Ca2+–calmodulin-dependent protein kinase expression and signalling in skeletal muscle during exercise

Ca²⁺ signalling is proposed to play an important role in skeletal muscle function during exercise. Here, we examined the expression of multifunctional Ca²⁺–calmodulin-dependent protein kinases (CaMK) in human skeletal muscle and show that CaMKII and CaMKK, but not CaMKI or CaMKIV, are expressed. Furthermore, the effect of exercise duration and intensity on skeletal muscle CaMKII activity and phosphorylation of downstream targets was examined. Eight healthy men exercised at ∼67% of peak pulmonary O₂ uptake     with muscle samples taken at rest and after 1, 10, 30, 60 and 90 min of exercise. Ten other men exercised for three consecutive 10 min bouts at 35%, 60% and 85%     with muscle samples taken at rest, at the end of each interval and 30 min post-exercise. There was a rapid and transient increase in autonomous CaMKII activity and CaMKII phosphorylation at Thr²⁸⁷ in skeletal muscle during exercise. Furthermore, the phosphorylation of phospholamban (PLN) at Thr¹⁷, which was identified as a CaMKII substrate in skeletal muscle, was rapidly (< 1 min) increased by exercise, and remained phosphorylated 5-fold above basal level during 90 min of exercise. The phosphorylation of serum response factor at Ser¹⁰³, a putative CaMKII substrate, was higher after 30 min of exercise. PLN phosphorylation at Thr¹⁷ was higher with increasing exercise intensities. These data indicate that CaMKII is the major multifunctional CaMK in skeletal muscle and its activation occurs rapidly and is sustained during continuous exercise, with the activation being greater during intense exercise.     

Exercise Increases Ca2+–Calmodulin-Dependent Protein Kinase II Activity in Human Skeletal Muscle

There is evidence in rodents that Ca²⁺-calmodulin-dependent protein kinase II (CaMKII) activity is higher in contracting skeletal muscle, and this kinase may regulate skeletal muscle function and metabolism during exercise. To investigate the effect of exercise on CaMKII in human skeletal muscle, healthy men (   n = 8  ) performed cycle ergometer exercise for 40 min at 76 ± 1 % peak pulmonary O₂ uptake (_O2peak), with skeletal muscle samples taken at rest and after 5 and 40 min of exercise. CaMKII expression and activities were examined by immunoblotting and in vitro kinase assays, respectively. There were no differences in maximal (+ Ca²⁺, CaM) CaMKII activity during exercise compared with rest. Autonomous (- Ca²⁺, CaM) CaMKII activity was 9 ± 1 % of maximal at rest, remained unchanged at 5 min, and increased to 17 ± 1 % (   P < 0.01  ) at 40 min. CaMKII autophosphorylation at Thr²⁸⁷ was 50-70 % higher during exercise, with no differences in CaMKII expression. The effect of maximal aerobic exercise on CaMKII was also examined (   n = 9  ), with 0.7- to 1.5-fold increases in autonomous CaMKII activity, but no change in maximal CaMKII activity. CaMKIV was not detected in human skeletal muscle. In summary, exercise increases the activity of CaMKII in skeletal muscle, suggesting that it may have a role in regulating skeletal muscle function and metabolism during exercise in humans.     

Regulation and function of Ca2+–calmodulin-dependent protein kinase II of fast-twitch rat skeletal muscle

The activation and function of Ca²⁺–calmodulin-dependent kinase II (CaMKII) in contracting rat skeletal muscle was examined. The increase in autonomous activity and phosphorylation at Thr²⁸⁷ of CaMKII of gastrocnemius muscle in response to contractions in situ was rapid and transient, peaking at 1–3 min, but reversed after 30 min of contractions. There was a positive correlation between CaMKII phosphorylation at Thr²⁸⁷ and autonomous CaMKII activity. In contrast to the rapid and transient increase in autonomous CaMKII activity, the phosphorylation of the putative CaMKII substrate trisk95/triadin was rapid and sustained during contractions. There were no changes in CaMKII activity and phosphorylation or trisk95 phosphorylation in the resting contralateral muscles during stimulation. When fast-twitch muscles were contracted ex vivo , CaMKII inhibition resulted in a greater magnitude of fatigue as well as blunted CaMKII and trisk95 phosphorylation, identifying trisk95 as a physiological CaMKII substrate. In summary, skeletal muscle CaMKII activation was rapid and sustained during exercise/contraction and is mediated by factors within the contracting muscle, probably through allosteric activation via Ca²⁺–CaM. CaMKII may signal through trisk95 to modulate Ca²⁺ release in fast-twitch rat skeletal muscle during exercise/contraction.     

Propagation in the transverse tubular system and voltage dependence of calcium release in normal and mdx mouse muscle fibres

Using a two-microelectrode voltage clamp technique, we investigated possible mechanisms underlying the impaired excitation–contraction coupling in skeletal muscle fibres of the mdx mouse, a model of the human disease Duchenne muscular dystrophy. We evaluated the role of the transverse tubular system (T-system) by using the potentiometric indicator di-8 ANEPPS, and that of the sarcoplasmic reticulum (SR) Ca²⁺ release by measuring Ca²⁺ transients with a low affinity indicator in the presence of high EGTA concentrations under voltage clamp conditions. We observed minimal differences in the T-system structure and the T-system electrical propagation was not different between normal and mdx mice. Whereas the maximum Ca²⁺ release elicited by voltage pulses was reduced by ∼67% in mdx fibres, in agreement with previous results obtained using AP stimulation, the voltage dependence of SR Ca²⁺ release was identical to that seen in normal fibres. Taken together, our data suggest that the intrinsic ability of the sarcoplasmic reticulum to release Ca²⁺ may be altered in the mdx mouse.     

Effect of endurance exercise training on Ca2+–calmodulin-dependent protein kinase II expression and signalling in skeletal muscle of humans

Here the hypothesis that skeletal muscle Ca²⁺–calmodulin-dependent kinase II (CaMKII) expression and signalling would be modified by endurance training was tested. Eight healthy, young men completed 3 weeks of one-legged endurance exercise training with muscle samples taken from both legs before training and 15 h after the last exercise bout. Along with an ∼40% increase in mitochondrial F₁-ATP synthase expression, there was an ∼1-fold increase in maximal CaMKII activity and CaMKII kinase isoform expression after training in the active leg only. Autonomous CaMKII activity and CaMKII autophosphorylation were increased to a similar extent. However, there was no change in α-CaMKII anchoring protein expression with training. Nor was there any change in expression or Thr¹⁷ phosphorylation of the CaMKII substrate phospholamban with training. However, another CaMKII substrate, serum response factor (SRF), had an ∼60% higher phosphorylation at Ser¹⁰³ after training, with no change in SRF expression. There were positive correlations between the increases in CaMKII expression and SRF phosphorylation as well as F₁ATPase expression with training. After training, there was an increase in cyclic-AMP response element binding protein phosphorylation at Ser¹³³, but not expression, in muscle of both legs. Taken together, skeletal muscle CaMKII kinase isoform expression and SRF phosphorylation is higher with endurance-type exercise training, adaptations that are restricted to active muscle. This may contribute to greater Ca²⁺ mediated regulation during exercise and the altered muscle phenotype with training.     

Reactive oxygen species reduce myofibrillar Ca2+ sensitivity in fatiguing mouse skeletal muscle at 37°C

The mechanisms of muscle fatigue were studied in small muscle bundles and single fibres isolated from the flexor digitorum brevis of the mouse. Fatigue caused by repeated isometric tetani was accelerated at body temperature (37°C) when compared to room temperature (22°C). The membrane-permeant reactive oxygen species (ROS) scavenger, Tiron (5 m m ), had no effect on the rate of fatigue at 22°C but slowed the rate of fatigue at 37°C to that observed at 22°C. Single fibres were microinjected with indo-1 to measure intracellular calcium. In the accelerated fatigue at 37°C the tetanic [Ca²⁺]_i did not change significantly and the decline of maximum Ca²⁺-activated force was similar to that observed at 22°C. The cause of the greater rate of fatigue at 37°C was a large fall in myofibrillar Ca²⁺ sensitivity. In the presence of Tiron, the large fall in Ca²⁺ sensitivity was abolished and the usual decline in tetanic [Ca²⁺]_i was observed. This study confirms the importance of ROS in fatigue at 37°C and shows that the mechanism of action of ROS is a decline in myofibrillar Ca²⁺ sensitivity.     

Local recovery of Ca2+ release in rat ventricular myocytes

Excitation–contraction coupling in the heart depends on the positive feedback process of Ca²⁺-induced Ca²⁺ release (CICR). While CICR provides for robust triggering of Ca²⁺ sparks, the mechanisms underlying their termination remain unknown. At present, it is unclear how a cluster of Ca²⁺ release channels (ryanodine receptors or RyRs) can be made to turn off when their activity is sustained by the Ca²⁺ release itself. We use a novel experimental approach to investigate indirectly this issue by exploring restitution of Ca²⁺ sparks. We exploit the fact that ryanodine can bind, nearly irreversibly, to an RyR subunit (monomer) and increase the open probability of the homotetrameric channel. By applying low concentrations of ryanodine to rat ventricular myocytes, we observe repeated activations of individual Ca²⁺ spark sites. Examination of these repetitive Ca²⁺ sparks reveals that spark amplitude recovers with a time constant of 91 ms whereas the sigmoidal recovery of triggering probability lags behind amplitude recovery by ∼80 ms. We conclude that restitution of Ca²⁺ sparks depends on local refilling of SR stores after depletion and may also depend on another time-dependent process such as recovery from inactivation or a slow conformational change after rebinding of Ca²⁺ to SR regulatory proteins.     

Presynaptic N-type and P/Q-type Ca2+ channels mediating synaptic transmission at the calyx of Held of mice

At the nerve terminal, both N- and P/Q-type Ca²⁺ channels mediate synaptic transmission, with their relative contribution varying between synapses and with postnatal age. To clarify functional significance of different presynaptic Ca²⁺ channel subtypes, we recorded N-type and P/Q-type Ca²⁺ currents directly from calyces of Held nerve terminals in α_1A-subunit-deficient mice and wild-type (WT) mice, respectively. The most prominent feature of P/Q-type Ca²⁺ currents was activity-dependent facilitation, which was absent for N-type Ca²⁺ currents. EPSCs mediated by P/Q-type Ca²⁺ currents showed less depression during high-frequency stimulation compared with those mediated by N-type Ca²⁺ currents. In addition, the maximal inhibition by the GABA_B receptor agonist baclofen was greater for EPSCs mediated by N-type channels than for those mediated by P/Q-type channels. These results suggest that the developmental switch of presynaptic Ca²⁺ channels from N- to P/Q-type may serve to increase synaptic efficacy at high frequencies of activity, securing high-fidelity synaptic transmission.     

Calcium oscillations in interstitial cells of the rabbit urethra

Measurements were made (using fast confocal microscopy) of intracellular Ca²⁺ levels in fluo-4 loaded interstitial cells isolated from the rabbit urethra. These cells exhibited regular Ca²⁺ oscillations which were associated with spontaneous transient inward currents recorded under voltage clamp. Interference with d - myo -inositol 1,4,5-trisphosphate (IP₃) induced Ca²⁺ release using 100 μ m 2-aminoethoxydiphenyl borate, and the phospholipase C (PLC) inhibitors 2-nitro-4-carboxyphenyl N , N -diphenylcarbamate and U73122 decreased the amplitude of spontaneous oscillations but did not abolish them. However, oscillations were abolished when ryanodine receptors were blocked with tetracaine or ryanodine. Oscillations ceased in the absence of external Ca²⁺, and frequency was directly proportional to the external Ca²⁺ concentration. Frequency of Ca²⁺ oscillation was reduced by SKF-96365, but not by nifedipine. Lanthanum and cadmium completely blocked oscillations. These results suggest that Ca²⁺ oscillations in isolated rabbit urethral interstitial cells are initiated by Ca²⁺ release from ryanodine-sensitive intracellular stores, that oscillation frequency is very sensitive to the external Ca²⁺ concentration and that conversion of the primary oscillation to a propagated Ca²⁺ wave depends upon IP₃-induced Ca²⁺ release.     

Calmodulin kinase modulates Ca2+ release in mouse skeletal muscle

Activation of the contractile machinery in skeletal muscle is initiated by the action-potential-induced release of Ca²⁺ from the sarcoplasmic reticulum (SR). Several proteins involved in SR Ca²⁺ release are affected by calmodulin kinase II (CaMKII)-induced phosphorylation in vitro , but the effect in the intact cell remains uncertain and is the focus of the present study. CaMKII inhibitory peptide or inactive control peptide was injected into single isolated fast-twitch fibres of mouse flexor digitorum brevis muscles, and the effect on free myoplasmic [Ca²⁺] ([Ca²⁺]_i) and force during different patterns of stimulation was measured. Injection of the inactive control peptide had no effect on any of the parameters measured. Conversely, injection of CaMKII inhibitory peptide decreased tetanic [Ca²⁺]_i by ≈25 %, but had no significant effect on the rate of SR Ca²⁺ uptake or the force-[Ca²⁺]_i relationship. Repeated tetanic stimulation resulted in increased tetanic [Ca²⁺]_i, and this increase was smaller after CaMKII inhibition. In conclusion, CaMKII-induced phosphorylation facilitates SR Ca²⁺ release in the basal state and during repeated contractions, providing a positive feedback between [Ca²⁺]_i and SR Ca²⁺ release.     

Exercise improves phosphatidylinositol-3,4,5-trisphosphate responsiveness of atypical protein kinase C and interacts with insulin signalling to peptide elongation in human skeletal muscle

We investigated if acute endurance-type exercise interacts with insulin-stimulated activation of atypical protein kinase C (aPKC) and insulin signalling to peptide chain elongation in human skeletal muscle. Four hours after acute one-legged exercise, insulin-induced glucose uptake was ∼80% higher ( N = 12, P < 0.05) in previously exercised muscle, measured during a euglycaemic–hyperinsulinaemic clamp (100 μU ml⁻¹). Insulin increased ( P < 0.05) both insulin receptor substrate (IRS)-1 and IRS-2 associated phosphatidylinositol (PI)-3 kinase activity and led to increased ( P < 0.001) phosphorylation of Akt on Ser⁴⁷³ and Thr³⁰⁸ in skeletal muscle. Interestingly, in response to prior exercise IRS-2-associated PI-3 kinase activity was higher ( P < 0.05) both at basal and during insulin stimulation. This coincided with correspondingly altered phosphorylation of the extracellular-regulated protein kinase 1/2 (ERK 1/2), p70S6 kinase (P70S6K), eukaryotic elongation factor 2 (eEF2) kinase and eEF2. aPKC was similarly activated by insulin in rested and exercised muscle, without detectable changes in aPKC Thr⁴¹⁰ phosphorylation. However, when adding phosphatidylinositol-3,4,5-triphosphate (PIP 3 ), the signalling product of PI-3 kinase, to basal muscle homogenates, aPKC was more potently activated ( P = 0.01) in previously exercised muscle. Collectively, this study shows that endurance-type exercise interacts with insulin signalling to peptide chain elongation. Although protein turnover was not evaluated, this suggests that capacity for protein synthesis after acute endurance-type exercise may be improved. Furthermore, endurance exercise increased the responsiveness of aPKC to PIP 3 providing a possible link to improved insulin-stimulated glucose uptake after exercise.     

Effect of exercise on protein kinase C activity and localization in human skeletal muscle

To investigate the effect of exercise on protein kinase C (PKC) activity and localization in human skeletal muscle, eight healthy men performed cycle ergometer exercise for 40 min at 76 ± 1% the peak pulmonary O₂ uptake     , with muscle samples obtained at rest and after 5 and 40 min of exercise. PKC expression, phosphorylation and activities were examined by immunoblotting and in vitro kinase assays of fractionated and whole tissue preparations. In response to exercise, total PKC activity was slightly higher at 40 min in an enriched membrane fraction, and using a pSer-PKC-substrate motif antibody it was revealed that exercise increased the serine phosphorylation of a ∼50 kDa protein. There were no changes in conventional PKC (cPKC) or PKCθ activities; however, atypical PKC (aPKC) activity was ∼70% higher at 5 and 40 min, and aPKC expression and Thr^410/403 phosphorylation were unaltered by exercise. There were no effects of exercise on the abundance of PKCα, PKCδ, PKCθ and aPKC within cytosolic or enriched membrane fractions of skeletal muscle. These data indicate that aPKC, but not cPKC or PKCθ, are activated by exercise in contracting muscle suggesting a potential role for aPKC in the regulation of skeletal muscle function and metabolism during exercise in humans.     

Signal transduction pathways and gating mechanisms of native TRP-like cation channels in vascular myocytes

Activation of Ca²⁺-permeable non-selective cation channels produces an increase in excitability of vascular smooth muscle cells which has an important role in vasoconstriction. These channels are activated by various physiological stimuli including vasoconstrictor agents such as noradrenaline, depletion of internal Ca²⁺ stores and cell stretching. In addition cation channels have been shown to be constitutively active and these channels are thought to contribute to resting membrane conductance and basal Ca²⁺ influx in vascular myocytes. Recent evidence has suggested that transient receptor potential (TRP) proteins represent strong candidates for these channels in the vasculature. This review discusses proposed signal transduction pathways and gating mechanisms which link physiological stimuli to opening of cation channels in vascular myocytes. It is apparent that G-protein-coupled pathways linked to stimulation of phospholipase activity have a profound effect on regulating channel activity and that generation of diacylglycerol (DAG) is a central event in these signalling cascades with this triglyceride having a pivotal role in gating cation channels via both PKC- independent and - dependent mechanisms. Moreover phosphorylation processes produced by stimulation of protein kinases have been proposed to have an important role in regulating cation channel activity.     

Dual Ca2+ modulation of glycinergic synaptic currents in rodent hypoglossal motoneurones

Glycinergic synapses are implicated in the coordination of reflex responses, sensory signal processing and pain sensation. Their activity is pre- and postsynaptically regulated, although mechanisms are poorly understood. Using patch-clamp recording and Ca²⁺ imaging in hypoglossal motoneurones from rat and mouse brainstem slices, we address here the role of cytoplasmic Ca²⁺ (Ca_i) in glycinergic synapse modulation. Ca²⁺ influx through voltage-gated or NMDA receptor channels caused powerful transient inhibition of glycinergic IPSCs. This effect was accompanied by an increase in both the failure rate and paired-pulse ratio, as well as a decrease in the frequency of mIPSCs, suggesting a presynaptic mechanism of depression. Inhibition was reduced by the cannabinoid receptor antagonist SR141716A and occluded by the agonist WIN55,212-2, indicating involvement of endocannabinoid retrograde signalling. Conversely, in the presence of SR141716A, glycinergic IPSCs were potentiated postsynaptically by glutamate or NMDA, displaying a Ca²⁺-dependent increase in amplitude and decay prolongation. Both presynaptic inhibition and postsynaptic potentiation were completely prevented by strong Ca_i buffering (20 m m BAPTA). Our findings demonstrate two independent mechanisms by which Ca²⁺ modulates glycinergic synaptic transmission: (i) presynaptic inhibition of glycine release and (ii) postsynaptic potentiation of GlyR-mediated responses. This dual Ca²⁺-induced regulation might be important for feedback control of neurotransmission in a variety of glycinergic networks in mammalian nervous systems.     

Lipid rafts, the sarcoplasmic reticulum and uterine calcium signalling: an integrated approach

The pathways involved in Ca²⁺ signalling in the uterus remain incompletely understood, impairing our ability to prevent preterm and difficult labours. In this review we focus on two elements in the pathway of Ca²⁺ signalling that have recently emerged as playing important roles: membrane lipid rafts and the sarcoplasmic reticulum. We examine the evidence for lipid rafts in the uterus and discuss their functional role. We suggest that the increases in cytosolic [Ca²⁺] and contractility that occur with raft disruption are due, at least in part, to effects on large conductance Ca²⁺-activated K⁺ (BK) channels that are localized to rafts. The role of the SR in contributing to subsarcolemmal cytosolic microdomains in uterus is evaluated, along with its interactions with ion channels on the plasma membrane. Thus, signalling microdomains play an important, but incompletely understood, role in the uterus, and integrating them into other Ca²⁺ signalling pathways is a challenge for further research. We suggest that the role of the SR changes in pregnancy, from promoting quiescence via BK channels or SR Ca²⁺ uptake, to promoting Ca²⁺ entry and contractility at term, and relate data on lipid rafts to clinical outcome in obese pregnant women.     

Exercise induces expression of leukaemia inhibitory factor in human skeletal muscle

The leukaemia inhibitory factor (LIF) belongs to the interleukin (IL)-6 cytokine superfamily and is constitutively expressed in skeletal muscle. We tested the hypothesis that LIF expression in human skeletal muscle is regulated by exercise. Fifteen healthy young male volunteers performed either 3 h of cycle ergometer exercise at ∼60% of     ( n = 8) or rested ( n = 7). Muscle biopsies were obtained from the vastus lateralis prior to exercise, immediately after exercise, and at 1.5, 3, 6 and 24 h post exercise. Control subjects had biopsy samples taken at the same time points as during the exercise trial. Skeletal muscle LIF mRNA increased immediately after the exercise and declined gradually during recovery. However, LIF protein was unchanged at the investigated time points. Moreover, we tested the hypothesis that LIF mRNA and protein expressions are modulated by calcium (Ca²⁺) in primary human skeletal myocytes. Treatment of myocytes with the Ca²⁺ ionophore, ionomycin, for 6 h resulted in an increase in both LIF mRNA and LIF protein levels. This finding suggests that Ca²⁺ may be involved in the regulation of LIF in endurance-exercised skeletal muscle. In conclusion, primary human skeletal myocytes have the capability to produce LIF in response to ionomycin stimulation and LIF mRNA levels increase in skeletal muscle following concentric exercise. The finding that the increase in LIF mRNA levels is not followed by a similar increase in skeletal muscle LIF protein suggests that other exercise stimuli or repetitive stimuli are necessary in order to induce a detectable accumulation of LIF protein.     

Olfactoretinal centrifugal input modulates zebrafish retinal ganglion cell activity: a possible role for dopamine-mediated Ca2+ signalling pathways

The vertebrate retina receives centrifugal input from the brain. In zebrafish, the major centrifugal input originates in the terminal nerve (TN). TN cell bodies are located in the olfactory bulb and ventral telencephalon. The TN projects axons to the retina where they branch in the inner plexiform layer (IPL) and synapse onto several inner retinal cell types, including dopaminergic interplexiform cells (DA-IPCs). This olfactoretinal centrifugal input plays a role in modulating retinal ganglion cell (RGC) activity, probably via dopamine-mediated Ca²⁺ signalling pathways. Normally, dopamine inhibits RGC firing by decreasing the inward Ca²⁺ current. Olfactory stimulation with amino acids decreases dopamine release in the retina, thereby reducing dopaminergic inhibition of RGCs. This model of olfacto-visual integration was directly tested by recording single-unit RGC activity in response to olfactory stimulation in the presence or absence of dopamine receptor blockers. Stimulation of the olfactory neurones increased RGC activity. However, this effect diminished when the dopamine D1 receptors were pharmacologically blocked. In isolated RGCs, the application of dopamine or a dopamine D1 receptor agonist decreased voltage-activated Ca²⁺ current and lowered Ca²⁺ influx. Together, the data suggest that olfactory input has a modulatory effect on RGC firing, and that this effect is mediated by dopamine D1 receptor-coupled Ca²⁺ signalling pathways.     

Ca2+–calmodulin-dependent myosin light chain kinase is essential for activation of TRPC5 channels expressed in HEK293 cells

Mammalian homologues of Drosophila transient receptor potential (TRP) proteins are responsible for receptor-activated Ca²⁺ influx in vertebrate cells. We previously reported the involvement of intracellular Ca²⁺ in the receptor-mediated activation of mammalian canonical transient receptor potential 5 (TRPC5) channels. Here we investigated the role of calmodulin, an important sensor of changes in intracellular Ca²⁺, and its downstream cascades in the activation of recombinant TRPC5 channels in human embryonic kidney (HEK) 293 cells. Ca²⁺ entry through TRPC5 channels, induced upon stimulation of the G-protein-coupled ATP receptor, was abolished by treatment with W-13, an inhibitor of calmodulin. ML-9 and wortmannin, inhibitors of Ca²⁺–calmodulin-dependent myosin light chain kinase (MLCK), and the expression of a dominant-negative mutant of MLCK inhibited the TRPC5 channel activity, revealing an essential role of MLCK in maintaining TRPC5 channel activity. It is important to note that ML-9 impaired the plasma membrane localization of TRPC5 channels. Furthermore, TRPC5 channel activity measured using the whole-cell patch-clamp technique was inhibited by ML-9, whereas TRPC5 channel activity observed in the cell-excised, inside-out patch was unaffected by ML-9. An antibody that recognizes phosphorylated myosin light chain (MLC) revealed that the basal level of phosphorylated MLC under unstimulated conditions was reduced by ML-9 in HEK293 cells. These findings strongly suggest that intracellular Ca²⁺–calmodulin constitutively activates MLCK, thereby maintaining TRPC5 channel activity through the promotion of plasma membrane TRPC5 channel distribution under the control of phosphorylation/dephosphorylation equilibrium of MLC.     

Effects of acute and chronic endurance exercise on mitochondrial uncoupling in human skeletal muscle

Mitochondrial proteins such as uncoupling protein 3 (UCP3) and adenine nucleotide translocase (ANT) may mediate back-leakage of protons and serve as uncouplers of oxidative phosphorylation. We hypothesized that UCP3 and ANT increase after prolonged exercise and/or endurance training, resulting in increased uncoupled respiration (UCR). Subjects were investigated with muscle biopsies before and after acute exercise (75 min of cycling at 70% of     ) or 6 weeks endurance training. Mitochondria were isolated and respiration measured in the absence (UCR or state 4) and presence of ADP (coupled respiration or state 3). Protein expression of UCP3 and ANT was measured with Western blotting. After endurance training     , citrate synthase activity (CS), state 3 respiration and ANT increased by 24, 47, 40 and 95%, respectively (all   P < 0.05  ), whereas UCP3 remained unchanged. When expressed per unit of CS (a marker of mitochondrial volume) UCP3 and UCR decreased by 54% and 18%  ( P < 0.05)  . CS increased by 43% after acute exercise and remained elevated after 3 h of recovery  ( P < 0.05)  , whereas the other muscle parameters remained unchanged. An intriguing finding was that acute exercise reversibly enhanced the capacity of mitochondria to accumulate Ca²⁺  ( P < 0.05)  before opening of permeability transition pores. In conclusion, UCP3 protein and UCR decrease after endurance training when related to mitochondrial volume. These changes may prevent excessive basal thermogenesis. Acute exercise enhances mitochondrial resistance to Ca²⁺ overload but does not influence UCR or protein expression of UCP3 and ANT. The increased Ca²⁺ resistance may prevent mitochondrial degradation and the mechanism needs to be further explored.