Cytoplasmic ATP Research Articles

Transsarcolemmal sodium-calcium exchange and myocardial oxygen consumption in isolated rat ventricular myocytes

We studied oxygen consumption and energy metabolism in isolated rat ventricular myocytes which were subjected to an abrupt change in the cation composition of the extracellular medium (‘transition’); extracellular [Na +] was decreased either alone or in combination with a change of [K +] or [Ca 2+]. The magnitude of change of the cation concentration(s) was varied. The respiratory rate (vO 2) of myocytes changed biphasically after such a transition. vO 2 increases to a maximum after about 25 to 30 s and returns to almost control after 180 to 200 s. vO 2-max depends on both the nature of the cation(s) of which the concentration(s) are varied and on the magnitude of these change(s); vO 2-max can almost be as high as that induced by uncoupling of oxidative phosphorylation with DNP. The free energy of hydrolysis of cytoplasmic ATP hardly decreases after transition. Cell viability remains unaltered, although an increasingly larger fraction of rodshaped cells transform to a hypercontracted state with increasing magnitude of the extracellular ion concentration change. Reversal of the ionic change or addition of EGTA at 30 s after transition accelerates the return of vO 2 to the value prior to transition. In the presence of ouabain, vO 2-max is higher and return to control is slower and incomplete. The total amount of oxygen consumption after transition, is linearly related to the initial change of the free energy of the Na +/Ca 2+-exchanger caused by the cation concentration change(s); this relationship does not depend on the nature of the cation(s) changed. We conclude that the transient increase of vO 2 after transition is regulated by intracellular free [Ca 2+], which transiently increases. This transient increase is caused by change of the thermodynamic driving force on the Na +/Ca 2+-exchanger after transition.

Transmitter glutamate release from isolated nerve terminals: evidence for biphasic release and triggering by localized Ca2+.

The kinetics of Ca2(+)-dependent release of glutamate from guinea-pig cerebrocortical synaptosomes evoked by KCl or 4-aminopyridine are investigated using a continuous fluorimetric assay. Release by both agents is biphasic, with a rapid phase complete within 2 s followed by a more extensive slow phase with a half-maximal release in 52 s for KCl-evoked release and greater than 120 s for 4-aminopyridine-evoked release. The two phases of glutamate release may reflect a dual localization of releasable vesicles at the active zone and in the bulk cytoplasm. Decreasing depolarization depresses the extent rather than increasing the time for half-maximal Ca2(+)-dependent release. Both the fast and the slow phases of glutamate release require external Ca2+ and cytoplasmic ATP. KCl depolarization produces a transient "spike" of cytoplasmic free Ca2+ [( Ca2+]c), which recovers to a plateau; the major component of glutamate release occurs during this plateau. Predepolarization in the absence of added external Ca2+, to inhibit transient Ca2+ channels, does not affect the subsequent glutamate release evoked by Ca2+ readdition. Thus, release involves primarily noninactivating Ca2+ channels. For a given increase in [Ca2+]c, KCl and 4-aminopyridine cause equal release of glutamate, while ionomycin releases much less glutamate. This lowered efficiency is not due to ATP depletion. It is concluded that glutamate exocytosis is evoked by localized Ca2+ entering through noninactivating voltage-dependent Ca2+ channels and that nonlocalized Ca2+ entry with ionomycin is inefficient.

Influence of cytoplasmic ATP reduction on catecholamine synthesis in cultured bovine adrenal chromaffin cells

To elucidate a possible role of cytoplasmic ATP in the regulation of catecholamine biosynthesis, cultured bovine adrenal chromaffin cells were pretreated with metabolic inhibitors in the glucose-free medium, and the cytoplasmic concentration of ATP was determined by measuring the amount of ATP released by permeabilization of the plasma membranes with digitonin. Catecholamine biosynthetic activity in these metabolic inhibitor-treated cells was determined by measuring the formation of [(14)C]catecholamines from l-[(14)C]tyrosine. The cytoplasmic concentration of ATP was reduced by pretreatment of the cells with 2,4-dinitrophenol (DNP), oligomycin, carbonyl cyanide m-chlorophenylhydrazone (CCCP) and antimycin. These metabolic inhibitors reduced [(14)C]catecholamine formation without any significant alteration in [(14)C]tyrosine uptake into the cells. In addition, the formation of [(14)C]catecholamines stimulated by high K(+) or cyclic AMP was also reduced by these inhibitors. In contrast, both 2-deoxyglucose (2-DG) and NaN(3) failed to cause any substantial effect on the rate of [(14)C]catecholamine formation as well as the ATP concentration in the cell cytoplasm. While, none of these inhibitors caused a direct inhibitory action on tyrosine hydroxylase prepared from bovine adrenal medulla. The results presented here suggest the possibility that ATP in the cell cytoplasm plays an important role as a factor regulating the catecholamine biosynthetic activity in the adrenal chromaffin cell.

ATP inhibits smooth muscle Ca 2+ -activated K + channels

There has been much recent interest in the roles played by smooth-muscle K+ channels in protecting cells against ischemic and anoxic insults and in therapeutic vaso- and bronchodilation (Buckingham 1990; Longmore & Weston 1990). A K+ channel, which is uniquely sensitive to cytoplasmic ATP (KATP), has been identified as a likely candidate for mediating these important functions (Standen et al. 1989). We now show, by using electrophysiological techniques in three different types of smooth muscle, that a large-conductance voltage and Ca2(+)-sensitive channel, otherwise indistinguishable from the the large-conductance Ca2(+)-activated K+ channel (BK channel), is also sensitive to cytoplasmic ATP and cromakalim. ATP, in a dose-dependent manner, decreased the probability of channel opening (Po) of rabbit aortic, rabbit tracheal and pig coronary artery BK channels with a Ki of 0.2-0.6 mM. Cromakalim, 10 microM, partially reversed the ATP induced inhibition and increased Po. Our observations raise the possibility that the ubiquitous BK channel may play a role during pathophysiological events.

An ATP, calcium and voltage sensitive potassium channel in porcine coronary artery smooth muscle cells

There is increasing interest in the roles played by potassium channels of smooth muscle in protecting against ischemic and anoxic insults. Hence, potassium-selective channels were studied in freshly dispersed porcine coronary artery smooth muscle cells using the inside-out variant of the patch-clamp technique. The most abundant potassium channel had a conductance of 148 pS in a 5.4 140 mM K + gradient, at 0 mV, and was regulated by cytoplasmic ATP (0.05 – 3.0 mM), cytoplasmic Ca 2+ (0.1–10 μM) and voltage. ATP and AMP-PNP (0.5 mM) reduced the probability of channel opening ( P 0) by 87 and 92%, respectively. This inhibition was partially reversed by the addition of 0.5 mM ADP. ADP on its own (2 mM) reduced P 0 by 46%. It appears, therefore, that this channel shares properties with both the ATP-sensitive and the calcium-regulated potassium channels, raising the possibility that it plays a central role in the regulation of coronary blood flow.

Phosphate Uptake by Excised Maize Root Tips Studied by in Vivo31P Nuclear Magnetic Resonance Spectroscopy

The extent of phosphate uptake measured by the relative changes in cytoplasmic Pi, vacuolar Pi, ATP, glucose-6-phosphate, and UDPG was determined using in vivo(31)P nuclear magnetic resonance spectroscopy. Maize (Zea mays) root tips were perfused with a solution containing 0.5 or 1.0 millimolar phosphate at pH approximately 6.5 under different conditions. In the aerated state, phosphate uptake resulted in a significant increase (>80%) in vacuolar Pi, but cytoplasmic Pi only transiently increased by 10%. Under N(2), the cytoplasmic Pi increased approximately 150% which could be attributed to a large extent to the breakdown of ATP, sugar phosphates and UDPG. Vacuolar Pi increased but only to the extent of approximately 10% of that seen under aerobic conditions. 2-deoxyglucose pretreatment was utilized to decrease the level of cytoplasmic Pi. When pretreated with the 2-deoxyglucose, the excised maize roots absorbed phosphate from the perfusate with a significant increase in the cytoplasmic Pi. The increase could only be traced to external phosphate since the concentrations of other phosphorus containing species remained constant during the uptake period. With 2-deoxyglucose pretreatment, phosphate uptake under anaerobic conditions was substantially inhibited with only the vacuolar phosphate showing a slight increase. When roots were treated with carbonyl cyanide m-chlorophenyl hydrazone, no detectable Pi uptake was found. These results were used to propose a H(+)-ATPase related transport mechanism for phosphate uptake and compartmentation in corn root cells.

ATP-sensitive K + channels reveal the effects of intracellular chloride variations on cytoplasmic ATP concentrations and mitochondrial function

Replacement of intracellular Cl − by impermeant anions, as well as treatment of insulinoma cells by the Cl − channel blocker, NPPB, leads to activation of ATP-dependent K + (K ATP) channels. Activation of K ATP channels by Cl − substitution is eliminated (i) when intracellular ATP is replaced by non-hydrolyzable ATP analogs, (ii) when the perfusion medium contains an ATP regenerating system, (iii) when the mitochondrial ATPase is blocked by oligomycin. Dinitrophenol and GDP have the same activating effects on K ATP channels as NPPB or intracellular Cl − substitution. Our interpretation of the results is that NPPB and intracellular Cl − replacement produce an uncoupling of oxidative phosphorylation by acting on mitochondrial anion channels, which leads to rapid degradation of ATP and to activation of K ATP channels. K ATP channels are useful sensors of cytoplasmic ATP variations.

Membrane transport of potassium ions in erythrocytes of the american black bear, Ursus americanus

1. 1. Membrane transport of K ions was investigated in red blood cells of bears by methods of measurement of unidirectional isotopic fluxes. 2. 2. Unlike red cells of dogs, red cells of bears exhibited a significant, though small, component of ouabain-sensitive K influx. 3. 3. Ouabain-insensitive K influx, as in other carnivore cells, was activated by swelling and inhibited by shrinkage. Swelling-induced K influx was dependent upon presence of chloride ions but was not inhibited by furosemide or bumetanide. 4. 4. Ouabain-sensitive K influx was largest with ATP and with high concentration of Na in the cell, but it persisted in the absence of cytoplasmic Na or ATP. It was also resistant to the drug, harmaline, at a concentration that in other cells fully inhibits ouabain-sensitive K influx. 5. 5. It was concluded that under such adverse conditions ouabain-sensitive K influx represents another mode of the Na/K pump not fully described elsewhere. 6. 6. Also, as in low K red cells of sheep and goat, apparent absence of Na/K pump activity in carnivore red cells may represent suppression rather than elimination of activity. 7. 7. Ouabain-insensitive K influx showed a seasonal pattern with minima occurring in early winter, earlier than for the minimum observed in Na influx. 8. 8. Ouabain-sensitive K influx tended to be lower in the hibernation season of the bear, but the seasonal pattern was not consistent.

Sites of adenosine formation, action and inactivation in the brain

Two extreme and non-exclusive hypotheses can be put forward to account for the largely inhibitory actions of adenosine on neural activity. Adenosine formation may be linked to the balance between formation and degradation of cytoplasmic ATP and hence only be indirectly related to neurotransmission. Adenosine formed by this route must then diffuse out of the cell to reach extracellular receptor sites. It may then act on the adenosine forming cells, neighbouring neurones or other cells including the cerebral vasculature to restore energy balance. Alternatively, adenosine formation may be directly linked to neurotransmission, probably by secretion of adenine nucleotides followed by ecto-nucleotidase activity. This adenosine then acts directly at extracellular receptors to modulate the activity of the neurones from which it arises. Localization of the sites of adenosine formation, action and inactivation in the brain could help, in principle, to distinguish these two hypotheses, or to define the neurones to which each hypothesis applies. Nagy and co-workers (Neurochem. Int.16, 211-221, 1990) review what has been learnt of the location in the brain of adenosine receptors, the nucleoside transporter and adenosine deaminase. In this critique, we attempt to place these important elements of the pathway of adenosine formation, action and inactivation in a wider perspective.

Oscillatory mode of calcium signaling in rat pancreatic acinar cells

Cytoplasmic free calcium concentration ((Ca2+]i) was evaluated by dual-wavelength microspectrofluorometry of fura-2-loaded individual rat pancreatic acinar cells. Resting [Ca2+]i in unstimulated acini was 94.1 +/- 4.1 nM. Stimulation with high concentrations of cholecystokinin (CCK, 100 pM to 1 nM) led to an immediate rise in [Ca2+]i to 400-1,000 nM followed by a fall within 2-5 min to a plateau only slightly above the prestimulation level. Lower and more physiological concentrations of CCK (1-30 pM), after a latent period of 60-90 s, induced a smaller sustained increase in [Ca2+]i (30-40 nM) with superimposed repetitive transient [Ca2+]i spikes. These oscillations averaged 120-150 nM in amplitude, occurred at a frequency which averaged 1.5 times/min, and were maintained as long as the stimulus was applied. Similar [Ca2+]i oscillations were observed when acini were stimulated with submaximal concentrations of carbamylcholine (0.1-1 microM) and neuromedin C (0.1-1 nM). Intracellular Ca2+ stores were not depleted during [Ca2+] oscillations, since a subsequent increase to 1 nM CCK led to an immediate rise in [Ca2+]i indistinguishable from the response of cells initially stimulated at this concentration. Although extracellular Ca2+ was required for maintenance of frequency of the spikes, the major source of Ca2+ utilized for oscillations was intracellular, since elimination of medium Ca2+ or Ca2+ entry blockade with lanthanum failed to inhibit oscillations. Vasoactive intestinal polypeptide (10 nM) and high K+ (50 mM) did not affect [Ca2+]i oscillations. Antimycin (10 microM), which depletes cytoplasmic ATP, increased basal [Ca2+]i and inhibited the oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolite-regulated ATP-sensitive K+ channel in human pancreatic islet cells.

In patch-clamped surface cells of human islets, we identified an inwardly rectifying, voltage-independent K+ channel that may be a crucial link between substrate metabolism and depolarization-induced insulin secretion. It is the major channel open at rest. It closes on exposure of the cell to secretagogue concentrations of glucose or other metabolic fuels and oral hypoglycemic sulfonylureas but reopens on addition of either a metabolic inhibitor that prevents substrate utilization or the hyperglycemic sulfonamide diazoxide. Onset of electrical activity coincides with channel closure by the secretagogues. In excised patches, the activity of this channel is inhibited at its cytoplasmic surface by ATP. These results suggest that in humans, as in rodents, 1) rises in cytoplasmic ATP levels during substrate metabolism trigger K+-channel closure and cell depolarization and 2) clinically useful sulfonamides modulate glucose-induced insulin secretion, in part by affecting a readily identifiable resting conductance pathway for K+.

Stimulation of respiration by mitogens in rat thymocytes is independent of mitochondrial calcium.

The role of calcium in the control of respiration by the mitogen concanavalin A (ConA) was investigated in rat thymocytes. ConA induced an increase in both mitochondrial respiration and the mitochondrial calcium pool. The stimulation of respiration was shown to be independent of the increase in mitochondrial calcium: the calcium pool declined after 3 min, whereas the respiration increase was persistent, and was not affected by depletion of the calcium pool or by buffering intracellular Ca2+ transients with quin2. The mitogen phytohaemagglutinin stimulated respiration to the same extent as ConA, but did not increase the mitochondrial calcium pool. In addition, respiration was unaffected by changes in the mitochondrial calcium pool induced by increasing or decreasing extracellular calcium. These results indicate that control of respiration is not located in the Ca2+-sensitive mitochondrial dehydrogenases. The ConA-induced increase in respiration could be blocked by oligomycin, suggesting control by cytoplasmic ATP turnover, and was not associated with detectable changes in NAD(P)H fluorescence, indicating a balance between increased electron transfer and increased supply of reduced substrates.

Use of actin-bound adenosine 5′-diphosphate as a method to determine the specific 32P-radioactivity of the γ-phosphoryl group of adenosine 5′-triphosphate in a highly compartmentalized cell, the platelet

Determination of the specific 32P-radioactivity of cytoplasmic ATP in 32P- P i-labeled platelets is complicated by the presence of a large pool of metabolically inactive, granule-stored nucleotides. Moreover, our data show that the specific 32P-radioactivity of cytoplasmic ATP is severely underestimated when determined in platelets after the complete secretion of granule-stored nucleotides, possibly due to isotopic dilution with granule-stored phosphate. As F-actin-bound ADP is ethanol-insoluble, this pool can be readily separated from the other nucleotide pools in platelets. Here we show that the specific 32P-radioactivity of F-actin-bound ADP accurately reflects that of the γ-phosphoryl group of cytoplasmic ATP. During uptake of 32P- P i by human platelets the specific 32P-radioactivity of F-actin-bound ADP equals that of the monoester phosphates of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, which are in metabolic equilibrium with cytoplasmic ATP. Therefore, this method enables the determination of the specific 32P-radioactivity of the γ-phosphoryl group of cytoplasmic ATP in platelets even under short-term labeling conditions.

Pharmacology of K+ channels in the plasmalemma of the green algaChara corallina

The outer membranes of plant cells contain channels which are highly selective for K+. However, many of their properties and their similarities to K+ channels found in animal cells had not previously been established. The channels open when the cells are depolarized in solutions with a high K+/Ca2+ ratio. In this work, the pharmacology of a previously identified plant K+ channel was examined. This survey showed that the channels have many properties which are similar to those of high-conductance Ca2+-activated K+ channels (highG K+(Ca2+)). K+ currents inChara were reduced by TEA+, Na+, Cs+, Ba2+, decamethonium and quinine, all inhibitors of, among other things, highG K+(Ca2+) channels. Tetracaine also inhibited K+ currentsChara, but its effect on most types of K+ channels in animal tissues is unknown. The currents were not inhibited by 4-aminopyridine (4AP), caffeine, tolbutamide, dendrotoxin, apamin or tubocurarine, which do not inhibit highG K+(Ca2+) channels, but affect other classes of K+ channels. The channels were “locked open” by 4AP, in a remarkably similar manner to that reported for K+(Ca2+) channels of a molluscan neuron. No evidence for the role of the inositol cycle in channel behavior was found, but its role in K+ channel control in animal cells is obscure. Potassium conductance was slightly decreased upon reduction of cytoplasmic ATP levels by cyanide + salicylhydroxamic acid (SHAM), consistent with channel control by phosphorylation. The anomalously strong voltage dependence of blockade by some ions (e.g. Cs+) is consistent with the channels being multiion pores. However, the channels also demonstrate some differences from the highG K+(Ca2+) channels found in animal tissues. The venom of the scorption,Leiurus quinquestriatus (LQV), and a protein component, charybdotoxin (CTX), an apparently specific inhibitor of highG K+(Ca2+) channels in various animal tissues, had no effect on the K+ channels in theChara plasmalemma. Als,, pinacidil, an antihypertensive drug which may increase highG K+(Ca2+) channel activity had no effect on the channels inChara. Although the described properties of theChara K+ channels are most similar to those of high conductance K+(Ca2+) in animal cells, the effects of CTX and pinacidil are notably different; the channels are clearly of a different structure to those found in animal cells, but are possibly related.

Import of honeybee prepromelittin into the endoplasmic reticulum: energy requirements for membrane insertion.

The import of small precursor proteins, derived from the honeybee secretory protein prepromelittin, into dog pancreas microsomes is independent of signal recognition particle (SRP) and docking protein, but requires that charged amino acids at the amino terminus of the mature part are counterbalanced by amino acids with the opposite charge at the carboxy terminus. The import pathway of such precursor proteins was resolved into two sequential steps: (i) binding of precursors to microsomes, and (ii) insertion of precursors into the membrane. Formation of an intramolecular disulfide bridge within the mature part of these precursor proteins allowed association of the oxidized precursors with the microsomal membrane but reversibly inhibited their membrane insertion. Furthermore, membrane insertion was inhibited by ATP depletion. Different prepromelittin derivatives were found to depend on ATP to varying degrees. We conclude that insertion of prepromelittin-derived precursor proteins into microsomal membranes involves a competent conformation of the precursor proteins and that, in general, this is accomplished with the help of both a cytoplasmic component and ATP.

Nitrite Uptake into Intact Pea Chloroplasts

The uptake of nitrite into intact pea chloroplasts was observed and its relationship with internal nitrite reduction was assessed. Net nitrite uptake exhibited saturation kinetics and an alkaline pH preference. This evidence questioned the accepted major role for neutral HNO(2) permeation and its reported influence on photosynthesis. The possible involvement of a nitrite permeation channel or transport protein is discussed. Net nitrite uptake curves were closely comparable with those for nitrite reduction within the chloroplast. Net nitrite uptake into chloroplasts was profoundly influenced by darkness, incubation temperature, and plant nitrate nutrition. Of several inorganic salts tested, sulfite was the only anion to exhibit a distinct inhibition of nitrite uptake. In contrast, nitrite uptake could be stimulated by the presence of certain cations, particularly at acidic pH values. It was concluded that nitrite uptake was closely related to stromal pH, internal nitrite accumulation, and nitrite reduction. The possible dependence of nitrite reduction on nitrite uptake rather than electron transport is discussed. External ATP and NAD(P)H did not significantly affect net nitrite uptake. This suggested that cytoplasmic ATP or reductant could not directly support nitrite uptake and, possibly, nitrite assimilation.

Requirement of ATP for exocytotic release of catecholamines from digitonin-permeabilized adrenal chromaffin cells.

Cultured chromaffin cells were preincubated with digitonin to deplete endogenous ATP from the cell cytoplasm. Catecholamine release from these digitonin-pretreated cells was then studied in the presence and absence of exogenous ATP to elucidate a possible involvement of the cytoplasmic ATP in the exocytotic process. The preincubation of the cells with digitonin in the ATP-free permeabilizing medium resulted in a marked decline of the releasing response to a calcium challenge. Furthermore, the declined activity of catecholamine release caused by digitonin pretreatment was restored by the presence of ATP, but not by other adenine nucleotides, and this recovery was observed in a manner dependent on the concentration of ATP. These findings, therefore, seem to indicate that a decrease in the releasing activity of the digitonin-pretreated cells may be due to the removal of endogenous ATP from the cytoplasmic space of the cells, thus suggesting that the cytoplasmic ATP may be involved in the exocytotic mechanism of catecholamine secretion.

Influence of reduction of cytoplasmic ATP on catecholamine secretion from intact and digitonin-permeabilized adrenal chromaffin cells.

The effect of the metabolic inhibitor 2,4-dinitrophenol (DNP) on the concentration of cytoplasmic ATP and the activity of catecholamine secretion was studied using cultured bovine adrenal chromaffin cells. The pretreatment of the cells with DNP resulted in a decrease in the concentration of cytoplasmic ATP. Catecholamine secretion evoked by either carbamylcholine or high K+ was also reduced by DNP pretreatment. Oligomycin caused a significant decrease in both cytoplasmic ATP concentration and catecholamine secretion. In contrast, neither the ATP level nor the secretory response was affected by 2-deoxyglucose. Similarly, catecholamine release from digitonin-permeabilized chromaffin cells in response to a direct calcium challenge was reduced by pretreatment of the cells with DNP, and this reduction was partially prevented by the presence of ATP in the permeabilizing medium. These results suggest that ATP in the cell cytoplasm may play an important role in the regulation of catecholamine secretion as a factor modulating the activity of the exocytotic process in the adrenal chromaffin cell.

Influence of Reduction of Cytoplasmic ATP on Catecholamine Secretion from Intact and Digitonin-Permeabilized Adrenal Chromaffin Cells

The effect of the metabolic inhibitor 2,4-dinitrophenol (DNP) on the concentration of cytoplasmic ATP and the activity of catecholamine secretion was studied using cultured bovine adrenal chromaffin cells. The pretreatment of the cells with DNP resulted in a decrease in the concentration of cytoplasmic ATP. Catecholamine secretion evoked by either carbamylcholine or high K+ was also reduced by DNP pretreatment. Oligomycin caused a significant decrease in both cytoplasmic ATP concentration and catecholamine secretion. In contrast, neither the ATP level nor the secretory response was affected by 2-deoxyglucose. Similarly, catecholamine release from digitonin-permeabilized chromaffin cells in response to a direct calcium challenge was reduced by pretreatment of the cells with DNP, and this reduction was partially prevented by the presence of ATP in the perme-abilizing medium. These results suggest that ATP in the cell cytoplasm may play an important role in the regulation of catecholamine secretion as a factor modulating the activity of the exocytotic process in the adrenal chromaffin cell.

Solute accumulation and electrical membrane potential in Agrobacterium tumefaciens-induced crown galls in Kalanchoë daigremontiana leaves

Leaves of Kalanchoë daigremontiana were infected with Agrobacterium tumefaciens strain C58 in order to determine the relative contributions of energy-dependent ion uptake to the observed higher solute concentrations in crown-gall tissue compared with unaffected tissue. The tumor tissue exhibited the following characteristics with respect to unaffected tissue: the content of most solutes was much higher: Na +, K +, Cl –, soluble P i, total N, protein, soluble amino acids, and soluble sugars. Yet NH − 4 and starch content was less. Malate did not fluctuate in a typical CAM rhythm and was lower. The respiration rate on a cytoplasmic volume basis was similar. Photosynthetic rates were much lower. The cytoplasmic ATP concentration was even less, while that of NAD(P)H was higher. Electrical membrane potential was lower (− 184 mV tumor vs. − 223 mV control) and was composed of a higher energy-independent component (− 125 vs. − 98 mV) and a smaller energy-dependent component (− 59 vs. − 125 mV). The response of the membrane potential upon addition of neutral, acidic and basic amino acids including the opines octopine and nopaline was similar in both tissue types. It is suggested that the stronger accumulation of solutes in tumor vs. mesophyll cells cannot be due to thermodynamically more favourable conditions at the plasmalemma, but more probably to a hormone-regulated solute transport across the tonoplast.