Effects of N-ethylmaleimide on ouabain-insensitive cation fluxes in human red cell ghosts

Lysosomal sulphate transport is dependent upon sulphydryl groups

Using thiol blocking agents, we examined the role of sulphydryl groups for function of the lysosomal sulphate transport system. Monothiol binding reagents, p-hydroxymercuribenzoic acid (p-HMB) and p-chloromercuribenzene sulphonic acid (p-CMBS), dithiol binding reagents such as CuCl2, the alkylating agent, N-ethylmaleimide (NEM), and NADH all inhibited lysosomal sulphate transport. The inhibitory effects of NEM and Cu2+ were not additive, suggesting that they both act upon the same critical sulphydryl group(s). Unlike the case for NEM, the inhibitory effects of Cu2+ were reversed by the reducing agent, dithiothreitol. Exposure to NEM resulted in a seven-fold increase in Km to 867 microM versus a control value of 126 microM and a modest decrease in Vmax to 99 pmolperunit beta-hexosaminidase per 30 s versus a control value of 129 pmolperunit beta-hexosaminidase per 30 s. Similar although somewhat less dramatic results were obtained using Cu2+ with an increase of Km to 448 microM and a Vmax of 77 pmolperunit beta-hexosaminidase per 30 s. The sulphate transport activity of detergent solubilized lysosomal membranes could be bound to a p-chloromercuribenzoic acid (p-CMB)-Sepharose sulphydryl affinity resin and eluted with mercaptoethanol. Sulphydryl groups thus appear to play a role in sulphate transport through effects on substrate affinity. Sulphydryl-binding appears to be a strategy that may be useful for purification of the transporter.

ATP stimulates lysosomal sulphate transport at neutral pH: evidence for phosphorylation of the lysosomal sulphate carrier

ATP markedly stimulated sulphate uptake by rat liver lysosomes that had been treated with N-ethylmaleimide to block the effects of the lysosomal proton-translocating ATPase (H+-ATPase). Maximal stimulation required millimolar concentrations of ATP and neutral buffer pH. ATP-stimulated transport exhibited saturation kinetics with a Km of 175 microM, identical with the Km for lysosomal sulphate uptake at pH 5.0, a process that does not require ATP. The requirement for ATP was specific: other nucleotides such as AMP, ADP, CTP, GTP, ITP and UTP failed to stimulate transport. Adenosine 5'-[beta,gamma-imido]triphosphate, the non-hydrolysable analogue of ATP, also failed to stimulate sulphate uptake, suggesting a requirement for ATP hydrolysis. Lysosomal pH, membrane potential and glucose transport were unchanged by the presence of ATP under the experimental conditions, consistent with a direct effect of ATP on the sulphate transporter. Exposure of lysosomes to protein kinase A and protein kinase C inhibitors did not alter the stimulation of sulphate transport by ATP. The lysosomal sulphate transport protein might be subject to regulation by a phosphorylation pathway that is not dependent on protein kinase A or protein kinase C.

Prejunctional alpha 2-adrenoceptors in mouse atria function through G-proteins which are sensitive to N-ethylmaleimide, but not pertussis toxin

1. The identity of the G-proteins involved in prejunctional alpha 2-adrenoceptor signal transduction in mouse atria was examined by use of the G-protein inactivators N-ethylmaleimide and pertussis toxin. 2. The alpha 2-adrenoceptor partial agonist clonidine (0.03 microM) inhibited the electrical stimulation-induced (S-I) outflow of radioactivity from mouse atria which were incubated with [3H]-noradrenaline and stimulated at 5 Hz. The partial alpha 2-adrenoceptor agonist St 363 (10 microM) inhibited the S-I outflow of radioactivity at the lower stimulation frequency of 2.5 Hz. The inhibitory effects of these compounds were not altered in mice pretreated with pertussis toxin (1.5 micrograms, i.v.). 3. The alpha 2-adrenoceptor antagonist, idazoxan (0.1 microM), increased the S-I outflow of radioactivity from mouse atria stimulated at 5 Hz, and this effect was not altered in atria from mice pretreated with pertussis toxin. 4. The inhibitory effects of clonidine and St 363 and the facilitatory effect of idazoxan on the S-I outflow of radioactivity from mouse atria were significantly less in atria incubated with N-ethylmaleimide (NEM, 3 microM) for 60 min before the [3H]-noradrenaline incubation. 5. The results suggest that prejunctional alpha 2-adrenoceptors in mouse atria function through G-proteins which are NEM-sensitive, but pertussis toxin insensitive.

Erythrocyte K-Cl cotransport: properties and regulation

Erythrocytes possess a Cl-dependent, Na-independent K transport system cotransporting K and Cl in a 1:1 stoichiometry that is membrane potential independent. This K-Cl cotransporter is stimulated by cell swelling, acidification, Mg depletion, and thiol modification. Cell shrinkage, elevation of cellular divalent ions, thiol alkylation, phosphatase inhibitors, and derivatives of certain loop diuretics and stilbenes are inhibitory. Thus regulation of K-Cl cotransport at the membrane and cytoplasmic levels is highly complex. Basal K-Cl cotransport decreases with cellular maturation, whereas its modes of stimulation and inhibition are variable between species. The physiological inactivation appears to be prevented in low-K animal erythrocytes. In certain human hemoglobinopathies, K-Cl cotransport may be the cause of cellular dehydration and volume decrease. K-Cl cotransport occurs also in nonerythroid cells, such as in epithelial and liver cells of other species. At the threshold of molecular characterization, this comprehensive review places our present understanding of the mechanisms modulating K-Cl cotransport physiologically and pathophysiologically into kinetic and thermodynamic perspectives.

Na+/K+/Cl- cotransport in resealed ghosts from erythrocytes of the Milan hypertensive rats

The erythrocytes (RBC) of the Milan hypertensive rats (MHS) have a smaller volume and faster Na+/K+/Cl- cotransport than RBC from normotensive controls (MNS). The difference in Na+/K+/Cl- cotransport is no longer present in inside-out Vesicles (IOV) of RBC membrane. To differentiate between cytoplasmic or membrane skeleton abnormalities as possible causes of these differences. Resealed ghosts (RG) were used to measure ion transport systems. The following results have been obtained: (1) RG from MHS have a smaller volume than MNS (mean +/- S.E. 20.7 +/- 0.45 vs. 22.09 +/- 0.42 fl, P < 0.05). (2) RG showed a bumetanide-sensitive Na efflux that retains the characteristics of the Na+/K+/Cl- cotransport of the original RBC: it is K(+)- and Cl(-)-sensitive and dependent on the intracellular Na+ concentration. (3) The Na+/K+/Cl- cotransport was faster in RG from MHS than in those from MNS (mean +/- S.E. 0.095 +/- 0.01 vs. 0.066 +/- 0.01 rate constant h-1, P < 0.01). These results, together with those of IOV, support the hypothesis that an abnormality in the membrane skeletal proteins may play a role in the different Na+/K+/Cl- cotransport modulation between MHS and MNS erythrocytes.

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

Prejunctional beta-adrenoceptors, angiotensin II and neuropeptide Y receptors on sympathetic nerves in mouse atria are linked to N-ethylmaleimide-susceptible G-proteins

We used the alkylating agent N-ethylmaleimide in order to investigate G-proteins linked to release-modulating prejunctional receptors of sympathetic nerves in mouse atria incubated with [3H]-noradrenaline. The receptors tested were facilitatory beta-adrenoceptors and angiotensin II receptors and inhibitory neuropeptide Y receptors. In order to evaluate the specificity of the N-ethylmaleimide treatment, we tested N-ethylmaleimide against the second messenger pathways that are linked to beta-adrenoceptors (adenylate cyclase) and angiotensin II (protein kinase C). The results show that a 60-min preincubation with N-ethylmaleimide (3 microM) abolished the facilitatory effect of isoprenaline (0.1 microM) and angiotensin II (0.1 microM) on the stimulation-induced release of noradrenaline and reduced the inhibitory action of neuropeptide Y (0.3 microM). N-ethylmaleimide had no effect on the stimulatory action of either phorbol dibutyrate (0.01, 0.1 microM), forskolin (10 microM), or a combination of 8-bromo adenosine-3'5'-monophosphate (90 microM) and 3-isobutyl-1-methylxanthine (100 microM). However, at a higher concentration (10 microM), N-ethylmaleimide reduced the facilitatory effect of phorbol dibutyrate (0.1 microM) and the combination of 8-bromo adenosine-3',5'-monophosphate (90 microM) and 3-isobutyl-1-methylxanthine (100 microM). This suggests that N-ethylmaleimide at 3 microM but not 10 microM was selective for receptor-mediated modulation of noradrenaline release without directly affecting the adenylate cyclase (forskolin, 8-bromo adenosine-3',5'-monophosphate + 3-isobutyl-1-methylxanthine) or protein kinase C (phorbol dibutyrate) transduction pathways. In atria from mice pretreated with pertussis toxin (1.5 micrograms/mouse), N-ethylmaleimide preincubation (1 and 3 microM) resulted in a more pronounced reduction of the inhibitory action of neuropeptide Y (0.3 microM). The nature of this interaction is unclear. Since N-ethylmaleimide has been shown in other studies to inactivate G-proteins, the inhibitory effect of N-ethylmaleimide on prejunctional beta-adrenoceptors, angiotensin II receptors and neuropeptide Y receptors of sympathetic nerves may suggest that G-proteins are involved with these receptors, although other effects of N-ethylmaleimide on the receptor coupling processes cannot be ruled out. Moreover, it appears that the concentration of N-ethylmaleimide used is critical since a higher concentration (10 microM) resulted in non-specific effects on signal transduction mechanisms in the present experimental conditions.

Volume-sensitive, Cl-dependent K transport in resealed human erythrocyte ghosts

Potassium influx and efflux in Cl and NO3 media were measured in resealed ghosts prepared from human red cells. Cl-dependent K influx was three times that in intact cells and, as in intact cells, was partially supported by Br but not by thiocyanate (SCN). In other properties, this flux differed from that in intact cells: substitution of N-methylglucamine for Na did not decrease but rather increased Cl-dependent K influx, the affinity for external K was reduced, with a Km of 21.3 +/- 12.5 mM, and inhibition by furosemide and bumetanide was incomplete. Furosemide at 1 mM inhibited Cl-dependent influx by 26 and 51% at 4 and 20 mM K, respectively. Bumetanide inhibited Cl-dependent K influx by 0 and 55% at concentrations of 10 microM and 1 mM, respectively, in 4 mM K, with no further inhibition at 20 mM K. Neither the magnitude nor the properties of the flux were altered by preparing ghosts in the presence of 1,4-dithiothreitol, indicating that sulfhydryl oxidation was not responsible for the altered flux in ghosts. Treatment with N-ethylmaleimide (NEM) either before or after ghost preparation did not increase Cl-dependent K influx. However, Cl-dependent influx in ghosts could be augmented by increasing ghost volume or ATP content. Resealed human erythrocyte ghosts thus exhibit a volume- and ATP-sensitive, Cl-dependent K flux that differs substantially from the putative Na-K-Cl cotransport in intact cells in that it is independent of Na, is relatively resistant to furosemide and bumetanide, and has a low affinity for K.(ABSTRACT TRUNCATED AT 250 WORDS)

Properties of K+ transport in resealed human erythrocyte ghosts

We report here our studies on K+ transport in resealed human red cell ghosts (RG) in the presence of 0.1 mM ouabain and 0.01 mM bumetanide, inhibitors of the Na+-K+ pump and Na+-K+-Cl- cotransport, respectively. RG were obtained with the gel-filtration method. K+ efflux from RG was dependent on the pH used in the lysis buffer and increased when the pH used in the lysis buffer and increased when the pH was raised from 5.5 to 8.0. As in intact red cells, RG made from cells of the least dense fraction had a much higher K+ efflux than RG made from cells of the densest fraction. This K+ flux is volume independent and increases when the pH of the flux medium is increased from 6.0 to 8.0. K+ efflux (60-70%) at pH 7.40 from RG made from cells of the least dense fraction is inhibited when Cl- is substituted by nitrate or when the ghosts are resealed in the absence of ATP. This chloride- and ATP-dependent component is markedly reduced in RG made from cells of the densest fraction. An increase in the internal Mg2+ concentration in RG from the least dense fraction induced marked inhibition of K+ efflux. Contrary to intact cells, N-ethylmaleimide (NEM) did not affect K+ efflux from RG. Thus the effects of pH, osmolarity, and NEM on K+ transport in RG are markedly different from those reported in intact erythrocytes.

Thiol-dependent passive K/Cl transport in sheep red cells: VII. Volume-independent freezing by iodoacetamide, and sulfhydryl group heterogeneity

The sulfhydryl (SH) reagent iodoacetamide (IAAM) inhibits stimulation of Cl-dependent K transport in low K (LK) sheep red cells by another SH reagent, N-ethylmaleimide (NEM), without itself activating this transport pathway (J. Membrane Biol., 1983, 73:257-261). We now report that IAAM alone, acting with a kinetic slower than NEM, sharply reduced the capability of the Cl-dependent K transport system to regulate its activity in response to cell volume changes. This effect of IAAM did not depend on the cell volume maintained during chemical treatment, a fact ruling out that the reactivity of the SH groups with IAAM was a function of the volume-dependent turnover rate of the transporter. On the other hand, the prevention of the NEM-stimulatory effect on Cl-dependent K transport was found to be volume-dependent since 1) the rate with which IAAM blocked the subsequent NEM action was twice as fast in cells swollen in 250 mOSM as opposed to cells shrunken in 370 mOSM media, and 2) the dose response of the IAAM effect was different in swollen and shrunken cells. The action of IAAM with or without subsequent treatment with NEM seemed to be independent of cellular ATP which is required for full expression of the stimulatory modification of Cl-dependent K transport by NEM (Am. J. Physiol., 1983, 245:C445-C448). Clusters of SH groups on the Cl-dependent K transporter apparently react differently with IAAM and NEM when separately applied but, used in combination, reflect a complex volume-dependent effect that may reveal a "volume-sensing" component of the transport molecule.

Effects of N-ethylmaleimide and forskolin on noradrenaline release from rat hippocampal slices. Evidence that prejunctional adenosine and alpha-receptors are linked to N-proteins but not to adenylate cyclase

N-ethylmaleimide (NEM) treatment has been shown to inactivate regulatory GTP-binding N (G)-proteins in many preparations, including slices of rat hippocampus. NEM-treatment (100 microM for 15 min) has been used to examine the possible involvement of a N-protein in the prejunctional inhibitory effect of an adenosine analogue, R-PIA acting on A1-receptors, and of clonidine acting on alpha 2-adrenoceptors in this tissue. NEM treatment significantly enhanced basal overflow of [3H]NA and the overflow stimulated by low (0.3 Hz) frequency stimulation, but not the overflow stimulated by higher (1-10 Hz) frequency stimulation. The prejunctional inhibitory effect of R-PIA (1 microM) on NA release, stimulated by a 3 Hz stimulation, was abolished by NEM pretreatment, which also eliminated the dose-dependent prejunctional effect of clonidine and reduced the facilitatory effect of yohimbine. Forskolin had a small, but significant stimulatory effect on NA overflow, but did not reduce the prejunctional inhibitory effect of R-PIA. The adenylate cyclase inhibitor SQ 22, 536 did not reduce NA overflow. These results show that NEM blocks a critical step in the prejunctional action of both adenosine- and alpha 2-receptor agonists, which may be a N-protein. The possibility is discussed that the prejunctional A1- and alpha 2-receptors couple to a N-protein that controls a different effector than adenylate cyclase.