Constitutively active Src tyrosine kinase changes gating of HCN4 channels through direct binding to the channel proteins

Cardiac pacemaker current, if, is generated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Our previous studies demonstrated that altered tyrosine phosphorylation can modulate the properties of both if and HCN channels. To assess a hypothesis that the intracellular tyrosine kinase Src may play a role in modulation by tyrosine phosphorylation of if, we cotransfected HEK293 cells with HCN4 and Src proteins. When HCN4 was cotransfected with a constitutively activated Src protein (Src529), the resultant voltage-dependent HCN4 activation was positively shifted (HCN4: V1/2 = -93 mV; Src529: V1/2 = -80 mV). The activation kinetics were accelerated at some potentials but not over the entire voltage range tested (eg, at -95 mV, tau_act(HCN4) = 3,243 ms; tau_act(Src529) = 1,113 ms). When HCN4 was cotransfected with a dominant negative Src protein (Src296), the HCN4 activation was shifted more negative to a smaller degree (HCN4: V1/2 = -93 mV; Src296: V1/2 = -98 mV; statistically insignificant) and the activation kinetics were slowed at most test potentials (eg, at -95 mV, tau_act(Src296) = 7,396 ms). Neither Src529 nor Src296 significantly altered HCN4 current density. Coimmunoprecipitation experiments revealed that Src forms a complex with HCN4 in HEK293 cells and in rat ventricular myocytes. Our data provide a novel mechanism of if regulation by Src tyrosine phosphorylation.

Tissue-engineered myocardial patch derived from extracellular matrix provides regional mechanical function

BACKGROUND

Extracellular matrix (ECM), a tissue-engineered scaffold, recently demonstrated cardiomyocyte population after myocardial implantation. Surgical restoration of myocardium frequently uses Dacron as a myocardial patch. We hypothesized that an ECM-derived myocardial patch would provide a mechanical benefit not seen with Dacron.

METHODS AND RESULTS

Using a canine model, a full thickness defect in the right ventricle was repaired with either Dacron or ECM. A third group had no surgery and determined baseline RV function. Eight weeks later, global systolic function was assessed by the preload recruitable stroke work relationship. Regional systolic function was measured by systolic area contraction (SAC), calculated by high density mechanical mapping. Tau was used to assess global diastolic function. Recoil rate and diastolic shear were used as measures of regional diastolic function. After functional data acquisition, tissue was fixed for histological evaluation. Global systolic and diastolic functions were similar at baseline and after ECM and Dacron implantation. Regional systolic function was greater in the ECM group compared with the Dacron group (SAC: 4.1+/-0.9% versus -1.8+/-1.1, P<0.05). Regional diastolic function was also greater in the ECM group (recoil rate (degrees sec(-1)): -44+/-7 versus -17+/-2, ECM versus Dacron; P<0.05). Immunohistochemical analysis revealed cardiomyocytes in the ECM implant region, a finding not seen with Dacron.

CONCLUSIONS

At 8 weeks, an ECM-derived tissue-engineered myocardial patch provides regional mechanical function, likely related to cardiomyocyte population. These results are in sharp contrast to Dacron, a commonly used myocardial patch.

Transmural gradients in Na/K pump activity and [Na+]I in canine ventricle

There are well-documented differences in ion channel activity and action potential shape between epicardial (EPI), midmyocardial (MID), and endocardial (ENDO) ventricular myocytes. The purpose of this study was to determine if differences exist in Na/K pump activity. The whole cell patch-clamp was used to measure Na/K pump current (I(P)) and inward background Na(+)-current (I(inb)) in cells isolated from canine left ventricle. All currents were normalized to membrane capacitance. I(P) was measured as the current blocked by a saturating concentration of dihydro-ouabain. [Na(+)](i) was measured using SBFI-AM. I(P)(ENDO) (0.34 +/- 0.04 pA/pF, n = 17) was smaller than I(P)(EPI) (0.68 +/- 0.09 pA/pF, n = 38); the ratio was 0.50 with I(P)(MID) being intermediate (0.53 +/- 0.13 pA/pF, n = 19). The dependence of I(P) on [Na(+)](i) or voltage was essentially identical in EPI and ENDO (half-maximal activation at 9-10 mM [Na(+)](i) or approximately -90 mV). Increasing [K(+)](o) from 5.4 to 15 mM caused both I(P)(ENDO) and I(P)(EPI) to increase, but the ratio remained approximately 0.5. I(inb) in EPI and ENDO were nearly identical ( approximately 0.6 pA/pF). Physiological [Na(+)](i) was lower in EPI (7 +/- 2 mM, n = 31) than ENDO (12 +/- 3 mM, n = 29), with MID being intermediate (9 +/- 3 mM, n = 22). When cells were paced at 2 Hz, [Na(+)](i) increased but the differences persisted (ENDO 14 +/- 3 mM, n = 10; EPI 9 +/- 2 mM, n = 10; and MID intermediate, 11 +/- 2 mM, n = 9). Based on these results, the larger I(P) in EPI appears to reflect a higher maximum turnover rate, which implies either a larger number of active pumps or a higher turnover rate per pump protein. The transmural gradient in [Na(+)](i) means physiological I(P) is approximately uniform across the ventricular wall, whereas transporters that utilize the transmembrane electrochemical gradient for Na(+), such as Na/Ca exchange, have a larger driving force in EPI than ENDO.

Pacemaker current and automatic rhythms: toward a molecular understanding

The ionic basis of automaticity in the sinoatrial node and His-Purkinje system, the primary and secondary cardiac pacemaking regions, is discussed. Consideration is given to potential targets for pharmacologic or genetic therapies of rhythm disorders. An ideal target would be an ion channel that functions only during diastole, so that action potential repolarization is not affected, and one that exhibits regional differences in expression and/or function so that the primary and secondary pacemakers can be selectively targeted. The so-called pacemaker current, If, generated by the HCN gene family, best fits these criteria. The biophysical and molecular characteristics of this current are reviewed, and progress to date in developing selective pharmacologic agents targeting If and in using gene and cell-based therapies to modulate the current are reviewed.

Molecular/genetic determinants of repolarization and their modification by environmental stress

Although a variety of factors, inherited or environmental, can influence expression of ion channel proteins to impact on repolarization, that environment can affect genetic determinants of repolarization for intervals of varying duration is a concept that is not as generally appreciated as it should be. In the following pages we review the molecular/genetic determinants of cardiac repolarization and summarize how pathologic events and environmental intrusions can affect these determinants. Understanding the chains of events involved should yield insights into both the causes and potential avenues of treatment for abnormalities of repolarization.

Cardiac memory ... new insights into molecular mechanisms

'Cardiac memory' describes an electrocardiographic T wave vector change, recorded during normal sinus rhythm that reflects the QRS complex vector during prior periods of ventricular pacing or arrhythmia. In this brief review we consider the mechanisms responsible for cardiac memory, which offer a unique window for relating molecular determinants of repolarization to their expression in the function of ion channels and in the electrophysiology of the heart. Understanding the steps that translate the molecular mechanisms for memory into clinical expression in this relatively straightforward model facilitates our comprehension of the complex pathways that order normal cardiac repolarization and repolarization changes.

A transgenic mouse model of heart failure using inducible Galpha q

Receptors coupled to Galpha q play a key role in the development of heart failure. Studies using genetically modified mice suggest that Galpha q mediates a hypertrophic response in cardiac myocytes. Galpha q signaling in these models is modified during early growth and development, whereas most heart failure in humans occurs after cardiac damage sustained during adulthood. To determine the phenotype of animals that express increased Galpha q signaling only as adults, we generated transgenic mice that express a silent Galpha q protein (Galpha qQ209L-hbER) in cardiac myocytes that can be activated by tamoxifen. Following drug treatment to activate Galpha q Q209L-hbER, these mice rapidly develop a dilated cardiomyopathy and heart failure. This phenotype does not appear to involve myocyte hypertrophy but is associated with dephosphorylation of phospholamban (PLB), decreased sarcoplasmic reticulum Ca2+-ATPase activity, and a decrease in L-type Ca2+ current density. Changes in Ca2+ handling and decreased cardiac contractility are apparent 1 week after Galpha qQ209L-hbER activation. In contrast, transgenic mice that express an inducible Galpha q mutant that cannot activate phospholipase Cbeta (PLCbeta) do not develop heart failure or changes in PLB phosphorylation, but do show decreased L-type Ca2+ current density. These results demonstrate that activation of Galpha q in cardiac myocytes of adult mice causes a dilated cardiomyopathy that requires the activation of PLCbeta. However, increased PLCbeta signaling is not required for all of the Galpha q-induced cardiac abnormalities.

Galpha q inhibits cardiac L-type Ca2+ channels through phosphatidylinositol 3-kinase

Cardiac myocyte contractility is initiated by Ca2+ entry through the voltage-dependent L-type Ca2+ channel (LTCC). To study the effect of Galpha q on the cardiac LTCC, we utilized two transgenic mouse lines that selectively express inducible Galpha q-estrogen receptor hormone-binding domain fusion proteins (Galpha qQ209L-hbER or Galpha qQ209L-AA-hbER) in cardiac myocytes. Both of these proteins inhibit phosphatidylinositol (PI) 3-kinase (PI3K) signaling, but Galpha qQ209L-AA-hbER cannot activate the canonical Galpha q effector phospholipase Cbeta (PLCbeta). L-type Ca2+ current (I(Ca,L)) density measured by whole-cell patch clamping was reduced by more than 50% in myocytes from both Galpha q animals as compared with wild-type cells, suggesting that inhibition of the LTCC by Galpha q does not require PLCbeta. To investigate the role of PI3K in this inhibitory effect, I(Ca,L) was measured in the presence of various phosphoinositides infused through the patch pipette. Infusion of PI 3,4,5-trisphosphate (PI(3,4,5)P3) into wild-type myocytes did not affect I(Ca,L), but it fully restored I(Ca,L) density in both Galpha q transgenic myocytes to wild-type levels. By contrast, PI 4,5-bisphosphate (PI(4,5)P2) or PI 3,5-bisphosphate had no effect. Infusion with p110beta/p85alpha or p110gamma PI3K in the presence of PI(4,5)P2 also restored I(Ca,L) density to wild-type levels. Last, infusion of either PTEN, a PI(3,4,5)P3 phosphatase, or the pleckstrin homology domain of Grp1, which sequesters PI(3,4,5)P3, reduced the peak I(Ca,L) density in wild-type myocytes by approximately 30%. Taken together, these results strongly suggest that the inhibitory effect of Galpha q on the cardiac LTCC is mediated by inhibition of PI3K.

Connexin-specific cell-to-cell transfer of short interfering RNA by gap junctions

The purpose of this study was to determine whether oligonucleotides the size of siRNA are permeable to gap junctions and whether a specific siRNA for DNA polymerase beta (pol beta) can move from one cell to another via gap junctions, thus allowing one cell to inhibit gene expression in another cell directly. To test this hypothesis, fluorescently labelled oligonucleotides (morpholinos) 12, 16 and 24 nucleotides in length were synthesized and introduced into one cell of a pair using a patch pipette. These probes moved from cell to cell through gap junctions composed of connexin 43 (Cx43). Moreover, the rate of transfer declined with increasing length of the oligonucleotide. To test whether siRNA for pol beta was permeable to gap junctions we used three cell lines: (1) NRK cells that endogenously express Cx43; (2) Mbeta16tsA cells, which express Cx32 and Cx26 but not Cx43; and (3) connexin-deficient N2A cells. NRK and Mbeta16tsA cells were each divided into two groups, one of which was stably transfected to express a small hairpin RNA (shRNA), which gives rise to siRNA that targets pol beta. These two pol beta knockdown cell lines (NRK-kcdc and Mbeta16tsA-kcdc) were co-cultured with labelled wild type, NRK-wt or Mbeta16tsA-wt cells or N2A cells. The levels of pol beta mRNA and protein were determined by semiquantitative RT-PCR and immunoblotting. Co-culture of Mbeta16tsA-kcdc cells with Mbeta16tsA-wt, N2A or NRK-wt cells had no effect on pol beta levels in these cells. Similarly, co-culture of NRK-kcdc with N2A cells had no effect on pol beta levels in the N2A cells. In contrast, co-culture of NRK-kcdc with NRK-wt cells resulted in a significant reduction in pol beta in the wt cells. The inability of Mbeta16tsA-kcdc cells to transfer siRNA is consistent with the fact that oligonucleotides of the 12 nucleotide length were not permeable to Cx32/Cx26 channels. This suggested that Cx43 but not Cx32/Cx26 channels allowed the cell-to-cell movement of the siRNA. These results support the novel hypothesis that non-hybridized and possible hybridized forms of siRNA can move between mammalian cells through connexin-specific gap junctions.

Adult human stem cells as a platform for gene therapy: Fabricating a biological pacemaker

Extract: Gene therapy and stem cells have become buzz words for future medical management. However, to date attempts to apply these technologies therapeutically have met with little success and, in the case of embryonic stem cells, have engendered political and philosophical debate. In this paper, we emphasize progress in one area of gene/stem cell therapy: the use of adult human mesenchymal stem cells (hMSCs, precursor cells for tissues such as skin, bone and muscle that are found in bone marrow) as a platform for delivering genes. These cells are adult hMSCs, and therefore not subject to the restrictions placed by some societies on use of human embryonic stem cells. In addition, they can be loaded with genes using a technique known as electroporation, by which an electrical charge temporarily makes the cell membrane permeable. This technique carries none of the risks that accompany the use of viral vectors, another method of gene delivery. As an example of the utility of hMSCs as platforms for gene delivery, we will discuss the fabrication of biological pacemakers.

Angiotensin Receptor Type 1 Forms a Complex with the Transient Outward Potassium Channel Kv4.3 and Regulates Its Gating Properties and Intracellular Localization

We report a novel signal transduction complex of the angiotensin receptor type 1. In this complex the angiotensin receptor type 1 associates with the potassium channel alpha-subunit Kv4.3 and regulates its intracellular distribution and gating properties. Co-localization of Kv4.3 with angiotensin receptor type 1 and fluorescent resonance energy transfer between those two proteins labeled with cyan and yellow-green variants of green fluorescent protein revealed that Kv4.3 and angiotensin receptor type I are located in close proximity to each other in the cell. The angiotensin receptor type 1 also co-immunoprecipitates with Kv4.3 from canine ventricle or when co-expressed with Kv4.3 and its beta-subunit KChIP2 in human embryonic kidney 293 cells. Treatment of the cells with angiotensin II results in the internalization of Kv4.3 in a complex with the angiotensin receptor type 1. When stimulated with angiotensin II, angiotensin receptors type 1 modulate gating properties of the remaining Kv4.3 channels on the cell surface by shifting their activation voltage threshold to more positive values. We hypothesize that the angiotensin receptor type 1 provides its internalization molecular scaffold to Kv4.3 and in this way regulates the cell surface representation of the ion channel.

MiRP1 Modulates HCN2 Channel Expression and Gating in Cardiac Myocytes

MinK-related protein (MiRP1 or KCNE2) interacts with the hyperpolarization-activated, cyclic nucleotide-gated (HCN) family of pacemaker channels to alter channel gating in heterologous expression systems. Given the high expression levels of MiRP1 and HCN subunits in the cardiac sinoatrial node and the contribution of pacemaker channel function to impulse initiation in that tissue, such an interaction could be of considerable physiological significance. However, the functional evidence for MiRP1/HCN interactions in heterologous expression studies has been accompanied by inconsistencies between studies in terms of the specific effects on channel function. To evaluate the effect of MiRP1 on HCN expression and function in a physiological context, we used an adenovirus approach to overexpress a hemagglutinin (HA)-tagged MiRP1 (HAMiRP1) and HCN2 in neonatal rat ventricular myocytes, a cell type that expresses both MiRP1 and HCN2 message at low levels. HA-MiRP1 co-expression with HCN2 resulted in a 4-fold increase in maximal conductance of pacemaker currents compared with HCN2 expression alone. HCN2 activation and deactivation kinetics also changed, being significantly more rapid for voltages between -60 and -95 mV when HA-MiRP1 was co-expressed with HCN2. However, the voltage dependence of activation was not affected. Co-immunoprecipitation experiments demonstrated that expressed HA-MiRP1 and HCN2, as well as endogenous MiRP1 and HCN2, co-assemble in ventricular myocytes. The results indicate that MiRP1 acts as a beta subunit for HCN2 pacemaker channel subunits and alters channel gating at physiologically relevant voltages in cardiac cells.

Genes, stem cells and biological pacemakers

The advent of gene therapy and cell therapy has led to reconsideration of standard therapies for cardiac disease. One such area of reconsideration is that of the cardiac pacemaker, which has been the mainstay of treatment for high-degree heart block and sinoatrial node dysfunction. Over the past five years, gene therapy has been used to explore the overexpression of beta(2)-adrenergic receptors, the down-regulation of inward rectifier current, and the overexpression of pacemaker current as potential sources of biological pacemakers. Cell therapy approaches have explored the "forcing" of embryonic stem cells to evolve along cardiac (and specifically pacemaker) cell lines and the use of adult mesenchymal stem cells as platforms for delivery of specific gene therapies. This review considers the strengths and weaknesses of each of the approaches used to date and attempts to look to the future of biological alternatives to electronic pacemakers.

Human mesenchymal stem cells make cardiac connexins and form functional gap junctions

Human mesenchymal stem cells (hMSCs) are a multipotent cell population with the potential to be a cellular repair or delivery system provided that they communicate with target cells such as cardiac myocytes via gap junctions. Immunostaining revealed typical punctate staining for Cx43 and Cx40 along regions of intimate cell-to-cell contact between hMSCs. The staining patterns for Cx45 rather were typified by granular cytoplasmic staining. hMSCs exhibited cell-to-cell coupling to each other, to HeLa cells transfected with Cx40, Cx43 and Cx45 and to acutely isolated canine ventricular myocytes. The junctional currents (I(j)) recorded between hMSC pairs exhibited quasi-symmetrical and asymmetrical voltage (V(j)) dependence. I(j) records from hMSC-HeLaCx43 and hMSC-HeLaCx40 cell pairs also showed symmetrical and asymmetrical V(j) dependence, while hMSC-HeLaCx45 pairs always produced asymmetrical I(j) with pronounced V(j) gating when the Cx45 side was negative. Symmetrical I(j) suggests that the dominant functional channel is homotypic, while the asymmetrical I(j) suggests the activity of another channel type (heterotypic, heteromeric or both). The hMSCs exhibited a spectrum of single channels with transition conductances (gamma(j)) of 30-80 pS. The macroscopic I(j) obtained from hMSC-cardiac myocyte cell pairs exhibited asymmetrical V(j) dependence, while single channel events revealed gamma(j) of the size range 40-100 pS. hMSC coupling via gap junctions to other cell types provides the basis for considering them as a therapeutic repair or cellular delivery system to syncytia such as the myocardium.

Human mesenchymal stem cells as a gene delivery system to create cardiac pacemakers

We tested the ability of human mesenchymal stem cells (hMSCs) to deliver a biological pacemaker to the heart. hMSCs transfected with a cardiac pacemaker gene, mHCN2, by electroporation expressed high levels of Cs+-sensitive current (31.1+/-3.8 pA/pF at -150 mV) activating in the diastolic potential range with reversal potential of -37.5+/-1.0 mV, confirming the expressed current as I(f)-like. The expressed current responded to isoproterenol with an 11-mV positive shift in activation. Acetylcholine had no direct effect, but in the presence of isoproterenol, shifted activation 15 mV negative. Transfected hMSCs influenced beating rate in vitro when plated onto a localized region of a coverslip and overlaid with neonatal rat ventricular myocytes. The coculture beating rate was 93+/-16 bpm when hMSCs were transfected with control plasmid (expressing only EGFP) and 161+/-4 bpm when hMSCs were expressing both EGFP+mHCN2 (P<0.05). We next injected 10(6) hMSCs transfected with either control plasmid or mHCN2 gene construct subepicardially in the canine left ventricular wall in situ. During sinus arrest, all control (EGFP) hearts had spontaneous rhythms (45+/-1 bpm, 2 of right-sided origin and 2 of left). In the EGFP+mHCN2 group, 5 of 6 animals developed spontaneous rhythms of left-sided origin (rate=61+/-5 bpm; P<0.05). Moreover, immunostaining of the injected regions demonstrated the presence of hMSCs forming gap junctions with adjacent myocytes. These findings demonstrate that genetically modified hMSCs can express functional HCN2 channels in vitro and in vivo, mimicking overexpression of HCN2 genes in cardiac myocytes, and represent a novel delivery system for pacemaker genes into the heart or other electrical syncytia.

Biological Pacemaker Implanted in Canine Left Bundle Branch Provides Ventricular Escape Rhythms That Have Physiologically Acceptable Rates

BACKGROUND

We hypothesized that administration of the HCN2 gene to the left bundle-branch (LBB) system of intact dogs would provide pacemaker function in the physiological range of heart rates.

METHODS AND RESULTS

An adenoviral construct incorporating HCN2 and green fluorescent protein (GFP) as a marker was injected via catheter under fluoroscopic control into the posterior division of the LBB. Controls were injected with an adenoviral construct of GFP alone or saline. Animals were monitored electrocardiographically for up to 7 days after surgery, at which time they were anesthetized and subjected to vagal stimulation to permit emergence of escape pacemakers. Hearts were then removed and injection sites visually identified and removed for microelectrode study of action potentials, patch clamp studies of pacemaker current, and/or immunohistochemical studies of HCN2. For 48 hours postoperatively, 7 of 7 animals subjected to 24-hour ECG monitoring showed multiple ventricular premature depolarizations and/or ventricular tachycardia attributable to injection-induced injury. Thereafter, sinus rhythm prevailed. During vagal stimulation, HCN2-injected dogs showed rhythms originating from the left ventricle, the rate of which was significantly more rapid than in the controls. Excised posterior divisions of the LBB from HCN2-injected animals manifested automatic rates significantly greater than the controls. Isolated tissues showed immunohistochemical and biophysical evidence of overexpressed HCN2.

CONCLUSIONS

A gene-therapy approach for induction of biological pacemaker activity within the LBB system provides ventricular escape rhythms that have physiologically acceptable rates. Long-term stability and feasibility of the approach remain to be tested.

Tyrosine kinase inhibition differentially regulates heterologously expressed HCN channels

The HCN ion channel subunit gene family encodes hyperpolarization-activated cation channels that are permeable to Na(+) and K(+). There are four members of this channel family, three of which, HCN1, HCN2, and HCN4, are expressed in the heart. Current evidence suggests that the HCN ion channel subunit family is the molecular correlate of the alpha subunit of the cardiac pacemaker current i(f). Our previous work has shown that HCN4 is the dominant isoform expressed in the rabbit sinoatrial (SA) node and that changes in tyrosine phosphorylation, either by kinase inhibition or growth factor activation, lead to changes in rabbit SA node i(f) conductance with no change in voltage dependence. In the present study we investigate the actions of genistein, a tyrosine kinase inhibitor, on heterologously expressed HCN currents in Xenopus oocytes. Genistein had no effect on HCN1-induced currents, but reduced whole-cell currents induced by HCN2 or HCN4 and slowed activation kinetics at voltages near the midpoint of activation. In the case of HCN2 there was also a negative shift in the voltage dependence of activation that accompanies the current reduction. We have shown previously that HCN2 is the dominant isoform expressed in rat ventricular myocytes. The above results predict that genistein should reduce i(f) in the rat ventricle and cause a negative shift of voltage dependence and kinetics of activation. We tested this hypothesis by studying the effects of genistein on isolated rat ventricular myocytes. Genistein significantly reduced i(f) current density (pA/pF) (control: 12.2+/-1.8; genistein: 3.5+/-0.5; washout: 7.7+/-0.8; n=10), and caused a negative shift of the midpoint of activation by 14 mV (-133+/-1 mV for genistein and -119+/-1 mV for washout, n=7) with no change in slope factor. Our results thus suggest that i(f) in the heart and i(f)-like currents in other tissues can be regulated differentially by tyrosine phosphorylation based on isoform expression patterns.

Recreating the biological pacemaker

In recent years, several groups have reported a variety of strategies for developing biological pacemakers whose ultimate function would be to supplement/replace electronic pacemakers. Strategies have included gene therapy using naked plasmids or viral vectors and cell therapy for which both adult human mesenchymal stem cells (hMSCs) and human embryonic stem cells have been employed. This article reviews the various approaches and summarizes our own research in which the pacemaker gene, HCN2, is administered via viral vector or in an hMSC platform to produce pacemaker function in the intact canine heart.

Both metabolic inhibition and mitochondrial K(ATP) channel opening are myoprotective and initiate a compensatory sarcolemmal outward membrane current

BACKGROUND

Blockade of oxidative phosphorylation may activate ATP sensitive mitochondrial potassium (mitoK(ATP)) channels. We examined whether both metabolic inhibition and mitoK(ATP) channel openers protect both the whole organ and isolated cells from ischemia.

METHODS AND RESULTS

Using a Langendorff preparation, one group of isolated rabbit hearts were exposed to ischemic preconditioning (IPC) via 2 episodes of flow interruption. The second group of hearts was preconditioned with 2 episodes of either the metabolic inhibitor, sodium cyanide (NaCN), or the mitoK(ATP) channel opener, diazoxide. The third group of hearts was exposed to the mitoK(ATP) channel inhibitor, 5-hydroxydecanoic acid (5-HD) prior to preconditioning with NaCN, diazoxide or IPC. Controls had no drug infused. Then, ischemia was induced in all hearts by left anterior descending coronary artery occlusion and infarct size was determined. Compared with controls (40+/-3%), infarct size was significantly reduced in hearts preconditioned with NaCN, diazoxide or IPC (18+/-3%, 26+/-3%, 21+/-2%, respectively; P<0.05 versus control). These reductions were reversed by 5-HD (36+/-3%, 33+/-2%, 37+/-2%; NaCN, diazoxide, IPC, respectively). Secondly, whole cell patch clamped isolated guinea pig ventricular myocytes were preconditioned with 2 episodes of either NaCN or diazoxide followed by Tyrodes perfusion with membrane potential set to -70 mV. Control cells were exposed to Tyrodes solution. All cells were then clamped to -20 mV and exposed to NaCN, which caused induction of an outward potassium current. Compared with controls, the average time to induction of the outward current was significantly reduced in cells preconditioned with either brief application of NaCN (11.6+/-1.8 versus 5.1+/-1.0 minutes, control versus NaCN, P<0.05) or diazoxide (5.5+/-1.4 versus 2.0+/-0.8 minutes, control versus diazoxide, P<0.05).

CONCLUSIONS

Preconditioning protects the heart through mitoK(ATP). This protection also alters a surface membrane current, which may be important in myocardial protection.

Role of L-type calcium channels in pacing-induced short-term and long-term cardiac memory in canine heart

BACKGROUND

We tested the hypothesis that ICa,L is important to the development of cardiac memory.

METHODS AND RESULTS

The effects of L-type Ca2+ channel blockade and beta-blockade were tested on acutely anesthetized and on chronically instrumented, conscious dogs. Short-term memory (STM) was induced by 2 hours of ventricular pacing and long-term memory (LTM) by ventricular pacing for 21 days. STM dogs received placebo, nifedipine, or propranolol, and LTM dogs received placebo, atenolol, or amlodipine. AT1 receptor blockade (candesartan) and ACE inhibition (trandolapril) were also tested in LTM. Microelectrodes were used to record transmembrane potentials from isolated epicardial and endocardial slabs using a protocol simulating STM in intact animals. Left ventricular epicardial myocytes from LTM or sham control dogs were dissociated, and ICa,L was recorded (whole-cell patch-clamp technique). Evolution of STM and LTM was attenuated by ICa,L blockers but not beta-blockers. Neither AT1 receptor blockade nor ACE inhibition suppressed LTM. In microelectrode experiments, pacing induced an epicardial-endocardial gradient change mimicking STM that was suppressed by nifedipine. In patch-clamp experiments, peak ICa,L density in LTM and control were equivalent, but activation was more positive and time constants of inactivation longer in LTM (P<0.05).

CONCLUSIONS

ICa,L blockade but not beta-adrenergic blockade suppresses cardiac memory. LTM evolution is unaffected by angiotensin II blockade and is associated with altered ICa,L kinetics.

Simultaneous assessment of myocardial perfusion and function during mental stress in patients with chronic coronary artery disease

BACKGROUND

Mental stress (MS) is an important provocateur of myocardial ischemia in many patients with chronic coronary artery disease. The majority of laboratory assessments of ischemia in response to MS have included measurements of either myocardial perfusion or function alone. We performed this study to determine the relationship between alterations in perfusion and ventricular function during MS. Methods and results Twenty-eight patients with reversible perfusion defects on exercise or pharmacologic stress myocardial perfusion imaging (MPI) underwent simultaneous technetium 99m sestamibi single photon emission computed tomography (SPECT) MPI and transthoracic echocardiography at rest and during MS according to a mental arithmetic protocol. In all cases the MS study was performed within 4 weeks of the initial exercise or pharmacologic MPI that demonstrated ischemia. SPECT studies were analyzed visually with the use of a 13-segment model and quantitatively by semiautomated circumferential profile analysis. Echocardiograms were graded on a segmental model for regional wall motion on a 4-point scale. Of 28 patients, 18 (64%) had perfusion defects and/or left ventricular dysfunction develop during MS: 9 (32%) had myocardial perfusion defects develop, 6 (21%) had regional or global left ventricular dysfunction develop, and 3 (11%) had both perfusion defects and left ventricular dysfunction develop. The overall concordance between perfusion and function criteria for ischemia during MS was only 46%. Among 9 patients with MS-induced left ventricular dysfunction, 5 had new regional wall motion abnormalities and 4 had a global decrement in function. In patients with MS-induced ischemia by SPECT, the number of reversible perfusion defects was similar during both MS and exercise/pharmacologic stress (2.8 +/- 2.0 vs 3.5 +/- 1.8, P =.41). Hemodynamic changes during MS were similar whether patients were divided on the basis of perfusion defects or left ventricular dysfunction during MS.

CONCLUSIONS

These data indicate the feasibility of simultaneous assessment of perfusion and function responses during MS. Flow and function responses to MS are frequently not concordant. These data suggest that MS-induced changes in perfusion may represent a different phenomenon than MS-induced changes in left ventricular function (either globally or regionally).

Calcium channel heterogeneity in canine left ventricular myocytes

Regional variations in the electrophysiological properties of myocytes across the left ventricular wall play an important role in both the normal physiology of the heart and the genesis of arrhythmias. To investigate the possible contributions of calcium channels to transmural electrical heterogeneity, whole-cell patch-clamp recordings were made from isolated canine epicardial and endocardial left ventricular myocytes. Two major differences in Ca2+ channel properties were found between epi- and endocardial cells. First, the L-type Ca2+ current was larger in endocardial than in epicardial myocytes. The average peak current density at +10 mV in endocardial myocytes was 3.4 +/- 0.2 pA pF-1, and was 45 % higher than that in epicardium (2.3 +/- 0.1 pA pF-1). The kinetic properties of the L-type current in epi- and endocardial cells were not significantly different. Second, a low-threshold, rapidly activating and inactivating Ca2+ current that resembled the T-type current was present in all endocardial myocytes but was small or absent in epicardial myocytes. This T-like current had an average peak density of 0.5 pA pF-1 at -40 mV in endocardial cells. In most endocardial cells the T-like Ca2+ current comprised two components: a Ni2+-sensitive T-type current and a tetrodotoxin-sensitive Ca2+ current. We conclude that there are considerable regional variations in the density and properties of Ca2+ channels across the canine left ventricular wall. These variations may contribute to the overall transmural electrical heterogeneity.

Expression and function of a biological pacemaker in canine heart

BACKGROUND

We hypothesized that localized overexpression of the hyperpolarization-activated, cyclic nucleotide-gated (HCN2) pacemaker current isoform in canine left atrium (LA) would constitute a novel biological pacemaker.

METHODS AND RESULTS

Adenoviral constructs of mouse HCN2 and green fluorescent protein (GFP) or GFP alone were injected into LA, terminal studies performed 3 to 4 days later, hearts removed, and myocytes examined for native and expressed pacemaker current (I(f)). Spontaneous LA rhythms occurred after vagal stimulation-induced sinus arrest in 4 of 4 HCN2+GFP dogs and 0 of 3 GFP dogs (P<0.05). Native I(f) in nonexpressed atrial myocytes was 7+/-4 pA at -130 mV (n=5), whereas HCN2+GFP LA had expressed pacemaker current (I(HCN2)) of 3823+/-713 pA at -125 mV (n=10) and 768+/-365 pA at -85 mV.

CONCLUSIONS

HCN2 overexpression provides an I(f)-based pacemaker sufficient to drive the heart when injected into a localized region of atrium, offering a promising gene therapy for pacemaker disease.