogenous calmodulin. The ability of exogenously added calmodulin to stimulate the rate of ATP-dependent Ca2+ transport by vascular smooth muscle plasma membranes suggests that calmodulin may play a role in lowering the cytoplasmic concentration of ionized calcium during vasodilatation. An Mg2+-independent, but not an Mg2+-dependent high-affinity Ca2+-ATPase, was identified in the plasma membranes. This may be separate from the plasmalemmal Ca2+-pump.     

An endothelin A receptor antagonist induces dilatation of sinusoidal endothelial fenestrae: implications for endothelin-1 in hepatic microcirculation

Background Sinusoidal endothelial fenestrae (SEF) regulate the sinusoidal circulation by altering their diameter and number. This study documented the effects of endothelin (ET) receptor antagonists on SEF and hepatic microcirculation. Methods The portal pressure and hepatic tissue blood flow were measured with a hydromanometer and a laser Doppler blood flow meter, respectively. BQ-123 (ET_A receptor antagonist) or BQ-788 (ET_B receptor antagonist) was continuously infused into normal rats at the rate of 10 nmol/min for 10 min. The sinusoids were observed at 60 min after the infusion by scanning electron microscopy. The localization of ET-1 and ET_A and ET_B receptors was examined by the indirect immunoperoxidase method. Results When BQ-123 was infused, the portal pressure gradually decreased with time, and it showed a significant reduction compared with the control groups. On the other hand, a decrease in portal pressure was not evident in the BQ-788-infused groups. Hepatic tissue blood flow was maintained at the value prior to the infusion in both groups. BQ-123 also caused a marked dilatation of the SEF. The diameters of the SEF after BQ-123 infusion were almost three times those of normal SEF. ET-1 was evenly present along the sinusoidal walls, and the reaction products of the ET_A receptors were recognized along the portal vein and in the sinusoidal cells, that is, the hepatic stellate cells and endothelial cells. Conclusions Action of ET-1 via the ET_A receptors may regulate the size of SEF in addition to hepatic microcirculation.     

高血压患者红细胞ATP含量、红细胞膜ATP酶活性与钙、镁代谢的研究

目的:探讨原发性高血压(EH)患者红细胞ATP含量,红细胞膜Na~ ,K~ -ATP酶和Ca~(2 ),Mg~(2 )-ATP酶的活性与红细胞内钙、镁含量之间的关系及意义。方法:观察EH患者和正常对照红细胞内ca~(2 ),Mg~(2 )水平,红细胞内ATP含量,红细胞膜Na~ ,K~ -ATP酶和Ca~(2 ),Mg~(2 )-ATP酶的活性变化。结果:原发性高血压患者与正常对照相比,红细胞膜Na~ ,K~ -ATP酶、Ca~(2 ),Mg~(2 )-ATP酶活性和红细胞内Mg~(2 )含量减低,红细胞内Ca~(2 )含量升高,(P均<0.05)。红细胞内ATP含量在原发性高血压患者和正常对照之间无显著性差别。原发性高血压患者中,平均动脉压与Na~ ,K~ -ATP酶和Ca~(2 ),Mg~(2 )-ATP酶活性负相关(P<0.01),与红细胞内钙正相关(P<0.01);Ca2~(2 ),Mg~(2 )-ATP酶的活性与红细胞内Ca~(2 )含量负相关(P<0.01);红细胞内ATP含量与红细胞内镁含量正相关(P<0.05),与红细胞膜Na~ ,K~ -ATP酶、ca~(2 ),Mg~(2 )-ATP酶活性之间无统计学联系。结论:高血压的发生与细胞膜离子的主动转运失常而致细胞内Ca~(2 )水平升高密切相关,而细胞内Mg~(2 )含量与细胞糖代谢有关。     

(Ca2++Mg2+)-ATPase activity of sickle cell membranes: decreased activation by red blood cell cytoplasmic activator.

Human red blood cells (RBCs) contain a cytoplasmic protein that activates membrane-bound (Ca2+ + Mg2+)-ATPase and the transport of Ca2+. The (Ca2+ + Mg2+)-ATPase of sickle cells showed a less than normal response to this activator. This was true whether the activator was obtained from normal or sickle cells. Activator present in sickle cell hemolysates fully activated the (Ca2+ + Mg2+)-ATPase of normal RBC membranes. These results demonstrate that membranes of sickle cells are defective in their response to the activator. Neither the apparent affinity for calcium nor the apparent affinity for activator was different comparing the (Ca2+ + Mg2+)-ATPase of sickle and normal membranes. Young, mature, and irreversibly sickled cells were separated by density gradient centrifugation, and membranes were prepared from each of these cell populations. No significant differences in ATPase activities were found based on cell age (density). The (Ca2+ + Mg2+)-ATPase of all populations of sickle cells showed a decreased response to the activator. Thus, it appears unlikely that the decreased response of the (Ca2+ + Mg2+)-ATPase of sickle cells is due to membrane damage caused by repeated sickling during the life-span of the cell. Reduced activation of (Ca2+ + Mg2+)-ATPase by the cytoplasmic activator may account for calcium accumulation in sickle cells.     

Ca2+-transport ATPases of vascular smooth muscle

To characterize the Ca2+-transport properties of the plasma membrane and of the endoplasmic reticulum of bovine pulmonary artery, membrane vesicles are subfractionated by a procedure of density-gradient centrifugation that takes advantage of the selective effect of digitonin on the density of plasma-membrane vesicles. The obtained endoplasmic-reticulum fraction contains hardly any plasma-membrane vesicles, whereas the plasma-membrane fraction is still contaminated by a substantial amount of endoplasmic-reticulum vesicles. An adenosine 5'-triphosphate (ATP) energized Ca2+-transport system and a Ca2+-stimulated ATPase activity are present in both subcellular fractions. The Ca2+ transport by the plasma membrane is catalyzed by a (Ca2+,Mg2+)-ATPase of Mr 130,000. It binds calmodulin and it has a low steady-state phosphoprotein intermediate level. The endoplasmic-reticulum vesicles contain a Ca2+-transport ATPase of Mr 100,000 that is characterized by a high steady-state phosphointermediate level. It is antigenically related to the Ca2+-pump protein of cardiac sarcoplasmic reticulum. Phospholamban, the regulatory protein of the Ca2+-transport enzyme of cardiac sarcoplasmic reticulum, is also present in the endoplasmic reticulum of the pulmonary artery. A comparison of these fractions with the previously characterized fractions from porcine gastric smooth muscle reveals important differences in the basal Mg2-ATPase activity, in the ratio of the (Ca2+,Mg2+)-ATPase of the plasmalemma to that of the endoplasmic reticulum, and in the ratio of the (Na+,K+)-ATPase activity to the plasmalemmal (Ca2+,Mg2+)-ATPase activity. These differences can be ascribed in part to the species and in part to the tissue. These data suggest that in the bovine pulmonary artery the Ca2+ extrusion via the ATP-dependent Ca2+ pump may have a less predominant role, and that the Ca2+ uptake by the endoplasmic reticulum, and possibly also the Ca2+ extrusion via the Na+-Ca2+ exchanger could be more important in this tissue than in the porcine stomach.     

Platelet and erythrocyte Mg2+, Ca2+, Na+, K+ and cell membrane adenosine triphosphatase activity in essential hypertension in blacks.

OBJECTIVE: To assess the relationship between intracellular Mg2+, Ca2+, Na+ and K+ and cell membrane adenosine triphosphatase (ATPase) activity in normotensive and hypertensive blacks. DESIGN: Intracellular cations and cell membrane ATPase activity were studied in black patients with untreated essential hypertension and age-, weight- and height-matched normotensive controls. Platelet, erythrocyte and serum Mg2+, Ca2+, Na+ and K+ levels as well as platelet and erythrocyte membrane Na+,K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase activities were measured in all subjects. METHODS: Intracellular Na+ and K+ were measured by flame photometry and Mg+ and Ca+ by atomic absorption spectrophotometry. Cell membrane ATPase activity was determined by a colorimetric method. RESULTS: The hypertensive group consistently demonstrated depressed activity of each ATPase studied, with significantly lower serum Mg2+, serum K+, erythrocyte Mg2+ and platelet Mg2+ levels compared with the normotensive group. Platelet Na+ and Ca2+ and erythrocyte Ca2+ were significantly elevated in the hypertensive group. In the hypertensive group, mean arterial pressure (MAP) was inversely correlated with platelet and erythrocyte membrane Na+,K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase. Serum Mg2+, serum Ca2+ and platelet Mg2+ were negatively correlated with MAP in the hypertensive group whilst erythrocyte and platelet Ca2+ were positively correlated. In the normotensive group, platelet Mg2+ and MAP were negatively, and erythrocyte Ca2+ and MAP, positively correlated. CONCLUSIONS: Black patients with essential hypertension have widespread depression of cell membrane Na+,K(+)-ATPase, Ca(2+)-ATPase and Mg(2+)-ATPase activities with serum and intracellular Mg2+ depletion and cytosolic Na+ and Ca2+ overload, which may reflect an underlying membrane abnormality in essential hypertension. These cellular abnormalities may be related to the defective transport mechanisms that in turn may be aggravated by Mg2+ depletion.     

A Ca++-Dependent and -Selective Ionophore as Part of the Ca++ + Mg++-Dependent Adenosinetriphosphatase of Sarcoplasmic Reticulum

Solubilized Ca(++) + Mg(++)-dependent adenosinetriphosphatase (EC 3.6.1.3; ATP diphosphohydrolase) from sarcoplasmic reticulum increased bimolecular lipid membrane (oxidized cholesterol) conductance several hundred-fold. The relative conductance change and the relative permeability elicited by this material has the following sequence: Ba(++) > Ca(++) > Sr(++) > Mg(++) > Mn(++) > Zn(++), Na(+), K(+), Cs(+), Li(+), and Rb(+). Zn(++) and Na(+) strongly inhibit the increase in Ca(++) conductance obtained with solubilized Ca(++) + Mg(++)-dependent adenosinetriphosphatase. The Ca(++)-ionophore is an integral part of the Ca(++) + Mg(++)-dependent adenosinetriphosphatase enzyme and may function as a Ca(++)-carrier in the overall Ca(++)-pump of sarcoplasmic reticulum.     

Regulation of Ca2+-Mg2+-ATPase activity in hepatocyte plasma membranes by vasopressin and phenylephrine

A high affinity Ca2+-stimulated, Mg2+-dependent ATPase (Ca2+-Mg2+-ATPase) was identified in microsomes and plasma membrane vesicles isolated from rat hepatocytes. The distribution of this enzyme was similar to that of the plasma membrane marker enzymes alkaline phosphodiesterase and 5'-nucleotidase. The Ca2+-Mg2+-ATPase had an apparent half-saturation constant of approximately 75 nM for Ca2+. After incubation of rat hepatocytes with 25 nM vasopressin for 3 min, the activity of Ca2+-Mg2+-ATPase was decreased 15-30%. The effect of vasopressin on the activity of this enzyme was near maximal after incubating hepatocytes with vasopressin for only 15 sec. The concentration of vasopressin needed for half-maximal inhibition of this enzyme in hepatocytes was approximately 6 nM. Treatment of the hepatocytes with 10 microM phenylephrine caused about a 10% decrease in ATPase activity while 10 nM glucagon or 200 microU/ml insulin did not affect the enzyme. These findings suggest that inhibition of the Ca2+-Mg2+-ATPase activity may be part of the mechanism by which vasopressin and alpha-adrenergic agonists elevate cytosolic Ca2+ in hepatocytes.     

A new role for follicle-stimulating hormone in the regulation of calcium flux in Sertoli cells: inhibition of Na+/Ca++ exchange

Elucidation of mechanisms regulating intracellular calcium levels in steroidogenic tissues is important for understanding control of cellular function. We have previously described FSH receptor-mediated flux of 45Ca++ into cultured rat Sertoli cells and receptor-enriched proteoliposomes via voltage-sensitive and voltage-independent calcium channels. In the present study, we report heretofore unrecognized inhibitory effects of FSH on Na+/Ca++ exchange in these two systems. An outwardly directed Na+ gradient, developed by preincubating Sertoli cell monolayers in buffer made hypertonic with NaCl, resulted in uptake of 45Ca++ that was unaffected by calcium channel blocking agents, ruthenium red or methoxyverapamil, but was enhanced by ouabain, a specific inhibitor of Na+/K(+)-ATPase. Sodium-dependent 45Ca++ flux into Sertoli cells was inhibited in a concentration-related manner by increased extracellular Na+ (up to 135 mM). FSH consistently and reproducibly (28.9 +/- 3.8%, 10 separate assays) reduced sodium-dependent 45Ca++ influx in the absence or presence of ouabain. A lesser effect on Na+/Ca++ exchange was seen when Li+ replaced Na+ in the preincubation buffer, and a marked reduction occurred when Sertoli cells were incubated in buffer containing KCl, presumably due to membrane depolarization. FSH-sensitive Na+/45Ca++ exchange was also observed when using FSH receptor-enriched proteoliposomes. Our earlier calcium channel studies indicated that FSH affects Ca++ entry into Sertoli cells via a receptor-mediated process. The results reported here demonstrate that the interaction of FSH with its receptor is associated with changes in Na+/Ca++ exchange as well, and suggest that this activity may also be involved in regulating intracellular free Ca++ levels in the Sertoli cell.