Myocardial infarction (MI) results in the loss of billions of cardiomyocytes (CMs), resulting in cardiac dysfunction. To re-muscularize injured myocardium, new CMs must be generated via renewed proliferation of surviving CMs. Approaches to induce proliferation of CMs after injury have been insufficient. Toward this end we are targeting the acetyltransferase Tip60, encoded by the Kat5 gene, based on the rationale that its pleiotropic functions combine to block CM proliferation at multiple checkpoints. We previously demonstrated that genetic depletion of Tip60 in a mouse model after MI reduces scarring, retains cardiac function, and activates the CM cell-cycle, although it remains unclear whether this culminates in the generation of daughter CMs. In order for pre-existing CMs in the adult heart to undergo proliferation, it has become accepted that they must first dedifferentiate, a process highlighted by loss of maturity, epithelial to mesenchymal transitioning (EMT), and reversion from fatty acid oxidation to glycolytic metabolism, accompanied by softening of the myocardial extracellular matrix (ECM). Based on recently published findings that Tip60 induces and maintains the differentiated state of hematopoietic stem cells and neurons via site-specific acetylation of the histone variant H2A.Z, we assessed levels of acetylated H2A.Z and dedifferentiation markers after depleting Tip60 in CMs post-MI. We report that genetic depletion of Tip60 from CMs after MI results in the near obliteration of acetylated H2A.Z in CM nuclei, accompanied by the altered expression of genes indicative of EMT induction, ECM softening, decreased fatty acid oxidation, and depressed expression of genes that regulate the TCA cycle. In accord with the possibility that site-specific acetylation of H2A.Z maintains adult CMs in a mature state of differentiation, CUT&Tag revealed enrichment of H2A.ZacK4/K7 in genetic motifs and in GO terms respectively associated with CM transcription factor binding and muscle development/differentiation. Along with our previous findings, these results support the notion that Tip60 has multiple targets in CMs that combine to maintain the differentiated state and prevent proliferation.

Cardiovascular disease is a leading cause of morbidity and mortality, and exercise-training (TRN) is known to reduce risk factors and protect the heart from ischemia and reperfusion injury. Though the cardioprotective effects of exercise are well-documented, underlying mechanisms are not well understood. This review highlights recent findings and focuses on cardiac factors with emphasis on K+ channel control of the action potential duration (APD), β-adrenergic and adenosine regulation of cardiomyocyte function, and mitochondrial Ca2+ regulation. TRN-induced prolongation and shortening of the APD at low and high activation rates, respectively, is discussed in the context of a reduced response of the sarcolemma delayed rectifier potassium channel (IK) and increased content and activation of the sarcolemma KATP channel. A proposed mechanism underlying the latter is presented, including the phosphatidylinositol-3kinase/protein kinase B pathway. TRN induced increases in cardiomyocyte contractility and the response to adrenergic agonists are discussed. The TRN-induced protection from reperfusion injury is highlighted by the increased content and activation of the sarcolemma KATP channel and the increased phosphorylated glycogen synthase kinase-3β, which aid in preventing mitochondrial Ca2+ overload and mitochondria-triggered apoptosis. Finally, a brief section is presented on the increased incidences of atrial fibrillation associated with age and in life-long exercisers.

Myocardial infarction (MI) leads to cardiomyocyte (CM) loss, resulting in cardiac dysfunction and heart failure. Remuscularization of injured myocardium requires proliferation of surviving CMs. However, approaches aimed at inducing CM regeneration following MI have been inadequate. To this end, we are targeting the acetyltransferase Tip60 (Tat-interactive protein 60 kD), a pleiotropic tumor suppressor encoded by the Kat5 gene. Using a murine genetic model, we previously reported that disruption of Kat5 promotes CM cell-cycle re-entry, and protects against the damaging effects of MI. It is becoming increasingly recognized that pre-existing CMs must undergo dedifferentiation accompanied by reversion to glycolytic metabolism in order to resume proliferation. The site-specific acetylation of the histone variant H2A.Z at lysine K4/K7 (H2A.Zac K4/K7 ) has been shown to maintain the differentiated state in hematopoietic stem cells (HSCs), neurons, and skeletal myocytes. Here, we report that Tip60 depletion post-MI results in the near-complete absence of H2A.Zac K4/K7 in CMs. This is associated with the enrichment of differentially expressed genes (DEGs) in epithelial to mesenchymal transitioning (EMT), cytoskeletal disassembly, extracellular matrix (ECM) softening, and metabolic reprogramming. To assess therapeutic relevance, we have begun to evaluate the potential cardioprotective effects of the anti-parasitic drug pentamidine, a known Tip60 inhibitor. Systemic administration of pentamidine on days 3-16 post-MI preserved cardiac function, accompanied by CM cell-cycle activation. These data suggest that pentamidine treatment enhances remuscularization, thereby promoting the retention of post-MI function, supporting the translational promise of targeting Tip60 as an innovative therapy for ischemic heart disease.

Injury from myocardial infarction (MI) and consequent post-MI remodeling is accompanied by massive loss of cardiomyocytes (CM), a cell type critical for contractile function that is for all practical purposes non-regenerable due to its profound state of proliferative senescence. Identification of factors that limit CM survival and/or constrain CM renewal provides potential therapeutic targets. Tip60, a pan-acetyltransferase encoded by the Kat5 gene, has been reported to activate apoptosis as well as multiple anti-proliferative pathways in non-cardiac cells; however, its role in CMs, wherein it is abundantly expressed, remains unknown. Here, using mice containing floxed Kat5 alleles and a tamoxifen-activated Myh6-MerCreMer recombinase transgene, we report that conditional depletion of Tip60 in CMs three days after MI induced by permanent coronary artery ligation greatly improves functional recovery for up to 28 days. This is accompanied by diminished scarring, activation of cell cycle transit markers in CMs within the infarct border and remote zones, reduced expression of cell-cycle inhibitors pAtm and p27, and reduced apoptosis in the remote regions. These findings implicate Tip60 as a novel, multifactorial target for limiting the damaging effects of ischemic heart disease.

ABSTRACT Pharmacologic strategies that target factors with both pro-apoptotic and anti-proliferative functions in cardiomyocytes (CMs) may be useful for the treatment of ischemic heart disease. One such multifunctional candidate for drug targeting is the acetyltransferase Tip60, which is known to acetylate both histone and non-histone protein targets that have been shown in cancer cells to promote apoptosis and to initiate the DNA damage response, thereby limiting cellular expansion. Using a murine model, we recently published findings demonstrating that CM-specific disruption of the Kat5 gene encoding Tip60 markedly protects against the damaging effects of myocardial infarction (MI). In the experiments described here, in lieu of genetic targeting, we administered TH1834, an experimental drug designed to specifically inhibit the acetyltransferase domain of Tip60. We report that, similar to the effect of disrupting the Kat5 gene, daily systemic administration of TH1834 beginning 3 days after induction of MI and continuing for 2 weeks of a 4-week timeline resulted in improved systolic function, reduced apoptosis and scarring, and increased activation of the CM cell cycle, effects accompanied by reduced expression of genes that promote apoptosis and inhibit the cell cycle and reduced levels of CMs exhibiting phosphorylated Atm. These results support the possibility that drugs that inhibit the acetyltransferase activity of Tip60 may be useful agents for the treatment of ischemic heart disease.

In recent decades, many cardiovascular diseases have been ascribed to acetylation-dependent regulatory mechanisms, as revealed by the beneficial effects of targeting de-acetylase proteins. By contrast, we are targeting the acetyltransferase Tip60 (Tat-interactive protein 60 kD), a pleiotropic tumor suppressor encoded by the Kat5 gene. Using a murine genetic model, we recently reported that cardiomyocyte (CM)-specific disruption of Kat5 markedly protected against the damaging effects of myocardial infarction (MI). To establish therapeutic relevance, we have begun to evaluate the potential cardioprotective effects of TH1834, a small MW drug designed to specifically target the acetyltransferase domain of Tip60. Daily systemic administration of TH1834 on days 3-16 post-MI efficiently preserved cardiac function for up to 28 days post-MI, which was accompanied by reduced scar formation, diminished CM apoptosis, and activation of CM cell-cycle in the absence of CM hypertrophy. Subsequent gene ontology enrichment analysis of transcriptome sequencing data revealed that, among a total of 2,051 differentially expressed genes, 154 cell-cycle regulators and 96 apoptotic markers were altered by TH1834 treatment at day 10 post-MI; significant transcriptional changes in genes involved in cardiac muscle contraction, CM differentiation processes, and responses to ischemia/hypoxia were also identified. These data demonstrate that TH1834 administration promotes retention of post-MI function via enhanced remuscularization and by inhibiting cell death, supporting the translational potential of targeting Tip60 as a novel treatment for ischemic heart disease.

Tip60, a pan-acetyltransferase encoded by the Kat5 gene, is enriched in the myocardium; however, its function in the heart is unknown. In cancer cells, Tip60 acetylates Atm (Ataxia-telangiectasia mutated), enabling its auto-phosphorylation (pAtm), which activates the DNA damage response (DDR). It was recently reported that activation of pAtm at the time of birth induces the DDR in cardiomyocytes (CMs), resulting in proliferative senescence. We therefore hypothesized that Tip60 initiates this process, and that depletion of Tip60 accordingly diminishes the DDR while extending the duration of CM cell-cycle activation. To test this hypothesis, an experimental model was used wherein a Myh6-driven Cre-recombinase transgene was activated on postnatal day 0 (P0) to recombine floxed Kat5 alleles and induce Tip60 depletion in neonatal CMs, without causing pathogenesis. Depletion of Tip60 resulted in reduced numbers of pAtm-positive CMs during the neonatal period, which correlated with reduced numbers of pH2A.X-positive CMs and decreased expression of genes encoding markers of the DDR as well as inflammation. This was accompanied by decreased expression of the cell-cycle inhibitors Meis1 and p27, activation of the cell-cycle in CMs, reduced CM size, and increased numbers of mononuclear/diploid CMs. Increased expression of fetal markers suggested that Tip60 depletion promotes a fetal-like proliferative state. Finally, infarction of Tip60-depleted hearts at P7 revealed improved cardiac function at P39 accompanied by reduced fibrosis, increased CM cell-cycle activation, and reduced apoptosis in the remote zone. These findings indicate that, among its pleiotropic functions, Tip60 induces the DDR in CMs, contributing to proliferative senescence.

ABSTRACT It is estimated that up to one billion cardiomyocytes (CMs) can be lost during myocardial infarction (MI), which results in contractile dysfunction, adverse ventricular remodeling, and systolic heart failure. Pharmacologic strategies that target factors having both pro-apoptotic and anti-proliferative functions in CMs may be useful for the treatment of ischemic heart disease. One such multifunctional candidate for drug targeting is the acetyltransferase Tip60, which is a member of the MYST family of acetyltransferases known to acetylate both histone and non-histone protein targets that have been shown in cultured cancer cells to promote apoptosis and to initiate the DNA damage response (DDR) thereby limiting cellular expansion. Using a murine model, we recently published findings demonstrating that CM-specific disruption of the Kat5 gene encoding Tip60 markedly protected against the damaging effects of MI. In the experiments described here, in lieu of genetic targeting, we administered TH1834, an experimental drug designed to specifically inhibit the acetyltransferase domain of Tip60. We report that, similar to the effect of disrupting the Kat5 gene, daily systemic administration of TH1834 beginning 3 days after induction of MI and continuing for two weeks of a 4-week timeline resulted in improved systolic function assessed by echocardiography, reduced apoptosis and scarring, and increased activation of the CM cell-cycle. Our results support that idea that drugs that inhibit the acetyltransferase activity of Tip60 may be useful agents for the treatment of ischemic heart disease.

Cardiac disease and injury are accompanied by significant cardiomyocyte (CM) loss. CMs are essentially non-regenerable, preventing re-muscularization and resulting in permanent dysfunction during the healing process. The identification of factors from multiple layers of inhibitors that induce and maintain CM’s pronounced state of proliferative senescence provides potential therapeutic targets. Tip60 ( T at- i nteractive p rotein, 60 kD), a pan-acetylase tumor suppressor encoded by the Kat5 gene, activates multiple anti-proliferative pathways in other cell types. Tip60 is known to acetylate Atm and p53, which respectively activate the DNA damage response and apoptosis. And, Tip60 regulates intracellular levels of p21 and Tert polymerase in a fashion designed to maintain proliferative senescence. Although these functions promote CM proliferative senescence and Tip60 is strongly expressed in the myocardium, whether Tip60 regulates these activities in CMs remains unknown. This study tests the hypothesis that Tip60 inhibits CM cell-cycle activation, promotes apoptosis in the myocardium, and prevents regeneration after cardiac injury. In adult mice containing LoxP -flanked Kat5 alleles and a tamoxifen-inducible Cre-recombinase transgene driven by Myh6 (Kat5 flox/flox;Myh6-merCremer ) , Tip60 depletion three days after myocardial infarction (MI) preserved cardiac function as assessed by echocardiography at 10, 21, and 28 days post-MI. Histologic evaluation revealed that depletion of Tip60 in CMs significantly diminished scarring and increased CM cell-cycle activation, as indicated by increased numbers of Ki67-, BrdU- and pH3-positive CMs at 28 days post-MI. This was accompanied by the presence of CMs in the infarct border zone expressing smooth muscle α-actin, indicative of CM de-differentiation, and by reduced apoptosis in the remote zone as assessed by TUNEL and cleaved caspase-3 staining, suggesting reduced cardiac remodeling. These findings suggest that genetic depletion of Tip60 protects CMs from ischemia-induced cell death while promoting CM proliferation, implicating Tip60 as a novel candidate for cardiac therapeutics.

Regeneration of muscle in the damaged myocardium is a major objective of cardiovascular research, for which purpose many investigators utilize mice containing transgenes encoding Cre-recombinase to recombine loxP-flanked target genes. An unfortunate side-effect of the Cre-loxP model is the propensity of Cre-recombinase to inflict off-target DNA damage, which has been documented in various eukaryotic cell-types including cardiomyocytes (CMs). In the heart, reported effects of Cre-recombinase include contractile dysfunction, fibrosis, cellular infiltration, and induction of the DNA damage response (DDR). During experiments on adult mice containing a widely used Myh6-merCremer transgene, the protein product of which is activated by tamoxifen, we observed large, transient off-target effects of merCremer, some of which have not been previously reported. On Day 3 after the first of three daily tamoxifen injections, immunofluorescent microscopy of heart sections revealed that the presence of merCremer protein in myonuclei was nearly uniform, thereafter diminishing to near extinction by Day 6; during this time, cardiac function was depressed as determined by echocardiography. On Day 5, peaks of apoptosis and expression of DDR regulatory genes were observed, highlighted by >25-fold increased expression of Brca1; concomitantly, the expression of genes encoding Cyclin A2, Cyclin B1 and Cdk1, which regulate the G2/S cell-cycle transition, were dramatically increased (>50-100-fold). Importantly, immunofluorescent staining revealed that this was accompanied by peaks of Ki67, 5'-bromodeoxyuridine, and phosphohistone H3 labeling in non-CMs, as well as CMs. We further document that tamoxifen-induced activation of merCremer exacerbates cardiac dysfunction following MI. These findings, when considered in the context of previous reports, indicate that the presence of merCremer in the nucleus induces DNA damage and unscheduled cell-cycle activation. Although these effects are transient, the inclusion of appropriate controls, coupled with an awareness of defects caused by Cre-recombinase, are required to avoid misinterpreting results when using Cre-loxP models for cardiac regeneration studies.

First Person is a series of interviews with the first authors of a selection of papers published in Disease Models & Mechanisms, helping early-career researchers promote themselves alongside their papers. Xinrui Wang is first author on 'Myh6-driven Cre recombinase activates the DNA damage response and the cell cycle in the myocardium in the absence of loxP sites', published in DMM. Xinrui is a postdoctoral fellow in the lab of John Auchampach at the Medical College of Wisconsin, Milwaukee, WI, USA, investigating cellular and molecular mechanisms of myocardium re-muscularization after injury.

Results demonstrate that exercise training (TRN) downregulates ventricular IKs channel current and the channel’s responsiveness to β-agonist factors mediated by TRN-induced decline in channel subunits KCNQ1 and KCNE1 and the A-kinase anchoring protein yotiao. The reduced IKs current helps explain the TRN-induced prolongation of the action potential in basal conditions and, coupled with previously reported upregulation of the KATP channel, results in a more efficient heart that is better able to respond to beat-by-beat changes in metabolism.

Tip60 ( T at i nteractive p rotein, 60 kD) is a tumor suppressor encoded by the Kat5 gene that functions as a pan-acetylase. Among its targets, Atm ( a taxia- t elangiectasia m utated) undergoes auto-phosphorylation (pATM) when acetylated by Tip60 to induce the DNA damage response (DDR). Because pAtm-induced activation of the DDR is known to cause proliferative senescence in neonatal cardiomyocytes (CMs), we hypothesize that Tip60 is one of the upstream components in this pathway. Using mice containing floxed Kat5 alleles and a tamoxifen-inducible Cre-recombinase transgene driven by Myh6 (Kat5 flox/flox;Myh6-merCremer ), we assessed the effect of depleting Tip60 from neonatal CMs via a single injection of tamoxifen on postnatal day 0 (P0). Immunofluorescent microscopy revealed reduced pAtm-positive CMs at three postnatal stages (P7, P12, P39), accompanied by activation of the cell-cycle as indicated by significantly increased percentages of 5’-bromodeoxyuridine (BrdU)-, phosphohistone H3 (pH3)- and Ki67-positive CMs. In addition to cell-cycle activation, Tip60 depletion promoted nuclear division followed by cytokinesis, as indicated by increased percentage of mononuclear diploid CMs at P12. Accordingly, qPCR revealed that Tip60 depletion increased expression of mRNAs encoding the fetal markers Nppa, Nppb, Myh7 , and Acta1 at P39, which in the absence of CM hypertrophy (assessed by WGA staining) suggested maintenance of a proliferation-competent state. These effects occurred concomitant with depressed levels of mRNAs encoding cell-cycle inhibitors ( Meis1, p27 ) and a trend toward increased levels of G 2 -phase cell-cycle activators ( cyclins A2, B1 ; Cdk1 ) in Tip60-depleted hearts. These findings suggest that depletion of Tip60 at birth inhibits the DDR and delays replicative senescence in neonatal CMs.

Tip60 (Tat interactive protein, 60 kD) is a tumor suppressor protein that acetylates histone and non‐histone proteins. Among the latter, ATM (Ataxia Telangiectasia Mutated), a kinase that induces the DNA damage response (DDR), is activated in cancer cells by Tip60‐mediated acetylation. We are investigating the function of Tip60 in the heart, using global and conditional knockout mouse models to disrupt the Kat5 gene encoding Tip60. Because global knockout (i.e. Tip60 −/− ) causes early embryonic lethality, we used Tip60 +/− heterozygotes to show that, when under cardiac stress, modest Tip60 depletion correlates with re‐activation of the cardiomyocyte (CM) cell‐cycle in the adult heart. More recently, using a knockout model wherein Tip60 is constitutively and specifically depleted in CMs, we observed CM dropout and lethality in three‐month‐old mice, wherein hearts exhibited features of mitotic catastrophe. Because the latter phenotype was preceded by increased CM density, it was suggested that Tip60‐depletion may extend the duration of CM proliferation during neonatal stages of heart development. Moreover, considered in the context of recent reports that ATM is activated at birth to induce the DDR, which in turn causes replicative senescence of CMs by mid‐neonatal stages, these findings suggest that Tip60 induces these events. To examine these possibilities we are conditionally depleting Tip60 in CMs of Tip60 F/F;Cre neonatal mice by injecting tamoxifen at postnatal day 0 (P0). To date, immunostaining of sections from neonatal hearts with antibodies that recognize 5′‐BrdU, phospho‐histone‐H3, and phospho‐ATM (pATM) has revealed that Tip60 depletion significantly increases CM proliferation as early as P7, by which time CM proliferation is normally diminished, and that this is accompanied by significantly decreased pATM levels, indicative of a diminished DDR. These findings provide further support for the notion that DDR activation at the time of birth induces replicative senescence of neonatal CMs, implicating Tip60 as the inducer of this process. Support or Funding Information NIH HL131788 (JL & JA), and a grant from the MCW Cardiovascular Center This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

The pathogenesis of myocardial infarction (MI) is largely attributed to the loss of cardiomyocytes (CMs) and their insufficient regeneration. Inducing the proliferation of pre-existing CMs has emerged as a potential therapeutic strategy for cardiac repair. Results in our laboratory indicate that Tip60 (Tat-interactive protein 60 kD), a pan-acetylase protein encoded by the Kat5 gene, inhibits CM proliferation consequent to its induction of the DNA damage response (DDR) at neonatal stages, which has recently been shown to cause CM replicative senescence. To determine whether Tip60 depletion permits re-entry of adult CMs into the cell-cycle and confers protection from MI, we are employing a line of Kat5 flox/flox mice wherein Tip60 is conditionally and specifically depleted in CMs via tamoxifen-induced activation of a Myh6 -driven merCremer transgene. In uninjured hearts, Tip60 depletion results in transient thickening of the left ventricular walls, accompanied by markedly increased expression of G 2 /M-phase cell cycle regulators (cyclins A2 & B1, Cdk1) and de-differentiation markers (Myh7, Osm, OsmR, Runx1), diminished CM size, decreased expression of cell-cycle inhibitors (p27, Meis1), and remarkable increases in Ki67 and pH3-positive CMs as well as non-CMs. In hearts infarcted by permanent ligation, tamoxifen administration increases fractional shortening, ejection fraction, and anterior wall thickening within 7 days, conditions that are sustained for at least 18 additional days, when reduced scarring is indicated by trichrome staining. Taken together, these results indicate that Tip60 depletion in adult heart may preserve cardiac performance after MI by inducing CM regeneration. These findings should help advance our understanding of the molecular mechanisms that keep CMs in replicative senescence, establishing a possible therapeutic target for maintaining and restoring cardiac muscle after MI.

Exercise training is known to protect the heart from ischemia and improve function during exercise by reducing cardiomyocyte action potential duration (APD) and increasing contractility. The cellular mechanisms involve β-adrenergic regulation and the ATP-sensitive K+(KATP) channel, but how each alters function of the left ventricle and sex specificity is unknown. To address this, female and male Sprague-Dawley rats were randomly assigned to wheel-running (TRN) or sedentary (SED) groups. After 6–8 wk of training, myocytes were isolated from the left ventricle and field stimulated at 1, 2, and 5 Hz. TRN significantly increased cardiomyocyte contractility, the kinetics of the Ca2+transient, and responsiveness to the adrenergic receptor agonist isoproterenol (ISO), as reflected by an increased sarcomere shortening. Importantly, we demonstrated a TRN-induced upregulation of KATPchannels, which was reflected by elevated content, current density, and the channel’s contribution to APD shortening at high activation rates and in the presence of the activator pinacidil. TRN induced increase in KATPcurrent occurred throughout the left ventricle, but channel subunit content showed regional specificity with increases in Kir6.2 in the apex and SUR2A in base regions. In summary, TRN elevated cardiomyocyte cross-bridge kinetics, Ca2+sensitivity, and the responsiveness of contractile function to β-adrenergic receptor stimulation in both sexes. Importantly, upregulation of the KATPchannel accelerates repolarization and shortens APD during stress and exercise. These adaptations have clinical importance, as increased contractility and reduced APD would help protect cardiac output and reduce intracellular Ca2+overload during stresses such as regional ischemia.NEW & NOTEWORTHY Our results demonstrate that regular exercise significantly increased ventricular myocyte shortening and relaxation velocity and the rate of rise in intracellular Ca2+transient and enhanced the response of biomechanics and Ca2+reuptake to β-adrenergic stimulation. Importantly, exercise training upregulated the cardiomyocyte sarcolemma ATP-sensitive K+channel across the left ventricle in both sexes, as reflected by elevated channel subunit content, current density, and the channel’s contribution to reduced action potential duration at high activation rates.

Regular exercise-training is known to reduce the incidences and severity of ischemic heart disease, but the mechanistic nature is unknown. We showed that regular exercise shortened the left ventricular action potential duration (APD) at high heart rate (HR), which would prevent Ca2+ overload and/or inadequate filling under conditions of metabolic stress. Here we test the hypothesis that exercise-training induced APD shortening at high HR is due to the up-regulation of ATP sensitive potassium channel (KATP). Female and male Sprague-Dawley rats were randomly assigned to voluntary wheel running or control groups. After 6-8 weeks training, cardiac myocytes were isolated from the apex and base regions of the left ventricle with collagenase–protease dispersion technique. APD was measured with glass micro-electrode, current-voltage relationship was recorded with discontinuous single electrode voltage clamp, and the expression level of KATP channel pore-forming subunit Kir6.2 and regulatory subunits SUR2A were determined by Western blots. At 1Hz, KATP activator pinacidil (100 µM) shortened APD in myocytes from both sexes and, the shortening was significantly greater in exercise-trained compared to sedentary groups. The KATP inhibitor glibenclamide (2 µM) prolonged the 10Hz APD more in exercise-trained than control rats. Regular exercise enhanced KATP outward current density in apex myocytes from both sexes; in base, trained females exhibited larger KATP inward current than controls. In both sexes, Kir6.2 expression was elevated by wheel-running in apex but not base region. Exercise-training increased SUR2A density in base region of both sexes, and eliminated its reginal difference in females. In conclusion, exercise-training induced APD shortening under energy demanding conditions (e.g. high HR) is caused at least in part by KATP up-regulation, an adaptation that reduces energy requirements for ion homeostasis and maintains a diastolic interval adequate for myocardial relaxation.

Regular exercise training is known to affect the action potential duration (APD) and improve heart function, but involvement of β-adrenergic receptor (β-AR) subtypes and/or the ATP-sensitive K+ (KATP) channel is unknown. To address this, female and male Sprague-Dawley rats were randomly assigned to voluntary wheel-running or control groups; they were anesthetized after 6-8 wk of training, and myocytes were isolated. Exercise training significantly increased APD of apex and base myocytes at 1 Hz and decreased APD at 10 Hz. Ca2+ transient durations reflected the changes in APD, while Ca2+ transient amplitudes were unaffected by wheel running. The nonselective β-AR agonist isoproterenol shortened the myocyte APD, an effect reduced by wheel running. The isoproterenol-induced shortening of APD was largely reversed by the selective β1-AR blocker atenolol, but not the β2-AR blocker ICI 118,551, providing evidence that wheel running reduced the sensitivity of the β1-AR. At 10 Hz, the KATP channel inhibitor glibenclamide prolonged the myocyte APD more in exercise-trained than control rats, implicating a role for this channel in the exercise-induced APD shortening at 10 Hz. A novel finding of this work was the dual importance of altered β1-AR responsiveness and KATP channel function in the training-induced regulation of APD. Of physiological importance to the beating heart, the reduced response to adrenergic agonists would enhance cardiac contractility at resting rates, where sympathetic drive is low, by prolonging APD and Ca2+ influx; during exercise, an increase in KATP channel activity would shorten APD and, thus, protect the heart against Ca2+ overload or inadequate filling.NEW & NOTEWORTHY Our data demonstrated that regular exercise prolonged the action potential and Ca2+ transient durations in myocytes isolated from apex and base regions at 1-Hz and shortened both at 10-Hz stimulation. Novel findings were that wheel running shifted the β-adrenergic receptor agonist dose-response curve rightward compared with controls by reducing β1-adrenergic receptor responsiveness and that, at the high activation rate, myocytes from trained animals showed higher KATP channel function.