Potentiation Of Contractility Research Articles

ATP2A2 rs3026468 does not associate with quadriceps contractile properties and acute muscle potentiation in humans.

The ATP2A2 gene encodes the SERCA protein required for active calcium reuptake to the sarcoplasmic reticulum in cardiac and slow-twitch skeletal muscle. The ATP2A2 rs3026468 variant has been associated with voluntary strength phenotypes in humans but requires further validation. Here we investigated a homogenous cohort of 80 young, healthy, active Caucasian males who were assessed for maximal isometric strength, voluntary activation, stimulated contractile properties, and muscle potentiation in the quadriceps. A dynamometer was used to record knee extensions, and electrical stimulation was applied to the thigh to elicit a twitch response. DNA was isolated from cheek swabs, and the rs3026468 genotypes were assessed by TaqMan primer quantitative PCR. The results show no association between ATP2A2 rs3026468 variants and muscle strength measures. We conclude there is no effect of the rs3026468 variant in our cohort and that functional influences do not likely contribute to contractile property differences in young healthy men.

The effects of KB-R7943, an inhibitor of reverse Na+/Ca2+ exchange, on the force of contraction of papillary muscles in the heart of the ground squirrel Spermophilus undulatus

We investigated the effect of KB-R7943, an inhibitor of the reverse mode of Na+/Ca2+ exchanger, on the force of isometric contractions, the contractile force–frequency relationship and post-rest potentiation (a qualitative parameter of Ca2+ levels in sarcoplasmic reticulum) in the right ventricle papillary muscles isolated from ground squirrel hearts during summer (June, n = 4) and autumn (October, n = 4) activities. In the presence of 1.8 mM Ca2+at 36°C, 1–1.5 hours-long treatment of the summer papillary muscles with KB-R7943 produced no significant effects on the contractile indices at the majority of stimulation frequencies. In the autumn papillary muscles KB-R7943 induced a 40–50% decrease in the force of contraction (negative inotropic effect) at low stimulation frequencies (0.1–0.3 Hz) without any significant effect at higher stimulation frequencies (0.4–3.0 Hz). Furthermore, in this group, KB-R7943 suppressed the post-rest potentiation of contractility by 50 ± 21% at pause durations exceeding 120 s. These observations indicate that KB-R7943 can affect Ca2+ levels in sarcoplasmic reticulum and that Na+/Ca2+ exchange may contribute to the physiological remodeling of intracellular Ca2+ homeostasis in myocardium of hibernating animals prior their transition to a hypometabolic torpid state.

Rat skeletal muscle contractility: The role of beta-adrenoceptors and nitric oxide system

Both beta-adrenoceptors and the nitric oxide system play a significant role in the modulation of skeletal muscle contractility. Skeletal muscle adrenoceptors mainly belong to beta2 subtype, while all three types of nitric oxide synthase may influence muscle contractility. The aim of our study was to investigate the possible interplay between beta-adrenoceptor agonists and nitric oxide system in the modulation of contractility of isolated rat hemidiaphragm. Adrenaline (0.05-1.5 μmol/L) and noradrenaline (1-5 μmol/L) given in a cumulative manner produced a concentration-related increase in Td. L-NAME (1, 3 and 10 mmol/L; 30 min of incubation) produced a significant, dose-dependent increase in Td of the muscle pretreated with cumulative concentrations of adrenaline (ΔTd up to 16%). When hemidiaphragm was pretreated with noradrenaline instead of adrenaline, L-NAME (3 mmol/L) it produced a similar potentiation of Td. In conclusion, nitric oxide seems to oppose beta-adrenoceptor potentiation of diaphragm contractility, and such an interaction depends on previous adrenoceptor stimulation. Nitric oxide probably decreases beta-adrenoceptor response via cGMP-induced stimulation of phosphodiesterase 2. The interaction between substances which modulate NO system activity and cAMP levels in the skeletal muscle may be of a great clinical importance for the treatment of certain respiratory and neurological diseases.

Redox modulation of diaphragm contractility: Interaction between DHPR and RyR channels

Previous reports indicate that reactive oxygen species (ROS) may modulate contractility in skeletal muscle. Although Ca 2+-sensitivity of the contractile apparatus appears to be a primary site of regulation, dihydropyridine receptor (DHPR or L-type Ca 2+ channels) and calcium efflux in isolated sarcoplasmic reticulum (SR) vesicles appear to be redox sensitive as well. However, DHPR as a target is poorly understood in intact muscles at body temperature, particularly in the diaphragm, a muscle more dependent on external Ca 2+ than locomotor muscles. Previously, we reported that oxidant challenge via xanthine oxidase (XO) alters the K + contractures in diaphragm fiber bundles, suggestive of a role of L-type Ca 2+ channels. Contractility of isolated rat diaphragm fiber bundles revealed a biphasic response to ROS challenge that was dose and time dependent. Potentiation of twitch and low-frequency diaphragm fiber bundle contractility with 0.02 U•ml -1 XO was reversible or partially preventable with washout, dithiothreitol, and the SOD/catalase mimetic EUK-134. The RyR antagonist ruthenium red inhibited xanthine oxidase-induced potentiation, while the RyR agonist caffeine elevated diaphragm twitch and low-frequency tension in a non-additive manner by 55% when introduced simultaneously with ROS challenge. The DHPR antagonist nitrendipine (15 μM) inhibited elevation in low-frequency diaphragm tension produced by ROS challenge. Caffeine threshold tension curves were shifted to the left with 0.02 U•ml -1 XO, but this effect was partially reversed with 15 μM nitrendipine. These results are consistent with the hypothesis that DHPR redox state and RyR function are modulated in an interactive manner, affecting contractility in intact diaphragm fiber bundles.

Effects of GBE50 on physiological characteristics and intracellular free calcium of myocardium

To investigate effects of GBE50 on the physiological characteristics and intracellular free calcium of myocardium. The preparations of isolated atria of guinea pigs were used for recording the myocardial physiological characteristics and the technique of Fluo-3/AM was used for measuring the concentration of free calcium in myocytes. GBE50 (10, 20, 40, 80, 160 mg x L(-1)) prominently inhibited the contractive force and decreased spontaneous beat of right atrium in concentration-dependent manner. GBE50 (10, 20, 40, 80, 160 mg x L(-1)) inhibited the contractive force of left atrium, and markedly decreased the post rest potentiation of myocardial contractility in left atrium in concentration-dependent manner. GBE50 (10, 20, 40, 80, 160 mg x L(-1)) prolonged the function refractory period of isolated guinea pig left atrium. 50 mg x L(-1) GBE50 decreased [Ca2+]i of guinea pig ventricular myocytes in normal Tyrode's solution by (46.53 +/- 8.44)% in 12 minutes. GBE50 induces inhibitory effects of the spontaneous beat, contractility, post-rest potentiation and prolonged the function refractory period. GBE50 may decrease [Ca2+]i of guinea pig ventricular myocytes.

Potentiation of adrenaline vasoconstrictor response by sub-threshold concentrations of U-46619 in human umbilical vein: Involvement of smooth muscle prostanoid TP α receptor isoform

Considering the potential physiological, pharmacological and therapeutic relevance of synergistic interaction of thromboxane A 2 with adrenaline at postjunctional receptor sites, we examined whether sub-threshold concentrations of thromboxane A 2 mimetic U-46619 (9,11-dideoxy-9α, 11α-methanoepoxy prostaglandin F 2α) could amplify adrenaline-induced contraction in human umbilical vein. The receptor involved in U-46619-induced potentiation of adrenaline contractility was also investigated. Umbilical cords ( n = 125) from healthy patients after full-term vaginal or caesarean deliveries were employed. The vein was dissected out of cords and rings used for isolated organ bath experiments or reverse transcription-polymerase chain reaction (RT-PCR) and Western blot. Presence of endothelium did not modify U-46619-induced contraction in human umbilical vein. Prostanoid TP-selective receptor antagonist, SQ-29548 (7-[3-[[2-[(phenylamino)carbonyl]hydrazino]methyl]-7-oxabicyclo[2.2.1]hept-2-yl]-[1S(1α,2α( Z),3α,4α)]-5-Heptenoic acid), inhibited U-46619-induced contraction (pA 2 = 8.22 ± 0.11). U-46619 sub-threshold concentrations (0.1–0.3 nM) potentiated adrenaline-vasoconstriction response in a concentration-dependent manner. SQ-29548 (0.1 μM) abolished this potentiation. Using RT-PCR, we found that human umbilical vein rings with or without endothelium express the prostanoid TP α, but not the prostanoid TP β receptor isoform. Western blot allowed the identification of proteins with an electrophoretic mobility (47- and 55-kDa) indistinguishable from human platelet prostanoid TP receptor, a rich source of prostanoid TP α receptor isoform. Collectively, present results demonstrate that prostanoid TP α is the major receptor isoform localized on smooth muscle cells which participate in both direct vasoconstriction and potentiating effects of U-46619 on adrenaline contractions in human umbilical vein. These results suggest that thromboxane A 2 may interact synergistically with adrenaline in pathophysiological situations that lead to an increase of its umbilical venous levels (e.g. preeclampsia associated with fetal distress) raising the possibility of vasoconstriction affecting fetal blood flow.

Force-frequency relationship and rest potentiation in papillary muscles of Siberian ground squirrel in the period of preparation to hibernation

activity to hibernation. To date, any published data on the nature of rhythm-inotropic relationship in the myocardium of hibernating animals in this period are completely absent. Experiments were performed on the right ventricular papillary muscle isolated from ground squirrels ( C. undulatus ). The isolation, stimulation, and contraction amplitude ( A ) measurement were performed as described previously [11] at 30 ± 1°e . The steady-state force‐frequency relationships were studied in the frequency range from 0.1 to 1.0 Hz. The contraction amplitude of papillary muscles at a frequency of 0.1 Hz was taken as 100%. The maximum rest potentiation of contractility (in the range of 1 to 60 s), sometimes up to 600 s, was determined as an increment in the amplitude of the first post-rest contraction ( A 1 ) relative to the amplitude of the preceding steady-state contraction ( A 0 ) at a stimulation frequency of 0.8 Hz and calculated by the formula (( A 1 – A 0 )/ A 0 ) × 100, %) [14]. The mechanical restitution curves were obtained by plotting the increment in A 1 versus pause duration. The data are expressed as the mean and square error of the mean ( p < 0.05).

Mechanomyographic determination of post-activation potentiation in myopathies

Objective There is a need to provide an index of muscle contractility in evaluating myopathies especially in the clinical setting. This study was conducted to investigate if the mechanomyogram post-activation potentiation (MMG-PAP) can be used as an index of muscle contractile force potentiation (force-PAP), if it differs between normal and myopathic muscles, and if it can reflect abnormalities in muscle fiber anatomy. Methods The correlation between MMG-PAP and force-PAP was evaluated in 12 normal subjects after maximum voluntary contraction (MVC) of the biceps brachii muscle. The same method was then applied to study MMG-PAP in 16 patients with myopathies, 16 disease and 25 normal controls. Mean fiber diameters and the proportions of type 1 and 2 fibers in biopsied biceps brachii muscle were determined and compared with MMG-PAP values. Results There was a significant positive correlation between force-PAP (197±148%) and MMG-PAP (135±68%) immediately after MVC ( P<0.05). The mean MMG-PAP in myopathies (66±53%) was significantly lower than those of the disease (128±34%; P<0.005) and normal controls (120±56%; P<0.005). Patients with non-dystrophic myopathies, including those with myositis, had significantly lower MMG-PAP values (38±20%; P<0.005) than those with muscular dystrophy (148±23%). MMG-PAP did not clearly correlate with either type 2 fiber atrophy or type 2 fiber disproportion based on muscle biopsy analysis of myopathic patients. Conclusions This study shows that MMG-PAP can be used as an index of muscle contractility and that it is significantly lower in non-dystrophic myopathies compared to normal subjects. MMG-PAP does not seem to reflect abnormal muscle fiber anatomy. Significance MMG-PAP may become a valuable non-invasive tool in augmenting routine clinical electrophysiologic studies especially in evaluating muscle contractility in myopathies.

Myosin light chain kinase and myosin phosphorylation effect frequency-dependent potentiation of skeletal muscle contraction

Repetitive stimulation potentiates contractile tension of fast-twitch skeletal muscle. We examined the role of myosin regulatory light chain (RLC) phosphorylation in this physiological response by ablating Ca(2+)/calmodulin-dependent skeletal muscle myosin light chain kinase (MLCK) gene expression. Western blot and quantitative-PCR showed that MLCK is expressed predominantly in fast-twitch skeletal muscle fibers with insignificant amounts in heart and smooth muscle. In contrast, smooth muscle MLCK had a more ubiquitous tissue distribution, with the greatest expression observed in smooth muscle tissue. Ablation of the MYLK2 gene in mice resulted in loss of skeletal muscle MLCK expression, with no change in smooth muscle MLCK expression. In isolated fast-twitch skeletal muscles from these knockout mice, there was no significant increase in RLC phosphorylation in response to repetitive electrical stimulation. Furthermore, isometric twitch-tension potentiation after a brief tetanus (posttetanic twitch potentiation) or low-frequency twitch potentiation (staircase) was attenuated relative to responses in muscles from wild-type mice. Interestingly, the site of phosphorylation of the small amount of monophosphorylated RLC in the knockout mice was the same site phosphorylated by MLCK, indicating a potential alternative signaling pathway affecting contractile potentiation. Loss of skeletal muscle MLCK expression had no effect on cardiac RLC phosphorylation. These results identify myosin light chain phosphorylation by the dedicated skeletal muscle Ca(2+)/calmodulin-dependent MLCK as a primary biochemical mechanism for tension potentiation due to repetitive stimulation in fast-twitch skeletal muscle.

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Caffeine is the most widely consumed psychoactive ‘drug’ in the world and probably one of the most commonly used stimulants in sports. This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects. Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body. Peak plasma concentrations after normal consumption are usually around 50 μM, and half-lives for elimination range between 2.5–10 h. The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine. There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors. Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo. These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions. Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data. The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational.

IGF-I-induced enhancement of contractile response in organ-cultured aortae from diabetic rats is mediated by sustained thromboxane A2 release from endothelial cells

We have investigated the mechanisms underlying the changes in vascular contractile responsiveness induced by insulin and IGF-I in established streptozotocin-induced diabetic rats. The contractile response to noradrenaline (NA) in organ-cultured diabetic rat aortae cultured with insulin or IGF-I was significantly greater than the corresponding responses in (a) diabetic rat aortae cultured in serum-free medium and (b) control rat aortae cultured with insulin or IGF-I. In aortae from which the endothelium was removed after organ culture the contractile response to NA was greater in those cultured with insulin or IGF-I than in those cultured in serum-free medium. This was not true of aortae endothelium denuded before organ culture. The IGF-I-induced enhancement was prevented by treatment with indomethacin (cyclo-oxygenase inhibitor), SQ29548 (thromboxane (TX) A2 receptor antagonist) or fregrelate (TXA2 synthase inhibitor). IGF-I-induced production of TXB2, a metabolite of TXA2, was greater in diabetic than in control aortae and was attenuated by endothelium denudation, indomethacin or AG1024 (IGF-I receptor inhibitor). The expression of the protein and mRNA for the IGF-I receptor (as assessed by RT-PCR and immunohistochemistry) was markedly increased within endothelial cells in diabetic aortae but only slightly increased within smooth muscle cells (versus control rat aortae). Thus, the NA-induced contractile response in aortae from diabetic rats was enhanced by both insulin and IGF-I and this enhancement may be mediated by sustained cyclo-oxygenase-dependent TXA2 production from endothelial cells. The observed enhancement of IGF-I receptor expression within endothelial cells may be causally related to the potentiation of vascular contractility and the increase in TXA2 production.

The influence of lidocaine and beta-blockers on L-NAME-induced potentiation of the isolated rat hemidiaphragm contractility

L-NAME (1-10 mM, 30 min of incubation) potentiates the tension developed (Td) in the isolated rat hemidiaphragm stimulated by direct subtetanic electrical stimulation (OSES) (F = 14 Hz). The aim of our study was to investigate the possible modulatory effect of local anaesthetic (lidocaine) and beta adrenergic blockers, propranolol and atenolol, on the L-NAME- induced potentiation of isometric contractility (1C). Lidocaine (0.1 mM, 5 min incubation) did not change Td by itself. Also, in the presence of lidocaine (0.1 mM, 5 min incubation), the stimulatory effect of L-NAME (3 mM, 30 min incubation) was preserved. In contrast, lidocaine itself (0.1 mM, 30 min incubation) produced an increase of Td up to 50%. The incubation of the muscle with both lidocaine (0.1 mM, 30 min incubation) and L-NAME (3 mM, 30 min incubation) produced an almost additive stimulatory effects, up to 90% on Td. Propranolol itself (1 µM, 5 min incubation) did not change Td. In the presence of propranolol, L-NAME maintained the stimulatory effect on Td. Propranolol (1 µM, 30 min incubation) did not change Td itself, but abolished the stimulatory effect of L-NAME. Atenolol (1 µM, 30 min incubation) neither significantly changed Td nor L-NAME-induced potentiation of Td.

Potentiation of atrial contractility by paired pacing augments ventricular preload and systolic performance

Background: Paired electrical stimulation and postextrasystolic potentiation (PESP) of contractility has been extensively studied in ventricular myocardium, but less is known about PESP of atrial contractility. Our aim was to determine whether PESP of atrial contractility could augment left ventricular (LV) preload and improve LV systolic performance. Methods and Results: A paired electrical stimulus closely following the pacing stimulus was applied to isolated atrial and ventricular myocardium from 4 dog hearts, and the interval dependent force potentiation was examined. In isolated atrial myocardium, paired pacing increased the active tension from a baseline of 1.36 ± 0.23 to 2.60 ± 0.57 g/mm2; in ventricular myocardium active tension increased from 2.58 ± 0.42 to 3.81 ± 0.27 g/mm2 (both P <.01). Then, LV pressure (micromanometer) and segment length (ultrasonic crystals) were measured in the intact hearts of 7 anesthetized dogs in which premature stimuli were applied to the atrium. In intact hearts, paired pacing of the atrium (coupling interval 200 ms) increased LV end-diastolic pressure from 3.8 ± 1.0 to 6.4 ± 1.0 mm Hg; systolic pressure increased from 105 ± 6 to 112 ± 7 mm Hg (both P <.05). LV pressure-length loop area (regional stroke work) increased 10.5 ± 0.2%. Conclusions: Isolated atrial myocardium exhibits substantial PESP of contractility, which is similar to ventricular myocardium. In the intact heart, atrial PESP augments LV systolic performance by effecting an increase in LV preload. This technique may provide a means of improving cardiac performance in patients with heart failure.

Effect of amiodarone on the dynamics of mechanical restitution of rat papillary muscles.

We studied the effect of amiodarone (class III antiarrhythmic drug) on the dynamics of mechanical restitution of rat papillary muscle. Amiodarone produced a weak negative inotropic effect and stimulated potentiation of contractility of the muscle preparation after short-term (4-60 sec) cessation of its electrical stimulation. On the other hand, the time of attaining half-maximum amplitude of contractions after amiodarone treatment did not differ from the control. Analysis of curves presenting the drop of potentiation of muscle preparation contractility after resumption of regular electrical stimulation after 60-sec arrest until attaining a stable level showed that the amplitude returned to the initial level by the 9thcontraction-relaxation cycle both in the control and after amiodarone treatment. Coefficient of the drop of contraction amplitude potentiation was virtually the same in the two groups. Presumably, amiodarone does not modulate calcium-binding capacity of the sarcoplasmic reticulum, but improves Ca retention in the sarcoplasmic reticulum stores.

Role of the Na/Ca exchanger in arrhythmias in compensated hypertrophy.

Sudden, presumably arrhythmic, death is common in heart failure patients. Although total mortality is highest in end-stage failure, the fraction of sudden death in total mortality is higher in the early stages. In each of these stages various, not necessarily identical, ionic mechanisms may contribute to arrhythmogenesis. Dogs with chronic complete atrioventricular block (6-8 weeks) have an increased risk for arrhythmias and sudden death and have compensated biventricular hypertrophy. In this animal model, Ca(2+) release from the sarcoplasmic reticulum (SR) is not reduced. For low frequencies of stimulation, the SR Ca(2+) content is increased, related to a higher activity of the Na/Ca exchanger. Spontaneous Ca(2+) release induces inward Na/Ca exchange current, which can lead to delayed afterdepolarizations (DADs) triggering a new action potential. Such arrhythmogenic DADs and ectopic beats also can be observed in vivo during monophasic action potential recording. They appear after pacing protocols, and/or administration of ouabain, which result in contractile potentiation, suggestive of a enhanced sarcoplasmic reticulum Ca(2+) content. Other arrhythmogenic mechanisms related to increased dispersion of repolarization also can be identified in vivo. Downregulation of delayed K(+) currents is an important factor in prolongation of action potentials. In conclusion, in this animal model of compensated hypertrophy, Ca(2+) handling is different from end-stage heart failure. It is possible that arrhythmogenic mechanisms related to a higher Ca(2+) load contribute to the high incidence of sudden death in stages of compensated hypertrophy before overt heart failure. However, more than one ionic remodeling process is likely to be present, and different cellular mechanisms of arrhythmias can coexist.

Cortisol-mediated potentiation of uterine artery contractility: effect of pregnancy.

During pregnancy, maternal plasma cortisol concentrations approximately double. Given that cortisol plays an important role in the regulation of vascular reactivity, the present study investigated the potential role of cortisol in potentiation of uterine artery (UA) contractility and tested the hypothesis that pregnancy downregulated the cortisol-mediated potentiation. In vitro cortisol treatment (3, 10, or 30 ng/ml for 24 h) produced a dose-dependent increase in norepinephrine (NE)-induced contractions in both nonpregnant and pregnant (138-143 days gestation) sheep UA. However, this cortisol-mediated response was significantly attenuated by approximately 50% in pregnant UA. The 11 beta-hydroxysteroid dehydrogenase (11-beta HSD) inhibitor carbenoxolone did not change the effect of cortisol in nonpregnant UA but abolished its effect in pregnant UA by increasing the NE pD(2) in control tissues from 6.20 +/- 0.05 to 6.59 +/- 0.11. The apparent dissociation constant value of NE alpha(1)-adrenoceptors was not changed by cortisol in pregnant UA but was decreased in nonpregnant UA. There was no difference in glucocorticoid receptor density between nonpregnant and pregnant UA. Cortisol significantly decreased endothelial nitric oxide (NO) synthase protein levels and NO release in both nonpregnant and pregnant UA, but the effect of cortisol was attenuated in pregnant UA by approximately 50%. Carbenoxolone alone had no effects on NO release in nonpregnant UA but was decreased in pregnant UA. These results suggest that cortisol potentiates NE-mediated contractions by decreasing NO release and increasing NE-binding affinity to alpha(1)-adrenoceptors in nonpregnant UA. Pregnancy attenuates UA sensitivity to cortisol, which may be mediated by increasing type-2 11-beta HSD activity in UA.

Targeting Ca2+ cycling proteins and the action potential in heart failure by gene transfer.

Cardiomyocytes isolated from failing human hearts are characterized by contractile dysfunction including prolonged relaxation, reduced systolic force and elevated diastolic force. These contractile abnormalities are paralleled by abnormal Ca2+ homeostasis such as reduced sarcoplasmic reticulum (SR) Ca2+ release, elevated diastolic Ca2+ and reduced rate of Ca2+ removal. In addition, failing human myocardium is characterized by a frequency-dependent decrease in systolic force and Ca2+ as opposed to normal myocardium where an increase in pacing rate results in potentiation of contractility and an increase in SR Ca2+ release. In the failing heart, the decrease in SR Ca2+ load has been linked to a decrease in SR Ca2+ ATPase (SERCA2a) function. We have recently shown that overexpression of SERCA2a by adenoviral gene transfer restores contractile function in cardiac myocytes from failing human hearts. In addition, we have shown that overexpression of SERCA2a in a model of pressure-overload hypertrophy in transition to failure improves contractile function and reserve in these animals. We are currently exploring the effect of long-term expression of SERCA2a in failing animals along with the energy cost of SERCA2a expression using NMR methods. We are also using a different strategy to improve SR Ca2+ ATPase activity which involves decreasing the expression of phospholamban by antisense strategies to enhance SR Ca2+ ATPase activity. The Na/Ca exchanger is also being targeted to enhance calcium removal in failing hearts. Action potential prolongation is attributed to reductions in transient outward current (Ito) density in human heart failure. This prolongation can alter contractility but can also cause afterdepolarization. Using gene transfer of various K channels responsible for Ito, we are investigating the molecular and the ionic basis of action potential prolongation in cardiac hypertrophy and failure and we are examining how intracellular calcium handling changes in response to alterations in action potential duration. Gene transfer, which serves initially as an experimental tool, may provide a novel therapeutic approach.

Origin of ischemia-induced phase 1b ventricular arrhythmias in pig hearts

ObjectivesThe goal of this study was to establish the role of ventricular filling on the 1b phase of ischemia-induced arrhythmias. BackgroundIschemia-induced ventricular arrhythmias occur in two phases. The mechanism of the initiation of delayed (1b) arrhythmias is unknown. The 1b arrhythmias (15 to 60 min of ischemia) are abundant in in situ hearts but scarce in isolated perfused hearts (with drained ventricles). MethodsLeft ventricular (LV) epicardial mapping (11 × 11 matrix, 5 mm interelectrode distance) of the initiation of delayed arrhythmias was performed in open-chested pigs (group A, n = 7) and isolated pig hearts without (group B, n = 8) and with a filled intraventricular balloon (group C, n = 5). ResultsThere were no differences in ischemic zone size between groups. The ischemia-induced rise in tissue impedance was similar in groups A and B. Arrhythmias were less frequent and less severe in group B than in groups A or C, with no differences between groups A and C. An epicardial focal origin was detected in 26% of all first beats, significantly more from the ischemic border than from elsewhere. During a pacing protocol with a long pause (a separate group of four isolated hearts with a balloon), more premature beats occurred in the first postpause interval than in any other interval. ConclusionsIn isolated hearts 1b arrhythmias were less frequent and less severe than in working preparations. Focal activity was documented in 26% of arrhythmias and emerged from the ischemic border. Postpause contractile potentiation was associated with more arrhythmias. Our study suggests that the initiation of ischemia-induced 1b arrhythmias is related to LV wall stress.

Interval-dependent potentiation of left ventricular contractility is preserved in patients with atrial fibrillation and depressed ejection fraction

Echocardiographic techniques were used to measure left ventricular isovolumic and ejection phase indexes of contractility in 54 patients with atrial fibrillation, and the relations between cycle lengths and contractility were compared in patients with normal and depressed ejection fractions. Data indicate that variations in contractility occur in a pattern that is consistent with postextrasystolic potentiation and that such interval-dependent potentiation is preserved in patients with atrial fibrillation and depressed ejection fraction.

Protein kinase C delta but not PKC epsilon activity is involved in contractile potentiation by endothelin-1 in the porcine coronary artery.

To clarify the mechanism of contractile strengthening by endothelin-1 (ET-1), we measured translocation of protein kinase C (PKC) from the cytosol to the membrane fraction in the porcine coronary artery. ET-1 potentiated the serotonin- (5-hydroxytryptamine, 5-HT) induced contraction without any additional increase in myosin light chain phosphorylation. Four PKC isoforms (alpha, beta1, delta, and zeta) were identified but not PKC epsilon. Only PKC delta was translocated from the cytosolic to the membrane fraction during the contractile potentiation by ET-1. Our results suggest that the activity of PKC delta but not PKC epsilon is involved in the contractile strengthening by ET-1 in the porcine coronary artery.