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L Varecka

Publications and source records attributed to L Varecka.

30 records · Page 2Linked to original sources
PubMed1994

Vanadate and fluoride activate red cell Na+ permeability by different mechanism.

Fluoride and vanadate are known to induce the Ca(2+)-dependent K+ efflux. We found that both agents concomitantly induced the Ca(2+)-dependent 22Na+ influx. The extent of the Ca(2+)-dependent 22Na+ influx induced by vanadate was very small in human red blood cells but clearly visible in guinea-pig red blood cells. The effect of fluoride has been studied in human red blood cells only. The 22Na+ influx induced by vanadate was inhibited by amiloride but was resistant to tetrodotoxin, whereas that induced by fluoride was resistant to amiloride but sensitive to tetrodotoxin. The effects of inhibitors indicate that vanadate activates the Na/H antiporter and that fluoride opens a tetrodotoxin-sensitive Na+ channel in red blood cells in the Ca(2+)-dependent manner. The results also indicate that both agents activate the Ca(2+)-dependent Na+ permeabilities by unknown auxiliary mechanisms.

Amiloride↗
PubMed1994

Characteristics of the passive 45Ca2+ transport by human blood platelets. Their dependence on the monovalent cation composition of the suspension media.

In the suspension of washed human platelets (32 degrees C, up to 30 min) we measured the uptake of 45Ca2+ in various media. We found that the substitution of Na+ by K+ or choline led to the stimulation of the uptake. According to the degree of the Na+ substitution we observed two phases of stimulation. The first one was observed in K(+)-containing media, but not in choline-containing media; the second one was observed in both K(+)- and choline-containing media. The transport in both phases was saturatable by Ca2+ and sensitive to other divalent cations. The saturating Ca2+ concentration and the sensitivity to other divalent cations were different in both phases. Dihydropyridine (nifedipine, nitrendipine) and phenylalkylamine (verapamil) calcium channel blockers were without effect in either medium up to 100 mumol/l. Indomethacin and acetylsalicylic acid were also without effect (100 mumol/l, and 1 mmol/l, respectively). On the other hand, veratridine (up to 75 mumol/l) stimulated the 45Ca2+ transport in Na(+)-rich medium but not in Na(+)-free medium. These results are in accordance with a notion that the first phase corresponds to a depolarization-stimulated Ca2+-transport, and the second one to the reversal of the Na/Ca exchange activity.

Aspirin↗
PubMed1990

Activation of red cell Ca2(+)-activated K+ channel by Ca2+ involves a temperature-dependent step.

We found that vanadate-induced 45Ca2+ uptake by red cells is maximal at 25 degrees C. At this temperature, the Cai-induced increase of the K+ permeability (the Gárdos effect) shows a lag (up to 8 min) which is not observed at 37 degrees C. This cannot be explained by the lack of availability of Ca2+ for the Ca2(+)-activated K+ channel, and suggests that its activation by Ca2+ is mediated by a temperature-dependent mechanism which remains unknown so far. The lag is not observed when the Gárdos effect was initiated by propranolol. This shows that the putative temperature-dependent step is different from chloride transport.

Biological Transport, Active↗
PubMed1990

[Characteristics of passive transport of 45Ca2(+) into the synaptosomes of the cerebral cortex in rats].

Transport of 45Ca2+ into synaptosomes was measured at 30 degrees C by rapid membrane filtration. The transport was saturable in high (135.10(-3) mol/l-1) or medium (25.10(-3) mol/l-1) K+ solutions with apparent KM values for 45Ca2+ of 0.45 and 0.1.10(-3) mol/l-1 respectively. At low K+ concentration (5.10(-3) mol/l-1) the transport was not clearly saturable by substrates. Vanadate had no effect on 45Ca2+ transport at any K+ concentration tested. Replacement of Na+ by other ions stimulated the 45Ca2+ transport with biphasic dependence on K+ concentration. Upon the substitution of choline for Na+, the dependence was monophasic and corresponded to Na/Ca antiporter reversal. The first stimulatory phase observed only in the presence of K+ corresponded to depolarization-activated transport. The phase itself occasionally showed a biphasic pattern with an extreme case of bell-shaped dependence on K+ concentration. The first phase set in immediately, whereas the onset of the second phase varied between the preparations. It could be shown that high K+ concentrations (as a rule approx. 50 mmol/l-1) may not be suitable for the study of depolarization-induced 45Ca2+ transport since they may induce NaůCa antiporter reversal. Also, it has been suggested that transport ATPases do not affect directly the basic parameters of passive 45Ca2+ transport into rat brain synaptosomes.

Animals↗
PubMed1987

Polymyxin B, a novel inhibitor of red cell Ca2+-activated K+ channel.

Polymyxin B (PXB), a cyclic peptide antibiotic, in concentrations 0.1-3.0 mg/ml (0.08-4.0 mmol/l), inhibited the K+ efflux induced by opening of the Ca2+-activated K+ channel (the Gárdos effect) in intact human red blood cells. The inhibition was observed when the Gárdos effect was elicited by Ca2+ in the presence of vanadate, or propranolol, in ATP-depleted cells, and in A23187-treated cells. The inhibition of the Gárdos effect is caused neither by the inhibition of the anion channel by PXB nor by the inhibition of Ca2+ entry. It can be ascribed to the inhibition of the Ca2+-activated K+ channel. The mechanism of the inhibition remains to be elucidated.

Calcimycin↗
PubMed1986

Inhibition by divalent cations and sulphydryl reagents of the passive Ca2+ transport in human red blood cells observed in the presence of vanadate.

The uptake of 45Ca2+ by human red blood cells induced by vanadate was found to be inhibited by a number of divalent cations. The following order of potencies was determined (in parentheses, IC50 in mmol/l): Cu2+ (0.006), Zn2+ (0.014), Cd2+ (0.030), Co2+ (0.20), Ni2+ (0.25), Mn2+ (8.0), Ba2+ (9.0), Sr2+ (14.0). The effects of Cu2+, Zn2+ and Cd2+ were biphasic--over a critical concentration their inhibitory potencies decreased, and finally, were lost. Besides Ca2+, Sr2+, Ba2+ and Mn2+ were also taken up, but only Ca2+ and Sr2+ were capable of eliciting the Gárdos effect. Ni2+ was not taken up. Several HS reagents also inhibited 45Ca2+ uptake. The following order of potencies was determined (in parentheses, IC50 in mmol/l): mersalyl (0.0025), 5,5'-dithiobis(2,2'-dinitrobenzoic acid) (0.011), p-chloromercuric acid (0.042), N-ethylmaleimide (2.0). The effects of all HS reagents except N-ethylmaleimide were biphasic. The biphasicity of the actions of the indicated agents was caused by the opening of a new pathway for 45Ca2+ entry which is different from that observed in the presence of vanadate alone, and is inhibited by low concentrations of these agents. The modified form of the anion channel seems to be identical with the former pathway. The last one is mediated by a transport protein which has an ionic specificity similar to Ca2+ channels in excitable tissues, and contains an HS group which is essential for the transport function.

Biological Transport↗
PubMed1982

Vanadate-induced movements of Ca2+ and K+ in human red blood cells.

Fresh human red blood cells become highly labeled with 45Ca2+ when exposed to 0.5 mM vanadate. The effect of vanadate requires its penetration into the cell, and is attributed to the inhibition of the outwardly directed Ca2+-pumping ATPase which would otherwise "mask" the uptake of 45Ca2+. Since the inhibition of the CA2+ pump by vanadate is not complete, a transmembrane Ca2+-Ca2+ exchange can be detected. The influx leg of the exchange is inhibited by verapamil, quinidine, and Co2+. This, as well as additional (kinetic) evidence, indicates that the influx of Ca2+ is a carrier-mediated process. Experiments in which the transmembrane K+ gradient has been abolished or decreased with ionophores, or by increasing the K+ concentration in the medium, suggest that the K+ gradient may play a role in the influx of Ca2+. The vanadate-induced accumulation of Ca2+ by red cells promotes a massive efflux of K+, indicating the activation of a Ca2+-sensitive K+-channel. The results indicate the occurrence of a slow cycling of Ca2+ across the red cell membrane. The influx leg of the cycle occurs through a verapamil-sensitive channel, and is possibly driven by the discharge of the transmembrane K+ gradient. The efflux leg of the cycle consists of the Ca2+-pumping ATPase.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗