Myokine Secretion following an Aerobic Exercise Intervention in Individuals with Type 2 Diabetes with or without Exercise Resistance

Type 2 diabetes (T2D) is characterized by muscle metabolic dysfunction that exercise can minimize, but some patients do not respond to an exercise intervention. Myokine secretion is intrinsically altered in patients with T2D, but the role of myokines in exercise resistance in this patient population has never been studied. We sought to determine if changes in myokine secretion were linked to the response to an exercise intervention in patients with T2D. The participants followed a 10-week aerobic exercise training intervention, and patients with T2D were grouped based on muscle mitochondrial function improvement (responders versus non-responders). We measured myokines in serum and cell-culture medium of myotubes derived from participants pre- and post-intervention and in response to an in vitro model of muscle contraction. We also quantified the expression of genes related to inflammation in the myotubes pre- and post-intervention. No significant differences were detected depending on T2D status or response to exercise in the biological markers measured, with the exception of modest differences in expression patterns for certain myokines (IL-1β, IL-8, IL-10, and IL-15). Further investigation into the molecular mechanisms involving myokines may explain exercise resistance with T2D; however, the role in metabolic adaptations to exercise in T2D requires further investigation.

Exercise training reduces circulating cytokines in male patients with coronary artery disease and type 2 diabetes: A pilot study

Low-grade inflammation is central to coronary artery disease (CAD) and type 2 diabetes (T2D) and is reduced by exercise training. The objective of this study was to compare the anti-inflammatory potential of moderate-to-vigorous intensity continuous training (MICT) and high-intensity interval training (HIIT) in patients with CAD with or without T2D. The design and setting of this study is based on a secondary analysis of registered randomized clinical trial NCT02765568. Male patients with CAD were randomly assigned to either MICT or HIIT, with subgroups divided according to T2D status (non-T2D-HIIT n = 14 and non-T2D-MICT n = 13; T2D-HIIT n = 6 and T2D-MICT n = 5). The intervention was a 12-week cardiovascular rehabilitation program consisting of either MICT or HIIT (twice weekly sessions) and circulating cytokines measured pre- and post-training as inflammatory markers. The co-occurrence of CAD and T2D was associated with increased plasma IL-8 (p = 0.0331). There was an interaction between T2D and the effect of the training interventions on plasma FGF21 (p = 0.0368) and IL-6 (p = 0.0385), which were further reduced in the T2D groups. An interaction between T2D, training modalities, and the effect of time (p = 0.0415) was detected for SPARC, with HIIT increasing circulating concentrations in the control group, while lowering them in the T2D group, and the inverse occurring with MICT. The interventions also reduced plasma FGF21 (p = 0.0030), IL-6 (p = 0.0101), IL-8 (p = 0.0087), IL-10 (p < 0.0001), and IL-18 (p = 0.0009) irrespective of training modality or T2D status. HIIT and MICT resulted in similar reductions in circulating cytokines known to be increased in the context of low-grade inflammation in CAD patients, an effect more pronounced in patients with T2D for FGF21 and IL-6.

The bidirectional relationship between AMPK pathway activation and myokine secretion in skeletal muscle: How it affects energy metabolism

Myokines are peptides and proteins secreted by skeletal muscle cells, into the interstitium, or in the blood. Their regulation may be dependent or independent of muscle contraction to induce a variety of metabolic effects. Numerous myokines have been implicated in influencing energy metabolism via AMP-activated protein kinase (AMPK) signalling. As AMPK is centrally involved in glucose and lipid metabolism, it is important to understand how myokines influence its signalling, and vice versa. Such insight will better elucidate the mechanism of metabolic regulation during exercise and at rest. This review encompasses the latest research conducted on the relationship between AMPK signalling and myokines within skeletal muscles via autocrine or paracrine signalling.

Placental superoxide dismutase 3 mediates benefits of maternal exercise on offspring health

Poor maternal diet increases the risk of obesity and type 2 diabetes in offspring, adding to the ever-increasing prevalence of these diseases. In contrast, we find that maternal exercise improves the metabolic health of offspring, and here, we demonstrate that this occurs through a vitamin D receptor-mediated increase in placental superoxide dismutase 3 (SOD3) expression and secretion. SOD3 activates an AMPK/TET signaling axis in fetal offspring liver, resulting in DNA demethylation at the promoters of glucose metabolic genes, enhancing liver function, and improving glucose tolerance. In humans, SOD3 is upregulated in serum and placenta from physically active pregnant women. The discovery of maternal exercise-induced cross talk between placenta-derived SOD3 and offspring liver provides a central mechanism for improved offspring metabolic health. These findings may lead to novel therapeutic approaches to limit the transmission of metabolic disease to the next generation.

Effects of PCB126 on Adipose-to-Muscle Communication in an in Vitro Model

BACKGROUND

Exposure to coplanar polychlorinated biphenyls (PCBs) is linked to the development of insulin resistance. Previous studies suggested PCB126 alters muscle mitochondrial function through an indirect mechanism. Given that PCBs are stored in fat, we hypothesized that PCB126 alters adipokine secretion, which in turn affects muscle metabolism.

OBJECTIVES

We determined a) the impacts of PCB126 exposure on adipocyte cytokine/adipokine secretion in vitro; b) whether adipocyte-derived factors alter glucose metabolism and mitochondrial function in myotubes when exposed to PCB126; and c) whether preestablished insulin resistance alters the metabolic responses of adipocytes exposed to PCB126 and the communication between adipocytes and myotubes.

METHODS

3T3-L1 adipocytes were exposed to PCB126 (1-100 nM) in two insulin sensitivity conditions [insulin sensitive (IS) and insulin resistant (IR) adipocytes], followed by the measurement of secreted adipokines, mitochondrial function, and insulin-stimulated glucose uptake. Communication between adipocytes and myotubes was reproduced by exposing C2C12 myotubes or mouse primary myotubes to conditioned medium (CM) derived from IS or IR 3T3-L1 adipocytes exposed to PCB126. Mitochondrial function and insulin-stimulated glucose uptake were then determined in myotubes.

RESULTS

IR 3T3-L1 adipocytes treated with PCB126 had significantly higher adipokine (adiponectin, IL-6, MCP-1, TNF-α) secretion and lower mitochondrial function, glucose uptake, and glycolysis. However, PCB126 did not significantly alter these parameters in IS adipocytes. Altered energy metabolism in IR 3T3-L1 adipocytes was linked to lower phosphorylation of AMP-activated protein kinase (p-AMPK) and higher superoxide dismutase 2 levels, an enzyme involved in reactive oxygen species detoxification. Myotubes exposed to the CM from PCB126-treated IR adipocytes had lower glucose uptake, with no alteration in glycolysis or mitochondrial function. Interestingly, p-AMPK levels were higher in myotubes exposed to the CM of PCB126-treated IR adipocytes.

DISCUSSION

Taken together, these data suggest that increased adipokine secretion from IR adipocytes exposed to PCB126 might explain impaired glucose uptake in myotubes. https://doi.org/10.1289/EHP7058.

Pimozide Alleviates Hyperglycemia in Diet-Induced Obesity by Inhibiting Skeletal Muscle Ketone Oxidation

Perturbations in carbohydrate, lipid, and protein metabolism contribute to obesity-induced type 2 diabetes (T2D), though whether alterations in ketone body metabolism influence T2D pathology is unknown. We report here that activity of the rate-limiting enzyme for ketone body oxidation, succinyl-CoA:3-ketoacid-CoA transferase (SCOT/Oxct1), is increased in muscles of obese mice. We also found that the diphenylbutylpiperidine pimozide, which is approved to suppress tics in individuals with Tourette syndrome, is a SCOT antagonist. Pimozide treatment reversed obesity-induced hyperglycemia in mice, which was phenocopied in mice with muscle-specific Oxct1/SCOT deficiency. These actions were dependent on pyruvate dehydrogenase (PDH/Pdha1) activity, the rate-limiting enzyme of glucose oxidation, as pimozide failed to alleviate hyperglycemia in obese mice with a muscle-specific Pdha1/PDH deficiency. This work defines a fundamental contribution of enhanced ketone body oxidation to the pathology of obesity-induced T2D, while suggesting pharmacological SCOT inhibition as a new class of anti-diabetes therapy.

Plasma Myokine Concentrations After Acute Exercise in Non-obese and Obese Sedentary Women

Exercise and physical activity levels influence myokine release from skeletal muscle and contribute to circulating concentrations. Indeed, many myokines, including interleukin (IL)-6, IL-15, secreted protein acidic rich in cysteine (SPARC), and fibroblast growth factor (FGF) 21 are higher in the circulation after an exercise bout. Since these peptides modulate muscle metabolism and can also be targeted toward other tissues to induce adaptations to energy demand, they are of great interest regarding metabolic diseases. Therefore, we set out to compare, in six women with obesity (BMI ≥30 kg/m²) and five healthy women (BMI 22–29.9 kg/m²), the effect of an acute bout of moderate-intensity, continuous cycling exercise (60 min, 60% VO₂peak) on the release of myokines (IL-6, IL-8, IL-10, IL-13, IL-15, SPARC, and FGF21) in plasma for a 24-h time course. We found that plasma IL-8 and SPARC levels were reduced in the group of women with obesity, whereas plasma IL-13 concentrations were elevated in comparison to non-obese women both before and after the exercise bout. We also found that plasma FGF21 concentration during the 24 h following the bout of exercise was regulated differently in the non-obese in comparison to obese women. Plasma concentrations of FGF21, IL-6, IL-8, IL-15, and IL-18 were regulated by acute exercise. Our results confirm the results of others concerning exercise regulation of circulating myokines while providing insight into the time course of myokine release in circulation after an acute exercise bout and the differences in circulating myokines after exercise in women with or without obesity.

The effects of acute BPA exposure on skeletal muscle mitochondrial function and glucose metabolism

Bisphenol A (BPA) is an environmental pollutant that has been associated with adverse health effects including skeletal muscle insulin resistance, a major contributor to the pathogenesis of type 2 diabetes (T2D). Early mitochondrial dysfunction and oxidative stress are linked to impaired glucose metabolism in skeletal muscle. In this study, we investigated the effects of BPA on skeletal muscle mitochondrial function and insulin sensitivity. L6 myotubes were treated with BPA (1 nM-10⁵ nM) during the last 24 h of differentiation. Following exposure to 10⁵ nM of BPA, resting and maximal oxygen consumption rates were decreased, whereas mitochondrial proton leak was increased. Overall metabolic activity, measured by redox ability, was decreased in L6 myotubes exposed to 10⁵ nM of BPA. At this concentration, insulin-stimulated glucose uptake was increased, which corresponded to an increased phosphorylation of the insulin signaling protein Akt, and increased glycolysis measured by extracellular acidification rate (ECAR). Acute BPA exposure did not alter levels of oxidative stress markers in muscle cells, but significantly increased mitochondrial proton leak, which is known to be involved in decreased ROS production. The effects of BPA on glucose uptake, but not mitochondrial function, were reversed by the use of an estrogen receptor antagonist. These results suggest that acute exposure of L6 myotubes at only high concentrations of BPA alters glucose metabolism, which is likely a compensatory response to reduced mitochondrial energy production capacity.

Examination of the Myokine Response in Pregnant and Non-pregnant Women Following an Acute Bout of Moderate-Intensity Walking

Background

It is recommended that women accumulate 150-min of weekly moderate-intensity physical activity (MPA) when pregnant. Engaging in regular physical activity (PA) confers many health benefits to both the mother and the fetus. However, the molecular mechanisms by which these health benefits are bestowed are not well understood. One potential factor that may be contributing to the observed benefits is myokines, which are small peptides secreted by skeletal muscles. In the non-pregnant population, myokines are believed to be involved in the molecular mechanisms resulting from PA. The objective of this study was to characterize and compare the myokine profile of pregnant and non-pregnant women, after an acute bout of MPA.

Methods

Pregnant (n = 13) and non-pregnant (n = 17) women were recruited from the Ottawa region to undergo a treadmill walking session at moderate-intensity (40–60% heart rate reserve). Pre- and post-exercise serum samples were taken, and a set of 15 myokines were analyzed although only 10 were detected. IL-6 was analyzed using a high-sensitivity assay, while FGF21, EPO, BDNF, Fractalkine, IL-15, SPARC, FABP-3, FSTL-1, and oncostatin were analyzed using various multiplex assays.

Results

The pregnant and non-pregnant groups did not differ in terms of age, height, non/pre-pregnancy weight, BMI, and resting heart rate. Baseline levels of EPO and oncostatin were higher in the pregnant group while FGF21 was higher in the non-pregnant group. Circulating levels of three myokines, FGF21, EPO, and IL-15 significantly increased in response to the acute exercise in the pregnant group. Non-pregnant women exhibited an increase in three myokines, FABP-3, FSTL-1, and oncostatin, while one myokine, EPO, decreased post-exercise. SPARC, fractalkine and BDNF were shown to increase post-exercise regardless of pregnancy status while the response for BDNF was more pronounced in the non-pregnant group.

Conclusion

This is the first study examining myokine response following an acute bout of PA in pregnancy. Moderate intensity PA, which is recommended during pregnancy, elicited an increase in four myokines post-compared to pre-exercise in the pregnant group. Further research is warranted to understand the role of myokines in pregnancy.

Role of myokines in the development of skeletal muscle insulin resistance and related metabolic defects in type 2 diabetes

Due to its mass, skeletal muscle is the major site of glucose uptake and an important tissue in the development of type 2 diabetes (T2D). Muscles of patients with T2D are affected with insulin resistance and mitochondrial dysfunction, which result in impaired glucose and fatty acid metabolism. A well-established method of managing the muscle metabolic defects occurring in T2D is physical exercise. During exercise, muscles contract and secrete factors called myokines which can act in an autocrine/paracrine fashion to improve muscle energy metabolism. In patients with T2D, plasma levels as well as muscle levels (mRNA and protein) of some myokines are upregulated, while others are downregulated. The signalling pathways of certain myokines are also altered in skeletal muscle of patients with T2D. Taken together, these findings suggest that myokine secretion is an important factor contributing to the development of muscle metabolic defects during T2D. It is also of interest considering that lack of physical activity is closely linked to the occurrence of this disease. The causal relationships between sedentary behavior, factors secreted by skeletal muscle at rest and during contraction and the development of T2D remain to be elucidated. Many myokines shown to influence muscle energy metabolism still have not been characterized in the context of T2D in skeletal muscle specifically. The purpose of this review is to highlight what is known and what remains to be determined regarding myokine secretion in patients with T2D to uncover potential therapeutic targets for the management of this disease.

Polychlorinated biphenyl 126 exposure in rats alters skeletal muscle mitochondrial function

In the past few years, polychlorinated biphenyls (PCBs), a class of environmental pollutants, have been associated with metabolism dysregulation. Muscle is one of the key regulators of metabolism because of its mass and its important role in terms of glucose consumption and glucose storage. It has been shown that muscle alterations, such as oxidative stress and mitochondrial dysfunction, contribute significantly to the development of metabolic diseases. No study has yet investigated the toxicological effect of PCBs on muscle mitochondrial function and oxidative stress in vivo. The aim of this study was to assess the effect of PCB126 in vivo exposure (single dose of 1.05 μmol/kg) on muscle mitochondrial function and oxidative stress in rats. PCB126-treated rats showed a marked increase in Cyp1a1 mRNA levels in skeletal muscles in association with a 40% reduction in state 3 oxygen consumption rate measured with complex I substrates in permeabilized muscle fibers. Furthermore, PCB126 exposure altered the expression of some enzymes involved in ROS detoxification such as catalase and glutaredoxin 2. Our results highlight for the first time a toxic effect of coplanar PCBs on skeletal muscle mitochondrial function and oxidative stress. This suggests that acute PCB exposure, by affecting muscle metabolism, could contribute to the development of metabolic disorders. Studies are needed to determine if lower-level but longer-term PCB exposure exhibits the same effect.

IL-15 improves skeletal muscle oxidative metabolism and glucose uptake in association with increased respiratory chain supercomplex formation and AMPK pathway activation

BACKGROUND

IL-15 is believed to play a role in the beneficial impact of exercise on muscle energy metabolism. However, previous studies have generally used supraphysiological levels of IL-15 that do not represent contraction-induced IL-15 secretion.

METHODS

L6 myotubes were treated acutely (3 h) and chronically (48 h) with concentrations of IL-15 mimicking circulating (1-10 pg/ml) and muscle interstitial (100 pg/ml -20 ng/ml) IL-15 levels with the aim to better understand its autocrine/paracrine role on muscle glucose uptake and mitochondrial function.

RESULTS

Acute exposure to IL-15 levels representing muscle interstitial IL-15 increased basal glucose uptake without affecting insulin sensitivity. This was accompanied by increased mitochondrial oxidative functions in association with increased AMPK pathway and formation of complex III-containing supercomplexes. Conversely, chronic IL-15 exposure resulted in a biphasic effect on mitochondrial oxidative functions and ETC supercomplex formation was increased with low IL-15 levels but decreased with higher IL-15 concentrations. The AMPK pathway was activated only by high levels of chronic IL-15 treatment. Similar results were obtained in skeletal muscle from muscle-specific IL-15 overexpressing mice that show very high circulating IL-15 levels.

CONCLUSIONS

Acute IL-15 treatment that mimics local IL-15 concentrations enhances muscle glucose uptake and mitochondrial oxidative functions. That mitochondria respond differently to different levels of IL-15 during chronic treatments indicates that IL-15 might activate two different pathways in muscle depending on IL-15 concentrations.

GENERAL SIGNIFICANCE

Our results suggest that IL-15 may act in an autocrine/paracrine fashion and be, at least in part, involved in the positive effect of exercise on muscle energy metabolism.

Contribution of cytosolic cysteine residues to the gating properties of the Kir2.1 inward rectifier

The topological model proposed for the Kir2.1 inward rectifier predicts that seven of the channel 13 cysteine residues are distributed along the N- and C-terminus regions, with some of the residues comprised within highly conserved domains involved in channel gating. To determine if cytosolic cysteine residues contribute to the gating properties of Kir2.1, each of the N- and C-terminus cysteines was mutated into either a polar (S, D, N), an aliphatic (A,V, L), or an aromatic (W) residue. Our patch-clamp measurements show that with the exception of C76 and C311, the mutation of individual cytosolic cysteine to serine (S) did not significantly affect the single-channel conductance nor the channel open probability. However, mutating C76 to a charged or polar residue resulted either in an absence of channel activity or a decrease in open probability. In turn, the mutations C311S (polar), C311R (charged), and to a lesser degree C311A (aliphatic) led to an increase of the channel mean closed time due to the appearance of long closed time intervals (T(c) >or= 500 ms) and to a reduction of the reactivation by ATP of rundown Kir2.1 channels. These changes could be correlated with a weakening of the interaction between Kir2.1 and PIP(2), with C311R and C311S being more potent at modulating the Kir2.1-PIP(2) interaction than C311A. The present work supports, therefore, molecular models whereby the gating properties of Kir2.1 depend on the presence of nonpolar or neutral residues at positions 76 and 311, with C311 modulating the interaction between Kir2.1 and PIP(2).

Role of aspartate residues in Ca(2+) affinity and permeation of the distal ECaC1

The Ca(2+) affinity and permeation of the epithelial Ca(2+) channel (ECaC1) were investigated after expression in Xenopus oocytes. ECaC1 displayed anomalous mole-fraction effects. Extracellular Ca(2+) and Mg(2+) reversibly inhibited ECaC1 whole cell Li(+) currents: IC(50) = 2.2 +/- 0.4 microM (n = 9) and 235 +/- 35 microM (n = 10), respectively. These values compare well with the Ca(2+) affinity of the L-type voltage-gated Ca(2+) (Ca(V)1.2) channel measured under the same conditions, suggesting that high-affinity Ca(2+) binding is a well-conserved feature of epithelial and voltage-gated Ca(2+) channels. Neutralization of D550 and E535 in the pore region had no significant effect on Ca(2+) and Mg(2+) affinities. In contrast, neutralization of D542 significantly decreased Ca(2+) affinity (IC(50) = 1.1 +/- 0.2 mM, n = 6) and Mg(2+) affinity (IC(50) > 25 +/- 3 mM, n = 4). Despite a 1,000-fold decrease in Ca(2+) affinity in D542N, Ca(2+) permeation properties and the Ca(2+)-to-Ba(2+) conductance ratio remained comparable to values for wild-type ECaC1. Together, our observations suggest that D542 plays a critical role in Ca(2+) affinity but not in Ca(2+) permeation in ECaC1.

Abnormal Ca2+ signalling in vascular endothelial cells from spontaneously hypertensive rats: role of free radicals

OBJECTIVE

To test the hypothesis that the Ca2+ signal transduction process in endothelial cells from genetically hypertensive rats (SHR) is affected by an overproduction of free radicals.

METHODS

The Ca2+ response to the inositol 1,4,5-triphosphate (IP3) mobilizing agonist, ATP, was measured using the fluorescent probe, fura-2, in endothelial cells from Sprague-Dawley rats, and in young and age-matched genetically hypertensive rats (SHR). The effect of free radicals and reducing agents on the intracellular release of Ca2+ and IP3productionwas determined in resting and ATP-stimulated cells. Experiments were also performed to compare the level of expression and enzymatic activity of catalase and superoxide dismutase (SOD) in endothelial cells from SHR and Sprague-Dawley rats.

RESULTS

The exposure of aortic endothelial cells from Sprague-Dawley rats to the free-radical generating system, hypoxanthine + xanthine oxidase (HX/XO), caused a time- and concentration-dependent inhibition of the ATP-induced Ca2+ response. A similar HX/XO-dependent inhibition was also observed in Sprague-Dawley cells stimulated with the endoplasmic reticulum Ca2+-ATPase inhibitor, thapsigargin. Incubation with the antioxidative enzymes, catalase and SOD, had no effect on the ATP-induced Ca2+ release in Sprague-Dawley cells, but led to a strong increase in the internal release of Ca2+ in cells from adult (12 weeks old) or young (3 weeks old) SHR. The effect of antioxidants was not related either to an enhancement of the ATP-induced production of IP3, or to a lower expression and activity of SOD and catalase.

CONCLUSION

The present work provides evidence that the Ca2+ signalling process in SHR endothelial cells is affected by an overproduction of free radicals, resulting in a depletion of releasable Ca2+ from IP3-sensitive and insensitive Ca2+ pools. These results point towards a beneficial action of antioxidants on Ca2+ signalling in endothelial cells from models of hypertension.

pH and external Ca(2+) regulation of a small conductance Cl(-) channel in kidney distal tubule

A single channel characterization of the Cl(-) channels in distal nephron was undertaken using vesicles prepared from plasma membranes of isolated rabbit distal tubules. The presence in this vesicle preparation of ClC-K type Cl(-) channels was first established by immunodetection using an antibody raised against ClC-K isoforms. A ClC-K1 based functional characterization was next performed by investigating the pH and external Ca(2+) regulation of a small conductance Cl(-) channel which we identified previously by channel incorporation experiments. Acidification of the cis (external) solution from pH 7.4 to 6.5 led to a dose-dependent inhibition of the channel open probability P(O). Similarly, changing the trans pH from 7.4 to 6.8 resulted in a 4-fold decrease of the channel P(O) with no effect on the channel conductance. Channel activity also appeared to be regulated by cis (external) Ca(2+) concentration, with a dose-dependent increase in channel activity as a function of the cis Ca(2+) concentration. It is concluded on the basis of these results that the small conductance Cl(-) channel present in rabbit distal tubules is functionally equivalent to the ClC-K1 channel in the rat. In addition, the present work constitutes the first single channel evidence for a chloride channel regulated by external Ca(2+).

Molecular characterization of an inwardly rectifying K+ channel from HeLa cells

Previous patch-clamp studies have shown that the potassium permeability of the plasma membrane in HeLa cells, a cell line derived from an epidermoid carcinoma of the cervix, is controlled by various K+-selective pores including an IRK1 type inwardly rectifying K+ channel. We used the sequence previously reported for the human heart Kir2.1 channel to design a RT-PCR strategy for cloning the IRK1 channel in HeLa cells. A full-length clone of 1.3 kb was obtained that was identical to the human cardiac Kir2.1 inward rectifier. The nature of the cloned channel was also confirmed in a Northern blot analysis where a signal of 5.3 kb corresponding to the molecular weight expected for a Kir2.1 channel transcript was identified not only in HeLa cells, but also in WI-38, ECV304 and bovine aortic endothelial cells. The HeLa IRK1 channel cDNA was subcloned in an expression vector (pMT21) and injected into Xenopus oocytes. Cell-attached and inside-out single channel recordings obtained from injected oocytes provided evidence for a voltage-independent K+-selective channel with current/voltage characteristics typical of a strong inward rectifier. The single channel conductance for inward currents measured in 200 mm K2SO4 conditions was estimated at 40 +/- 1 pS (n = 3), for applied voltages ranging from -100 to -160 mV, in agreement with the unitary conductance for the IRK1 channel identified in HeLa cells. In addition, the single channel conductance for inward currents, Gamma, was found to vary as a function of alphaK, the external K+ ion activity, according to Gamma = Gamma0 [alphaK]delta with Gamma0 = 3.3 pS and delta = 0.5. Single channel recordings from injected oocytes also provided evidence of a voltage-dependent block by external Cs+ and Ba2+. The presence of 500 micron Cs+ caused a voltage-dependent flickering, typical of a fast channel blocking process which resulted in a reduction of the channel open probability at increasingly negative membrane potential values. The fractional electrical distance computed for the Cs+ blocking site was greater than 1 indicating a multiple ion channel occupation. In contrast, external Ba2+ at concentrations ranging from 25 to 100 micron caused a slow channel block, consistent with the binding of a single Ba2+ ion at a site located at half the membrane span. It is concluded on the basis of these observations that HeLa cells expressed a Kir2.1 type inwardly rectifying channel likely to be involved in maintaining and regulating the cell resting potential.

Single-channel characterization of the pharmacological properties of the K(Ca2+) channel of intermediate conductance in bovine aortic endothelial cells

The pharmacological profile of a voltage-independent Ca2+-activated potassium channel of intermediate conductance (IK(Ca2+)) present in bovine aortic endothelial cells (BAEC) was investigated in a series of inside-out and outside-out patch-clamp experiments. Channel inhibition was observed in response to external application of ChTX with a half inhibition concentration of 3.3 +/- 0.3 nm (n = 4). This channel was insensitive to IbTX, but channel block was detected following external application of MgTX and StK leading to the rank order toxin potency ChTX > StK > MgTX >>IbTX. A reduction of the channel unitary current amplitude was also measured in the presence of external TEA, with half reduction occurring at 23 +/- 3 mm TEA (n = 3). The effect of TEA was voltage insensitive, an indication that TEA may bind to a site located on external side of the pore region of this channel. Similarly, the addition of d-TC to the external medium caused a reduction of the channel unitary current amplitude with half reduction at 4.4 +/- 0.3 mm (n = 4). In contrast, application of d-TC to the bathing medium in inside-out experiments led to the appearance of long silent periods, typical of a slow blocking process. Finally, the IK(Ca2+) in BAEC was found to be inhibited by NS1619, an activator of the Ca2+-activated potassium channel of large conductance (Maxi K(Ca2+)), with a half inhibition value of 11 +/- 0.8 micron (n = 4). These results provide evidence for a pharmacological profile distinct from that reported for the Maxi K(Ca2+) channel, with some features attributed to the voltage-gated KV1.2 potassium channel.

Characterization of a chloride-selective channel from rough endoplasmic reticulum membranes of rat hepatocytes: evidence for a block by phosphate

We have characterized the conduction and blocking properties of a chloride channel from rough endoplasmic reticulum membranes of rat hepatocytes after incorporation into a planar lipid bilayer. Our experiments revealed the existence of a channel with a mean conductance of 164 +/- 5 pS in symmetrical 200 mm KCl solutions. We determined that the channel was ten times more permeable for Cl- than for K+, calculated from the reversal potential using the Goldman-Hodgkin-Katz equation. The channel was voltage dependent, with an open probability value ranging from 0.9 at -20 mV to 0.4 at +60 mV. In addition to its fully open state, the channel could also enter a flickering state, which appeared to involve rapid transitions to zero current level. Our results showed a decrease of the channel mean open time combined with an increase of the channel mean closed time at positive potentials. An analysis of the dwell time distributions for the open and closed intervals led to the conclusion that the observed fluctuation pattern was compatible with a kinetic scheme containing a single open state and a minimum of three closed states. The permeability sequence for test halides determined from reversal potentials was Br- > Cl- > I- approximately F-. The voltage dependence of the open probability was modified by the presence of halides in trans with a sequence reflecting the permeability sequence, suggesting that permeant anions such as Br- and Cl- have access to an internal site capable of controlling channel gating. Adding NPPB to the cis chamber inhibited the channel activity by increasing fast flickering and generating long silent periods, whereas channel activity was not affected by 50 microM DNDS in trans. The channel was reversibly inhibited by adding phosphate to the trans chamber. The inhibitory effect of phosphate was voltage-dependent and could be reversed by addition of Cl-. Our results suggest that channel block involves the interaction of HPO2-4 with a site located at 70% of the membrane span.

Evidence from incorporation experiments for an anionic channel of small conductance at the apical membrane of the rabbit distal tubule

Many of the hormone-regulated ion transport processes in distal nephron involve transcellular pathways which require a passive entry of ions at the apical membrane of the distal tubule cells. To investigate molecular mechanisms underlying the ionic permeability of the distal tubule apical membrane, a study was undertaken in which vesicles prepared from apical membranes from isolated rabbit distal tubules were fused onto a planar lipid bilayer. These experiments led to the identification of several ionic channels including a Cl(-)-permeable channel of 14 pS with a Na+ over Cl- permeability ratio, PNa/PCl < 0.09. The open channel probability (Po) showed a weak voltage dependency with Po increasing slightly at negative potential values (intracellular (trans) relative to extracellular (cis) for right-side-out vesicles). Channel activity was inhibited by NPPB at high concentrations (> 100 microM) and by DIDS (300 microM). A small inhibitory effect was also observed in the presence of DPC at concentrations ranging from 200 microM to 500 microM. The presence of SO4(2-) (32 mmol/l) in the trans solution caused a complete inhibition of channel activity, but no modification of channel behaviour was observed with the non-selective channel blocking agent gadolinium (Gd3+) at 100 microM. Finally, addition of the catalytic subunit of protein kinase A into the trans chamber (60 U/ml to 80 U/ml) led to an increase in channel activity characterized by a greater number of active channels coupled to an increase of the individual channel open probability. The action of the protein kinase A could be cancelled by the addition of a non specific protein phosphatase, such as alkaline phosphatase. Our results suggest that the apical membrane of the rabbit distal tubule contains a Cl- permeable channel of small conductance the activity of which may be modulated by hormones linked to the adenylate cyclase pathway.

Effects of halothane and isoflurane on bradykinin-evoked Ca2+ influx inbovine aortic endothelial cells

BACKGROUND

Volatile anesthetics, such as halothane and isoflurane, have been reported to affect the endothelium mediated relaxation of vascular smooth muscle cells. Because the activity of the constitutive nitric oxide synthase in endothelial cells depends on the availability of intracellular Ca2+, there is a definite possibility that the observed inhibitory effect of volatile anesthetics involves an action on the agonist-evoked internal Ca2+ mobilization and/or Ca2+ influx in these cells. Therefore, a study was undertaken to determine how halothane and isoflurane affect the Ca2+ signalling process in vascular endothelial cells.

METHODS

The effect of halothane and isoflurane on the Ca2+ response to bradykinin of bovine aortic endothelial (BAE) cells was investigated using the fluorescent Ca2+ indicator fura-2. Halothane or isoflurane was applied either to resting cells or after bradykinin stimulation. The agonist-evoked Ca2+ influx in BAE cells was estimated by measuring either the rate of fura-2 quenching induced by Mn2+ or the increase in cytosolic Ca2+ concentration initiated after readmission of external Ca2+ after a brief exposure of the cells to a Ca(2+)-free external medium. The effects of halothane on cell potential and intracellular Ca2+ concentration were measured in cell-attached patch-clamp experiments in which a calcium-activated K+ channel and an inward rectifying Ca(2+)-independent K+ channel were used as probes to simultaneously monitor the intracellular Ca2+ concentration and the cell transmembrane potential. In addition, combined fura-2 and patch-clamp cell-attached recordings were carried out, to correlate the variations in internal Ca2+ caused by halothane and the activity of the Ca(2+)-dependent K+ channels, which are known in BAE cells to regulate intracellular potential. Finally, a direct action of halothane and isoflurane on the gating properties of the Ca(2+)-activated K+ channel present in these cells was investigated in patch-excised inside-out experiments.

RESULTS

The results of the current study indicate that the initial Ca2+ increase in response to bradykinin stimulation is not affected by halothane, but that pulse applications of halothane (0.4-2 mM) or isoflurane (0.5-1 mM) reversibly reduce the sustained cytosolic Ca2+ increase initiated either by bradykinin or by the Ca2+ pump inhibitor thapsigargin. In addition, halothane appeared to dose-dependently inhibit the Ca2+ influx evoked by bradykinin, and to cause, concomitant to a decrease in cytosolic Ca2+ concentration, a depolarization of the cell potential. Halothane failed, however, to affect internal Ca2+ concentration in thapsigargin-treated endothelial cells, which were depolarized using a high K+ external solution. Finally, halothane and isoflurane decreased the open probability of the Ca(2+)-dependent K+ channel present in these cells.

CONCLUSIONS

These observations suggest that the effects of halothane and isoflurane on Ca2+ homeostasis in BAE cells reflect, for the most part, a reduction of the thapsigargin- or bradykinin-evoked Ca2+ influx, which would be consequent to a cellular depolarization caused by an inhibition of the Ca(2+)-dependent K+ channel activity initiated after cell stimulation.

Isolation of single mammalian proximal tubule cells: effects of hypotonic shocks on cell yield and function

Nord et al. [Am J Physiol 1986; 250:F539-F550] proposed a method to give a high yield of proximal tubule cells by exposing a suspension of rabbit cortical tubules to a hypotonic shock in calcium-free media. The present study describes the effects of both amplitude and duration of the hypotonic treatment on some transport-related characteristics of individual cells as compared to the starting tubule suspension. The averaged cell yield increased by an order of magnitude when the osmolality of the hypotonic solution was varied in four steps from 200 (C200 cells) to 70 mosm/kg H2O (C70 cells) while the proportion of trypan blue-positive cells progressively decreased from 33% for C200 cells to 9.5% for C70 cells. An increase in duration of the hypotonic shock from 0.5 to 6 min did not change the cell yield of C200 cells while it significantly increased that of C70 cells by 61%. Basal and ouabain-sensitive oxygen consumption (QO2) increased by 57 and 155%, respectively, from C70 to C200 cells but was approximately one order of magnitude smaller than the QO2 measured for tubule suspension. Intracellular ATP content averaged 5.5 +/- 0.8 nmol/mg for the starting tubule suspension, 4.6 +/- 0.8 nmol/mg for C70 cells but only 1.3 +/- 0.1 nmol/mg for C200 cells. The maximal velocity for phloridzin-sensitive alpha-methyl glucose transport averaged 13.7 +/- 1.7 nmol min-1 mg-1 for C70 cells and only 6.3 +/- 1.3 nmol min-1 mg-1 for C200 cells which is approximately one order of magnitude smaller than what can be expected from a tubule presenting a good access to luminal membrane. We conclude from these results that, in the process of isolating individual cells from a polarized epithelium, membrane transport rates have decreased by one order of magnitude and this reduction is intensified by a large hypotonic shock. In comparison with C200 cells, the cells obtained with a large hypotonic shock give a high yield, a larger proportion of trypan blue-negative cells and their lower overall transport rate allows the cells to maintain a better electrochemical gradient for Na and a higher intracellular ATP level.

Histamine-evoked Ca2+ oscillations in HeLa cells are sensitive to methylxanthines but insensitive to ryanodine

The relative contribution of inositol-trisphosphate(InsP3)-sensitive and InsP3-insensitive Ca2+ stores to the agonist-evoked oscillatory release of Ca2+ in HeLa cells was investigated using fura-2 cytosolic Ca2+ measurements and whole-cell recordings of Ca(2+)-activated K+ currents [K(Ca2+)]. The experimental approach chosen consisted in studying the effects on Ca2+ oscillations of a variety of pharmacological agents such as ryanodine, ruthenium red, caffeine and theophylline, which are known to affect the Ca2+ channels responsible for Ca(2+)-induced Ca2+ release (CICR) in excitable cells. The results obtained essentially indicate (a) that neither ryanodine nor ruthenium red affects the generation of periodic K(Ca2+) current pulses in whole-cell experiments, and (b) that histamine-induced Ca2+ oscillations are inhibited by caffeine and theophylline in a dose-dependent manner. However, these methylxanthines were unable, at concentrations ranging from 0.1 mM to 10 mM, either to mobilize Ca2+ from internal stores or to block the initial Ca2+ rise evoked by histamine. In addition, both methylxanthines showed at high concentrations (10-20 mM) a moderate inhibitory action on the production of InsP3 induced by histamine. This effect was not essential to the action of caffeine on the oscillatory release of Ca2+, since an inhibition by caffeine of InsP3-induced Ca2+ oscillations was still observed in whole-cell experiments where the InsP3 concentration was kept constant. The results also show (c) that the application of either caffeine or theophylline during histamine stimulation leads systematically to an increased Ca2+ sequestration in InsP3-sensitive Ca2+ pools, the effect observed with theophylline being stronger than that resulting from the application of caffeine, and finally (d) that the action of caffeine and theophylline is not related to an increase in cAMP concentration since neither forskolin (10-50 microM) nor 8-Br-cAMP (1 mM) caused an inhibition of the InsP3-induced Ca2+ oscillations. It is concluded on the basis of these results that the agonist-evoked Ca2+ oscillations in HeLa cells do not involve directly or indirectly a ryanodine-sensitive Ca(2+)-release channel with CICR properties, but rather arise from a control by Ca2+ of the InsP3 Ca(2+)-release process.

Lepidopteran-specific crystal toxins from Bacillus thuringiensis form cation- and anion-selective channels in planar lipid bilayers

Previous studies in our laboratory have shown that CryIC, a lepidopteran-specific toxin from Bacillus thuringiensis, triggers calcium and chloride channel activity in SF-9 cells (Spodoptera frugiperda, fall armyworm). Chloride currents were also observed in SF-9 membrane patches upon addition of CryIC toxin to the cytoplasmic side of the membrane. In the present study the ability of activated CryIC toxin to form channels was investigated in a receptor-free, artificial phospholipid membrane system. We demonstrate that this toxin can partition in planar lipid bilayers and form ion-selective channels with a large range of conductances. These channels display complex activity patterns, often possess subconducting states and are selective to either anions or cations. These properties appeared to be pH dependent. At pH 9.5, cation-selective channels of 100 to 200 pS were most frequently observed. Among the channels recorded at pH 6.0, a 25-35 pS anion-selective channel was often seen at pH 6.0, with permeation and kinetic properties similar to those of the channels previously observed in cultured lepidopteran cells under comparable pH environment and for the same CryIC toxin doses. We conclude that insertion of CryIC toxin in SF-9 cell native membranes and in artificial planar phospholipid bilayers may result from an identical lipid-protein interaction mechanism.

Early response of cultured lepidopteran cells to exposure to delta-endotoxin from Bacillus thuringiensis: involvement of calcium and anionic channels

The role of ion channels in the initial steps following exposure of SF-9 lepidopteran insect cells in culture to the delta-endotoxin CryIC from the insecticidal bacterium Bacillus thuringiensis was investigated using single ionic channel measurements and microspectrofluorescence of the calcium-sensitive probe fura-2. It was found that: (1) the toxin triggers an immediate rise in intracellular calcium; (2) the surge is due to calcium entering the cells via calcium channels; (3) the toxin recruits or introduces anionic channels in the cell's plasma membrane in a time-dependent manner. These channels, not seen in the absence of the toxin, are induced by toxin exposure to either side of the cell membrane. They have a conductance of 26 picosiemens (pS) and are mainly permeable to chloride. This study provides the first evidence of the primary role of calcium and chloride ions in the action of delta-endotoxin on cultured insect cells.

Single-channel and Fura-2 analysis of internal Ca2+ oscillations in HeLa cells: contribution of the receptor-evoked Ca2+ influx and effect of internal pH

Patch-clamp and Fura-2 experiments were performed in order to investigate the calcium oscillations due to H1 receptor stimulation in HeLa cells. The cytosolic calcium fluctuations occurring directly at the plasma membrane inner face were detected by measuring the activity of calcium-dependent potassium channels. This method also allowed measurement of changes in intracellular potential using as indicator the amplitude of the channel current jump. The average internal calcium concentration was obtained from Fura-2 experiments carried out at either the single-cell level or from a small population of cells in monolayer. The results indicate that the internal calcium oscillations in HeLa cells arise from a biphasic process with an initial phase independent of the presence of external calcium. External calcium was found, however, to become essential once the regular oscillatory process has been established. Removing external calcium after this initial phase produced a rapid decay in the burst frequency and eventually a complete abolition of the oscillations. In addition, the calcium oscillations occurring during the external-calcium-dependent phase could be blocked by calcium entry blockers such as Co2+ or La3+, or abolished by perfusing the external medium with a high-K+ solution. Experiments were also performed in which the cell internal pH (pHi) was changed by removing the external bicarbonate or by adding NH4Cl to the bathing solution. The results obtained under these conditions indicate that an increase in internal pH abolishes selectively the appearance of calcium spikes without increasing the basal calcium level, while a cellular acidification maintains or stimulates the calcium oscillatory process. It was also observed that the inhibitory effect of alkaline pH was independent of external calcium, and that calcium oscillations could always be seen at alkaline pH during the initial phase of histamine stimulation. On the basis of these results, it is proposed that the internal calcium oscillations in HeLa cells depend on the release of calcium from internal pools, which are reloaded via a pH-dependent mechanism. Part of the calcium sequestration occurring during the oscillatory process would be carried out, however, by pH-insensitive calcium compartments.

Membrane crosstalk in the mammalian proximal tubule during alterations in transepithelial sodium transport

The present paper examines the effects of reduced transepithelial Na transport (JNa) on membrane electrophysiological parameters in proximal convoluted tubules and the possible role of cytosolic calcium concentration ([Ca]i) in the regulation of basolateral membrane K conductance (GK). When JNa was reduced by elimination of glucose and alanine and replacement of 100 mM sodium with N-methyl-D-glucamine from the luminal perfusate, basolateral membrane potential (VBL) hyperpolarized transiently by 12.6 mV and the ratio of apical to basolateral membrane resistance (RA/RBL) doubled. The apparent transference number for K at the basolateral membrane (GK/Gcell) decreased from 0.13 to 0.08 in the first 4 min following reductions in JNa. The elimination of Na-alanine and Na-glucose cotransport was responsible for the initial hyperpolarization and increase in RA/RBL, whereas the resultant decrease in the cellular concentrations of glucose and alanine, together with the reductions in GK, could elicit the secondary VBL depolarization. Measurement of [Ca]i with the fluorescent probe fura-2 during reductions in JNa revealed that [Ca]i increased by an average of 12%, a value very similar to the average reduction in cellular volume (13%) measured using morphometric techniques. The observation that [Ca]i increased while GK was decreasing is inconsistent with the effect of [Ca]i on putative basolateral Ca-activated K channel. We believe that [Ca]i changes passively (at least in the first few minutes) in response to a decrease in cell volume occurring as a consequence of reductions in JNa and that some as yet unidentified volume-sensitive mechanism is responsible for the regulation of GK.