Transcriptomic profile of the mechanosensitive ion channelome in human cardiac fibroblasts

Human cardiac fibroblasts (HCFs) have mRNA transcripts that encode different mechanosensitive ion channels and channel regulatory proteins whose functions are not known yet. The primary goal of this work was to define the mechanosensitive ion channelome of HCFs. The most common type of cationic channel is the transient receptor potential (TRP) family, which is followed by the TWIK-related K+ channel (TREK), transmembrane protein 63 (TMEM63), and PIEZO channel (PIEZO) families. In the sodium-dependent NON-voltage-gated channel (SCNN) subfamily, only SCNN1D was shown to be highly expressed. Particular members of the acid-sensing ion channel (ASIC) (ASIC1 and ASIC3) subfamilies were also significantly expressed. The transcripts per kilobase million (TPMs) for Piezo 2 were almost 100 times less abundant than those for Piezo 1. The tandem of P domains in a weak inward rectifying K+ channel (TWIK)-2 channel, TWIK-related acid-sensitive K+ channel (TASK)-5, TASK-1, and the TWIK-related K1 (TREK-1) channel were the four most prevalent types in the K2P subfamily. The highest expression in the TRPP subfamily was found for PKD2 and PKD1, while in the TRPM subfamily, it was found for TRPM4, TRPM7, and TRPM3. TRPV2, TRPV4, TRPV3, and TRPV6 (all members of the TRPV subfamily) were also substantially expressed. A strong expression of the TRPC1, TRPC4, TRPC6, and TRPC2 channels and all members of the TRPML subfamily (MCOLN1, MCOLN2, and MCOLN3) was also shown. In terms of the transmembrane protein 16 (TMEM16) family, the HCFs demonstrated significant expression of the TMEM16H, TMEM16F, TMEM16J, TMEM16A, and TMEM16G channels. TMC3 is the most expressed channel in HCFs of all known members of the transmembrane channel-like protein (TMC) family. This analysis of the mechanosensitive ionic channel transcriptome in HCFs: (1) agrees with previously documented findings that all currently identified mechanosensitive channels play a significant and well recognized physiological function in elucidating the mechanosensitive characteristics of HCFs; (2) supports earlier preliminary reports that point to the most common expression of the TRP mechanosensitive family in HCFs; and (3) points to other new mechanosensitive channels (TRPC1, TRPC2, TWIK-2, TMEM16A, ASIC1, and ASIC3).

Anticarcinogenic Potency of EF24: An Overview of Its Pharmacokinetics, Efficacy, Mechanism of Action, and Nanoformulation for Drug Delivery

EF24, a synthetic monocarbonyl analog of curcumin, shows significant potential as an anticancer agent with both chemopreventive and chemotherapeutic properties. It exhibits rapid absorption, extensive tissue distribution, and efficient metabolism, ensuring optimal bioavailability and sustained exposure of the target tissues. The ability of EF24 to penetrate biological barriers and accumulate at tumor sites makes it advantageous for effective cancer treatment. Studies have demonstrated EF24's remarkable efficacy against various cancers, including breast, lung, prostate, colon, and pancreatic cancer. The unique mechanism of action of EF24 involves modulation of the nuclear factor-kappa B (NF-κB) and nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathways, disrupting cancer-promoting inflammation and oxidative stress. EF24 inhibits tumor growth by inducing cell cycle arrest and apoptosis, mainly through inhibiting the NF-κB pathway and by regulating key genes by modulating microRNA (miRNA) expression or the proteasomal pathway. In summary, EF24 is a promising anticancer compound with a unique mechanism of action that makes it effective against various cancers. Its ability to enhance the effects of conventional therapies, coupled with improvements in drug delivery systems, could make it a valuable asset in cancer treatment. However, addressing its solubility and stability challenges will be crucial for its successful clinical application.

Tricyclic hydrocarbon fluorene attenuates ventricular ionic currents and pressure development in the navaga cod

The release of polycyclic aromatic hydrocarbons (PAHs) into the environment due to oil and diesel fuel spills is a serious threat to Arctic fish populations. PAHs produce multiple toxic effects in fish, but disturbance of electrical and contractile activity of the heart seems to be the most negative effect. Our study focused on the effects of fluorene, a tricyclic PAH resembling the well-investigated tricyclic phenanthrene, on major ionic currents and action potential (AP) waveform in isolated ventricular myocytes and on contractile activity in isolated whole hearts of polar navaga cod (Eleginus nawaga). Among the studied currents, the repolarizing rapid delayed rectifier K+ current IKr demonstrated the highest sensitivity to fluorene with IC50 of 0.54 μM. The depolarizing inward currents, INa and ICaL, were inhibited with 10 μM fluorene by 20.2 ± 2.8 % and 27.9 ± 8.4 %, respectively, thereby being much less sensitive to fluorene than IKr. Inward rectifier IK1 current was insensitive to fluorene (up to 10 μM). While 3 μM fluorene prolonged APs, 10 μM also slowed the AP upstroke. Resting membrane potential was not affected by any tested concentrations. In isolated heart experiments 10 μM fluorene caused modest depression of ventricular contractile activity. Thus, we have demonstrated that fluorene, a tricyclic PAH present in high quantities in crude oil, strongly impacts electrical activity with only slight effects on contractile activity in the heart of the polar fish, the navaga cod.

Positive Tetrahydrocurcumin-Associated Brain-Related Metabolomic Implications

Tetrahydrocurcumin (THC) is a metabolite of curcumin (CUR). It shares many of CUR's beneficial biological activities in addition to being more water-soluble, chemically stable, and bioavailable compared to CUR. However, its mechanisms of action have not been fully elucidated. This paper addresses the preventive role of THC on various brain dysfunctions as well as its effects on brain redox processes, traumatic brain injury, ischemia-reperfusion injury, Alzheimer's disease, and Parkinson's disease in various animal or cell culture models. In addition to its strong antioxidant properties, the effects of THC on the reduction of amyloid β aggregates are also well documented. The therapeutic potential of THC to treat patterns of mitochondrial brain dysmorphic dysfunction is also addressed and thoroughly reviewed, as is evidence from experimental studies about the mechanism of mitochondrial failure during cerebral ischemia/reperfusion injury. THC treatment also results in a dose-dependent decrease in ERK-mediated phosphorylation of GRASP65, which prevents further compartmentalization of the Golgi apparatus. The PI3K/AKT signaling pathway is possibly the most involved mechanism in the anti-apoptotic effect of THC. Overall, studies in various animal models of different brain disorders suggest that THC can be used as a dietary supplement to protect against traumatic brain injury and even improve brain function in Alzheimer's and Parkinson's diseases. We suggest further preclinical studies be conducted to demonstrate the brain-protective, anti-amyloid, and anti-Parkinson effects of THC. Application of the methods used in the currently reviewed studies would be useful and should help define doses and methods of THC administration in different disease conditions.

Myocardial infarction and oxidative damage in animal models: objective and expectations from the application of cysteine derivatives

Reactive oxygen species (ROS) and associated oxidative stress are the main contributors to pathophysiological changes following myocardial infarction (MI), which is the principal cause of death from cardiovascular disease. The glutathione (GSH)/glutathione peroxidase (GPx) system appears to be the main and most active cardiac antioxidant mechanism. Hence, enhancement of the myocardial GSH system might have protective effects in the setting of MI. It follows that by increasing antioxidant capacity, the heart will be able to reduce the damage associated with MI and even prevent/weaken the occurrence of oxidative stress, which is highly ranked among the factors responsible for the occurrence of acute MI. For these reasons, the primary goal of future investigations should be to address the effects of different antioxidative compounds and especially cysteine derivatives like N-acetyl cysteine (NAC) and L-2-oxothiazolidine-4-carboxylic acid (OTC) as precursors responsible for the enhancement of the GSH-related antioxidant system's capacity. It is assumed that this will lay down the basis for elucidation of the mechanisms throughout which applicable doses of OTC will manifest a potentially positive impact in the reduction of adverse effects of acute MI. The inclusion of OTC in the models for prediction of the distribution of oxygen in infarcted animal hearts can help to upgrade existing computational models. Such a model would be based on computational geometries of the heart, but the inclusion of biochemical redox features in addition to angiogenic therapy, despite improvement of the post-infarcted oxygenated outcome could enhance the accuracy of the predictive values of oxygenation.

Efficacy of the monocarbonyl curcumin analog C66 in the reduction of diabetes-associated cardiovascular and kidney complications

Curcumin is a polyphenolic compound derived from turmeric that has potential beneficial properties for cardiovascular and renal diseases and is relatively safe and inexpensive. However, the application of curcumin is rather problematic due to its chemical instability and low bioavailability. The experimental results showed improved chemical stability and potent pharmacokinetics of one of its analogs – (2E,6E)-2,6-bis[(2-trifluoromethyl)benzylidene]cyclohexanone (C66). There are several advantages of C66, like its synthetic accessibility, structural simplicity, improved chemical stability (in vitro and in vivo), presence of two reactive electrophilic centers, and good electron-accepting capacity. Considering these characteristics, we reviewed the literature on the application of C66 in resolving diabetes-associated cardiovascular and renal complications in animal models. We also summarized the mechanisms by which C66 is preventing the release of pro-oxidative and pro-inflammatory molecules in the priming and in activation stage of cardiomyopathy, renal fibrosis, and diabetic nephropathy. The cardiovascular protective effect of C66 against diabetes-induced oxidative damage is Nrf2 mediated but mainly dependent on JNK2. In general, C66 causes inhibition of JNK2, which reduces cardiac inflammation, fibrosis, oxidative stress, and apoptosis in the settings of diabetic cardiomyopathy. C66 exerts a powerful antifibrotic effect by reducing inflammation-related factors (MCP-1, NF-κB, TNF-α, IL-1β, COX-2, and CAV-1) and inducing the expression of anti-inflammatory factors (HO-1 and NEDD4), as well as targeting TGF-β/SMADs, MAPK/ERK, and PPAR-γ pathways in animal models of diabetic nephropathy. Based on the available evidence, C66 is becoming a promising drug candidate for improving cardiovascular and renal health.

VIP/PACAP signaling as an alternative target during hyperoxic exposure in preterm newborns

The use of oxygen therapy (high doses of oxygen - hyperoxia) in the treatment of premature infants results in their survival. However, it also results in a high incidence of chronic lung disease known as bronchopulmonary dysplasia, a disease in which airway hyper-responsiveness and pulmonary hypertension are well known as consequences. In our previous studies, we have shown that hyperoxia causes airway hyper-reactivity, characterized by an increased constrictive and impaired airway smooth muscle relaxation due to a reduced release of relaxant molecules such as nitric oxide, measured under in vivo and in vitro conditions (extra- and intrapulmonary) airways. In addition, the relaxation pathway of the vasoactive intestinal peptide (VIP) and/or pituitary adenylate cyclase activating peptide (PACAP) is another part of this system that plays an important role in the airway caliber. Peptide, which activates VIP cyclase and pituitary adenylate cyclase, has prolonged airway smooth muscle activity. It has long been known that VIP inhibits airway smooth muscle cell proliferation in a mouse model of asthma, but there is no data about its role in the regulation of airway and tracheal smooth muscle contractility during hyperoxic exposure of preterm newborns.

Cardiomyocytes' prolonged IL-2 incubation induces enhancement in L-type Ca2+ channels mediated by inhibitory-kappaB kinase/nuclear factor-kappaB signalling

The main objective of this study was to determine the primary intracellular signalling pathway affected by prolonged (2 hours) incubation in interleukin-2 (IL-2). Based on the inflammatory nature of IL-2, priority was given to the involvement of inhibitory-kappaB kinase/nuclear factor-kappaB (IKK/NF-κB) signalling. All of the experiments were performed on freshly prepared cardiomyocytes isolated from rat left ventricles. After isolation, the whole-cell voltage-clamp recordings were performed on single cells. After 2 hours of incubation in IL-2, the current at 0 mV was approximately 100% higher than at the start of the incubation. ACHP, a highly specific kinase β inhibitor, in a concentration of 10 nmol/L, caused significant reduction in the ICa,L . IL-2 (2 ng/mL) in the presence of 0.1 μmol/L IMD-0354 as a specific inhibitor of IKKβ, caused nearly no changes in the ICa,L . IL-2 (3 ng/mL) induced a significant increase in phosphorylated NF-κB p65. The cardiomyocytes incubated in a Kraftbrühe solution containing IL-2 plus PDTC as a specific inhibitor of inducible nitric oxide synthase (iNOS) for 2 hours had a similar ICa,L increase compared to the cells incubated only in IL-2. IL-2-induced enhancement in L-type Ca2+ channels was mediated by IKK/NF-κB signalling, but not via iNOS-mRNA signalling.

Curcumin analogs (B2BrBC and C66) supplementation attenuates airway hyperreactivity and promote airway relaxation in neonatal rats exposed to hyperoxia

BACKGROUND

This study was undertaken to test the hypothesis that the newly synthesized curcuminoids B2BrBC and C66 supplementation will overcome hyperoxia-induced tracheal hyperreactivity and impairment of relaxation of tracheal smooth muscle (TSM).

MATERIALS AND METHODS

Rat pups (P5) were exposed to hyperoxia (>95% O2 ) or normoxia for 7 days. At P12, tracheal cylinders were used to study in vitro contractile responses induced by methacholine (10-8 -10-4 M) or relaxation induced by electrical field stimulation (5-60 V) in the presence/absence of B2BrBC or C66, or to study the direct relaxant effects elicited by both analogs.

RESULTS

Hyperoxia significantly increased contraction and decreased relaxation of TSM compared to normoxia controls. Presence of B2BrBC or C66 normalized both contractile and relaxant responses altered by hyperoxia. Both, curcuminoids directly induced dose-dependent relaxation of preconstricted TSM. Supplementation of hyperoxic animals with B2BrBC or C66, significantly increased catalase activity. Lung TNF-α was significantly increased in hyperoxia-exposed animals. Both curcumin analogs attenuated increases in TNF-α in hyperoxic animals.

CONCLUSION

We show that B2BrBC and C66 provide protection against adverse contractility and relaxant effect of hyperoxia on TSM, and whole lung inflammation. Both analogs induced direct relaxation of TSM. Through restoration of catalase activity in hyperoxia, we speculate that analogs are protective against hyperoxia-induced tracheal hyperreactivity by augmenting H2 O2 catabolism. Neonatal hyperoxia induces increased tracheal contractility, attenuates tracheal relaxation, diminishes lung antioxidant capacity, and increases lung inflammation, while monocarbonyl CUR analogs were protective of these adverse effects of hyperoxia. Analogs may be promising new therapies for neonatal hyperoxic airway and lung disease.

Association between IL-18/18R gene polymorphisms and coronary artery disease: influence of IL-18/18R genetic variants on cytokine expression

Purpose

The present study investigated the influence of IL-18/18R genetic variants on cytokine expression in patients with stable coronary artery disease (CAD).

Materials and methods

The polymorphisms rs1946518, rs187238, rs326, rs1169288, and rs183130 were determined in patients with and without CAD. Circulating cytokine levels were measured immunologically.

Results

The rs1946518-GG genotype shows higher IL-18 concentration in the group with CAD, but still not significant. The TG genotype from rs1946518 in carriers with CAD showed a significant decrease in relation to the pro-inflammatory cytokines IL-6, IL-8, and IL-18. The decreases of IL-6 and IL-8 were also specific for rs187238 CAD carriers with the GC genotype. The CAD carriers with the AA genotype from rs326 in the IL-18R gene showed significant increase in IL-8 and IL-18 in comparison with those without CAD. Regarding rs1169288 from the IL-18R gene, IL-8 showed a T allele-dependent increase. In the last rs183130 polymorphism of the IL-18R gene, the pro-inflammatory onset showed a C allele-dependent disease-associated decrease in IL-8 CC and IL-6 CT carriers. In contrast, the CAD CT carriers in relation to IL-8 showed significant increase.

Conclusions

Most of the IL-18/18R single-nucleotide polymorphisms were mainly associated with pro-inflammatory cytokines. It is surmised that these associations between some pro-inflammatory cytokines (mainly IL-8) and some IL-18R genotypes in the subjects with CAD from this study are most likely based on inflammatory-induced upregulation of IL-18R expression.

Association between interleukin-6/6R gene polymorphisms and coronary artery disease in Russian population: influence of interleukin-6/6R gene polymorphisms on inflammatory markers

This study determined the genotype effects of interleukin (IL)-6/IL-6R single nucleotide polymorphisms (SNPs) on circulating levels of different cytokines in healthy and coronary artery disease (CAD) patients with different allele frequencies. In the control patients, rs1800795 showed significant differences in IL-18 concentrations between CC and CG and CC and GG genotypes (P=0.003 and 0.004, respectively). Furthermore, circulatory IL-1β was significantly different between GC and GG genotypes from the same SNP (P=0.038). In the diseased patients, significance was determined only for IL-2 (P=0.021) between the C and G homozygote allele carriers of rs1800795. The diseased GC and GG genotype carriers were statistically different for IL-2 (P=0.049) from the rs1800796 and for IL-4 (P=0.049) from the rs2228044. IL-4 was also statistically significant between the GC and CC genotypes from the rs2228043 of the IL-6R gene (P=0.025). The last combination of genotypes in the same gene for the same SNP was statistically significant for IL-10 (P=0.036). According to the logistic regression, only gender (odds ratio [OR] =2.43) and triglycerides (OR =1.98) could be taken as determinants of CAD, while examined SNPs genotypes were not identified as risk factors for CAD. In general, the IL-6 polymorphism genotypes were mainly associated with inflammatory cytokines, while the IL-6R polymorphism genotypes were associated with anti-inflammatory cytokines.

Diadenosine pentaphosphate affects electrical activity in guinea pig atrium via activation of potassium acetylcholine-dependent inward rectifier

Diadenosine pentaphosphate (Ap5A) belongs to the family of diadenosine polyphosphates, endogenously produced compounds that affect vascular tone and cardiac performance when released from platelets. The previous findings indicate that Ap5A shortens action potentials (APs) in rat myocardium via activation of purine P2 receptors. The present study demonstrates alternative mechanism of Ap5A electrophysiological effects found in guinea pig myocardium. Ap5A (10-4 M) shortens APs in guinea pig working atrial myocardium and slows down pacemaker activity in the sinoatrial node. P1 receptors antagonist DPCPX (10-7 M) or selective GIRK channels blocker tertiapin (10-6 M) completely abolished all Ap5A effects, while P2 blocker PPADS (10-4 M) was ineffective. Patch-clamp experiments revealed potassium inward rectifier current activated by Ap5A in guinea pig atrial myocytes. The current was abolished by DPCPX or tertiapin and therefore was considered as potassium acetylcholine-dependent inward rectifier (I KACh). Thus, unlike rat, in guinea pig atrium Ap5A produces activation of P1 receptors and subsequent opening of KACh channels leading to negative effects on cardiac electrical activity.

Maximum heart rate in brown trout (Salmo trutta fario) is not limited by firing rate of pacemaker cells

Temperature-induced changes in cardiac output (Q̇) in fish are largely dependent on thermal modulation of heart rate ( fH), and at high temperatures Q̇ collapses due to heat-dependent depression of fH. This study tests the hypothesis that firing rate of sinoatrial pacemaker cells sets the upper thermal limit of fH in vivo. To this end, temperature dependence of action potential (AP) frequency of enzymatically isolated pacemaker cells (pacemaker rate, fPM), spontaneous beating rate of isolated sinoatrial preparations ( fSA), and in vivo fH of the cold-acclimated (4°C) brown trout ( Salmo trutta fario) were compared under acute thermal challenges. With rising temperature, fPM steadily increased because of the acceleration of diastolic depolarization and shortening of AP duration up to the break point temperature (TBP) of 24.0 ± 0.37°C, at which point the electrical activity abruptly ceased. The maximum fPM at TBP was much higher [193 ± 21.0 beats per minute (bpm)] than the peak fSA (94.3 ± 6.0 bpm at 24.1°C) or peak fH (76.7 ± 2.4 at 15.7 ± 0.82°C) ( P < 0.05). These findings strongly suggest that the frequency generator of the sinoatrial pacemaker cells does not limit fH at high temperatures in the brown trout in vivo.

Serum interleukin-6: Association with circulating cytokine serum levels in patients with sinus arrhythmia and patients with coronary artery disease

In this study, we were focused on the differences between certain circulating cytokine levels in patients with or without sinus arrhythmia, according to the median IL-6 level. All patients were stable with regards to symptoms and therapy for at least one month prior to the measurements conducted within this study.Exclusion criteria were: patients with sleep apnea, asthma, respiratory insufficiency of any genesis, active infection, allergy, inflammatory diseases, cancer, diabetes of any type and treatment with anti-inflammatory drugs. The study was approved by the Institutional Review Board. All recruited patients gave their verbal and written consent for participation in the study. The study group consisted of 74 patients divided into two groups: with (38) and without sinus arrhythmia but with diagnosed coronary artery disease (36). Sinus arrhythmia was confirmed by 24h Holter monitoring. From all test parameters only cytokines IL-2, IL-8, IL-10, IL-17 and IL-18, showed statistically significant increasing in patients with statistically higher IL-6 levels. It is possible that IL-6 may not be a marker for the selection of patients with sinus arrhythmia or coronary artery disease. The findings indicate that IL-6 represents a reliable indicator for increased expression of IL-2, IL-8, IL-10, IL-17 and IL-18 in patients with sinus arrhythmia or coronary artery disease. Further studies in a large number of patients would be necessary to confirm our observations.

Influence of mechanical stress on fibroblast-myocyte interactions in mammalian heart

Cardiac fibroblasts are an essential component of cardiac tissue. These cells not only produce the extracellular matrix, but also are electrically and mechanically coupled with cardiomyocytes. In this way, fibroblasts can influence the electrical activity of cardiomyocytes. Cardiac fibroblasts cannot generate action potentials, but their membrane potential is controlled by mechanical stretch or compression of the surrounding myocardium which in turn affects their interaction with myocytes and the way myocytes respond to mechanical stress. This review discusses the electrical properties of cardiac fibroblasts, the present evidence of fibroblast-myocyte coupling and the way in which these cells respond to mechanical stress. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

Single mechano-gated channels activated by mechanical deformation of acutely isolated cardiac fibroblasts from rats

AIM

Mechanosensitive conductances were reported in cardiac fibroblasts, but the properties of single channels mediating their mechanosensitivity remain uncharacterized. The aim of this work was to investigate single mechano-gated channels (MGCs) activated by mechanical deformations of cardiac fibroblasts.

METHODS

Currents through single MGCs and mechanosensitive whole-cell currents were recorded from isolated rat atrial fibroblasts using the cell-attached and whole-cell patch-clamp configurations respectively. Defined mechanical stress was applied via the patch pipette used for the whole-cell recordings.

RESULTS

Under resting conditions occasional short openings of two types of single MGCs with conductances of 43 and 87 pS were observed. Both types of channels displayed a linear current-voltage relationship with the reversal potential around 0 mV. Small (1 microm) mechanical deformations affected neither single nor whole-cell mechano-gated currents. Cell compressions (2, 3 and 4 microm) augmented the whole-cell currents and increased the frequency and duration of single channel openings. Cell stretches (2, 3 and 4 microm) inactivated the whole-cell currents and abolished the activity of single MGCs. Gd(3+) (8 microm) blocked the whole-cell currents within 5 min. No single channel activity was observed in the cell-attached mode when Gd(3+) was added to the intrapipette solution. Cytochalasin D and colchicine (100 microm each) completely blocked both the whole-cell and single channel currents.

CONCLUSIONS

These findings show that rat atrial fibroblasts express two types of MGCs whose activity is governed by cell deformation. We conclude that fibroblasts can sense the direction of applied stress and contribute to mechano-electrical coupling in the heart.

Enhanced L-type calcium currents in cardiomyocytes from transgenic rats overexpressing SERCA2a

BACKGROUND

Previous research reported that transgenic rats overexpressing the sarco(endo)plasmic reticulum Ca(2+)-ATPase SERCA2a exhibit improved contractile function of the myocardium. Furthermore, impaired Ca(2+) uptake and reduced relaxation rates in rats with diabetic cardiomyopathy were partially rescued by transgenic expression of SERCA2a in the heart.

OBJECTIVE

To explore whether enhanced Ca(2+) cycling in the cardiomyocytes of SERCA2a transgenic rats is associated with changes in L-type Ca(2+) (I(Ca-L)) currents.

METHODS

The patch-clamp technique was used to measure whole-cell currents in cardiomyocytes from transgenic rats overexpressing SERCA2a and from wild-type (nontransgenic) animals.

RESULTS

The amplitudes of I(Ca-L) currents at depolarizing pulses ranging from -45 mV to 0 mV (350 ms duration, 1 Hz) were significantly higher in cardiomyocytes of SERCA2a transgenic rats than in nontransgenic rats (1985±48 pA [n=32] versus 1612±55 pA [n=28], respectively). The inactivation kinetics of I(Ca-L) showed subtle differences with increased tau fast and tau slow decay constants in cardiomyocytes of SERCA2a transgenic animals. Beta-adrenergic stimulation with 50 nM isoproterenol reduced tau fast and tau slow decay constants in cardiomyocytes of transgenic rats to values that were not significantly different from those in normal cardiomyocytes. Furthermore, isoproterenol enhanced I(Ca-L) currents 3.2-fold and 2.3-fold in cardiomyocytes with and without the SERCA2a transgene, respectively, and this effect was abolished by buffering intracellular Ca(2+) with BAPTA.

CONCLUSIONS

These findings indicate that enhanced Ca(2+) cycling in the hearts of SERCA2a transgenic rats, both under normal conditions and during beta-adrenergic stimulation, involves changes in I(Ca-L) currents. Modified I(Ca-L) kinetics may contribute, to some extent, to the improved contractile function of the myocardium of transgenic rats.

Electrical interaction of mechanosensitive fibroblasts and myocytes in the heart

Fibroblasts in the heart can respond to mechanical deformation of the plasma membrane with characteristic changes of their membrane potential. Membrane depolarization of the fibroblasts occurs during the myocardial contractions and is caused by an influx of cations, mainly of sodium ions, into the cells. Conversely, application of mechanical stretch to the cells, i.e., during diastolic relaxation of the myocardium, will hyperpolarize the membrane potential of the fibroblasts due to reduced sodium entry. Thus, cardiac fibroblasts can function as mechano-electric transducers that are possibly involved in the mechano-electric feedback mechanism of the heart. Mechano-electric feedback refers to the phenomenon, that the cardiac mechanical environment, which depends on the variable filling pressure of the ventricles, modulates the electrical function of the heart. Increased sensitivity of the cardiac fibroblasts to mechanical forces may contribute to the electrical instability and arrhythmic disposition of the heart after myocardial infarction. Novel findings indicate that these processes involve the intercellular transfer of electrical signals between fibroblasts and cardiomyocytes via gap junctions. In this article we will discuss the recent progress in the electrophysiology of cardiac fibroblasts. The main focus will be on the intercellular pathways through which fibroblasts and cardiomyocytes communicate with each other.

Characterization of stretch-activated ion currents in isolated atrial myocytes from human hearts

To explore further the mechanisms that may underlie cardiac arrhythmia, we analysed stretch-activated ion currents in human atrial myocytes. Longitudinal stretch of freshly isolated atrial myocytes prolonged the duration of action potentials, depolarized the resting membrane potential and caused extra action potentials. Under voltage-clamp conditions, the amplitude of stretch-induced transmembrane currents increased reversibly with the intensity of stretch. Stretch-activated currents ( I(SAC)) had a reversal potential of 0 mV and were insensitive to substitution of Cl(-) with aspartate ions in the extracellular fluid. I(SAC) was suppressed by 5 micro M gadolinium (Gd(3+)). Furthermore, mechanical stretch decreased transmembrane ion fluxes through L-type calcium channels (I(Ca,L)). This reduction of I(Ca,L) was inhibited by dialysing the cells for 5 min with 5 mM BAPTA prior to application of stretch. In contrast, both BAPTA and removal of Ca(2+) from the extracellular bathing solution had no significant effect on stretch activation of I(SAC). These findings suggest that non-selective cation channels in human atrial myocytes are sensitive to mechanical stimulation. We propose that activation of transmembrane influx of cations, preferentially Na(+), by local stretch may play a role in cardiac arrhythmia.

Differential effects of stretch and compression on membrane currents and [Na+]c in ventricular myocytes

Mechano-electrical feedback was studied in the single ventricular myocytes. A small fraction (approximately 10%) of the cell surface could be stretched or compressed by a glass stylus. Stretch depolarised, shortened the action potential and induced extra systoles. Stretch activated non-selective cation currents (I(ns)) showed a linear voltage dependence, a reversal potential of 0 mV, a pure cation selectivity, and were blocked by 8 microM Gd(3+) or 30 microM streptomycin. Stretch reduced Ca(2+) and K(+) (I(K)) currents. Local compression of broadwise attached cells activated I(K) but not I(ns). Cytochalasin D or colchicin, thought to disrupt the cytoskeleton, suppressed the mechanosensitivity of I(ns) and I(K). During stretch, the cytosolic sodium concentration increased with spatial heterogeneities, local hotspots with [Na(+)](c)>24 mM appeared close to surface membrane and t-tubules (pseudoratiometric imaging using Sodium Green fluorescence). Electronprobe microanalysis confirmed this result and indicated that stretch increased total sodium [Na] in cell compartments such as mitochondria, nuclear envelope and nucleus. Our results obtained by local stretch differ from those obtained by end-to-end stretch (literature). We speculate that channels may be activated not only by axial but also by shear stress, and, that stretch can activate channels outside the deformed sarcomeres via second messenger.

Cardiac fibroblasts and the mechano-electric feedback mechanism in healthy and diseased hearts

Cardiac arrhythmia is a serious clinical condition, which is frequently associated with abnormalities of mechanical loading and changes in wall tension of the heart. Recent novel findings suggest that fibroblasts may function as mechano-electric transducers in healthy and diseased hearts. Cardiac fibroblasts are electrically non-excitable cells that respond to spontaneous contractions of the myocardium with rhythmical changes of their resting membrane potential. This phenomenon is referred to as mechanically induced potential (MIP) and has been implicated in the mechano-electric feedback mechanism of the heart. Mechano-electric feedback is thought to adjust the frequency of spontaneous myocardial contractions to changes in wall tension, which may result from variable filling pressure. Electrophysiological recordings of single atrial fibroblasts indicate that mechanical compression of the cells may activate a non-selective cation conductance leading to depolarisation of the membrane potential. Reduced amplitudes of MIPs due to pharmacological disruption of F-actin and tubulin suggest a role for the cytoskeleton in the mechano-electric signal transduction process. Enhanced sensitivity of the membrane potential of the fibroblasts to mechanical stretch after myocardial infarction correlates with depression of heart rates. It is assumed that altered electrical function of cardiac fibroblasts may contribute to the increased risk of post-infarct arrhythmia.

Ion selectivity of stretch-activated cation currents in mouse ventricular myocytes

Stretch-activated non-selective cation currents ( I(SAC)) constitute a mechanism that can induce cardiac arrhythmias. We studied I(SAC) in mouse ventricular myocytes by stretching part of the cell surface between the patch-pipette and a motor-driven glass stylus. In non-clamped cells, local stretch depolarised and induced after-depolarisations and extrasystoles. In voltage-clamped cells (K(+) currents suppressed) I(SAC) activated by local stretch had a nearly linear voltage dependence and reversed polarity between -12 and 0 mV. Conductance G(SAC) increased with the extent of local stretch. I(SAC) was not a Cl(-) current (insensitivity to replacement of Cl(-) by aspartate(-)). I(SAC) was not a Ca(2+)-activated current (insensitivity to 5 mM intracellular BAPTA). G(SAC) was blocked by 5 micro M GdCl(3) or by 75 mM extracellular (e.c.) CaCl(2). Removal of e.c. CaCl(2) increased G(SAC) 2.5-fold, as if G(SAC) were sensitive to Ca(2+) and Gd(3+). Replacement of 150 mM e.c. Na(+) by 150 mM Cs(+), Li(+), tetraethylammonium (TEA(+)) or N-methyl d-glucosamine (NMDG(+)) yielded currents that suggested for the conductance a selectivity G(Cs)> G(Na)> G(Li)> G(TEA)> G(NMDG). I(SAC) was suppressed by cytochalasin D, as if an intact F-actin cytoskeleton were necessary for activation of I(SAC).

Mechanically induced potentials in atrial fibroblasts from rat hearts are sensitive to hypoxia/reoxygenation

Membrane potential changes of atrial fibroblasts in response to mechanical stress have been considered to modulate the rhythmic electrical activity of healthy hearts. Our recent findings suggest that cardiac arrhythmia after infarction is related to enhanced susceptibility of the fibroblasts to physical stretch. In this study, we analysed the effect of hypoxia/reoxygenation, which are major components of tissue ischemia/reperfusion, on the membrane potential of atrial fibroblasts. Intracellular microelectrode recordings were performed together with isometric force measurements on isometrically contracting right atrial tissue preparations from adult rats. Lowering the oxygen tension in the perfusate from 80 kPa to 3.5 kPa reduced active force development and decreased the resting membrane potential of the cardiac fibroblasts from -23+/-5 mV to -5+/-2 mV ( n=35). Application of gadolinium (40 microM) to inhibit non-selective cation channels prevented hypoxia-induced membrane depolarization of the fibroblasts. Reoxygenation of the myocardial tissue resulted in a transient increase of the resting membrane potential to maximally -60+/-8 mV. These findings indicate that transmembrane currents in atrial fibroblasts are sensitive to changes in tissue oxygenation. In conclusion, altered electro-mechanical function of the ischemic heart may possibly involve changes of the membrane potential of the cardiac fibroblasts.

Activation and inactivation of a non-selective cation conductance by local mechanical deformation of acutely isolated cardiac fibroblasts

OBJECTIVE

We describe mechanically induced non-selective cation currents in isolated rat atrial fibroblasts, which might play a role as a substrate for mechano-electrical feedback in the heart.

METHODS

Isolated fibroblasts were used for voltage-clamp analysis of ionic currents generating mechanically-induced potentials. Fibroblasts were mechanically deformed (compressed or stretched) by two patch-pipettes.

RESULTS

These cells had a resting potential (E(0)) of -37+/-3 mV and an input resistance of 514+/-11 M(Omega). At intracellular pCa 7 (patch-pipette solution), compression of 2 or 3 microm shifted E(0) from -36+/-7 to -17+/-3 mV, and to -10+/-2 mV. Compression by 2 or 3 microm induced a negative difference current (at -45 mV -0.06+/-0.02 and -0.20+/-0.04 nA, respectively) with a reversal potential (E(rev)) of approx. 0 mV. The currents were carried by Na(+), K(+) and Cs(+) ions, and were blocked by application of 8 microM Gd(3+). Stretch of 2 or 3 microm hyperpolarized E(0) from -34+/-4 to -45+/-5, and to -61+/-7 mV and induced a positive difference current (at -45 mV: 0.04+/-0.02 and 0.18+/-0.03 nA) with an E(rev) close to 0 mV. Application of Gd(3+) shifted E(0) to potentials as negative as E(K) (-90+/-4 mV). Cell dialysis with 5 mM BAPTA (pCa 8) or 5 mM Ca(2+)/EGTA (pCa 6) had no influence on non-selective cation currents suggesting that Ca(2+) dependent conductances are unlikely to contribute.

CONCLUSION

Compression of the isolated cardiac fibroblast caused depolarization of the membrane by activating inward currents through a non-selective cation conductance (G(ns)). Stretch hyperpolarizes the fibroblast, however, not by Ca(2+) activation of K(+)-conductance. Ion selectivity, E(rev,) and Gd(3+)-sensitivity of stretch suppressed currents suggest that stretch reduces G(ns) that is activated by compression.

A possible role for atrial fibroblasts in postinfarction bradycardia

Atrial fibroblasts are considered to modulate the contractile activity of the heart in response to mechanical stretch. In this study we examined whether atrial fibroblasts are possibly involved in bradyarrhythmia, which is a severe complication after myocardial infarction. For this purpose, transmembrane electrical potentials were recorded in cardiac fibroblasts near the sinoatrial node from sham-operated rats and from rats with myocardial infarction. Twenty days after infarction due to coronary artery ligation, the right atrial tissue weights and the sensitivity of the fibroblast membrane potential to mechanical stretch correlated positively with the infarct size. Cardiac growth was enhanced, but the stretch sensitivity and the resting membrane potential of the atrial fibroblasts declined between 8 and 30 days after infarction. The frequency of spontaneous atrial contractions was significantly reduced 8 days after myocardial infarction and recovered in parallel with the membrane potential of the fibroblasts. These findings suggest that changes in the susceptibility of atrial fibroblasts to mechanical stretch may contribute to bradyarrhythmia during postinfarct remodeling of the heart.

Inotropic response to beta-adrenergic receptor stimulation and anti-adrenergic effect of ACh in endothelial NO synthase-deficient mouse hearts

1. The functional consequences of a lack of endothelial nitric oxide synthase (eNOS) on left ventricular force development and the anti-adrenergic effect of acetylcholine (ACh) were investigated in isolated hearts and cardiomyocytes from wild type (WT) and eNOS knockout (eNOS-/-) mice. 2.eNOS expression in cardiac myocytes accounted for 20 % of total cardiac eNOS (Western blot analysis). These results were confirmed by RT-PCR analysis. 3. In the unstimulated perfused heart, the left ventricular pressure (LVP) and maximal rate of left ventricular force development (dP/dtmax) of eNOS-/- hearts were not significantly different from those of WT hearts (LVP: 97 +/- 11 mmHg WT vs. 111 +/- 11 mmHg eNOS-/-; dP/dtmax: 3700 +/- 712 mmHg s(-1) WT vs. 4493 +/- 320 mmHg s)-1) eNOS-/-). 4. The dobutamine (10-300 nM)-induced increase in LVP was enhanced in eNOS-/- hearts. In contrast, L-type Ca2+ currents (ICa,L) in isolated cardiomyocytes of WT and eNOS-/- hearts showed no differences after beta-adrenergic stimulation. Dibutyryl-cGMP (50 microM) reduced basal ICa,L in WT cells to 72 +/- 12 % while eNOS-/- ICa,L was insensitive to the drug. The pre-stimulated ICa,L (30 nM isoproterenol) was attenuated by dibutyryl-cGMP in WT and eNOS-/- cells to the same extent. 5. The Ca2+ (1.5-4.5 mM)-induced increase in inotropy was not different between the two experimental groups and beta-adrenergic receptor density was increased by 50% in eNOS-/- hearts. 6. The contractile effects of dobutamine could be inhibited almost completely by ACh or adenosine. The extent of the anti-adrenergic effect of both compounds was identical in WT and eNOS-/- hearts. Measurement of ICa,L in isolated cardiac myocytes yielded similar results. 7. These data demonstrate that in the adult mouse (1) lack of eNOS is associated with increased cardiac contractile force in response to beta-adrenergic stimulation and with elevated -adrenergic receptor density, (2) the unaltered response of ICa,L in eNOS-/- cardiac myocytes to beta-adrenergic stimulation suggests that endothelium-derived NO is important in mediating the whole-organ effects and (3) eNOS is unimportant for the anti-adrenergic effect of ACh and adenosine.

Mechanically induced potentials in rat atrial fibroblasts depend on actin and tubulin polymerisation

When atrial tissue contracts, mechanically induced potentials (MIPs) are generated in fibroblasts, presumably by activation of a non-selective cation conductance Gns. Non-stimulated atrial fibroblasts had a mean (+/-SD) membrane potential (Em) of -22 +/- 2 mV and an input resistance of 510 +/- 10 MS. MIP amplitude (AMIP) was 38+/-4 mV when current injection had polarised Em to Vm = -50 mV. The slope of the function relating AMIP to Vm can be regarded as a mechanosensitive factor (Xms) that describes the relative increase in Gns during a MIP. Putative involvement of cytoskeletal fibres in activation of Gns was studied by delivering drugs from the intracellular recording microelectrode. Destabilisation of F-actin by 0.2 mM cytochalasin D reduced AMIP from 38 to 16 mV and Xms from 5 to 1.8. Destabilisation of tubulin with 0.2 mM colchicine reduced AMIP to 21 mV and Xms to 2.1. The combination colchicine plus cytochalasin D reduced AMIP to 9 mV and Xms to 1.4. Promoting F-actin stability with exogenous adenosine 5'-triphosphate (ATP) increased AMIP and Xms and attenuated the effects of cytochalasin D. Similarly, facilitation of tubulin stability with guanosine 5'-triphosphate (GTP) or taxol increased AMIP and Xms and attenuated the effects of colchicine. The results suggest that transfer of mechanical energy from the deformed fibroblast surface to the Gns channel protein depends on intact F-actin and tubulin fibres.

Stretch-activated currents in ventricular myocytes: amplitude and arrhythmogenic effects increase with hypertrophy

BACKGROUND

Mechanical dilation of the human ventricle is known to induce arrhythmias, the underlying ionic mechanisms, however, remain to be clarified.

METHODS

Ventricular myocytes isolated from human, guinea-pig or rat hearts were stretched between the patch electrode and a glass stylus.

RESULTS

Local stretch prolonged the action potential, depolarized the resting membrane and caused extra systoles. Under voltage-clamp conditions, stretch activated several ionic current components. The most prominent current was a stretch activated current (I(SAC)) through non-selective cation channels. I(SAC) followed a linear voltage-dependence, reversed polarity close to 0 mV and was suppressed by 5 microM Gd(3+). During stretch, I(SAC) became steady within 200 ms. I(SAC) did not inactivate and it completely disappeared upon relaxation. Stretch-sensitivity was evaluated from the slope of I(SAC) versus amplitude of stretch. Stretch sensitivity was 75 pA/microm in myocytes from young (3 month), 143 pA/microm in myocytes from old (15 months), and 306 pA/microm in hypertrophied myocytes from old (15 months) spontaneously hypertensive animals. Stretch sensitivity was 262 pA/microm in hypertrophied myocytes from human failing hearts, and it was 143 pA/microm in guinea-pig ventricular myocytes.

CONCLUSIONS

Local stretch of adult single ventricular myocytes can induce arrhythmias that resemble surface-recordings from whole hearts. Stretch modulates multiple current components, I(SAC) being the current with the largest arrhythmogenic potential. Stretch-sensitivity of I(SAC) is higher in hypertrophied than in control myocytes as can be expected from the observation that hypertrophy and failure increase the risk of stretch-induced arrhythmias.

Mechano-electric feedback in right atrium after left ventricular infarction in rats

Left ventricular myocardial infarction (MI) can lead to alterations in hemodynamic load conditions, thereby inducing right atrial hypertrophy and dilatation associated with phenotypic modulation of cardiomyocytes, electrical abnormalities, rhythm disturbances, and atrial fibrillation. However, there is limited information on the electrophysiological basis for these events. We investigated whether atrial stretch in the setting of chronic MI modulates the electrophysiological properties of cardiomyocytes via "mechano-electric feedback", providing a mechanism for atrial arrhythmia after ventricular infarction. Five weeks after left ventricular MI (n=37), action potentials (AP) were measured in right atrial tissue preparations using a current clamp scheme, and compared to sham-operated rats (SO, n=10). Contractile activity was recorded at a preload of 1 mN, and sustained stretch was applied via a micrometer. In SO, stretch of 1.75 mN shortened repolarization at 50% and prolonged it at 90%. In MI, mechanically-induced electrical alterations were observed at a significantly lower level of stretch than in SO (0.19 mN). Sustained stretch in MI prolonged AP at 90% repolarization giving rise to stretch-activated depolarizations (SAD) near 90% repolarization (SAD90). When reaching threshold for premature APs, electrical phenomena similar to atrial fibrillations were seen in some preparations. Moreover, we observed APs with prolonged duration at 25%, 50%, and 90% repolarization where stretch induced SAD near 50%. Gadolinium used at a concentration to inhibit stretch-activated channels (40microM) suppressed mechanically-induced electrical events. In conclusion, increased susceptibility after MI to mechanical stretch may predispose atrial cardiomyocytes to arrhythmia. These mechano-electrical alterations are sensitive to gadolinium suggesting involvement of stretch-activated ion channels.

Effects of metoprolol and ramipril on action potentials after myocardial infarction in rats

The effects of chronic treatment with the beta-adrenoceptor antagonist metoprolol, the angiotensin converting enzyme inhibitor ramipril, their combination, or placebo on action potential configuration 6 weeks after myocardial infarction in rats were studied. Action potentials were measured in isolated left ventricular posterior papillary muscles and compared with action potentials from a sham operated group without infarction. After infarction, the action potential amplitude was reduced and this phenomenon was partially reversed by metoprolol- and ramipril-treatment. Prolonged repolarisation after infarction compared to sham operated animals was additionally delayed after metoprolol treatment. Thus, metoprolol extends the refractory period, which may counteract tachyarrhythmia.

Mechanoelectric feedback after left ventricular infarction in rats

BACKGROUND

Myocardial infarction can lead to electrical abnormalities and rhythm disturbances. However, there is limited data on the electrophysiological basis for these events. Since regional contraction abnormalities feature prominently in infarction, we investigated whether stretch of myocardium from the infarction borderzone can modulate the electrophysiological properties of cardiomyocytes via mechanoelectric feedback providing a mechanism for post-infarction arrhythmia.

METHODS

Five weeks after experimental myocardial infarction (MI) in rats due to ligation of the left coronary artery (n = 26) or after sham operation (SO, n = 16), action potentials (AP) were measured in left ventricular preparations from the infarction borderzone. Sustained stretch was applied via a micrometer.

RESULTS

Preparations from MI generated spontaneous electrical and contractile activity. Cardiomyocytes from MI had a comparable AP amplitude, a more negative resting membrane potential, and a prolonged AP duration (APD) when compared to SO. In SO, stretch of 150 microns increased the APD90. This was associated with stretch activated depolarizations near APD90 (SAD-90). In MI, significantly lower stretch, of only 20 microns, elicited SAD-90s, or SADs near APD50 (SAD-50). Stretch-induced events were suppressed by gadolinium, at a concentration (40 microM) normally used to inhibit stretch-activated channels.

CONCLUSION

After MI, SADs are generated in the infarction borderzone at lower degrees of stretch. Increased sensitivity of the membrane potential of cardiac myocytes to mechanical stimuli may contribute to the high risk of arrhythmia after infarction. These SADs may involve the opening of stretch-activated channels.

Mechanically induced potentials in fibroblasts from human right atrium

It has been shown that cardiac fibroblasts of the human heart are electrically non-excitable and mechanosensitive. The resting membrane potential of these cells is -15.9+/-2.1 mV and the membrane resistance is 4.1+/-0.1 G[Omega]. Rhythmic contractions of the myocardium associated with stretch of the surrounding tissue produce reversible changes in the membrane potential of cardiac fibroblasts. These mechanically induced potentials (MIPs) follow the rhythm of myocardial contractions. Simultaneous recording of the action potential of cardiomyocytes and MIPs of cardiac fibroblasts demonstrates a delay of 40.0+/-0.4 ms after the action potential before the appearance of the MIP. Contraction produces a MIP which is more positive or more negative than the reversal potential - the membrane potential due to current injection at which the MIP reverses its direction. Regardless of the initial orientation of the MIP, intracellular polarization increases the amplitude towards the reversal potential if the background MIP had depolarized the membrane or away from the reversal potential if the initial background MIP had hyperpolarized the membrane. Artificial intracellular polarization changed the amplitude but not the frequency of the MIP. The pool of electrically non-excitable mechanosensitive cells, which change their electrical activity during contraction and relaxation of the heart, may play a role in the mechano-electrical feedback mechanism which has to be taken into account in the normal function of the heart as well as in pathological processes.

Electrophysiological properties of mechanosensitive atrial fibroblasts from chronic infarcted rat heart

Electrically non-excitable but mechanosensitive right-atrial fibroblasts are thought to be involved in the chronotropic response of the heart to stretch. After myocardial infarction, altered chronotropic response may be due to the remodeling process which also involves the right atrium. Remodeling is associated with the development of hypertrophy of cardiomyocytes and proliferation of fibroblasts. Electrical properties of atrial mechanosensitive fibroblasts from chronic infarcted hearts and their possible role for altered chronotropic response has not, to our knowledge, been studied until now. Thus, resting membrane potential as well as mechanically induced potential of fibroblasts, action potential (AP) of cardiomyocytes, spontaneous frequency and mechanical activity of preparations from the sinus node region were studied 10 weeks after myocardial infarction induced by ligation of the left coronary artery in rats. The prolongation of AP repolarization (increases in APD50 and APD90) correlated closely to the infarct size (IS) and the degree of hypertrophy, respectively. Along with increasing IS, membrane potentials of fibroblasts were shifted to more negative values, with a peak of frequency distribution at -15 mV (control and very small IS), at -35 mV (intermediate IS), and -55 mV (large IS), and spontaneous electrical activity was decreased. Membrane resistance of fibroblasts also correlated to IS and was eight-fold greater at large IS than in control. We hypothesize that, in the infarcted heart, increased membrane potential and membrane resistance of fibroblasts may alter electrical activity of neighbouring myocytes in the sinus-venosus region via intercellular electrical coupling.

Calcium and mechanically induced potentials in fibroblasts of rat atrium

OBJECTIVES

Electrically non-excitable cardiac fibroblasts in the sino-atrial node region are mechano-sensitive. Rhythmic contraction of adjacent myocardium, or artificial stretch of the tissue, produce a reversible change in the membrane potential: mechanically induced potentials (MIP). Stretch of normal cardiomyocytes can be associated with intracellular calcium changes. The purpose of this study is to use pharmacological interventions to investigate the possibility that stretch-induced Ca2+ entry through ion channels in the sarcolemma and Ca2+ release from internal stores play a role in MIP generation.

METHODS

Isolated spontaneously contracting or artificially stretched preparations of right atrium of rat heart were superfused with physiological solutions. An intracellular floating microelectrode recorded fibroblast MIPs and was also used for injection of current. A dye, Lucifer yellow, applied through the micropipette, identified recording sites. We assessed the role of extracellular Ca2+ using EGTA in the bathing solution. For the role of intracellular Ca2+ in the generation of MIP, several substances that influence [Ca2+]i handling were applied intracellularly by diffusion from the recording microelectrode. These include: BAPTA (to chelate intracellular Ca2+); BHQ, thapsigargin and CPA (to deplete Ca2+ from intracellular stores by inhibition of the endoplasmic reticulum (ER) ATP Ca2+ pump), and caffeine and ryanodine (to induce ER Ca2+ release).

RESULTS

All the pharmacological compounds which were introduced intracellulary, and EGTA applied extracellularly, decreased the amplitude of the MIP to variable degrees. Only thapsigargin induced a bi-phasic response with an initial increase in MIP amplitude, followed by a decrease. MIP duration was reduced by most interventions, exceptions being low extracellular Ca2+, BHQ and ryanodine. Short duration extracellular application of caffeine, which was added to the perfusate as a secondary contractile stimulus, partly restored the MIPs by activation of cardiac contraction. Intracellular current injection, before any intervention, linearly altered both membrane potential (Em) and MIP amplitude (Vm). Application of compounds listed above introduced non-linearity to the Em/Vm relationship.

CONCLUSION

We suggest that mechanically induced Ca2+ influx, induced through stretch-activated channels in the plasma membrane, and release of Ca2+ from the endoplasmic reticulum, play key roles in the mechanism of MIP generation. Further, our results demonstrate the existence of functional ryanodine/caffeine-sensitive Ca2+ stores in cardiac fibroblasts.