Inhibition of K+-sensitive p-nitrophenylphosphatase activity on rhesus-positive red cells after incubation with IgG-anti-rhesus-D

The incubation of ghosts derived from human Rhesus-positive red cells with IgG-anti-Rhesus-D inhibited the K⁺-sensitive p-nitrophenylphosphatase activity. This enzyme has a partial function in the (Na⁺ + K⁺)-ATPase system related to the phosphorylation step, which is important for active potassium transport through the red cell membrane. The specificity of the impairment by the antigen-antibody reaction in the Rhesus-D system was proved by the following controls. Ghosts obtained from Rhesus-negative red cells incubated by IgG-anti-Rhesus-D and those of Rhesus-positive red cells treated with non-immune serum did not show any reduction of the K⁺-p-nitrophenylphosphatase activity. The ghost preparation with lanthanum carried out after hypotonic hemolysis of the washed red cells in 2 mM LaCl₃ at pH 6 was the most suitable procedure to explore this topic in comparison to other techniques for preparing ghosts of red cells.     

Mechanism of Ca2+-dependent selective rapid K+-transport induced by propranolol in red cells

Summary Passive Ca²⁺ influx is gradually enhanced by 0.5 to 5mm propranolol in fresh and phosphate ester-depleted human red cells. In fresh cells the active Ca²⁺ efflux tends to counteract Ca²⁺ uptake. Membrane hyperpolarization, induced by the K⁺ transport that accompanies Ca²⁺ uptake, further enhances the rate of Ca² uptake. The dissociated, positively charged form of propranolol seems to be crucial in the increase of passive Ca²⁺ influx caused by the drug. The effect can be attributed to the release of structural Ca²⁺ from the membrane (lipids).The release of structural Ca²⁺ promotes the formation of the selectively K⁺-permeable membrane structure as well. The transitions of lipid structure responsible for the opening of the passive Ca²⁺ and K⁺ pathways, however, are not identical. The opening of the K⁺ pathways is prevented by certain highly lipid-soluble substances (chlorobutanol, heptanol, oligomycin, etc.), whereas the formation of the Ca²⁺ pathways is unaffected. Passive K⁺ transport is inhibited by high propranolol concentrations (more intensively at alkaline pH), whereas Ca²⁺ transport is promoted. A further difference between the passive K⁺ and Ca²⁺ pathways is that SH-proteins also seem to be involved in the formation of the K⁺ pathways, whereas they do not play a specific role in the opening of the passive Ca²⁺ channels. The additional Ca²⁺ binding that triggers the formation of the K⁺ pathways also seems to occur in the protein area of the inner membrane surface.     

Sucrose transport into vacuoles isolated from barley mesophyll protoplasts

Sucrose transport has been investigated in vacuoles isolated from barley mesophyll protoplasts. Rates of sucrose transfer across the tonoplast were even higher in vitro than in vivo indicating that the sucrose transport system had not suffered damage during isolation of the vacuoles. Sucrose transport is carrier-mediated as shown by substrate saturation of transport and sensitivity to a metabolic inhibitor and to competitive substrates. A number of sugars, in particular maltose and raffinose, decreased uptake of sucrose. Sorbitol was slowly taken up but had no effect on sucrose transport. The SH-reagent p-chloromercuribenzene sulfonate inhibited sucrose uptake completely. The apparent K_m of the carrier for sucrose uptake was 21 mM. Transport was neither influenced by ATP and pyrophosphate, with or without Mg²⁺ present, nor by protonophores and valinomycin (with K⁺ present). Apparently uptake was not energy dependent. Efflux experiments with preloaded vacuoles indicated that sucrose unloading from the isolated vavuoles is mediated by the same carrier which catalyses uptake. The vacuole of mesophyll cells appears to represent an intermediary storage compartment. Uptake of photosynthetic products into the vacuole during the light apparently minimizes osmotic swelling of the small cytosolic compartment of vacuolated leaf cells when photosynthetic productivity exceeds the capacity of the phloem for translocation of sugars.Abbreviations Hepes 4-(2-hydroxyethyl)-1-piperazincethane-sulfonic acid - pCMBS p-chloromercuribenzene sulfonate Dedicated to Professor Dr. W. Simonis on the occasion of his 75th birthday     

Transport Properties of the Tomato Fruit Tonoplast : III. Temperature Dependence of Calcium Transport

Calcium transport into tomato (Lycopersicon esculentum Mill, cv Castlemart) fruit tonoplast vesicles was studied. Calcium uptake was stimulated approximately 10-fold by MgATP. Two ATP-dependent Ca²⁺ transport activities could be resolved on the basis of sensitivity to nitrate and affinity for Ca²⁺. A low affinity Ca²⁺ uptake system (K_m > 200 micromolar) was inhibited by nitrate and ionophores and is thought to represent a tonoplast localized H⁺/Ca²⁺ antiport. A high affinity Ca²⁺ uptake system (K_m = 6 micromolar) was not inhibited by nitrate, had reduced sensitivity to ionophores, and appeared to be associated with a population of low density endoplasmic reticulum vesicles that contaminated the tonoplast-enriched membrane fraction. Arrhenius plots of the temperature dependence of Ca²⁺ transport in tomato membrane vesicles showed a sharp increase in activation energy at temperatures below 10 to 12°C that was not observed in red beet membrane vesicles. This low temperature effect on tonoplast Ca²⁺/H⁺ antiport activity could only by partially ascribed to an effect of low temperature on H⁺-ATPase activity, ATP-dependent H⁺ transport, passive H⁺ fluxes, or passive Ca²⁺ fluxes. These results suggest that low temperature directly affects Ca²⁺/H⁺ exchange across the tomato fruit tonoplast, resulting in an apparent change in activation energy for the transport reaction. This could result from a direct effect of temperature on the Ca²⁺/H⁺ exchange protein or by an indirect effect of temperature on lipid interactions with the Ca²⁺/H⁺ exchange protein.     

Simulation of intracellular Ca2+ oscillation in a sympathetic neurone

Three different theoretical models were considered for the mechanism of the oscillation of the intracellular free Ca²⁺ ([Ca²⁺]_i) linked to the K⁺ conductance of the plasma membrane (G_K) observed in bullfrog sympathetic ganglion cells. The models assumed a Ca²⁺-induced Ca²⁺ release mechanism, an active Ca²⁺ uptake mechanism at a Ca²⁺ reservoir site in the ganglion cell, and a Michaelis—Menten type relationship between [Ca²⁺]_i and G_K. Including both active and passive Ca²⁺ transport mechanisms at the plasma membrane, either a one-compartment model or a two-compartment model for the intracellular Ca²⁺ store reconstructed successfully the [Ca²⁺]_i oscillation and rhythmic membrane hyperpolarizations observed in the ganglion cell, and simulated most of their characteristics. On the other hand, a two-compartment model disregarding of Ca²⁺ transport at the plasma membrane failed to reproduce the oscillations of [Ca²⁺]_i and membrane potential.     

Specific modification of gastric K+-stimulated ATPase activity by thimerosal

Treatment of hog gastric microsomes with the sulfhydryl reagent, thimerosal (ethylmercurithiosalicylate), produced differential effects on the K⁺-ATPase and the K⁺-stimulated p-nitrophenylphosphatase activities. For example, exposure to 2 mM thimerosal for 3 min severely reduced the activity of K⁺-stimulated ATPase, while K⁺-p-nitrophenylphosphatase activity was enhanced 2- to 3-fold. Higher concentration of thimerosal, or longer incubation times, also led to inhibition of K⁺-p-nitrophenylphosphatase. The activated state of p-nitrophenylphosphatase could be sustained by a 20-fold, or greater, dilution of treated membranes, and could be reversed by reduction of membrane SH groups by exogenous thiols. Significant activation of K⁺-p-nitrophenylphosphatase was not produced by p-chloromercuribenzene sulfonate, p-chloromercuribenzoate or mersalyl; however, ethyl mercuric chloride had qualitatively similar activity effects as thimerosal. Kinetics of K⁺-p-nitrophenylphosphatase for thimerosal-treated membranes were altered as follows: V increased; K_m for p-nitrophenylphosphate unchanged for K_a for K⁺ increased. ATP, which is a potent inhibitor of K⁺-p-nitrophenylphosphatase activity in native membranes (K_I ≈ 200 μM). These data suggest that there are multiple SH groups which differentially influence the gastric K⁺-stimulated ATPase activity. Defined treatments with thimerosal are interpreted as an uncoupling of the K⁺-stimulated phosphatase component of the enzyme (for which p-nitrophenylphosphatase is a presumed model reaction). Such differential modifications can be usefully applied to the study of partial reactions of the enzyme and their specific role in the related H⁺-transport reaction.     

Extracellular potassium controls responsiveness of the noradrenergic cAMP system in the rat retina to fluphenazine

The effects of fluphenazine (FLU) on the noradrenaline (NA) induced cAMP-synthesis in intact rat retinae were studied as a function of extracellular K⁺- and Ca²⁺-ions. Thus NA-induced cAMP levels were measured after incubating intact rat retinae with 50 μM NA in the presence or absence of FLU and in the presence of 1 or 10 mM theophylline. Results were: (1) Experimental condition a: standard NA-responses were measured after incubating retinae at 0.75 mM Ca²⁺, at 10 mM theophylline, at 10 μM FLU and at 2 and 0 mM K⁺. FLU does not affect the NA-response at 2 mM K⁺ significantly; however, it inhibits the NA-response at 0 mM K⁺ in this condition. (2) Experimental condition b: NA-responses were measured after incubating retinae at 0.125 mM Ca²⁺, 10 mM theophylline, 10 μM FLU and at 2 and 0 mM K⁺. At 2 mM K⁺ FLU replaces a Ca²⁺ function probably connected with the synthesis part of the NA-cAMP system and NA-responses in this low Ca²⁺ condition are consequently enhanced by FLU; however, FLU inhibits the NA-response at 0 mM K⁺ in this condition. (3) Experimental condition c: NA-responses were measured after incubating retinae at 0.75 mM Ca²⁺, 1 mM theophylline, 10 μM FLU and at 2 and 0 mM K⁺. At 2 mM K⁺ FLU enhances the NA-response by further inhibition of the degradation part of the NA-cAMP system; FLU inhibits the NA-response at 0 mM K⁺ in this condition. (4) The inhibitions of the NA-responses by FLU at 0 mM K⁺ in all three conditions a, b and c showed an apparent K_m of 1 μM. (5) Low concentrations of K⁺ (0.4–0.8 mM) maintain the property of FLU to enhance the NA-responses at condition b (0.125 mM Ca²⁺) and at condition c (1 mM theophylline). Results suggest that the activation of NA-receptor coupled adenylate cyclases (NA-AC-ases) by NA, resulting in activation of phosphodiesterase activity by the NA-elevated cAMP-levels, is sustained by (a) membraneous factor(s) connected to the NA-receptor. This (these) factor(s) is (are) switched off in the absence of K⁺. Evidence has been presented, that Ca²⁺ and FLU do not have access to this intramembraneous factor-enzyme activating moiety of the NA-cAMP system at 0 mM K⁺. Between 0.4 and 0.8 mM K⁺ the factor-enzyme-NA-receptor complex is still intact.     

Release of Sucrose from Vicia faba L. Leaf Discs

The release of sucrose from leaf discs of Vicia faba L. to a bathing medium was studied for evidence of a relationship between this release and mesophyll export of photosynthate in vivo. Sucrose was released specifically over hexoses and represented over 85% of total photosynthate released. The sucrose appeared to be derived from the mesophyll tissue directly and release did not require concurrent photosynthesis. The data indicated two separate channels for sucrose release. The first was sensitive to inhibition by 1 millimolar p-chloromercuribenzenesulfonic acid and the second was promoted by lowering the Ca²⁺ concentration below 0.1 millimolar. Flow through both channels was about equal when tissue that had been actively photosynthesizing for several hours was used. The rate of release was not dependent on the extracellular pH, but was inhibited by 10 micromolar carbonylcyanide p-trifluromethoxyphenylhydrazone. Lowering the Ca²⁺ concentration below 0.1 millimolar or raising the K⁺ concentration above 100 millimolar stimulated sucrose release. The stimulation by high K⁺ was not reversed by adding Ca²⁺. The data supported the postulate that Ca²⁺ removal or K⁺ addition changed the permeability of the mesophyll plasma membrane to sucrose.     

Effect of yttrium on calcium-dependent processes in vertebrate myocardium

Inoptopic effect of yttrium acetate (Y³⁺) on myocardium of the marsh frog Rana ridibunda and its effect on ion transport across the inner mitochondrial membrane (IMM) of rat heart was studied. Y³⁺ was found to decrease the rate of heart contractions and to stimulate ion transport in the rat heart mitochondria in media with 10 mM glutamate and 2 mM malate. Presence of Y³⁺ induced inhibition of energy-dependent Ca²⁺ transport into mitochondria, which was expressed as a marked decrease of their swelling in the media containing 125 mM NH₄NO₃ and Ca²⁺ or 25 mM potassium acetate, 100 mM sucrose and Ca²⁺. It is suggested that the Y³⁺-induced decrease in rat muscle contractions is determined not only by direct suppressing effect of Y³⁺ on potential-modulated Ca²⁺-channels of pacemaker and contractile cardiomyocytes (CM), but also by its indirect effect on Ca²⁺-carrier in IMM. The data confirming that Y³⁺ activates energy-dependent K⁺ transport catalyzed by mitochondrial uniporter and blocks Ca²⁺-channels in the mitochondrial membrane are important for more complete understanding of mechanisms of the Y³⁺ action on vertebrates and human CM.     

Na+-Ca2+ exchange in the axolemma-rich membrane vesicle preparations from the walking-leg nerves of the american lobster

An axolemma-rich membrane vesicle fraction was prepared from the leg nerve of the lobster, Homerus americanus. In this preparation Ca²⁺ transport across the membrane was shown to require a Na⁺ gradient (Na⁺-Ca²⁺ exchange), and external K⁺ was found to facilitate this Na⁺-Ca²⁺ exchange activity. In addition, at high Ca²⁺ concentrations (20 mM) a Ca²⁺-Ca²⁺ exchange system was shown to operate, which is stimulated by Li⁺. The Na⁺-Ca²⁺ exchange system is capable of operating in the reverse direction, with Ca²⁺ uptake coupled with Na⁺ efflux. Such a vesicular preparation has the potential for providing useful experimental approaches to study the mechanism of this important Ca²⁺ extrusion system in the nervous system.     

Involvement of cation channels and NH4+-sensitive K+ transporters in Na+ uptake by cowpea roots under salinity

Na⁺ accumulation was investigated in the roots of 11-d-old cowpea [Vigna unguiculata (L.) Walp.] plants. The relative contribution of different membrane transporters on Na⁺ uptake was estimated by applying Ca²⁺, K⁺, NH₄ ⁺, and pharmacological inhibitors. Na⁺ accumulation into the root symplast was decreased by half in the presence of 1 mM Ca²⁺ and it was almost abolished by 100 mM K⁺. The inhibitory effect of external NH⁴⁺ on Na⁺ accumulation was more pronounced in the roots of NH₄ ⁺-free growing plants. Na⁺ accumulation was reduced about 73 % by 0.1 mM flufenamate and it was almost blocked by 2 mM quinine. In addition, 20 mM tetraethylammonium and 1.0 mM Cs⁺ decreased Na⁺ accumulation by 28 and 30 %, respectively. These results evidenced that low-affinity Na⁺ uptake by cowpea roots depends on Ca²⁺-sensitive and Ca²⁺-insensitive pathways. The Ca²⁺-sensitive pathway is probably mediated by nonselective cation channels and the Ca²⁺-insensitive one may involve K⁺ channels and to a lesser extent NH₄ ⁺-sensitive K⁺ transporters.     

Transformation of (Ca2+ + Mg2+)ATPase of calmodulin-depleted erythrocyte membranes into a high Ca2+-affinity form by Ca2+

Specific activity and Ca²⁺-affinity of (Ca²⁺+Mg²⁺)ATPase of calmodulin-depleted ghosts progressively increase during preincubation with 0.1–2 mM Ca²⁺. Concomitantly, the increment in ATPase activity caused by calmodulin and the binding of calmodulin to ghosts decrease. The effects of calcium ions are abolished by the addition of calmodulin. ATP protects the enzyme from a Ca²⁺-dependent decrease of the maximum activity but does not seem to influence the Ca²⁺-dependent transformation of the low Ca²⁺-affinity enzyme into a high Ca²⁺-affinity form.     

Transformation of (Ca + Mg)ATPase of calmodulin-depleted erythrocyte membranes into a high Ca-affinity form by Ca

Specific activity and Ca²⁺-affinity of (Ca²⁺+Mg²⁺)ATPase of calmodulin-depleted ghosts progressively increase during preincubation with 0.1–2 mM Ca²⁺. Concomitantly, the increment in ATPase activity caused by calmodulin and the binding of calmodulin to ghosts decrease. The effects of calcium ions are abolished by the addition of calmodulin. ATP protects the enzyme from a Ca²⁺-dependent decrease of the maximum activity but does not seem to influence the Ca²⁺-dependent transformation of the low Ca²⁺-affinity enzyme into a high Ca²⁺-affinity form.     

Ca-dependent inhibitory effects of Na and K on Ca transport in sarcoplasmic reticulum vesicles

Effects of Na⁺ and K⁺ on Ca²⁺ transport by sarcoplasmic reticulum vesicles were studied in a medium containing high Mg²⁺ and ATP (2mM) and low Ca²⁺ (0.44μM) concentrations. Under these conditions, Na⁺ and K⁺ inhibit Ca²⁺ uptake. ATPase activity and membrane phosphorylation by ATP. Since the concentrations of ATP and Ca²⁺ used are consistent with relaxation in vivo, the results suggest that under physiological resting conditions the Ca²⁺ pump of the sarcoplasmic reticulum operates below its maximal capacity.     

Ca2+-dependent inhibitory effects of Na+ and K− on Ca2+ transport in sarcoplasmic reticulum vesicles

Effects of Na⁺ and K⁺ on Ca²⁺ transport by sarcoplasmic reticulum vesicles were studied in a medium containing high Mg²⁺ and ATP (2mM) and low Ca²⁺ (0.44μM) concentrations. Under these conditions, Na⁺ and K⁺ inhibit Ca²⁺ uptake. ATPase activity and membrane phosphorylation by ATP. Since the concentrations of ATP and Ca²⁺ used are consistent with relaxation in vivo, the results suggest that under physiological resting conditions the Ca²⁺ pump of the sarcoplasmic reticulum operates below its maximal capacity.     

An electrogenic Na+/Ca2+ antiporter in addition to the Ca2+ pump in cardiac sarcolemma

Vesicles isolated from rat heart, particularly enriched in sarcolemma markers, were examined for their sidedness by investigation of side-specific interactions of modulators with the asymmetric (Na⁺ + K⁺)-ATPase and adenylate cyclase complex. The membrane preparation with the properties expected for inside-out vesicles showed the highest rate of ATP-driven Ca²⁺ transport. The Ca²⁺ pump was stimulated 1.7- and 2.1-fold by external Na⁺ and K⁺, respectively, the half-maximal activation occurring at 35 mM monovalent cation concentration. In vesicles loaded with Ca²⁺ by pump action in a medium containing 160 mM KCl, a slow spontaneous release of Ca²⁺ started after 2 min. The rate of this release could be dramatically increased by the addition of 40 mM NaCl to the external medium. In contrast, 40 mM KCl exerted no appreciable effect on vesicles loaded with Ca²⁺ in a medium containing 160 mM NaCl. Ca²⁺ movements were also studied in the absence of ATP and Mg²⁺. Vesicles containing an outwardly directed Na⁺ gradient showed the highest Ca²⁺ uptake activity. These findings suggested the operation of a Ca²⁺/Na⁺ antiporter in addition to the active Ca²⁺ pump in these sarcolemmal vesicles. A valinomycin-induced inward K⁺-diffusion potential stimulated the Na⁺- Ca²⁺ exchange, suggesting its electrogenic nature. If in the absence of ATP and Mg²⁺ the transmembrane Na_i⁺/Na_o⁺ gradient exceeded 160/15 mM concentrations, Ca²⁺ uptake could be stimulated by the addition of 5 mM oxalate, indicating Na⁺ gradient-induced Ca²⁺ uptake to be a translocation of Ca²⁺ to the lumen of the vesicle. A sarcoplasmic reticulum contamination, removed by further sucrose gradient fractionation, contained rather low Na⁺-Ca²⁺ exchange activity. This result suggests that the activity can be entirely accounted for by the sarcolemmal content of the cardiac membrane preparation.     

Effect of thiourea on PCMBS inhibition of osmotic water transport in human red cells

The organomercurial reagent p-chloromercuribenzene sulfonate (PCMBS) is an inhibitor of osmotic water permeability in the human red cell membrane. We have found that thiourea, when added along with PCMBS to a red cell suspension, interferes with this inhibition and at high enough concentrations prevents the inhibition from developing altogether. For a 2 mM PCMBS concentration K_i = ; 3 ± 1 mM. When thiourea is added at a later time, the PCMBS inhibition, which normally takes about 20 min to develop fully, is halted and remains fixed at the value attained by that time. Thiourea also inhibits the reversal of PCMBS inhibition by a 10 mM concentration of cysteine, the half-time for reversal increasing by more than an order of magnitude when [thiourea] = ; 50 mM. Possible implications for the nature of the water and urea transport pathways across the red cell membrane are discussed.     

Glucose-induced mobilisation of intracellular Ca2+ in depolarised pancreatic islets

Perifused rat pancreatic islets, prelabelled with ⁴⁵Ca, were exposed for 90 min to a medium containing 30 mM K⁺, 0.25 mM diazoxide and 0.5 mM EGTA, but deprived of CaCl₂. Either verapamil (0.05 mM) or Cd²⁺ (0.05 mM) were also present in the perifusate. Under these conditions a rise in D-glucose concentrations from either 2.8 to 16.7 mM or zero to 8.3 mM increased both ⁴⁵Ca outflow and insulin release, after an initial and transient decrease in effluent radioactivity. These findings suggest that, in islets depolarised by exposure to a high extracellular concentration of K⁺, D-glucose provokes an intracellular redistribution of Ca²⁺ ions and subsequent stimulation of insulin release. The functional response to D-glucose is apparently not attributable to either the closing of ATP-sensitive K⁺ channels, which were actually activated by diazoxide, or stimulation of Ca²⁺ influx, which was prevented by the absence of extracellular Ca²⁺. The present experimental design thus reveals a novel component of the glucose-induced remodelling of Ca²⁺ fluxes in islet cells. Such an effect might also be operative under physiological conditions, when the hexose leads to depolarisation of the islet B-cells.     

Cytoplasmic streaming and ionic regulation inNitella flexilis having artificial cell saps of low ionic strength

The cell sap of the internode ofNitella flexilis was replaced with the isotonic artificial pond water of high Ca²⁺-concentration (0.1 mM KCl, 0.1 mM NaCl, 10 mM CaCl₂ and 275 mM mannitol) and changes in osmotic value and concentrations of K⁺, Na⁺ and Cl⁻ of the cells were followed. When the operated cells were incubated in the artificial pond water containing 0.1 mM each of KCl, NaCl, CaCl₂, they survived for only a short period of time (<10 hr). The cells did not absorb ions from the artificial pond water and showed a conspicuous decrease in the rate of cytoplasmic streaming. In such cell the concentration of K⁺ in the protoplasm decreased significantly. In order to reverse normal concentration gradients of K⁺ and Na⁺ across the protoplasmic layer, the cells of low vacuolar ionic concentrations were incubated in the artificial cell sap (90 mM KCl, 40 mM NaCl, 15 mM CaCl₂, 10 mM MgCl₂). It was found that the cells rapidly absorbed much K⁺, Na⁺ and Cl⁻ and survived for a longer period (1–2 days). During this period the rate of cytoplasmic streaming was nearly normal. Furthermore, the cell lost much mannitol, indicating an enormous increase in permeability to it. Since both absorption of ions and leakage of mannitol at 1 C occurred at nearly the same rates as at 22 C, the processes are assumed to be passive.     

Effect of potential-dependent potassium uptake on calcium accumulation in rat brain mitochondria

The effect of potential-dependent potassium uptake at 0–120 mM K⁺ on matrix Ca²⁺ accumulation in rat brain mitochondria was studied. An increase in oxygen consumption and proton extrusion rates as well as increase in matrix pH with increase in K⁺ content in the medium was observed due to K⁺ uptake into the mitochondria. The accumulation of Ca²⁺ was shown to depend on K⁺ concentration in the medium. At K⁺ concentration ?30 mM, Ca²⁺ uptake is decreased due to K⁺-induced membrane depolarization, whereas at higher K⁺ concentrations, up to 120 mM K⁺, Ca²⁺ uptake is increased in spite of membrane depolarization caused by matrix alkalization due to K⁺ uptake. Mitochondrial K _ATP ⁺ -channel blockers (glibenclamide and 5-hydroxydecanoic acid) diminish K⁺ uptake as well as K⁺-induced depolarization and matrix alkalization, which results in attenuation of the potassium-induced effects on matrix Ca²⁺ uptake, i.e. increase in Ca²⁺ uptake at low K⁺ content in the medium due to the smaller membrane depolarization and decrease in Ca²⁺ uptake at high potassium concentrations because of restricted rise in matrix pH. The results show the importance of potential-dependent potassium uptake, and especially the K _ATP ⁺ channel, in the regulation of calcium accumulation in rat brain mitochondria.