The calcium channel antagonist diltiazem effectively blocks two types of potassium channels in the neuronal membranes

Effect of the Ca²⁺-channel antagonist diltiazem on potential-operated Ca²⁺ and K⁺ currents was studied on isolated edible snail neurons by a two-microelectrode patch-clamp technique. Diltiazem in a concentration of 0.1 mM inhibits Ca²⁺ current, high-threshold Ca²⁺-dependent K⁺ current, and Ca²⁺-independent K⁺ current and has no effect on low-threshold K⁺ current and leakage current. It is suggested that therapeutic effect of diltiazem is mediated through blockade of Ca²⁺ and K⁺ channels. Tranlated fromByulleten' Eksperimental'noi biologii i Meditsiny, Vol. 124, No. 9, pp. 271–274. September, 1997     

Effect of human defensins on the cytoplasmic Ca2+ content in platelets

The effect of the total fraction of human defensins (HNP-1, HNP-2, and HNP-3) on the cytoplasmic Ca²⁺ content ([Ca²⁺]_i) in the platelets of healthy donors was studied. At concentrations of 0.1–40 μg/ml and an incubation time of 10 min defensins have no effect on [Ca²⁺]_i in platelets labeled with Fura-2AM. However, at higher concentrations (100 μg/ml) they increased platelet [Ca²⁺]_i. In addition, defensins (40 μg/ml) inhibited the Ca²⁺ increase in platelets induced by thrombin, adenosine diphosphate, and the lipopolysaccharide ofS. typhimurium endotoxin. The most pronounced inhibitory effect was observed in a suspension of thrombin-stimulated platelets. It is shown that the effect of human defensins on the functional activity of platelets is due to the alterations in the intracellular Ca²⁺. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 118, N ^o 12, pp. 600–603, December, 1994     

Effect of cytosolic Mg2+ on mitochondrial Ca2+ signaling

Cytosolic Ca²⁺ signals are followed by mitochondrial Ca²⁺ uptake, which, in turn, modifies several biological processes. Mg²⁺ is known to inhibit Ca²⁺ uptake by isolated mitochondria, but its significance in intact cells has not been elucidated. In HEK293T cells, activation of purinergic receptors with extracellular ATP caused cytosolic Ca²⁺ signals associated with parallel changes in cytosolic [Mg²⁺]. Neither signals were affected by omitting bivalent cations from the extracellular medium. The effect of store-operated Ca²⁺ influx on cytosolic Mg²⁺ concentration ([Mg²⁺]_c) was negligible. Uncaged Ca²⁺ displaced Mg²⁺ from cytosolic binding sites, but for an equivalent Ca²⁺ signal, the change in [Mg²⁺] was significantly smaller than that measured after adding extracellular ATP. Inositol 1,4,5-trisphosphate mobilized Ca²⁺ and Mg²⁺ from internal stores in permeabilized cells. The increase of [Mg²⁺] in the range that occurred in ATP-stimulated cells inhibited mitochondrial Ca²⁺ uptake in permeabilized cells without affecting mitochondrial Ca²⁺ efflux. Therefore, the Mg²⁺ signal generated by Ca²⁺ mobilizing agonists may attenuate mitochondrial Ca²⁺ uptake.     

Effect of thapsigargin and caffeine on Ca2+ homeostasis in HeLa cells: implications for histamine-induced Ca2+ oscillations

Several studies have already established that the stimulation of H1 receptors by exogenous histamine induces intracellular Ca²⁺ oscillations in HeLa cells. The molecular mechanism underlying this oscillatory process remains, however, unclear. A series of fura-2 experiments was undertaken in which the nature of the Ca²⁺ pools involved in the histamine-induced Ca²⁺ oscillations was investigated using the tumour promoter agent thapsigargin (TG) and the Ca²⁺-induced Ca²⁺-release promoter, caffeine. The results obtained indicate first that TG causes a gradual increase in cytosolic Ca²⁺ without inducing internal Ca²⁺ oscillations, and second that TG and histamine share common internal Ca²⁺ storage sites. The latter conclusion was derived from experiments performed in the absence of external Ca²⁺, where the addition of TG before histamine resulted in a total inhibition of the Ca²⁺ response linked to H1 receptor stimulation, whereas the addition of histamine before TG decreased by more than 90% the TG-induced Ca²⁺ release. Finally, TG was found to inhibit irreversibly histamine-induced Ca²⁺ oscillations when added to the bathing medium during the oscillatory process. The effect of caffeine at concentrations ranging from 1 mM to 10 mM on intracellular Ca²⁺ homeostasis was also investigated. The results obtained show that caffeine does not affect systematically the internal Ca²⁺ concentration in resting and TG-stimulated HeLa cells, but increases the Ca²⁺ sequestration ability of inositol-trisphosphate (InsP ₃)-related Ca²⁺ stores. These results suggest either that TG acts on InsP ₃-sensitive as well as InsP ₃-insensitive Ca²⁺ pools, so that no final conclusion on the nature of the pools involved in Ca²⁺ spike generation can be currently drawn, or that the contribution of an InsP ₃-insensitive Ca²⁺-induced Ca²⁺-release process is not essential to the Ca²⁺ oscillation machinery in these cells. It is also concluded that a release of Ca²⁺ by caffeine may not be directly accessible to fura-2 measurements in this cellular preparation, but that the inhibitory effect of caffeine on the Ca²⁺ mobilization process triggered by InsP ₃ can be clearly documented using this experimental approach.     

Potassium transport through the erythrocyte plasma membrane in patients with insulin-dependent diabetes mellitus: Effects of insulin therapy

Plasma membrane potential and function of Ca²⁺-activated K⁺ channels of erythrocytes are studied in patients with insulin-dependent diabetes mellitus. A significant increase in membrane potential and in the degree of opening of Ca²⁺-activated K⁺ channels is revealed in comparison with erythrocytes of healthy donors. The degree of channel opening is higher in patients with nephropathy than in those with retinopathy, in which this parameter is normal. Insulin therapy normalize the erythrocyte plasma membrane potential in the patients and has no effect on the degree of channel opening. Ion transport disturbances and modulations of Na⁺, K⁺, and Ca²⁺ levels in erythrocytes may impair their function in insulin-dependent diabetes mellitus. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 122, No. 7, pp. 109–113, July, 1996     

Localized L-type calcium channels control exocytosis in cat chromaffin cells

Depolarizing 1-s pulses to 0 mV from a holding potential of −70 mV, induced whole-cell currents through Ca²⁺ channels (I _Ca) in patch-clamped cat adrenal medulla chromaffin cells. The dihydropyridine (DHP) furnidipine (3 μM) reduced the peak current by 47% and the late current by 80%. ω-Conotoxin GVIA (CgTx, 1 μM) reduced the peak I _Ca by 42% and the late I _Ca by 55%. Pulses (10 s duration) with 70 mM K⁺/2.5 mM Ca²⁺ solution (70 K⁺/2.5 Ca²⁺), applied to single fura-2-loaded cat chromaffin cells increased the cytosolic Ca²⁺ concentration ([Ca²⁺]_i from 0.1 to 2.21 μM; this increase was reduced by 43.7% by furnidipine and by 42.5% by CgTx. In the perfused cat adrenal gland, secretion evoked by 10-s pulses of 70 K⁺/2.5 Ca²⁺ was reduced by 25% by CgTx and by 96% by furnidipine. Similar results were obtained when secretion from superfused isolated cat adrenal chromaffin cells was studied and when using a tenfold lower [Ca²⁺]_o. The results are compatible with the existence of DHP-sensitive (L-type) as well as CgTx-sensitive (N-type) voltage-dependent Ca²⁺ channels in cat chromaffin cells. It seems, howevever, that though extracellular Ca²⁺ entry through both channel types leads to similar increments of averaged [Ca²⁺]_i, the control of catecholamine release is dominated only by Ca²⁺ entering through L-type Ca²⁺ channels. This supports the idea of a preferential segregation of L-type Ca²⁺ channels to localized “hot spots” in the plasmalemma of chromaffin cells where exocytosis occurs.     

Mobilization of intracellular Ca2+ induced by ADP and thrombin in platelets from diabetic patients with vascular complications

It is shown that the baseline level of cytoplasmic Ca²⁺ in platelets from diabetic patients is nearly 1.5 times as high as in healthy donors. The thrombin-induced increase of intracellular Ca²⁺ in patients with angiopathies is reliably lower than in the control, while in patients without angiopathies it is higher than that in donor platelets. The evevation of cytoplasmic Ca²⁺ induced by ADP is greater in both groups of patients. No Changes are found in the baseline level of intracellular Ca²⁺ or in the ADP-induced concentration of Ca²⁺ in platelets from diabetic patients during a 12-week course of insulin therapy. The intracellular Ca²⁺ does not rise after 2 weeks of insulin treatment in platelets from diabetic patients. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 121, N ^o 1, pp. 112–115, January, 1996     

Na/Ca exchangers in collecting cells of rat kidney

Single pieces of fura-2-loaded cortical collecting tubule (CCT) isolated either from normal or adrenalectomized (ADX) rats were superfused in vitro, and the cytosolic calcium concentration ([Ca²⁺]_i) was calculated from fluorescence recordings. The effects of altering the sodium gradient across cell membranes were investigated. Switching external sodium from 164 mM to 27 mM (low [Na⁺]_o) had little effect on [Ca²⁺]_i in normal tubules (106±9 versus 101±9 nM, n=15) whereas it resulted in a large peak of [Ca²⁺]_i in CCT from ADX-rats (270±32 versus 135±11 nM, n=21). Since CCT from ADX rats are known to have a reduced Na-pump activity, the effect of ouabain treatment on CCT from normal rats was also tested. When CCT from normal rats were exposed to 1 mM of ouabain in the presence of 164 mM of [Na⁺]_o, [Ca²⁺]_i increased only moderately (123±15 versus 111±11 nM, n=13); when the low [Na⁺]_o solution was applied to these ouabain-treated tubules, a large and transient increase in [Ca²⁺]_i was obtained (287±38 versus 123±15 nM, n=13). This response was absent with [Ca²⁺]_o=0. The data suggest the presence of 3 Na⁺/1 Ca²⁺ exchangers in cell membranes of rat CCT. The calcium flux equation derived by Läuger for the exchanger indicates a non-linear relationship between net calcium flux and driving force which could account for the difference observed here between the poor effect of applying either low [Na⁺]_o or ouabain alone and the large peak of [Ca²⁺]_i induced by combining these two conditions.     

Effects of isoproterenol and suphan on the concentration of free calcium ions in isolated cardiomyocytes: a comparative study

Isoproterenol and suphan, two cardioactive drugs with different mechanisms of action, are studiedin vitro for their effects on calcium homeostasis in myocardial cells. Isoproterenol lowers the basal Ca²⁺ level in resting cardiomyocytes and potentiates its rise in these cells after their induction. Suphan stimulates reversible elevation of the diastolic Ca²⁺ concentration, causing increased calcium accumulation in the sarcoplasmic reticulum of cardiomyocytes. In anin vitro model of hypoxia, the Ca response to isoproterenol is significantly reduced, whereas that to dibutyryl cAMP is retained. The effect of suphan on the Ca²⁺ content of cardiomyocytes exposed to “chemical” hypoxia is 30–50% higher than its effect on the Ca²⁺ content of intact cells. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 122, No. 8 pp. 170–172, August, 1996     

Acetylcholine induces Ca2+ oscillations via m3/m4 muscarinic receptors in the mouse oocyte

Changes in intracellular Ca²⁺ concentration are required for the activation of mammalian oocytes. They are caused mainly by Ca²⁺ release from the endoplasmic reticulum (ER) via InsP ₃ receptors (InsP ₃R). Several studies have reported that acetylcholine (ACh) is capable of triggering early activation events in mouse oocytes over-expressed with the m1 muscarinic ACh receptor (m1AChR). Here we examined which subtypes of the mAChR (m1 to m4) are involved in the generation of Ca²⁺ oscillations in native mouse oocytes. ACh (10 M) elicited regular Ca²⁺ oscillations similar to those induced by sperm in their temporal characteristics. The Ca²⁺ oscillations were abolished by application with atropine, the mAChR inhibitor. Within 1 min after treatment of ACh, intracellular Fluo-3 fluorescence intensity increased from 794±119 to 2023±755 (increase to 250% of original value), indicating a strong rise of cytosolic Ca²⁺ concentration. 4-DAMP mustard and Tropicamide, specific antagonists of m3AChR and m4AChR, completely abolished ACh-induced Ca²⁺ oscillations. In the ovulated oocytes, the expression of m3/m4 AChR was clearly detected by RT-PCR analysis. Furthermore, ACh-induced Ca²⁺ oscillations were also abolished or decreased by PLC inhibitors (U73122 or D609) and an InsP ₃-receptor antagonist (xestospongin C), confirming that ACh generates Ca²⁺ oscillations via the PLC-InsP ₃ (PI) pathway. These results strongly suggest that m3/m4AChR is coupled to the generation of Ca²⁺ oscillations mainly via the PI pathway in mouse oocytes.     

AlF 4 − induces Ca2+ oscillations in guinea-pig ileal smooth muscle

The effects of different compounds that inhibit the isolated plasma-membrane Ca²⁺/Mg²⁺-ATPase on the cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) and on the corresponding force development have been examined in smooth muscle of the longitudinal layer of the guinea-pig ileum. F^–, in the presence of Al³⁺, induced an increase of the resting force and of the amplitude of the superimposed phasic contractions. The increase of resting force was associated with an increased level of basal [Ca²⁺]_i while the phasic contractions were accompanied by concomitant oscillations in [Ca²⁺]_i. Comparable contractions could be induced by vanadate and the calmodulin antagonist calmidazolium. The oscillations of [Ca²⁺]_i and of force elicited by AlF ₄ ^– were not modified by adrenergic or cholinergic blocking agents but were inhibited by verapamil. These phasic contractions were not affected by depleting the intracellular Ca²⁺ stores with ryanodine. This finding excludes a cytosolic origin of these oscillations. However, hyperpolarization and complete depolarization of the cells inhibited the oscillations. It is concluded that AlF ₄ ^– , vanadate and calmidazolium induce cytoplasmic Ca²⁺ oscillations possibly by acting at the plasma membrane. Indeed all these substances affect by different mechanisms the isolated plasma-membrane Ca²⁺/Mg²⁺-ATPase. The generation of membrane-linked Ca²⁺ oscillations could therefore be related to an inhibition of the plasma-membrane Ca²⁺ pump resulting in an increase of [Ca²⁺]_i. This change in [Ca²⁺]_i could be responsible for the pronounced changes of the electrical and mechanical activity of this tissue.     

Ca2+ transport of the skeletal muscle sarcoplasmic reticulum during readaptation of rats after a 40-day load relief on the hind paws

The rate of Ca²⁺ accumulation in the sarcoplasmic reticulum of the skeletal muscle (m. gastrocnemius lateralis, m. vastus medialis, andm. soleus) is studied in rats under conditions of functional off-loading of the hind paws (suspending animals by the tail). The rate of Ca²⁺ transport in the sarcoplasmic reticulum is shown to be stepped up in all these muscles. In the sarcoplasmic reticulum ofm. gastrocnemius lateralis andm. vastus medialis the Ca²⁺ transport rate reliably drops, which does not occur inm. soleus. During a 2-week period of readaptation of animals suspended for 40 days, the Ca²⁺-transporting function of them. soleus sarcoplasmic reticulum gradually recovers to reach the control values, whereas the time course of recovery of Ca²⁺-pump activity in the sarcoplasmic reticulum ofm. gastrocnemius lateralis andm. vastus medialis has a phasic pattern. Translated fromByulleten' Eksperimental'noi Biologii i Meditsity, Vol. 118, N ^o 12, pp. 591–595, December, 1994 Presented by A. I. Grigor'ev, Member of the Russian Academy of Medical Sciences     

Activation and inactivation kinetics of a Ca2+-activated Cl- current: photolytic Ca2+ concentration and voltage jump experiments

The activation kinetics of the endogenous Ca²⁺-activated Cl⁻ current (I _Cl,Ca) from Xenopus oocytes was investigated in excised “giant” membrane patches with voltage and Ca²⁺ concentration jumps performed by the photolytic cleavage of the chelator DM-nitrophen. Currents generated by photolytic Ca²⁺ concentration jumps begin with a lag phase followed by an exponential rising phase. Both phases show little voltage dependence but are Ca²⁺-dependent. The lag phase decreases from about 10 ms after a small Ca²⁺ concentration jump (0.1 μM) to less than 1 ms after a saturating concentration jump (55 μM). The rate constant of the rising phase is half-maximal at about 5 μM. At saturating Ca²⁺ concentrations, the rate constant is 400 to 500 s⁻¹. The Ca²⁺ dependence of the stationary current can be described by the Hill equation with n=2.3 and K _0.5=0.5 μM. The amplitude of the stationary current decreases after the excision of the membrane patch with t _1/2≈5 min (run-down). The activation kinetics of the current elicited by a Ca²⁺ concentration jump is not affected by the run-down phenomenon. At low Ca²⁺ concentration (0.3 μM), voltage jumps induce a slowly activating current with voltage-independent time-course. Activation is preceded by an initial transient of about 1-ms duration. At saturating Ca²⁺ levels (1 mM), the initial transient decays to a stationary current. The transient can be explained by a voltage-dependent inactivation process. The experimental data reported here can be described by a linear five-state reaction model with two sequential voltage-dependent Ca²⁺-binding steps, followed by a voltage-independent rate-limiting transition to the open and a voltage-dependent transition to a closed, inactivated state.     

Bell-shaped activation of inositol-1,4,5-trisphosphate-induced Ca2+ release by thimerosal in permeabilized A7r5 smooth-muscle cells

There is no consensus about the different types of Ca²⁺ transport processes in the endoplasmic reticulum that are targeted by the sulphydryl reagent thimerosal. We have therefore investigated how thimerosal affects the various Ca²⁺ transport processes in permeabilized A7r5 smooth-muscle cells, using an unidirectional ⁴⁵Ca²⁺ flux technique. Thimerosal up to a concentration of 32 M did not have an effect on the passive ⁴⁵Ca²⁺ leak from the stores, while higher concentrations increased this aspecific leak. Thimerosal inhibited the endoplasmic reticulum Ca²⁺ pump with an EC⁵⁰ of 9 M. Thimerosal exerted a biphasic effect on the Ca²⁺ release induced by inositol 1,4,5-trisphosphate [Ins(1,4,5)P ₃] with a stimulation of the release at thimerosal concentrations below 10 M, and an inhibitory effect at higher concentrations. Thimerosal (2.5–250 M) did not exert an effect on the specific binding of [³H]Ins(1,4,5)P ₃ to its receptor, indicating that it probably did not act at the level of the binding site. This finding contrasts with the effect of the closely related sulphydryl reagent parachloromercuriphenylsulphonate, which, at high concentrations, inhibited [³H]Ins(1,4,5)P ₃ binding. The effects of thimerosal were largely prevented by the sulphydryl reducing agent dithiothreitol (3 mM). We conclude that thimerosal concentrations ranging from 0.32 to 1 M can stimulate the Ins(1,4,5)P ₃-induced Ca²⁺ release without inhibiting the Ca²⁺ pumps or without increasing the passive Ca²⁺ permeability of the endoplasmic reticulum.     

Effects of some steroid hormones on Ca2+ transport and oxidative metabolism of isolated mitochondria

Effects of prednisolone, estradiol, and testosterone on the transport of Ca²⁺ and the respiration induced by it in the heart and liver mitochondria of rats were studied. Prednisolone and testosterone were found to reduce the Ca-accumulating capacity of the mitochondria, the rates of ion entry and exit, and the rate of Ca²⁺-induced respiration. Estradiol, while inhibiting Ca²⁺ transport across mitochondrial membrane, did not influence the respiration in the phase of Ca²⁺ absorption, but accelerated it in the phase of ion exit. These data suggest that due to their lipophilic properties, the steroids become incorporated in the mitochondrial membrane, thereby changing its viscosity and permeability and limiting the mobility of transmitter proteins. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 118, N ^o 12, pp. 616–618, December, 1994     

Effect of activation of Ca2+, Mg2+-dependent endonuclease on proliferative response of human peripheral blood lymphocytes

Activity of Ca²⁺, Mg²⁺-dependent endonuclease in human peripheral blood mononuclears is studied. Intact cells exhibit extremely low activity of the enzyme. Treatment with the synthetic hexapeptide imunofan considerably stimulates endonuclease activity in mononuclears. This activation does not depend on additional protein synthesis. An 1-h incubation in the presence of cycloheximide also activates Ca²⁺, Mg²⁺-dependent endonuclease. These data suggest that imunofant and/or cycloheximide activate the apoptotic cascade. This leads to activation of endonuclease, which is not synthesizedde novo but persists in the cell in the form of inactive precursor. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 123, No. 5, pp. 535–537, May, 1997     

Evaluation of mag-fura-5, the new fluorescent indicator for free magnesium measurements

The new fluorescent indicator, mag-fura-5, was evaluated for its ability to measure accurately physiological changes in cytosolic free magnesium. The apparent dissociation constants (K _d) of the fluorochrome for Mg²⁺, Mg²⁺/EGTA and Ca²⁺/EGTA solutions were 14.7 mM, 15.4 mM, and 1.8 mM respectively. The calculated difference in the fluorescence ratios and in the resulting pMg between the standards with low-Ca²⁺ or low H⁺ backgrounds and the corresponding samples with approximately physiological levels were not significant. In contrast, the changes due to an increased Ca²⁺ or H⁺ content were statistically significant, with mean pMg differences of 0.10±0.09 (P<0.02) and 0.33±0.26 (P<0.01) respectively. Repetitive measurements on 3 consecutive days yielded comparable data with differences not exceeding 4%. Because of the good reproducibility, it is suggested that the new fluoresecent probe may be suitable for free cytosolic magnesium determinations in isolated cells.     

Action of stimulatory and inhibitory α-MSH secretagogues on spontaneous calcium oscillations in melanotrope cells of Xenopus laevis

The secretion of α-melanophore-stimulating hormone (α-MSH) from melanotrope cells in the pituitary gland of Xenopus laevis is regulated by various neural factors, both classical neurotransmitters and neuropeptides. The majority of these cells (80%) display spontaneous Ca²⁺ oscillations. In order to gain a better understanding of the external regulation of intracellular Ca²⁺ ([Ca²⁺]_i) in the melanotrope cell, we have examined the action of well known α-MSH secretagogues on the Ca²⁺ oscillations. It is shown that all secretagogues tested also control the oscillatory state of Xenopus melanotropes, that is, the secreto-inhibitors dopamine, isoguvacine (γ-aminobutyric acid, GABA_A agonist), baclofen (GABA_B agonist) and neuropeptide Y evoked a rapid quenching of the spontaneous Ca²⁺ oscillations, whereas the secreto-stimulant sauvagine, an amphibian peptide related to corticotropin releasing hormone, induced oscillatory activity in non-oscillating cells. Supporting argument is given for the idea that the regulation of Ca²⁺ oscillations is a focal point in the regulation of secretory activity of melanotrope cells. There was considerable heterogeneity among melanotrope cells in the threshold of their Ca²⁺ response to secretagogue treatment. This heterogeneity may be the basis for melanotrope cell recruitment observed during physiological adaptations of the animal to the light intensity of its background.     

Adaptation to periodic hypoxia and a diet supplemented with polyunsaturated class ω-3 fatty acids enhance the resistance of Ca2+ transport in the myocardial sarcoplasmic reticulum to free-radical oxidation

The relationship between the level of accumulation of lipid peroxidation products and the status of the Ca²⁺-transporting system in the sarcoplasmic reticulum of the rat myocardium is studied against the background of two cardioprotective factors, namely adaptation to periodic hypoxia and a diet enriched in polyunsaturated fatty acids of the ω-3 class. It is shown that the diet leads to an increase of level of lipid peroxidation products by 1.8 times in the heart and by 19 times in the liver, whereas a adaptation has no effect on the level of lipid peroxidation products in either of these organs. At the same time, the combined action of both factors considerably enhances the resistance of the myocardial Ca²⁺-transporting system to free radical-induced oxidation. Inin vitro experiments it is shown that adaptation to periodic hypoxia results in a more than twofold deceleration of Ca²⁺ transport inhibition during the oxidation induction by the Fe²⁺/ascorbate system; the diet causes a 3.5-fold deceleration of such inhibition. The results show that the accumulation of a high level of lipid peroxidation products is not always followed by damage to the Ca²⁺-transporting system in the myocardial sarcoplasmic reticulum. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 120, N ^o 7, pp. 42–45, July, 1995 Presented by G. N. Kryzhanovskii, Member of the Russian Academy of Medical Sciences     

A novel synergistic stimulation of Na+-transport across frog skin (Xenopus laevis) by external Cd2+- and Ca2+-ions

Isolated skin of the clawed frogXenopus laevis was mounted in an Ussing-chamber. The transcellular sodiumcurrent (I _Na) was identified either as amiloride-blockable (10^–3 mol/l) short-circuit current (I _SC), or by correctingI _SC for the shunt-current obtained with mucosal Tris. A dose of 10 mmol/l Cd²⁺ applied to the mucosal side increased the current by about 70%. The half-maximal effect was reached at a Cd²⁺-concentration of 2,6 mmol/l (in NaCl-Ringer). The quick and fully reversible effect of Cd²⁺ could not be seen when 10^–3 mol/l amiloride was placed in the outer, Na⁺-containing solution, nor when Na⁺ was replaced by Tris. This suggests that Cd²⁺ stimulatesI _Na. Cd²⁺ intefered with the Na⁺-current self-inhibition, and therefore with the saturation ofI _Na by increasing the apparent Michaelis constant (K _Na) of this process. The I _Na recline after stepping up mucosal [Na⁺] was much reduced in presence of Cd²⁺. Ca²⁺-ions on the mucosal side had an identical effect to Cd²⁺, and 10 mmol/l Ca²⁺ increaseI _Na by about 100%. The half-maximal effect was obtained with 4.4 mmol/l Ca²⁺. The mechanism ofI _Na-stimulation by Ca²⁺ did not seem to differ from that of Cd²⁺. Thus, although of low Na⁺-transport capacity,Xenopus skin appears to be as good a model for Na⁺-transporting epithelia asRanidae skin, with the exception of the calcium effect which, so far, has not been reported forRanidae.