Stretch-activated Currents Research Articles

Mechanisms of conduction slowing during myocardial stretch by ventricular volume loading in the rabbit

Acute ventricular loading by volume inflation reversibly slows epicardial electrical conduction, but the underlying mechanism remains unclear. This study investigated the potential contributions of stretch-activated currents, alterations in resting membrane potential, or changes in intercellular resistance and membrane capacitance. Conduction velocity was assessed using optical mapping of isolated rabbit hearts at end-diastolic pressures of 0 and 30 mmHg. The addition of 50 microM Gd3+ (a stretch-activated channel blocker) to the perfusate had no effect on slowing. The effect of volume loading on conduction velocity was independent of changes in resting membrane potential created by altering the perfusate potassium concentration between 1.5 and 8 mM. Bidomain model analysis of optically recorded membrane potential responses to a unipolar stimulus suggested that the cross-fiber space constant and membrane capacitance both increased with loading (21%, P = 0.006, and 56%, P = 0.004, respectively), and these changes, when implemented in a resistively coupled one-dimensional network model, were consistent with the observed slowing (14%, P = 0.005). In conclusion, conduction slowing during ventricular volume loading is not attributable to stretch-activated currents or altered resting membrane potential, but a reduction of intercellular resistance with a concurrent increase of effective membrane capacitance results in a net slowing of conduction.

Nav Channel Mechanosensitivity: Activation and Inactivation Accelerate Reversibly with Stretch

Voltage-gated sodium channels (Nav) are modulated by many bilayer mechanical amphiphiles, but whether, like other voltage-gated channels (Kv, HCN, Cav), they respond to physical bilayer deformations is unknown. We expressed human heart Nav1.5 pore α-subunit in oocytes (where, unlike αNav1.4, αNav1.5 exhibits normal kinetics) and measured small macroscopic currents in cell-attached patches. Pipette pressure was used to reversibly stretch the membrane for comparison of I Na( t) before, during, and after stretch. At all voltages, and in a dose-dependent fashion, stretch accelerated the I Na( t) time course. The sign of membrane curvature was not relevant. Typical stretch stimuli reversibly accelerated both activation and inactivation by ∼1.4-fold; normalization of peak I Na( t) followed by temporal scaling (∼1.30- to 1.85-fold) resulted in full overlap of the stretch/no-stretch traces. Evidently the rate-limiting outward voltage sensor motion in the Nav1.5 activation path (as in Kv1) accelerated with stretch. Stretch-accelerated inactivation occurred even with activation saturated, so an independently stretch-modulated inactivation transition is also a possibility. Since Nav1.5 channel-stretch modulation was both reliable and reversible, and required stretch stimuli no more intense than what typically activates putative mechanotransducer channels (e.g., stretch-activated TRPC1-based currents), Nav channels join the ranks of putative mechanotransducers. It is noteworthy that at voltages near the activation threshold, moderate stretch increased the peak I Na amplitude ∼1.5-fold. It will be important to determine whether stretch-modulated Nav current contributes to cardiac arrhythmias, to mechanosensory responses in interstitial cells of Cajal, to touch receptor responses, and to neuropathic (i.e., hypermechanosensitive) and/or normal pain reception.

Starring TREK-1

See related article, pages 176–184 “We are more alike than unlike, my dear Captain. I have pores, humans have pores.” Lieutenant Commander Data, Stardate 41209.2 Before 1996, all known mammalian K+ channels were classified into only two different structural families according to the number of transmembrane (TM) spanning and pore-forming (P) domains in their α subunit. One family is characterized by K+ channels composed of two TM domains and one P domain, and includes the inwardly rectifying and ATP-sensitive K+ channels. The second family represents K+ channels characterized by six or seven TM domains and one P domain, and includes the voltage-gated and Ca2+-activated K+ channels. To form one functional channel for either of these families, four α subunits assemble to establish one K+ permeable pore. In the mid 1990s, researchers took advantage of the fact that the P domain of each K+ channel’s pore-forming α subunit is highly conserved across species and represents a common structural motif.1 Genome searches for DNA sequences coding for the P domain revealed a unique K+ channel in yeast and Caenorhabditis elegans that contained two P domains within a single α subunit polypeptide having eight potential TM domains.2 The following year a human K+ channel was cloned that also showed the unique feature of two P domains, but displayed four TM domains in a single α subunit (Figure 1).3 The channel was given the name TWIK-1 ( T andem of P domains in a W eak I nward rectifying K + channel). Since the cloning of TWIK-1, a total of fifteen …

Mechanosensitive channel activity and F‐actin organization in cholesterol‐depleted human leukaemia cells

This study focuses on the functional role of cellular cholesterol in the regulation of mechanosensitive cation channels activated by stretch in human leukaemia K562 cells. The patch-clamp method was employed to examine the effect of methyl-beta-cyclodextrin (MbetaCD), a synthetic cholesterol-sequestering agent, on stretch-activated single currents. We found that cholesterol-depleting treatment with MbetaCD resulted in a suppression of the activity of mechanosensitive channels without a change in the unitary conductance. The probability that the channel was open significantly decreased after treatment with MbetaCD. Fluorescent microscopy revealed F-actin reorganization, possibly involving actin assembly, after incubation of the cells with MbetaCD. We suggest that suppression of mechanosensitive channel activation in cholesterol-depleted leukaemia cells is due to F-actin rearrangement, presumably induced by lipid raft destruction. Our observations are consistent with the notion that stretch-activated cation channels in eukaryotic cells are regulated by the membrane-cytoskeleton complex rather than by tension developed purely in the lipid bilayer.

Coupling a change in intraluminal pressure to vascular smooth muscle depolarization: still stretching for an explanation

pressure-induced (myogenic) constriction of arterioles is believed to be a fundamental contributor to the local regulation of microvascular blood flow and pressure. Myogenic constriction provides a level of tone upon which vasodilators can act to lower resistance and match blood flow to metabolic

Association between decreased intra-thoracic impedance and ventricular tachyarrhythmias

The increased risk of ventricular tachyarrhythmias (VT) in the congestive heart failure (CHF) population has been well described [ [1] Bigger Jr., J.T. Fleiss J.L. Kleiger R. Miller J.P. Rolnitzky L.M. The relationships among ventricular arrhythmias, left ventricular dysfunction, and mortality in the 2 years after myocardial infarction. Circulation. 1984; 69: 250-258 Crossref PubMed Scopus (1079) Google Scholar ]. The concept of mechanical–electrical feedback has been proposed as a potential mechanism for the observed clinical relationship between CHF and VT in multiple animal [ 2 Lab M.J. Mechanically dependent changes in action potentials recorded from the intact frog ventricle. Circ Res. 1978; 42: 519-528 Crossref PubMed Scopus (110) Google Scholar , 3 Franz M.R. Cima R. Wang D. Profitt D. Kurz R. Electrophysiological effects of myocardial stretch and mechanical determinants of stretch-activated arrhythmias. Circulation. 1992; 86: 968-978 Crossref PubMed Scopus (356) Google Scholar , 4 Hansen D.E. Craig C.S. Hondeghem L.M. Stretch-induced arrhythmias in the isolated canine ventricle. Evidence for the importance of mechanoelectrical feedback. Circulation. 1990; 81: 1094-1105 Crossref PubMed Scopus (253) Google Scholar , 5 Parker K.K. Lavelle J.A. Taylor L.K. Wang Z. Hansen D.E. Stretch-induced ventricular arrhythmias during acute ischemia and reperfusion. J Appl Physiol. 2004; 97: 377-383 Crossref PubMed Scopus (20) Google Scholar ] and computer [ 6 Healy S.N. McCulloch A.D. An ionic model of stretch-activated and stretch-modulated currents in rabbit ventricular myocytes. Europace. 2005; 7: 128-134 Crossref PubMed Scopus (39) Google Scholar , 7 Kohl P. Hunter P. Noble D. Stretch-induced changes in heart rate and rhythm: clinical observations, experiments and mathematical models. Prog Biophys Mol Biol. 1999; 71: 91-138 Crossref PubMed Scopus (228) Google Scholar ] models. Despite these reports, the dynamic relationship between pulmonary congestion/decompensated heart failure and VT in the clinical environment is not well understood. A new cardiac defibrillator/cardiac resynchronization therapy (CRT-D) system is capable of ambulatory monitoring of ventricular arrhythmias as well as intra-thoracic fluid using thoracic impedance. The system records a daily mean impedance value and calculates an index of net pulmonary fluid accumulation [ [8] Yu C.M. Wang L. Chau E. et al. Intrathoracic impedance monitoring in patients with heart failure: correlation with fluid status and feasibility of early warning preceding hospitalization. Circulation. 2005; 112: 841-848 Crossref PubMed Scopus (555) Google Scholar ]. An acute decrease in intra-thoracic impedance has been previously correlated with an increase in left ventricular filling pressure [ [8] Yu C.M. Wang L. Chau E. et al. Intrathoracic impedance monitoring in patients with heart failure: correlation with fluid status and feasibility of early warning preceding hospitalization. Circulation. 2005; 112: 841-848 Crossref PubMed Scopus (555) Google Scholar ]. We report a case of repeated VT episodes preceded by acutely lowered thoracic impedance.

Mechanoelectric feedback leads to conduction slowing and block in acutely dilated atria: a modeling study of cardiac electromechanics

Atrial fibrillation, a common cardiac arrhythmia, is promoted by atrial dilatation. Acute atrial dilatation may play a role in atrial arrhythmogenesis through mechanoelectric feedback. In experimental studies, conduction slowing and block have been observed in acutely dilated atria. In the present study, the influence of the stretch-activated current (I(sac)) on impulse propagation is investigated by means of computer simulations. Homogeneous and inhomogeneous atrial tissues are modeled by cardiac fibers composed of segments that are electrically and mechanically coupled. Active force is related to free Ca(2+) concentration and sarcomere length. Simulations of homogeneous and inhomogeneous cardiac fibers have been performed to quantify the relation between conduction velocity and I(sac) under stretch. In our model, conduction slowing and block are related to the amount of stretch and are enhanced by contraction of early-activated segments. Conduction block can be unidirectional in an inhomogeneous fiber and is promoted by a shorter stimulation interval. Slowing of conduction is explained by inactivation of Na(+) channels and a lower maximum upstroke velocity due to a depolarized resting membrane potential. Conduction block at shorter stimulation intervals is explained by a longer effective refractory period under stretch. Our observations are in agreement with experimental results and explain the large differences in intra-atrial conduction, as well as the increased inducibility of atrial fibrillation in acutely dilated atria.

Regulation of capacitative calcium entries by α1‐syntrophin: association of TRPC1 with dystrophin complex and the PDZ domain of α1‐syntrophin

Calcium mishandling in Duchenne dystrophic muscle suggested that dystrophin, a membrane-associated cytoskeleton protein, might regulate calcium signaling cascade such as calcium influx pathway. It was previously shown that abnormal calcium entries involve uncontrolled stretch-activated currents and store-operated Ca2+ currents supported by TRPC1 channels. Moreover, our recent work demonstrated that reintroduction of minidystrophin in dystrophic myotubes restores normal capacitative calcium entries (CCEs). However, until now, no molecular link between the dystrophin complex and calcium entry channels has been described. This study is the first to show by coimmunoprecipitation assays the molecular association of TRPC1 with dystrophin and alpha1-syntrophin in muscle cells. TRPC1 was also associated with alpha1-syntrophin in dystrophic muscle cells independently of dystrophin. Furthermore, glutathione S-transferase (GST) pull-down assays showed that TRPC1 binds to the alpha1-syntrophin PDZ domain. Transfected recombinant alpha1-syntrophin formed a complex with TRPC1 channels and restored normal CCEs in dystrophic muscle cells. We suggest that normal regulation of CCEs in skeletal muscle depends on the association between TRPC1 channels and alpha1-syntrophin that may anchor the store-operated channels to the dystrophin-associated protein complex (DAPC). The loss of this molecular association could participate in the calcium alterations observed in dystrophic muscle cells. This study provides a new model for the regulation of calcium influx by interaction with the scaffold of the DAPC in muscle cells.

TRPM7 is a stretch- and swelling-activated cation channel involved in volume regulation in human epithelial cells

Stretch- and swelling-activated cation (SSAC) channels play essential roles not only in sensing and transducing external mechanical stresses but also in regulating cell volume in living cells. However, the molecular nature of the SSAC channel has not been clarified. In human epithelial HeLa cells, single-channel recordings in cell-attached and inside-out patches revealed expression of a Mg(2+)- and Gd(3+)-sensitive nonselective cation channel that is exquisitely sensitive to membrane stretch. Whole cell recordings revealed that the macroscopic cationic currents exhibit transient receptor potential (TRP) melastatin (TRPM)7-like properties such as outward rectification and sensitivity to Mg(2+) and Gd(3+). The whole cell cation current was augmented by osmotic cell swelling. RT-PCR and Western blotting demonstrated molecular expression of TRPM7 in HeLa cells. Treatment with small interfering RNA (siRNA) targeted against TRPM7 led to abolition of single stretch-activated cation channel currents and of swelling-activated, whole cell cation currents in HeLa cells. The silencing of TRPM7 by siRNA reduced the rate of cell volume recovery after osmotic swelling. A similar inhibition of regulatory volume decrease was also observed when extracellular Ca(2+) was removed or Gd(3+) was applied. It is thus concluded that TRPM7 represents the SSAC channel endogenously expressed in HeLa cells and that, by serving as a swelling-induced Ca(2+) influx pathway, it plays an important role in cell volume regulation.

Shear stress predicts distribution of high strain spots on plaques in human coronary arteries

Although stretch-activated currents have been extensively studied in isolated cells and intact heart in the context of mechanoelectric feedback (MEF) in the heart, quantitative data regarding other mechanical parameters such as pressure, shear, bending, etc, are still lacking at the multicellular level. Cultured cardiac cell monolayers have been used increasingly in the past decade as an in vitro model for the studies of fundamental mechanisms that underlie normal and pathological electrophysiology at the tissue level. Optical mapping makes possible multisite recording and analysis of action potentials and wavefront propagation, suitable for monitoring the electrophysiological activity of the cardiac cell monolayer under a wide variety of controlled mechanical conditions. In this paper, we review methodologies that have been developed or could be used to mechanically perturb cell monolayers, and present some new results on the acute effects of pressure, shear stress and anisotropic strain on cultured neonatal rat ventricular myocyte (NRVM) monolayers.

Early afterdepolarizations produced by oxidative stress and stretch activated conductance in rat ventricular cells

Objective: To investigate the hypothesis that acute oxidative stress and abnormal stretch of the myocardium may contribute to the production of early-afterdepolarizations (EADs). Introduction: Mechanical stretch and oxidative stress have been shown to prolong action potential durations and produce EADs. Physically stretching isolated cardiac cells is technically difficult. To study the role of stretch and oxidative stress in arrhythmia development, we have incorporated a model of mechanical stretch into isolated rat ventricular cells. Methods: We adapted our coupling clamp circuit so that a model ionic current representing stretch-activated currents was injected into isolated rat ventricular cells and added as an additional ionic current in real-time. This current was calculated as ISAC GSAC*(Vm-ESAC), where GSAC is the stretch activated conductance, Vm is the membrane potential and ESAC is the reversal potential. Results: In control, application of GSAC did not produce sustained activity or EADs, although turn-on of GSAC did produce some transient activity at high levels (6.0 0.4 nS, n 7). In H2O2 solution, used to induce acute oxidative stress, the action potential prolonged but did not have EADs in the absence of GSAC. However, the combination of GSAC and H2O2 consistently produced EADs at lower levels of GSAC (3.2 0.5 nS, n 7, p 0.05). Pacing the cell at a faster rate also prolonged the action potential and promoted the development of EADs. Conclusions: We conclude that the effects of mechanical stretch on the production of cardiac arrhythmias from EADs may be enhanced in the presence of acute oxidative stress.

Actin cytoskeleton disassembly affects conductive properties of stretch-activated cation channels in leukaemia cells

Mechanosensitive channels in various eucaryotic cells are thought to be functionally and structurally coupled to the cortical cytoskeleton. However, the results of electrophysiological studies are rather controversial and the functional impact of cytoskeleton assembly–disassembly on stretch-activated channel properties remains unclear. Here, the possible involvement of cytoskeletal elements in the regulation of stretch-activated Ca 2+-permeable channels was studied in human leukaemia K562 cells with the use of agents that selectively modify the actin or tubulin system. F-actin disassembly resulted in a considerable reduction of the amplitude of stretch-activated currents without significant change in channel open probability. The effects of treatments with cytochalasins or latrunculin were principally similar, developed gradually and consisted a strong decrease of single channel conductance. Microtubule disruption did not affect stretch-activated channels. The data presented here are in principal agreement with the general conclusion that mechanosensitive channel functions are largely dependent on the integrity of the cortical actin cytoskeleton. Specifically, changes in conductive properties of the pore may provide an essential mechanism of channel regulation underlying functional modulation of membrane currents. Our results allow one to speculate that microfilament organization may be an important determinant in modulating biophysical characteristics of stretch-activated cation channels in cells of blood origin.

An ionic model of stretch-activated and stretch-modulated currents in rabbit ventricular myocytes

To develop an ionic model of stretch-activated and stretch-modulated currents in rabbit ventricular myocytes consistent with experimental observations, that can be used to investigate the role of these currents in intact myocardium. A non-specific cation-selective stretch-activated current I(ns), was incorporated into the Puglisi-Bers ionic model of epicardial, endocardial and midmyocardial ventricular myocytes. Using the model, we predict a reduction in action potential duration at 20% repolarization (APD(20)) and action potential amplitude, an elevated resting transmembrane potential and either an increase or decrease in APD(90), depending on the reversal potential of I(ns). A stretch-induced decrease in I(K1) (70%), plus a small I(ns) current (g(ns) = 10 pS), results in a reduction in APD(20) and increase in APD(90), and a reduced safety factor for conduction. Increasing I(K1) (150%) plus a large I(ns) current (g(ns) = 40 pS), also leads to a reduction in APD(20) and increase in APD(90), but with a greater safety factor. Endocardial and midmyocardial cells appear to be the most sensitive to stretch-induced changes in action potential. The addition of the K(+)-specific stretch-activated current (SAC) I(Ko) results in action potential shortening. Transmural heterogeneity of I(Ko) may reduce repolarization gradients in intact myocardium caused by intrinsic ion channel densities, nonuniform strains and electrotonic effects.

Inhibition of the Na +–H + exchanger with cariporide abolishes stretch-induced calcium but not sodium accumulation in mouse ventricular myocytes

We address the question whether activation of the sodium–proton exchanger (NHE) does contribute to the stretch-induced accumulation of intracellular sodium and calcium in mouse ventricular myocytes. NHE-blocker cariporide (10 μM) were applied to the bath for 10 min. Axial stretch was applied for 2 min by increasing the distance between an adherent glass stylus and the patch pipette by 20%. Myocytes (stimulated at 3 Hz) were shock-frozen in diastole and the membrane currents monitored till cryofixation. Controls were treated identically, but not stretched. Total sodium and calcium concentrations ([Na], [Ca] = sum of free and bound Na and Ca) were measured by electron probe microanalysis (EPMA) in peripheral and central cytosol, mitochondria, nucleus and nuclear envelope. Cariporide did not reduce the stretch-activated negative current. The stretch-induced rise in [Na] was not different in the presence and in the absence of cariporide. Cariporide significantly reduced diastolic [Ca] in the cytosol of stretched myocytes. Since cariporide does not prevent the stretch-induced [Na] accumulation, we suggest that not NHE but the stretch-activated streptomycin-sensitive current I SAC causes the well documented stretch-induced [Na] accumulation. The discovery that cariporide prevents the stretch-induced rise in cytosolic [Ca] demonstrates an important additional effect of the drug on calcium handling.

A patch-clamp investigation of membrane currents in a novel mammalian retinal ganglion cell line

We characterised membrane currents in undifferentiated RGC-5 cells, a cell line used in in vitro models of apoptosis and glaucoma. The cells were inexcitable, with no voltage-dependent Na + currents or action potentials. Some novel currents were observed including basal Cl − currents, inwardly rectifiying K + currents and Gd 3+ insensitive stretch-activated currents. Our results highlight the differences between the electrophysiological properties of undifferentiated RGC-5 cells and retinal ganglion cells.

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).

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.

Stretch-induced calcium release in smooth muscle.

Smooth muscle cells undergo substantial increases in length, passively stretching during increases in intraluminal pressure in vessels and hollow organs. Active contractile responses to counteract increased transmural pressure were first described almost a century ago (Bayliss, 1902) and several mechanisms have been advanced to explain this phenomenon. We report here that elongation of smooth muscle cells results in ryanodine receptor–mediated Ca2+ release in individual myocytes. Mechanical elongation of isolated, single urinary bladder myocytes to ∼120% of slack length (ΔL = 20) evoked Ca2+ release from intracellular stores in the form of single Ca2+ sparks and propagated Ca2+ waves. Ca2+ release was not due to calcium-induced calcium release, as release was observed in Ca2+-free extracellular solution and when free Ca2+ ions in the cytosol were strongly buffered to prevent increases in [Ca2+]i. Stretch-induced calcium release (SICR) was not affected by inhibition of InsP3R-mediated Ca2+ release, but was completely blocked by ryanodine. Release occurred in the absence of previously reported stretch-activated currents; however, SICR evoked calcium-activated chloride currents in the form of transient inward currents, suggesting a regulatory mechanism for the generation of spontaneous currents in smooth muscle. SICR was also observed in individual myocytes during stretch of intact urinary bladder smooth muscle segments. Thus, longitudinal stretch of smooth muscle cells induces Ca2+ release through gating of RYR. SICR may be an important component of the physiological response to increases in luminal pressure in smooth muscle tissues.

Characterization of stretch-activated cation current in coronary smooth muscle cells.

Stretch-activated ion currents were recorded from vascular smooth muscle (VSM) after enzymatic isolation of single cells from porcine coronary arterioles. Patch pipettes were used to record whole cell current and control cell length. Under voltage clamp in physiological saline solution, an inward cation current (I(CAT)) was activated by 105--135% longitudinal stretch. I(CAT) coincided with an increase in intracellular Ca(2+) concentration. Under current clamp, membrane depolarization was induced by stretch. The magnitude of I(CAT) varied from -0.8 to -6.9 pA/pF at a holding potential of -60 mV. I(CAT) was graded with stretch, inactivated on release, and could be repeatedly induced. A potassium current (I(K)) activated in unstretched cells by depolarization was also enhanced by stretch. In Ca(2+)-free bath solution, stretch-induced enhancement of I(K) was blocked, but I(CAT) was still present. Hexamethyleneamiloride (50 microM), a reputed inhibitor of mechanosensitive channels, blocked I(CAT) and the stretch-induced increase in I(K) but not basal I(K). Grammostolla spatulata venom (1:100,000) blocked basal I(K), blocked stretch-induced increases in I(K), and blocked I(CAT). Iberiotoxin, a specific Ca(2+)-activated K(+) channel blocker, did not alter I(CAT) but blocked the stretch-induced increase in I(K) and increased the magnitude of stretch-induced depolarization. We concluded that longitudinal stretch directly activates a cation current and secondarily activates a Ca(2+)-activated K(+) current in isolated coronary myocytes. Although these two currents would partially counteract each other, the predominance of I(CAT) at physiological potentials is likely to explain the depolarization and contraction observed in intact coronary VSM during pressure elevation.

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

Mechanical dilation of the human ventricle is known to induce arrhythmias, the underlying ionic mechanisms, however, remain to be clarified. Ventricular myocytes isolated from human, guinea-pig or rat hearts were stretched between the patch electrode and a glass stylus. 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. 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.