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PubMed1991

[The Ca(2+)-transport ATPases of smooth muscle].

The cytoplasmic Ca2+ concentration plays a central role in the contraction of smooth muscle cells. The concentration of cytoplasmic Ca2+ is determined for an important part by the operation of Ca(2+)-transport ATPases which extrude Ca2+ from the cell or accumulate the ion in the endoplasmic reticulum. The present work concerns the characterization of the Ca(2+)-transport ATPases of smooth muscle by biochemical, immunochemical and recombinant DNA techniques. This study also includes the investigation of the regulation of the Ca(2+)-transport ATPases and of the expression of associated Ca(2+)-binding proteins. Methods were developed for the purification of endoplasmic reticulum and plasma membranes from smooth-muscle cells. From the study of the phosphorylated transport intermediates and the proteolytic breakdown products, and by using polyclonal and monoclonal antibodies we could conclude that two different Ca(2+)-transport ATPases are expressed in smooth-muscle cells. Of each of these types of Ca(2+)-transport ATPases different isoforms exist. These isoforms were further characterized at the cDNA level and by generating isoform-specific antibodies. One isoform of the plasma-membrane Ca(2+)-pump and two different organellar-type Ca(2+)-pumps have been cloned and sequenced. In smooth-muscle cells, the primary RNA transcripts of the gene of the SERCA2 Ca(2+)-transport ATPase are alternatively processed in three different ways. In neural tissues even a fourth mode of splicing occurs. These different splice modes can be explained by the analysis of the exon/intron structure of the SERCA2 gene. The regulation of the alternative RNA splicing was studied on the stable muscle-cell line BC3H1 during induced myogenic differentiation. From this study we could conclude that the mechanisms responsible for active Ca(2+)-transport in smooth-muscle cells partially resemble those of non-muscle cells, and partially resemble the corresponding system in cardiac cells, but not those in skeletal muscle. A similar conclusion was reached concerning the regulation of the Ca(2+)-transport ATPase of the endoplasmic reticulum via the phosphorylation of phospholamban, and for the expression of the Ca(2+)-binding protein calsequestrin.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

[A preliminary study on the cellular mechanism of hypotensive effect of high calcium diet in spontaneously hypertensive rats].

Significant reduction of both systolic blood pressure and body weight could be observed in SHRs after being fed with high calcium diet for about 7 weeks (P less than 0.01), with some changes of characteristics in ion transport in RBCs. The intracellular content of Na+, K+, the basic and calmodulin-stimulated activity of calcium-pump, and the calcium membrane binding ability on RBCs in SHR were determined. The intracellular content of K+ and the calmodulin-stimulated activity of calcium-pump in SHR fed with high calcium diet were significantly higher than those in SHR fed with normal calcium diet (P less than 0.01). The ratio of intracellular Na+ to K+ and the calcium membrane binding ability were found to be significantly reduced in SHR fed with high calcium diet (P less than 0.05). The systolic blood pressure in SHR fed with high calcium diet was found to be correlated inversely with the calmodulin-stimulated activity of calcium-pump and the intracellular K+ content (r = -0.720, r = -0.663 respectively, P less than 0.01). Thus, the hypotensive effect of chronic high calcium diet may be mediated through the changes in plasma ion transport, which most likely resulted from the changes in composition and structure of plasma membrane. The exact mechanism concerning the reduced calcium membrane binding ability in SHR fed with high calcium diet still remained unknown.

Animals↗
PubMed1991

[The evaluation of the role of endogenous Ca-dependent regulators and protein kinases in activating and inhibiting ion-transport ATPases].

The data on hormonal regulation of ATP-driving ion pumps are contradictory depending on the object used: whether native cells or isolated membranes. To eliminate this contrariety, we studied the ion transporting ATPases in saponin-permeabilized cells in the presence of all endogenous regulators. In permeabilized erythrocytes we obtained the presence of Ca(2+)-dependent activation of Ca(2+)-ATPase by factor(s) not affected by calmodulin antagonist R24571. We obtained also Ca(2+)-dependent activation and inhibition of Na+,K(+)-ATPase. At a concentration of Mg(2+)-ions corresponding to the intracellular level (370 microM), the 0.5-0.7 microM Ca(2+)-activated Na+,K(+)-ATPase (up to 3-fold), whereas the 1-5 microM Ca2+ inhibited it. The cyclic AMP (10(-5) M) inhibited or eliminated Ca(2+)-dependent activation. The decrease in Mg(2+)-ion concentration to 50 microM eliminated the activation and strengthened the inhibition, which reached 100% at the 1-2 microM Ca2+ concentration. The washing of membranes with EGTA eliminated Ca2+ effects on Na+,K(+)-ATPase. These data suggest that the ion-transporting ATPases are activated or inhibited by Ca(2+)-dependent regulators whose activities may be changed by protein kinase catalysed phosphorylation.

Adenosine Triphosphatases↗
PubMed1991

Alterations in calcium homeostasis as a possible cause of ochratoxin A nephrotoxicity.

Disruption of calcium homeostasis, leading to a sustained increase in cytosolic calcium level, has been associated with cytotoxicity in response to a variety of agents in different cell types. We have observed that a single high dose or multiple lower doses of ochratoxin A administered to rats resulted in an increase in renal endoplasmic reticulum calcium pump activity. The increase was very rapid, being evident within 10 min of ochratoxin A administration and remained elevated for at least 6 h thereafter. Ochratoxin A also decreased renal mitochondrial state-3 respiration and calcium uptake. The latter may lead to an increase in cytosolic calcium level, and the increase in microsomal calcium uptake activity may be an attempt to restore calcium homeostasis. Repeated moderate doses of ochratoxin A led to an eventual decrease in microsomal calcium pump activity, and this could lead to even higher cytosolic calcium levels. Changes in the rate of microsomal calcium uptake correlated with changes in the steady-state levels of the phosphorylated Mg2+/Ca(2+)-ATPase intermediate, indicating that this enzyme is responsible for the calcium pump activity.

Animals↗
PubMed1977

Ca2+-activated ATPase of rat liver plasma membrane.

Rat liver plasma membranes hydrolyze ATP in the presence of Ca2+. The rate of hydrolysis is different when Mg2+ions are present in the incubation system. Several parameters differentiate Ca2+-ATPase from Mg2+-ATPase: a) the Km of ATP hydrolysis for Ca2+ (2.25 x 10(-4) M) is lower than for Mg2+ (2.14 x 10(-3) M); b) the shape of the activation curve is hyperbolic in the presence of Ca2+ and sigmoid in the presence of Mg2+; c) Mg2+-ATPase shows two different values of activation energy while Ca2+-ATPase presents only a single value; d) Ca2+-ATPase is inhibited, while Mg2+-ATPase is unaffected by cyclic AMP. Ca2+-ATPase is localized on the plasma membrane and is not inhibited by cysteine. It does not hydrolyze substrates different from nucleotides triphosphate, such as glucose-1-phosphate or alpha-glycero-phosphate. The enzyme is probably related to a mechanism of calcium transport.

Adenosine Triphosphate↗
PubMed1977

Studies on adenosine triphosphatase activity of rat cardiac myosin in isoproterenol-induced cardiac hypertrophy.

Ca2+- and K+ -activated ATPase activity of cardiac myosin from normal and hypertrophied rat hearts was investigated. Cardiac hypertrophy was induced by isoproterenol treatment. A nearly 40% increase in heart mass was seen after seven consecutive days of isoproterenol injection (5 mg/kg) as determined by either heart weight expressed as per cent of body weight or by dry heart weight and total protein content. The measurement of ATP hydrolysis revealed that cardiac myosin from isoproterenol-treated rats had a significant decrease (P less than 0.01) in Ca2+-activated ATPase activity at low ionic strength (0.05 M KCl) in the presence of 5 and 10 mM Ca2+. In contrast, in a high ionic strength medium (0.50 M KCl) the K+- and Ca2+-activated ATPase activity of myosin prepared from hypertrophied myocardium remained unchanged. Comparative analysis of protein present in the light chains of myosin showed no alteration in the proportion of LC1 to LC2 in the myosin from hypertrophied hearts, however, a decrease in the absorption of myosin in the u.v. region was observed. On the basis of our results one can hypothesize that there may be some conformational change in the myosin molecule from hypertrophied myocardium, thereby modifying both Ca2+-sensitivity and ATPase activity at a low KCl concentration.

Animals↗
PubMed1979

Toxic substances and cell membrane function.

The exposed location and functional importance of cell membranes make them particularly susceptible to the toxic effects of many chemicals. The likelihood of such effects has been appreciated for many years. However, the recent advent of new techniques has greatly increased our understanding of the complexities of membrane structure and function. These data make it quite clear that the interaction of toxic compounds with either the protein or the lipid component of cell membranes may substantially alter membrane function. This paper summarizes the current concepts of membrane structure and function and discusses the techniques currently in use to study cell membranes. Several examples are presented in which xenobiotics significantly alter membrane function. These include effects of heavy metals on passive ion permeability, impairment of osmoregulation and calcium transport by organochlorine pesticides, inhibition of the transport of neurotransmitter metabolites by phenoxyacetic acid herbicides in choroid plexus, and reduction in intestinal nutrient transport by heavy metals. Hence the study of the interactions of foreign compounds with membrane function may enhance our understanding of mechanisms both of toxicity and of basic membrane function.

2,4-Dichlorophenoxyacetic Acid↗
PubMed1979

Nerve cell calcium as a messenger for opiate and endorphin actions.

The studies outlined in this chapter provide convincing evidence for the role of calcium in pharmacological and biochemical actions of opiates on the central nervous system. The ability of the synaptosomes to accumulate excess calcium during narcotic exposure represents a singular major example of cellular adaptation at the level of the membrane. These findings together with the possibility that CDR and calcium may mediate a number of processes involving neurotransmitter activity provide many avenues for the future study of opiates and other central nervous system depressants.

Animals↗
PubMed1979

Na,K-ATPase in excitation-contraction coupling of vascular smooth muscle from cattle.

Lowering the extracellular K+ content from 6 to 0.6 mM causes a rise, and elevation from 6 to 8.5 mM a fall of 45Ca++ efflux from the vascular smooth muscle cells of the arteria carotis communis of cattle. In contrast, a level of 17 mM K+ has no influence. Removal of extracellular calcium does not block these effects. 10(-4) M ouabain also induces a rise in Ca++ efflux, additional potassium reduction then being without effect; 10(-9) M ouabain is of no influence. The 45Ca++ efflux kinetics correlates with the activity of the isolated Na,K-ATPase. Tonus increases of the vascular strips by 10(-4) M ouabain and potassium deficiency cannot be blocked by 4 mM lanthanum or removal of extracellular calcium. Unlike sodium, potassium stimulates the active Ca++ binding and the activity of the Ca-ATPase of the microsomal fraction. The ative Ca++ binding of the mitochondria is stimulated by both ions. It is postulated that the activity of the plasma membrane Na,K-pump is able to regulate the tonus of big arteries through alteration of Ca++ storage processes.

Animals↗
PubMed1979

[Biochemical mechanisms for the effect of alcohol on the brain].

This paper reviews and discusses basic knowledge of biochemical mechanisms of action of ethanol upon the central nervous system, the emphasis being on effects upon cerebral membrane structures and processes as well as mechanisms of chemical synaptic transmission. Results of detailed studies into material and steric changes in membranes, ion-dependent adenosine triphosphatases, variations in ion balance, effects upon cyclic nucleotides, influences on special transmitter systems, and mechanisms of formation of morphine-analogous condensation products are presented. In addition, open questions are derived and formulated in problem complexes.

Acetaldehyde↗
PubMed1979

[Studies of the presence of enzymes in various tissues of swine. 5. Studies of the activity and properties of adenosine triphosphatases in the pancreas].

Studies were conducted into the activities of magnesium-ATPase, sodium-potassium-ATPase, and HCO3-ATPase in homogenates of pancreas of 19 foetuses, with body weights between 340 g and 1,642 g, two piglets, with body weights of 9 kg and 10 kg, and four adult pigs. While general enzyme activity was low during foetal development, highest activies usually were recordable from HCO3-ATPase. High activities were recorded from both the piglets and adult pigs. The highest data, again, were recordable from HCO3-ATPase which is essential to the secretion of HCO3 ions. Maximum activity of magnesium-ATPase was based on an ATP-magnesium ratio of 1:1. HCO3-ATPase was best activated by 25 mM NaHCO3 and exhibited high stability to temperature. The activities of magnesium-ATPase and of HCO3-ATPase were inhibited by 10 mM of Rhodanid. Calcium-ATPase reached its maximum activity in response to 5 mM calcium concentration.

Adenosine Triphosphatases↗
PubMed1979

[Role of calcium in realization of nervous control during RNA synthesis in skeletal muscles].

The effects of Ca2+ on the RNA polymerase activity of the nuclei isolated from normal and denervated gastrocnemius muscles of the rabbit were studied. It was shown that 18 hrs after denervation the RNA synthesis in vitro, Ca2+ content and the Ca, Mg-ATPase activity of the nuclei are decreased. After addition of exogenous Ca2+ the incorporation of labelled UTP into the nuclei is stimulated in the denervated muscle and is inhibited in the control. Electrostimulation of the denervated muscle at the peripheral part of the sciatic nerve for 3 hrs increases both the RNA synthesis in the nuclei and the Ca2+ content, as well as the Ca, Mg-ATPase activity. Exogenous Ca2+ has an inhibitory effect on the nuclei of the stimulated muscle. The correlation established is indicative of participation of Ca2+ in the transmission of excitation in skeletal muscle sarcolemma to the processes occurring in nuclear structures.

Animals↗
PubMedDate not supplied

[The effect of catecholamines on the Ca2(+)-adenosinetriphosphatase of the sarcoplasmic reticulum in the skeletal muscles in chicken ontogeny].

It has been shown that the activity of Ca(2+)-ATPase increases during development. Epinephrine in vivo increases the activity of Ca(2+)-ATPase in chick skeletal muscles. The effect of hormone is lacking at embryonic stages of development and appears only before hatching. In the presence of exogenous protein kinase, cAMP also increases the activity of the enzyme, this effect being observed also in embryonic muscles. Lack of effect of epinephrine on Ca(2+)-ATPase in embryonic muscles is associated with non-reactivity of their adenylate cyclase to catecholamines. Ca(2+)-ATPase itself already at embryonic period is ready to react to cAMP. It is concluded that Ca(2+)-ATPase of sarcoplasmic reticulum is one of the sites of action of catecholamines on calcium metabolism in muscle cell and that this action is realized via the system adenylate cyclase-cAMP-protein kinase.

Animals↗