Treatment of muscle damage, induced by high intracellular Ca2+, with calmodulin antagonists

Modulation of 6-phosphofructo-1-kinase oligomeric equilibrium by calmodulin: formation of active dimers

Muscle 6-phospho-1-kinase (PFK) is the key regulatory enzyme of the glycolytic pathway and is a calmodulin-binding protein binding two calmodulin molecules per PFK protomer. This enzyme is characterized by a complex regulation that involves its allosteric behavior modulated by several ligands, which modulate the equilibrium between the active tetramers and the inactive dimers of the enzyme. Calmodulin is described to induce the dimerization of PFK, so inhibiting its catalytic activity. Here, we show that binding of calmodulin specifically to its higher-affinity site of PFK induce its dimerization without compromising enzyme catalytic activity forming a hitherto not described active dimmer of PFK. It is also shown that the dimerization is a Ca2+ -dependent event that responds to physiological intracellular Ca2+ concentrations and decrease the interaction of the enzyme to membrane site, which stimulate its catalytic activity. We propose that the effects of calmodulin on PFK reported here are of great physiological significance due to the response to physiological concentrations of Ca2+ and due to be in accordance to the known effects of calmodulin on cell ATP production. We also propose that calmodulin might affect the interaction of PFK to other cellular components as the cytoskeleton.

Excitation-induced Ca2+ influx and skeletal muscle cell damage

Excessive exercise may lead to skeletal muscle cell damage with degradation of cellular components and leakage of intracellular enzymes. Calcium has repeatedly been proposed to be involved in these processes. Studies have shown that the resting level of cytoplasmic Ca2+ increases up to threefold during long-term low-frequency stimulation. We have shown that electrical stimulation produces a marked increase in Ca2+ uptake and Ca2+ content in rat skeletal muscle, both in vivo and in vitro. Continuous stimulation for 240 min at 1 Hz results in an increased release (18-fold) of lactate dehydrogenase (LDH) from extensor digitorum longus (EDL) muscle. This was associated with an increased total Ca2+ content (185%), was augmented at high [Ca2+]o and suppressed at low [Ca2+]o. The release of LDH may reflect partial loss of sarcolemmal integrity as a result of degradation of membrane components by Ca2+-activated enzymes (e.g. calpain or phospholipase A2). After cessation of stimulation the increased release of LDH continues for at least 120 min. This is associated with an up to sevenfold increase in 45Ca uptake. The increased permeability to Ca2+ may further activate calpain and phospholipase A2 and accelerate the loss of membrane integrity. Stimulation-induced uptake of Ca2+ and release of LDH is most pronounced in EDL (mainly composed of fast-twitch fibres at variance with soleus which is mainly composed of slow-twitch fibres). This may account for the observation that prolonged exercise leads to preferential damage to fast-twitch fibres. We hypothesize that excessive exercise may lead to an intracellular accumulation of Ca2+ and increased cytoplasmic Ca2+ causing activation of self-accelerating degradative pathways leading to muscle damage.

Ca2+-induced changes in energy metabolism and viability of melanoma cells

Cancer cells are characterized by a high rate of glycolysis, which is their primary energy source. We show here that a rise in intracellular-free calcium ion (Ca2+), induced by Ca2+-ionophore A23187, exerted a deleterious effect on glycolysis and viability of B16 melanoma cells. Ca2+-ionophore caused a dose-dependent detachment of phosphofructokinase (EC 2.7.1.11), one of the key enzymes of glycolysis, from cytoskeleton. It also induced a decrease in the levels of glucose 1,6-bisphosphate and fructose 1,6-bisphosphate, the two stimulatory signal molecules of glycolysis. All these changes occurred at lower concentrations of the drug than those required to induce a reduction in viability of melanoma cells. We also found that low concentrations of Ca2+-ionophore induced an increase in adenosine 5'-triphosphate (ATP), which most probably resulted from the increase in mitochondrial-bound hexokinase, which reflects a defence mechanism. This mechanism can no longer operate at high concentrations of the Ca2+-ionophore, which causes a decrease in mitochondrial and cytosolic hexokinase, leading to a drastic fall in ATP and melanoma cell death. The present results suggest that drugs which are capable of inducing accumulation of intracellular-free Ca2+ in melanoma cells would cause a reduction in energy-producing systems, leading to melanoma cell death.

Effects of Ca(2+)-ionophore A23187 and calmodulin antagonists on regulatory mechanisms of glycolysis and cell viability of NIH-3T3 fibroblasts

We studied here, in NIH-3T3 fibroblasts, the effect of the Ca(2+)-ionophore A23187 (which is known to increase intracellular-free Ca(2+)) on the control of glycolysis and cell viability and the action of calmodulin antagonists. Time-response studies with Ca(2+)-ionophore A23187 have revealed dual effects on the distribution of phosphofructokinase (PFK) (EC 2.7.1.11), the rate-limiting enzyme of glycolysis, between the cytoskeletal and cytosolic (soluble) fractions of the cell. A short incubation (maximal effect after 7 min) caused an increase in cytoskeleton-bound PFK with a corresponding decrease in soluble activity. This leads to an enhancement of cytoskeletal glycolysis. A longer incubation with Ca(2+)-ionophore caused a reduction in both cytoskeletal and cytosolic PFK and cell death. Both the "physiological" and "pathological" phases of the Ca(2+)-induced changes in the distribution of PFK were prevented by treatment with three structurally different calmodulin antagonists, thioridazine, an antipsychotic phenothiazine, clotrimazole, from the group of antifungal azole derivatives that were recently recognized as calmodulin antagonists, and CGS 9343B, a more selective inhibitor of calmodulin activity. The longer incubation with Ca(2+)-ionophore also induced a decrease in the levels of glucose 1,6-bisphosphate and fructose 1,6-bisphosphate, the two allosteric stimulatory signal molecules of glycolysis. All these pathological changes preceded the reduction in cell viability, and a strong correlation was found between the fall in ATP and cell death. All three calmodulin antagonists prevented the pathological reduction in the levels of the allosteric effectors, ATP and cell viability. These experiments may throw light on the mechanisms underlying the therapeutic action of calmodulin antagonists that we previously found in treatment of the proliferating melanoma cells, on the one hand, and skin injuries, on the other hand.

Effects of Ca2+ on erythrocyte membrane skeleton-bound phosphofructokinase, ATP levels, and hemolysis

Erythrocyte Ca2+ overload is known to occur in several different disease states, and to affect the erythrocyte membrane deformability. We show here that an increase in intracellular Ca2+ concentration in erythrocytes, induced by ionomycin, caused a reduction in ATP levels. Concomitant to the fall in ATP, a marked activation of phosphofructokinase (PFK) (EC 2.7.1.11), the rate-limiting enzyme in glycolysis, in the membrane skeleton fraction occurred. The increase in the membrane skeleton-bound PFK activity was most probably mediated by Ca2+, as direct addition of Ca2+ to the membrane skeleton fraction from the erythrocyte induced an enhancement of the bound PFK activity. Time-response curves revealed that erythrocyte hemolysis did not occur during the first 30 min of incubation with ionomycin, when the membrane skeleton-bound PFK was activated. Longer incubation time resulted in solubilization of the membrane skeleton-bound PFK and a concomitant hemolysis of the erythrocytes. These results suggest that the Ca2+-induced activation of membrane skeleton-bound PFK, and thereby glycolysis, the sole source of energy in erythrocytes, may be a defense mechanism to surmount the damage induced by high Ca2+ levels.

Mechanisms underlying the slow recovery of force after fatigue: importance of intracellular calcium

Recovery of force production after an intense bout of activity may sometimes take several days, especially at low activation frequencies ('low frequency fatigue'). This slow recovery can also be observed in isolated muscle and single muscle fibres. The origin of the force deficit is failure of excitation-contraction coupling at the level of the triads. The most likely cause of the failure is an elevated intracellular Ca2+ level, but the site of action of Ca2+ is unclear. Available evidence does not support the involvement of Ca2+-activated proteases. Ca2+-induced damage to mitochondria or swelling of t-tubules do not seem to be causative factors. Other mechanisms are discussed, including possible detrimental effects of Ca2+-activated lipases, calmodulin, and reactive oxygen species.

Serotonin decreases cytoskeletal and cytosolic glycolytic enzymes and the levels of ATP and glucose 1,6-bisphosphate in skin, which is prevented by the calmodulin antagonists thioridazine and clotrimazole

Serotonin (5-hydroxytryptamine) is believed to play a pathogenic role in skin damage and various skin abnormalities; however, its mechanism of action remains unknown. We show here that intradermal injection of serotonin in rats induced a marked reduction in the activities of the glycolytic enzymes, phosphofructokinase (EC 2.7.1.11) and aldolase (EC 4.1.2.13), in both the cytoskeletal and cytosolic fractions from skin. Serotonin also decreased the levels of glucose 1,6-bisphosphate in skin, the powerful regulator of glucose metabolism. These serotonin-induced changes were accompanied by a marked decrease in ATP content in skin. All these pathological changes induced by serotonin were prevented by treatment with two structurally different calmodulin antagonists: thioridazine, an antipsychotic phenothiazine, or clotrimazole, from the group of the antifungal azole derivatives that were recently recognized as calmodulin antagonists. The present results suggest that calmodulin antagonists may be effective drugs in the treatment of skin damage under various pathological conditions and diseases in which serotonin levels are increased.

Effects of carbamylcholine and pyridostigmine on cytoskeleton-bound and cytosolic phosphofructokinase and ATP levels in different rat tissues

1. The effects of carbamylcholine (CaCh) (acetylcholine agonist) and pyridostigmine (Pyr) (acetylcholinesterase inhibitor), on the activity of cytoskeleton-bound and cytosolic phosphofructokinase (PFK), the rate-limiting enzyme in glycolysis, and ATP levels, were studied in rat tibialis anterior (TA) muscle, heart, and brain. 2. In the TA muscle, a marked (about three-fold) increase in the allosteric activity of cytosolic (soluble) PFK was found, 3-5 min following the injection of CaCh or Pyr. The intracellular distribution of the enzyme was not affected by both drugs. Stimulation of glycolysis in this muscle was also expressed by a significant increase in the concentrations of glycolytic intermediates and lactate. Glucose 1,6-bisphosphate (Glc-1,6-P2) levels were unchanged, whereas fructose-2,6-bisphosphate (Fru-2,6-P2) was increased. Glycogenolysis was also stimulated, as deduced from the decrease in glycogen content. The stimulation of glycolysis, induced by both drugs, was accompanied by an increase in ATP level in the TA muscle. 3. In contrast to the stimulatory action of CaCh or Pyr on glycolysis in the TA muscle, both drugs had no effect on cytosolic and cytoskeletal PFK in heart and brain. However, ATP content in both heart and brain was markedly reduced by these drugs, most probably due to their reported harmful effects on mitochondrial function, leading to tissue damage. 4. Electron microscopic studies of TA muscle and heart from rats treated with CaCh or Pyr, revealed severe damage of heart but no harmful effects on TA muscle, which is a muscle with high glycolytic and low oxidative capacity. The present experiments suggest that the accelerated glycolysis in this muscle induced by both drugs, supplies ATP, thus preventing muscle damage.

Differences between haloperidol- and pimozide-induced withdrawal syndrome: a role for Ca2+ channels

We investigated the behavioral and biochemical events appearing in rats after withdrawal for 24 h or 8-12 days from two classical neuroleptics, haloperidol and pimozide. The neuroleptics were given for 14 days alone or shortly after injection of the Ca2+ channel blocker nifedipine. We have found that withdrawal effects after haloperidol and pimozide were different. After haloperidol treatment we observed an increase in cortical Ca2+ channel and limbic dopamine D1 receptor density and an increase in spontaneous motor activity and apomorphine-induced hyperactivity and stereotypy. In contrast no biochemical changes were observed during pimozide withdrawal, and locomotor activity and responses to apomorphine were depressed. Co-administration of nifedipine with haloperidol prevented the observed biochemical and behavioral symptoms of withdrawal. Nifedipine administration did not change the depressant effects of pimozide. Our results suggest that the voltage-dependent Ca2+ channel is involved in the observed withdrawal syndrome of neuroleptics, and that the absence of this syndrome after pimozide may be related to its considerable Ca2+ channel-blocking properties.

Rapid stimulatory effect of insulin on binding of glycolytic enzymes to cytoskeleton of C-6 glial cells, and the antagonistic action of calmodulin inhibitors

Insulin was shown in our previous experiments to induce an increase in binding of glycolytic enzymes to muscle cytoskeleton. We show here the same stimulatory effect of insulin in C-6 glial cells in culture. In these cells, like in muscle, a short time of incubation with insulin (1-10 min) induced an increase in cytoskeleton bound phosphofructokinase and aldolase. This stimulatory effect of insulin could be prevented by treatment with calmodulin antagonists trifluoperazine, thioridazine or CGS 9343 B (a potent and selective inhibitor of calmodulin activity), which strongly suggests that calmodulin is involved in this action of insulin. Our previous experiments have shown that growth factors and Ca(2+) also induce a rapid, calmodulin-mediated stimulation of binding of glycolytic enzymes to cytoskeleton. The present and previous results suggest that the rapid binding of glycolytic enzymes to cytoskeleton, may be a general mechanism, in different cells, in signal transduction of insulin, growth factors and other Ca(2+) -mobilizing hormones. The accelerated cytoskeletal glycolysis will supply local ATP, which is required for the rapid cytoskeletal-membrane rearrangements following the binding of hormone to its receptor.

Serotonin-induced decrease in brain ATP, stimulation of brain anaerobic glycolysis and elevation of plasma hemoglobin; the protective action of calmodulin antagonists

1. Injection of serotonin (5-hydroxytryptamine) to rats, induced a dramatic fall in brain ATP level, accompanied by an increase in P(i). Concomitant to these changes, the activity of cytosolic phosphofructokinase, the rate-limiting enzyme of glycolysis, was significantly enhanced. Stimulation of anaerobic glycolysis was also reflected by a marked increase in lactate content in brain. 2. Brain glucose 1,6-bisphosphate level was decreased, whereas fructose 2,6-bisphosphate was unaffected by serotonin. 3. All these serotonin-induced changes in brain, which are characteristic for cerebral ischemia, were prevented by treatment with the calmodulin (CaM) antagonists, trifluoperazine or thioridazine. 4. Injection of serotonin also induced a marked elevation of plasma hemoglobin, reflecting lysed erythrocytes, which was also prevented by treatment with the CaM antagonists. 5. The present results suggest that CaM antagonists may be effective drugs in treatment of many pathological conditions and diseases in which plasma serotonin levels are known to increase.