Kinetics of Cl-dependent K fluxes in hyposmotically swollen low K sheep erythrocytes

A detailed kinetic study of K:Cl cotransport in hyposmotically swollen low K sheep red blood cells was carried out to characterize the nature of the outwardly poised carrier. The kinetic parameters were determined from the rate of K efflux and influx under zero-K-trans conditions in red cells with cellular K altered by the nystatin method and with different extracellular K or Rb concentrations. Although apparent affinities for efflux and influx were quite similar, the maximal velocity for K efflux was approximately two times greater than for influx. Furthermore, at thermodynamic equilibrium (i.e., when the ion product of K and Cl within the cell was equal to that outside) a temperature-dependent net K efflux was observed, approaching zero only when the external product reached approximately two times the internal product. The binding order of the ions to the transporter was asymmetric, being ordered outside (Cl binding first, followed by K) and random inside. K efflux but not influx was trans-inhibited by KCl. Trans inhibition of K efflux was used to verify the order of binding outside: trans inhibition by external Cl occurred in the absence of external K, but not vice versa. Thus K:Cl cotransport is kinetically asymmetric in hyposmotically swollen low K sheep red cells.     

Transport and control of Ca2+ by pigeon erythrocytes. III. A 'paradoxical' expulsion of Ca2+ induced by a low dose of A23187 at 0 degrees C

Pigeon erythrocytes expelled preloaded 45Ca2+ in response to a low dose of A23187 at 0 degrees C. We call this phenomenon 'paradoxical' expulsion. Within the first minute, 1.85 +/- 0.38 mumol/l cell water was expelled; after that the internal 45Ca2+ began to rise. The rises in Ca2+ uptake with and without A23187 addition were essentially paralleled. No premonitory rise of 45Ca2+ upon the addition of A23187 was observed. Expulsion of 45Ca2+ in response to A23187 was probably by the action of the Ca2+ pump and not by Na+-Ca2+ exchange since vanadate inhibited, but K+ replacement of Na+ in the medium had no effect. Lysophosphatidylcholine (lysoPC) caused an abrupt increase in 45Ca2+ influx by cells at 0 degrees C and was dose dependent. However, a very low dose of lysoPC induced expulsion of preloaded 45Ca2+ similar to that by A23187, the response was fast and transitory, without any premonitory rise in 45Ca2+ uptake. The results lend support to the suggestion that the signal to which cells respond may be a sudden change in Ca2+ influx per se rather than a change in internal Ca2+ concentration. These features of 'paradoxical' 45Ca2+ expulsion induced by A23187 and lysoPC are not expected from mass-action equilibria but, instead, agree with the characteristics of an energy-dissipating control mechanism.     

A chloride dependent K+ flux induced by N-ethylmaleimide in genetically low K+ sheep and goat erythrocytes

In genetically low K⁺ but not in high K⁺ red cells of sheep and goat N-ethylmaleimide induced a ouabain insensitive K⁺ flux as measured by tracer influx or net efflux methods. The augmented K⁺ flux was observed in Cl^? or Br^? but not in NO₃^?, SO₄^2? or PO₄^2? media. The action of N-ethylmaleimide was distinct from that of parachloromercuribenzoate or its sulfonic acid derivative which increased both passive K⁺ and Na⁺ movements across the red cell membrane. The instantaneous selective action of N-ethylmaleimide suggests that sulfhydryl groups control a $\text{K}{}^{\mathrm{+}}\text{Cl}{}^{\mathrm{?}}$ transport system which, associated with the low K⁺ gene, is apparently functionally silent in adult ruminant red cells.     

Kinetic comparison of ouabain-resistant K:CI fluxes (K:Cl [Co]-transport) stimulated in sheep erythrocytes by membrane thiol oxidation and alkylation

Summary The stimulatory effects of two thiol (SH) group oxidants, methylmethane thiosulfonate (MMTS) and diazene dicarboxylic acid bis [N,N-dimethylamide] (diamide), on the kinetics of ouabain-resistant (OR) K:Cl [co]-transport in low K (LK) sheep red blood cells were compared with the effects of alkylating agents, notably N-ethylmaleimide (NEM). At low concentrations, both MMTS and diamide stimulated K:CI [co]-transportv and with a latency period, as measured by OR zero-trans K efflux and OR uptake of external Rb, Rb_o, as K congener in Cl and NO₃ media. At high concentrations the effect of diamide saturated, and that of MMTS disappeared. The stimulatory effect of MMTS was partially reversed by the reducing agent dithiothreitol (DTT) known to fully restore the diamide-activated K flux (Lauf, J. Memb. Biol. 101:179–188, 1988). In diamide pre-equilibrated LK sheep red cells, the K_m of K:Cl [co]-transport for external Cl, Cl_o, was 84.3 mM, and 18.7 mM for Rb_o, with nearly identical V_max values around 4 mmol Rb/L cells × h for K (Rb) fluxes in Cl and after correction for the small Cl-independent component. Zero net K (Rb) flux existed at K_c (cell K)/Rb_o concentration ratios, [K]_c/[Rb]_c, of 0.8 i.e. when the electrochemical driving forces across the membrane were about equal. The measured K efflux/Rb influx ratios were almost twice those predicted from [K]_c/[Rb]_o and the Cl equilibrium potential suggesting that the diamide-stimulated K (Rb) flux may occur through non-diffusional, carrier-mediated transport. The effects of NEM and of A23187 plus/minus Ca or chelators on K: [co]Cl-transport (Lauf, Am. J. Physiol. 249:C271–278, 1985) consisted primarily of V_max changes. Thus, all chemical interventions resulted in an increase of the number of actively transporting K:Cl [co]-transport units or an augmented turnover number per existing site.     

Effects of adenosine diphosphate on Ca2+ fluxes and Ca2+ accumulation of sarcoplasmic reticulum

Ca2+ transport by sarcoplasmic reticulum vesicles was examined by incubating sarcoplasmic reticulum vesicles (0.15 mg/ml) at 37 degrees C in, either normal medium that contained 0.15 M sucrose, 0.1 M KCl, 60 microM CaCl2, 2.5 mM ATP and 30 mM Tes at pH 6.8, or a modified medium for elimination of ADP formed from ATP hydrolysis by including, in addition, 3.6 mM phosphocreatine and 33 U/ml of creatine phosphokinase. In normal medium, Ca2+ uptake of sarcoplasmic reticulum vesicles reached a plateau of about 100 nmol/mg. In modified medium, after this phase of Ca2+ uptake, a second phase of Ca2+ accumulation was initiated and reached a plateau of about 300 nmol/mg. The second phase of Ca2+ accumulation was accompanied by phosphate uptake and could be inhibited by ADP. Since, under these experimental conditions, there was no significant difference of the rates of ATP hydrolysis in normal medium and modified medium, extra Ca2+ uptake in modified medium but not in normal medium could not be explained by different phosphate accumulation in the two media. Unidirectional Ca2+ influx of sarcoplasmic reticulum near steady state of Ca2+ uptake was measured by pulse labeling with 45Ca2+. The Ca2+ efflux rate was then determined by subtracting the net uptake from the influx rate. At the first plateau of Ca2+ uptake in normal medium, Ca2+ influx was balanced by Ca2+ efflux with an exchange rate of 240 nmol/mg per min. This exchange rate was maintained relatively constant at the plateau phase. In modified medium, the Ca2+ exchange rate at the first plateau of Ca2+ uptake was about half of that in normal medium. When the second phase of Ca2+ uptake was initiated, both the influx and efflux rates started to increase and reached a similar exchange rate as observed in normal medium. Also, during the second phase of Ca2+ uptake, the difference between the influx and efflux rates continued to increase until the second plateau phase was approached. In conditions where the formation of ADP and inorganic phosphate was minimized by using a low concentration of sarcoplasmic (7.5 micrograms/ml) and/or using acetyl phosphate instead of ATP, the second phase of Ca2+ uptake was also observed. These data suggest that the Ca2+ load attained by sarcoplasmic reticulum vesicles during active transport is modulated by ADP accumulated from ATP hydrolysis. ADP probably exerts its effect by facilitating Ca2+ efflux, which subsequently stimulates Ca2+ exchange.     

Regulation of Ca2+ fluxes in rat liver mitochondria by Ca2+. Effects on Ca2+ distribution

Rat liver mitochondria are able to temporarily lower the steady-state concentration of external Ca²⁺ after having accumulated a pulse of added Ca²⁺. This has been attributed to inhibition of a putative -modulated efflux pathway [Bernardi, P. (1984)Biochim. Biophys. Acta 766, 277–282]. On the other hand, the rebounding could be due to stimulation of the uniporter by Ca²⁺ [Kröner, H. (1987)Biol. Chem. Hoppe-Seyler 369, 149–155]. By measuring unidirectional Ca²⁺ fluxes, it was found that the uniporter was stimulated during the rebounding peak both under Bernardi's and Kröner's conditions, while no effects on the efflux could be demonstrated. The rate of unidirectional efflux of Ca²⁺ was not affected by inhibition of the uniporter. It appears likely that the rebounding is due to stimulation of the uniporter rather than inhibition of efflux.     

Effects of Ca2+ and ionophore A23187 on protein synthesis in intact rabbit reticulocytes

1. Amino acid incorporation in intact rabbit reticulocytes was unaffected by depletion of Ca2+ with EGTA. 2. The Ca2+ ionophore A23187 strongly inhibited incorporation in reticulocytes incubated in 1 mM Ca2+ but not in EGTA. Polysomal profiles and average ribosomal transit times of cells treated with Ca2+ ionophore at 1 mM Ca2+ were characteristic of translational elongation block. 3. The behavior of reticulocytes with respect to Ca2+ and A23187 contrasts with that of nucleated cells possessing endoplasmic reticulum in which protein synthesis is inhibited at translational initiation by either Ca2+ depletion or by exposure to Ca2+ ionophore.     

Comparative effects of H+ and Ca2+ on large-conductance Ca2+- and voltage-gated Slo1 K+ channels

Large-conductance Ca²⁺- and voltage-gated Slo1 BK channels are allosterically activated by depolarization and intracellular ligands such as Ca²⁺. Of the two high-affinity Ca²⁺ sensors present in the channel, the RCK1 sensor also mediates H⁺-dependent activation of the channel. In this study, we examined the comparative mechanisms of the channel activation by Ca²⁺ and H⁺. Steady-state macroscopic conductance-voltage measurements as well as single-channel openings at negative voltages where voltage-sensor activation is negligible showed that at respective saturating concentrations Ca²⁺ is more effective in relative stabilization of the open conformation than H⁺. Calculations using the Debye-Hückel formulation suggest that small structural changes in the RCK1 sensor, on the order of few angstroms, may accompany the H⁺-mediated opening of the channel. While the efficacy of H+ in activation of the channel is less than that of Ca²⁺, H⁺ more effectively accelerates the activation kinetics when examined at the concentrations equipotent on macroscopic voltage-dependent activation. The RCK1 sensor therefore is capable of transducing the nature of the ligand bound and transmits qualitatively different information to the channel's permeation gate.     

Characteristics of the volume- and chloride-dependent K transport in human erythrocytes homozygous for hemoglobin C

Summary In human red cells homozygous for hemoglobin C (CC), cell swelling and acid pH increase K efflux and net K loss in the presence of ouabain (0.1mm) and bumetanide. We report herein, that K influx is also dependent on cell volume in CC cells: cell swelling induces a marked increase in the maximal rate (from 6 to 18 mmol/liter cell × hr) and in the affinity for external K (from 77±16mm to 28±3mm) of K influx. When the external K concentration is varied from 0 to 140mm, K efflux from CC and normal control cells is unaffected. Thus, K/K exchange is not a major component of this K movement. K transport through the pathway of CC cells is dependent on the presence of chloride or bromide; substitution with nitrate, acetate or thiocyanate inhibits the volume- and pH-dependent K efflux. When CC cells are separated according to density, a sizable volume-dependent component of K efflux can be identified in all the fractions and is the most active in the least dense fraction. N-ethylmaleimide (NEM) markedly stimulates K efflux from CC cells in chloride but not in nitrate media, and this effect is present in all the fractions of CC cells separated according to density. The persistence of this transport system in denser CC cells suggests that not only cell age, but also the presence of the positively charged C hemoglobin is an important determinant of the activity of this system. These data also indicate that the K transport pathway of CC cells is not an electrodiffusional process and is coupled to chloride.     

Modulation of Ca2+-activated K+ channels of human erythrocytes by endogenous protein kinase C

Single IK(Ca) channels of human erythrocytes were studied with the patch-clamp technique to define their modulation by endogenous protein kinase C (PKC). The perfusion of the cytoplasmic side of freshly excised patches with the PKC activator, phorbol 12-myristate 13-acetate (PMA), inhibited channel activity. This effect was blocked by PKC(19-31), a peptide inhibitor specific for PKC. Similar results were obtained by perfusing the membrane patches with the structurally unrelated PKC activator 1-oleoyl-2-acetylglycerol (OAG). Blocking of this effect was induced by perfusion with PKC(19-31) or chelerythrine. Channel activity was not inhibited by the PMA analog 4alpha-phorbol 12,13-didecanoate (4alphaPDD), which has no effect on PKC. Activation of endogenous cAMP-dependent protein kinase (PKA), which is known to up-modulate IK(Ca) channels, restored channel activity previously inhibited by OAG. The application of OAG induced a reversible reduction of channel activity previously up-modulated by the activation of PKA, indicating that the effects of the two kinases are commutative, and antagonistic. Kinetic analysis showed that down-regulation by PKC mainly changes the opening frequency without significantly affecting mean channel open time and conductance. These results provide evidence that an endogenous PKC down-modulates the activity of native IK(Ca) channels of human erythrocytes. Our results show that PKA and PKC signal transduction pathways integrate their effects, determining the open probability of the IK(Ca) channels.     

Effects of phalloidin on K+-dependent, Ca2+-independent neurotransmitter efflux and K+-facilitated, Ca2+-dependent neurotransmitter release.

Phalloidin tightly binds to actin and converts soluble actin into depolymerization-resistant actin filaments. Phalloidin promotes the potassium-dependent, calcium-independent efflux of γ-amino butyric acid and nore-pinephrine from synaptosomes but inhibits the potassium-facilitated, calcium-dependent release of these neurotransmitters. This suggests that an actomyosin system is involved in synaptic transmission.     

Na+- and cGMP-induced Ca2+ fluxes in frog rod photoreceptors

We have examined the Ca2+ content and pathways of Ca2+ transport in frog rod outer segments using the Ca2+-indicating dye arsenazo III. The experiments employed suspensions of outer segments of truncated, but physiologically functional, frog rods (OS-IS), intact isolated outer segments (intact OS), and leaky outer segments (leaky OS with a plasma membrane leaky to small solutes, but with sealed disk membranes). We observed the following. Intact OS or OS-IS isolated and purified in Percoll-Ringer's solution contained an average of 2.2 mM total Ca2+, while leaky OS contained 2.0 mM total Ca2+. This suggests that most of the Ca2+ in OS-IS is contained inside OS disks. Phosphodiesterase inhibitors increased the Ca2+ content to approximately 4.2 mM in intact OS or OS-IS, whereas the Ca2+ content of leaky OS was not altered. Na-Ca exchange was the dominant pathway for Ca2+ efflux in both intact and leaky OS/OS-IS. The rate of Na-Ca exchange in intact OS/OS-IS was half-maximal between 30 and 50 mM Na+; at 50 mM Na+, this amounted to 5.8 X 10(7) Ca2+/OS X s or 0.05 mM total Ca2+/s. This is much larger than the Ca2+ component of the dark current. Other alkali cations could not replace Na+ in Na-Ca exchange in either OS-IS or leaky OS. They inhibited the rate of Na-Ca exchange (K greater than or equal to Rb greater than Cs greater than or equal to Li greater than TMA) and, as the inhibition became greater, a delay developed in the onset of Na-Ca exchange. The inhibition of Na-Ca exchange by alkali cations correlates with the prolonged duration of the photoresponse induced by these cations (Hodgkin, A. L., P. A. McNaughton, and B. J. Nunn. 1985. Journal of Physiology. 358:447-468). In addition to Na-Ca exchange, disk membranes in leaky OS showed a second pathway of Ca2+ transport activated by cyclic GMP (cGMP). The cGMP-activated pathway required the presence of alkali cations and had a maximal rate of 9.7 X 10(6) Ca2+/OS X s. cGMP caused the release of only 30% of the total Ca2+ from leaky OS. The rate of Na-Ca exchange in leaky OS amounted to 1.9 X 10(7) Ca2+/OS X s.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effects of phorbol ester, diacylglycerol, phospholipase C and Ca2+ ionophore on protein phosphorylation in human and sheep erythrocytes.

Treatment of human or sheep erythrocytes with PMA (phorbol myristate acetate) enhanced [32P]phosphate labelling of membrane polypeptides of approx. 100, 80 and 46 kDa. The 80 kDa and 46 kDa polypeptides coincided with bands 4.1 and 4.9 respectively on Coomassie-Blue-stained gels. Similar but smaller effects were obtained by treating human cells with 1-oleoyl-2-acetyl-rac-glycerol (OAG), exogenous bacterial phospholipase C or ionophore A23187 + Ca2+, each of which treatments would be expected to raise the concentration of membrane diacylglycerol. In contrast, sheep cells, which do not increase their content of diacylglycerol when treated with phospholipase C or A23187 + Ca2+, only showed enhanced phosphorylation with OAG. Neither human nor sheep cells showed any enhanced [32P]phosphate labelling of phosphoproteins when treated with 1-mono-oleoyl-rac-glycerol. It is concluded that diacylglycerol from a variety of sources can activate erythrocyte protein kinase C, but that the most effective diacylglycerol is that derived from endogenous polyphosphoinositides. In contrast with bacterial phospholipase C and A23187, which stimulate synthesis of phosphatidate by increasing the cell-membrane content of diacylglycerol in human erythrocytes, PMA, OAG or 1-mono-oleoyl-rac-glycerol caused no change in phospholipid metabolism.     

Effects of BAY K 8644, nifedipine, and low Ca2+ on halothane and caffeine potentiation.

The purpose of this investigation was to examine the effects of the Ca2+ agonist BAY K 8644 and the Ca2+ antagonist nifedipine on halothane- and caffeine-induced twitch potentiation of mammalian skeletal muscle. Muscle fiber bundles were taken from normal Landrace pigs and exposed to BAY K 8644 (10 microM), nifedipine (1 microM), and low Ca2+ media administered alone and in combination with halothane (3%) or with increasing concentrations of caffeine (0.5-8.0 mM). Both BAY K 8644 and halothane potentiated twitches by approximately 80%; when they were administered in combination, twitch potentiation was nearly double that caused by either drug alone. In the presence of nifedipine, halothane increased twitches by less than 30%. Low Ca2+ significantly depressed twitches by approximately 25% but also inhibited halothane's inotropic effect. BAY K 8644 augmented caffeine potentiation but only at low caffeine concentrations (0.5-2.0 mM). Nifedipine and low Ca2+ failed to inhibit caffeine's inotropic effects. These results suggest that halothane potentiates twitches via a mechanism that involves or is influenced by extracellular Ca2+.