Calcium transport system: a comparative study in different cells

Calcium entry blockade and excitation contraction coupling in the cardiovascular system (with an attempt of pharmacological classification)

Calcium may be considered as the final intracellular messenger of excitation-contraction coupling. In this report the main mechanisms involved in the cellular regulation of calcium movements are reviewed. Most of the pharmacological agents actually available for therapy interfere with the processes responsible for increase in cytoplasmic activator calcium. Study of the relation between contraction and calcium movements shows that blockade of calcium entry may cause inhibition of contraction. This has allowed to illustrate some of the characteristics of calcium channels in smooth muscle. Potential-operated channels are activated by changes in membrane potential, that can be evoked by K-rich solutions. They are completely blocked by calcium antagonistic dihydropyridines and diphenylpiperazines. This blockade shows (time-)-use-dependence. They are opened by calcium agonistic dihydropyridines. Receptor-operated channels are activated by the interaction of an agonist with its receptors; they are incompletely blocked by calcium antagonistic dihydropyridines and diphenylpiperazines. Endothelium appears to modulate the response of the adjacent smooth muscle to vasoconstrictors. This may be accounted for by modulation of calcium metabolism, both at the level of membrane channels and of intracellular stores. As a consequence, endothelium modulates the action of calcium entry blockers. Study of isolated human tissues illustrates difference in drug sensitivity between cardiac and smooth muscle. Human isolated coronary arteries are sensitive to nifedipine concentrations found in the blood of patients receiving therapeutic regimen. These concentrations do not alter the contractility of the isolated human myocardium. Differences between drugs may be explained assuming that they may interact with one of the states (closed, open, inactivated) of the Ca channels. This also helps the understanding of tissue selectivity of some compounds. A pharmacological classification of calcium entry blockers may be proposed. This classification is a rationale to differentiate between the various clinical indications of drugs affecting calcium movements.

Calcium bonding in enamel fluid and driving force for enamel mineralization in the secretory stage of amelogenesis

A large portion (about 85%) of the calcium present in the fluid of porcine enamel in the secretory stage of amelogenesis appears to be in a non-ionic form. Calcium is probably bound in the fluid by peptides derived from the enzymatic degradation of the amelogenins present in the matrix or by other ligands of an as-yet-undetermined nature. The apparent dissociation constant of the hypothesized complex was determined by titration of a small aliquot of fluid (50 μL) with 10 mmol/L CaCl2 solution, adding 2 μL at a time. The initial composition of the fluid was determined analytically. The ionic calcium activity, (Ca 2+), and the pH value along the titration were determined by a specific ion and a glass electrode, respectively, with the same reference micro-electrode. The concentration of bound Ca, [B], was related to the (Ca2+) by the equation [B] = N(Ca2+)/ {K + (Ca2+)}, in which K is the dissociation constant of the Ca-ligand complex and N is the number of binding sites per liter of fluid. The values of K and N were obtained by a non-linear least-squares procedure. The value of K, 7.47 × 10-5 mol/L, indicates that the binding bond is rather weak, a feature that seems advantageous if complexed calcium acts as a reservoir and/or fine control of calcium levels in the fluid during enamel formation. The effects that the total concentration of calcium, [Ca]T, the total concentration of P, the concentration of ligand, and the pH value have on the degree of saturation (DS) were investigated by simulation models. It is concluded that the factor that most affects the DS is [Ca]T. Some physicochemical aspects of amelogenesis are discussed on the basis of present knowledge and the reported results.

Calcium-activated ATPase of the human placenta: identification, characterization, and functional involvement in calcium transport

A specific, membrane-bound, Ca2+-activated and Mg2+-dependent adenosine triphosphatase (ATPase) activity is present in the human term placenta. The enzyme activity is fractionated electrophoretically into two distinct forms which correspond to molecular weights of 120,000 and 145,000. Cytohistochemistry localized the Ca2+-ATPase to the chorionic villi of the placental labyrinth, and specific staining was primarily associated with the syncytio- and cytotrophoblast layers as well as the perivascular cells. The enzyme activity is inhibited by phenothiazin and erythrosin B which also significantly inhibit active calcium in vitro by placental microsomal membrane vesicles.

Toxic and teratologic effects of verapamil on early chick embryos: evidence for the involvement of calcium in neural tube closure

Toxic and teratologic effects of verapamil, a calcium antagonist, on chick embryos explanted at stage 8 (four-somite stage) and cultured for 6-8 hours were investigated. In general, embryos responded to verapamil in a dose-related manner. Concentrations lower than 2 micrograms/ml had no apparent effect on the development of embryos. A concentration of 15 micrograms/ml significantly increased the incidence of embryos (approximately 80% of viable embryos) with neural tube closure defects and less numerous somites. Higher concentrations (e.g., 30 micrograms/ml) were embryotoxic and over 90% of the embryos were either severely malformed or dead after 8 hours of incubation. Compared to controls, verapamil-treated neuroepithelial cells had smoother apical surfaces and less conspicuous microfilament bundles. The deleterious effects of verapamil (15 micrograms/ml) could be reversed by subculturing the affected embryos, within 3 hours of treatment, on nutrient medium alone or on nutrient medium containing 25 micrograms/ml chlorotetracycline (CTC), a calcium agonist, the latter being more effective provided that treatment did not exceed 4 hours. Exposure of the developing neuroepithelium to 15 micrograms/ml verapamil for 3-4 hours resulted in a significant reduction in free Ca2+ levels, as revealed by the pyroantimonate precipitation method, throughout neuroepithelial cells. Overall results suggest that verapamil causes neural tube closure defects by reducing intracellular free Ca2+ levels, thereby relaxing apical microfilament bundles of developing neuroepithelial cells.

Ultrahistochemical localization of Na+-K+ ATPase, Ca2+-ATPase and alkaline phosphatase activity in a calcium-transporting epithelium of a crustacean during moulting

Periodical changes in Na+-K+-ATPase, Ca2+-ATPase and non-specific alkaline-phosphatase activity were observed using cytochemical techniques in the posterior caeca of the crustacean amphipod, Orchestia cavimana, during the moult cycle. These changes were considered in relation to the calcium transport mechanisms in the posterior caecal epithelium. For both ATPases as well as alkaline phosphatase, the specific reaction products were most intense during the pre-exuvial period, i.e. when calcium is slowly transported against a concentration gradient: the localization of Na+-K+-ATPase activity in microvilli and the upper extracellular channels strongly supports the hypothesis that this enzyme is involved in an indirect, sodium-dependent mechanism for the transport of calcium. The detection of Ca2+-ATPase activity in microvilli would seem to indicate that this enzyme plays a role in the direct, active extrusion of Ca2+ at this level. Although the role of alkaline phosphatase in the transport of calcium remains unclear, the histochemical detection of this enzymatic activity throughout the apical part of the caecal epithelium suggests that this enzyme may be involved in calcium secretion. In post-exuvial period, we found only weak specific reaction products, thus indicating a reduced active calcium transport as these ions are rapidly reabsorbed down the concentration gradient.

Vesicular demyelination induced by raised intracellular calcium

Incubation of nerve with high concentrations of the divalent cation ionophore A23187 produces myelin vesiculation (Schlaepfer 1977). This observation has now been extended using segments of rat ventral or dorsal root incubated with high (19 microM, 10 micrograms/ml) or low (1-1.5 microM) concentrations of A23187, or another divalent ionophore, ionomycin. Low concentrations of A23187 induced no vesiculation within a 2-h period. However, subsequent incubation of these roots in fresh, ionophore-free medium for 20 h, resulted in a prominent vesicular demyelination at the Schmidt-Lanterman incisures and paranodes of many fibres. At this time (22 h) the Schwann cells associated with some demyelinating internodes appeared vital upon ultrastructural examination: the cells also excluded the nuclear dye nigrosin. High concentrations of A23187 induced a similar vesicular demyelination in affected fibres within only 15-20 min. While the Schwann cells continued to exclude nigrosin for a further 4 h, their ultrastructural appearance indicated that they were probably in the early stages of necrosis. Incubation of moribund root with the ionophore produced no myelin vesiculation. At all ionophore concentrations, the myelin vesiculation was dependent upon the presence of extracellular Ca2+, and could be modulated in severity by varying this concentration. Other divalent cations (Ba2+, Co2+, Mg2+, Mn2+, Ni2+, Sr2+) could not substitute for Ca2+. The vesiculation induced by A23187 could be entirely prevented by the addition of Zn2+ (greater than or equal to 1 microM), Ni2+ (greater than or equal to 1-10 microM), Co2+ (greater than or equal to 100 microM) or Mn2+ (greater than or equal to 100 microM) to the bathing medium. A23187 applied to only part of an isolated internode resulted in a localization of the myelin disruption to that region. Ionomycin (greater than or equal to 1 microM), an ionophore with a greater selectivity for Ca2+ than A23187, also induced a prompt Ca2+-dependent myelin vesiculation. We conclude that vesicular demyelination can be initiated in vital Schwann cells by a raised intracellular Ca2+ concentration. Such demyelination does not necessarily lead to Schwann cell death. The possible relevance of the findings to vesicular demyelinating neuropathies is discussed, and a hypothesis regarding the mechanism of demyelination is advanced.

Microvillar elongation following parthenogenetic activation of sea urchin eggs

Parthenogenetic activation of unfertilized sea urchin eggs with ammonium chloride at pH 8.0 resulted in a slow, but dramatic, reorganization of surface microvilli in four species of sea urchin eggs. Following NH4Cl treatment, elongation of microvilli on the egg surface was observed concomitant with the formation of microfilament bundles within the microvillar cores. A minimum of 2 h of treatment was required for elongation and microfilament bundle formation to occur. The maintenance of elongated microvilli was pH-sensitive; removal of the activating agent resulted in the retraction of extended microvilli while readdition of NH4Cl caused microvilli to elongate again. Accompanying microvillar elongation in activated eggs, there was an increased calcium uptake as measured by 45Ca uptake. Blocking calcium uptake by incubation in lanthanum chloride or zero-calcium seawater containing 2 mM EGTA prevented microvillar elongation. These results suggested that elongation of microvilli following parthenogenetic activation by NH4Cl is pH- and calcium-dependent and is similar to that observed during normal fertilization.

Effects of extracellular calcium and of light adaptation on the response to dim light in honey bee drone photoreceptors

Light responses in honey bee drone photoreceptors were recorded with intracellular micro-electrodes in superfused slices of retina. The effects of changes in extracellular calcium on the size and the shape of the response to dim light were studied and compared with the effects of light adaptation. Dim light stimuli were used so that the amplitude of the response was linearly related to the number of the photons absorbed, the effects of voltage-dependent mechanisms were negligible and no detectable light adaptation was produced by the stimulus. Lowering the extracellular calcium concentration increased the amplitude and the duration of the response. Raising the extracellular calcium concentration produced the opposite effects. Changing the extracellular calcium concentration modified the response without altering either the linearity of the intensity--response relation or the resting membrane potential in the dark. Light adaptation decreased the amplitude and the duration of the response in a manner that could be quantitatively simulated, in the same photoreceptors, by an increase in the extracellular calcium concentration. Changing the extracellular calcium concentration, or light-adapting the preparation, modified the response without altering its early depolarizing phase. Lowering external calcium either did not affect, or slightly increased, the maximum rate of the light-induced depolarization; raising external calcium, or light-adapting the preparation, either did not affect, or slightly decreased, the maximum rate of the light-induced depolarization. The experimental data can be quantitatively described by a mathematical model with the basic assumption that calcium acts in the process of light adaptation by decreasing the mean open time of the light-activated channels.

Ca2+, Mg2+ and (adenosine diphosphate ribose)n in cellular response to irradiation

A model of cellular response to irradiation involving adenosine diphosphate ribosylation (ADP-ribosylation) is proposed. Its main assumptions are (a) control of accessibility of sites for ADP-ribosylation in chromatin by free Ca2+/Mg2+ ratio; and (b) regulation of the Ca2+/Mg2+ ratio by factors affecting intracellular free Ca2+ concentration; the regulation would be mediated by mitochondria. The model seeks to explain the mechanism of action of radiomodifiers such as caffeine, local anaesthetics, polyamines and 2,4-dinitrophenol.

Mode of action of staphylococcal leukocidin: effects of the S and F components on the activities of membrane-associated enzymes of rabbit polymorphonuclear leukocytes

The cytotoxic action of the S component of leukocidin from Staphylococcus aureus on rabbit polymorphonuclear leukocytes was supported by the following observations, (i) Leukocytes displayed a large chemotactic response to the S component (10(-10) M) as well as to the chemotactic factor N-formylmethionylleucylphenylalanine (10(-11) M). (ii) The S component stimulated high levels of phospholipase A2 activity in the cell membranes, with concomitant synthesis and release of prostaglandins. (iii) Uptake of 45Ca into leukocytes exposed to the S component was about double the rate of uptake into untreated cells. The increased 45Ca uptake into the cells was not inhibited by trifluoperazine and ruthenium red. (iv) Indomethacin and alloxazine, which had no effects on the binding of the S component to the cells, attenuated markedly the stimulation of phospholipase A2 activity, the syntheses of prostaglandins, and the increased uptake of 45Ca caused by the S component. The F component of leukocidin, bound to rabbit leukocytes with the aid of the S component, rapidly induced complete release of 86Rb from preloaded leukocytes. This release resulted from stimulation of ouabain-insensitive (Na+ + K+)-adenosine triphosphatase activity and inhibition of cyclic AMP-dependent protein kinase.

Differentiation of Ca2+ pumps linked to plasma membrane and endoplasmic reticulum in the microsomal fraction from intestinal smooth muscle

ATP promotes 45Ca uptake by the microsomal fraction from the longitudinal smooth muscle of guinea-pig ileum and this uptake is stimulated by oxalate. As the microsomal fraction is made up of various subcellular entities, we examined the localization of the Ca2+-transport activity by density gradient centrifugation, taking advantage of the selective effect of digitonin (at low concentration) on the density of plasmalemmal elements. When the 45Ca-uptake activity was measured in the absence of oxalate, its behavior in subfractionation experiments closely paralleled that of the plasmalemmal marker 5'-nucleotidase. In contrast, the additional Ca2+-transport activity elicited by oxalate behaved like NADH-cytochrome c reductase, a putative endoplasmic reticulum marker. The endoplasmic reticulum vesicles constituted only a small part of the membranes in the microsomal fraction, which explains that their Ca2+-storage capacity was not detectable in the absence of Ca2+-trapping agent. Low digitonin concentrations selectively increased the Ca2+ permeability of the plasmalemmal vesicles. The two Ca2+-transport activities were further differentiated by their distinct sensitivity of K+, vanadate and calmodulin. In this respect, the oxalate-insensitive and oxalate-stimulated Ca2+-transport systems resembled, respectively, the sarcolemmal and sarcoplasmic reticulum Ca2+ pumps in cardiac and skeletal muscle, in accordance with the subcellular locations established by density gradient centrifugation.

A calmodulin-stimulated Ca2+ pump in rat aorta plasma membranes

An ATP-driven Ca2+-transport system has been characterized in a microsomal fraction from rat aorta. Calmodulin enhanced 2.5-fold 45Ca accumulation by EGTA-treated microsomes incubated with 10 microM Ca2+ (in the absence of oxalate) by increasing markedly the apparent affinity of the transport system for Ca2+. The ionophore A23187 induced a rapid release of the sequestered 45Ca. The vesicles that took up 45Ca were distributed like plasmalemmal marker enzymes when the microsomal fraction was subfractionated by density gradient centrifugation. In particular, these vesicles were markedly shifted towards higher equilibrium densities after addition to the microsomes of 0.2 mg digitonin/mg protein before isopycnic centrifugation. We conclude that the calmodulin-stimulated Ca2+ pump associated with the microsomal fraction is located in plasmalemmal elements.