Changes in the fluctuation of the contraction rhythm of spontaneously beating cardiac myocytes in cultures with and without cardiac fibroblasts

The heart functions as a syncytium of cardiac myocytes and surrounding supportive non-myocytes such as fibroblasts. There is a possibility that a variety of non-myocyte-derived factors affect the maturation of cardiac myocytes in the development of the heart. Cultured neonatal cardiac myocytes contract spontaneously and cyclically. The fluctuation of beating rhythm varies depending on the strength of coupling through gap junctions among cardiac myocytes, indicating that the development of intercellular communication via gap junctions is crucial to the stability of contraction rhythm in cardiac myocytes. In this study, we aimed at elucidating whether and how cardiac fibroblasts affect the development of cardiac myocytes from the point of view of the changes in the fluctuation of the contraction rhythm of cardiac myocytes in cardiac myocyte–fibroblast co-cultures. The present study suggested that cardiac fibroblasts co-cultured with cardiac myocytes enhanced the intercellular communication among myocytes via gap junctions, thereby stabilizing the spontaneous contraction rhythm of cultured cardiac myocytes.     

Fluctuations of Contraction Rhythm During Simulated Ischemia/Reperfusion in Cultured Cardiac Myocytes from Neonatal Rats

Cardiac ischemia results in a rapid decrease of intracellular pH and in the rise of intracellular Ca 2+ , changes that have been shown to reduce intercellular communication via gap junctions (GJ) between cardiac myocytes. Ischemia also results in electrical instability probably caused by the reduced GJ permeability contributing to an increased vulnerability to arrhythmias. This study aims at elucidating whether the fluctuations of contraction rhythm of spontaneously beating cardiac myocytes in culture changes during simulated ischemia/reperfusion. The coefficient of variation (CV) of contraction intervals, reflecting the fluctuation of contraction rhythm, increased significantly during simulated ischemia/reperfusion. However, the contraction rhythm of the cardiac myocytes in an aggregate remained synchronized during simulated ischemia/reperfusion. In contrast, pharmacological blockade of GJ with 12-doxyl stearic acid, a blocker of GJ permeability, resulted in the de-synchronization of contraction rhythm and in an increase in the CV of contraction intervals in normoxic conditions. The present findings lead to the suggestion that GJ remained open during simulated ischemia/reperfusion, and that a mechanism other than electrical uncoupling between myocytes contributed to the observed increase in the fluctuation of beating rhythm during ischemia.     

Possible involvement of ATP-purinoceptor signalling in the intercellular synchronization of intracellular Ca2+ oscillation in cultured cardiac myocytes

Isolated and cultured neonatal cardiac myocytes contract spontaneously and cyclically. The contraction rhythms of two isolated cardiac myocytes, each of which beats at different frequencies at first, become synchronized after the establishment of mutual contacts, suggesting that mutual entrainment occurs due to electrical and/or mechanical interactions between two myocytes. The intracellular concentration of free Ca(2+) also changes rhythmically in association with the rhythmic contraction of myocytes (Ca(2+) oscillation), and such a Ca(2+) oscillation was also synchronized among cultured cardiac myocytes. In this study, we investigated whether intercellular communication other than via gap junctions was involved in the intercellular synchronization of intracellular Ca(2+) oscillation in spontaneously beating cultured cardiac myocytes. Treatment with either blockers of gap junction channels or an un-coupler of E-C coupling did not affect the intercellular synchronization of Ca(2+) oscillation. In contrast, treatment with a blocker of P2 purinoceptors resulted in the asynchronization of Ca(2+) oscillatory rhythms among cardiac myocytes. The present study suggested that the extracellular ATP-purinoceptor system was responsible for the intercellular synchronization of Ca(2+) oscillation among cardiac myocytes.     

心肌细胞自发性搏动节律的分岔和混沌现象

心脏的节律是复杂的、非线性的;其节律复杂性的起源是多层次的。实验观察了心肌细胞自发性搏动节律的模式,以及改变细胞间耦合强度时节律的转化规律。表明在以正常灌流液灌流状态下,心肌细胞表现为多种不同的节律模式,可以是周期的,也可以是非周期的。当细胞间耦合强度下降时,心肌细胞节律发生转化,并经倍周期分岔进入混沌节律。实验结果有助于更好地理解心脏节律复杂性的起源。     

Rhythmic contraction and intracellular Ca2 + oscillatory rhythm in spontaneously beating cultured cardiac myocytes

Cultured cardiac myocytes from neonatal rats show spontaneous and rhythmic contractions. The intracellular concentration of free Ca^2 + also changes rhythmically, associated with the rhythmic contraction of myocytes (Ca^2 + oscillation). This study aims to elucidate whether spontaneous rhythmic contraction affects the dynamics of intracellular Ca^2 + oscillation in cultured cardiac myocytes. In cultures at four days in vitro (4 DIV), spontaneous Ca^2 + oscillation was synchronized among myocytes. Treatment of cultures with an uncoupler of E - C coupling resulted in a cessation of the spontaneous contraction of cardiac myocytes, but did not abolish the Ca^2 + oscillation. The intercellular synchronization of intracellular Ca^2 + oscillation persisted, and both the intervals and the fluctuation of the oscillation tended to increase after the termination of rhythmic contraction. The present study demonstrated that mechanical factors associated with rhythmic contraction did not affect the intercellular synchronization of intracellular Ca^2 + oscillation, but possibly contributed to the stability of the oscillatory rhythm.     

Carbachol-induced Suppression of Contraction Rhythm in Spontaneously Beating Cultured Cardiac Myocytes From Neonatal Rats

Nitric oxide (NO) and reactive oxygen species (ROS) are known to play various functional and pathophysiological roles as an intracellular messenger in the heart. In this study, we investigated whether the increased production of NO and/or ROS was involved in the cholinergic regulation of rhythmic contraction in spontaneously beating cultured cardiac myocytes from neonatal rats. Exposure of cultures to carbachol, an agonist of muscarinic acetylcholine receptors (mAchR), produced a dose-dependent decrease in the beat rate of cultured cardiac myocytes, and such a effect was significantly attenuated by pre-treatment with an NOS inhibitor, as well as an NO scavenger. In addition, exposure to an NO donor (SNAP) also decreased the beat rate dose-dependently. Carbachol- or SNAP-induced suppression of the contraction rhythm was significantly attenuated by co-treatment with 5-hydroxydecanoate (5-HD). In contrast, treatment with diazoxide decreased the beat rate dose-dependently. Carbachol treatment increased the intensity of 2',7'-dichlorodihydrofluorescein fluorescence, suggesting that the production of ROS was enhanced by the treatment. In addition, the carbachol- or diazoxide-induced suppression of contraction rhythm was attenuated by co-treatment with 2-mercaptopropionyl glycine, a scavenger of ROS. The present study has suggested that the mAchR-NO-mitoK ATP -ROS pathway is a factor responsible for carbachol-induced suppression of contraction rhythm in cultured cardiac myocytes.     

The Expression, Phosphorylation, and Localization of Connexin 43 and Gap-Junctional Intercellular Communication during the Establishment of a Synchronized Contraction of Cultured Neonatal Rat Cardiac Myocytes

We analyzed the expression, phosphorylation, and localization of the major cardiac gap-junction protein connexin 43 (Cx43) during the establishment of a synchronized contraction in confluent monolayers of primary cultured neonatal rat cardiac myocytes, combined with a functional assay of gap junctions by the microinjection-dye transfer method. Monitoring of the beating rate and synchronization by Fotonic Sensor showed that at Day 1 of culture cardiac myocytes contracted spontaneously but irregularly, that the contractile rate increased with culture time, and that a synchronized contraction was gradually formed. At Day 7, the confluent cells exhibited synchronous contraction with a relatively constant rate (125 ± 20 beats/min). Cardiac myocytes expressed a large amount of Cx43 mRNA even at Day 1 and maintained the expression until at least Day 7. Immunofluorescence of Cx43 showed that the localization of Cx43-positive spots was mostly restricted to cell-cell contacts between myocytes and that few Cx43-positive spots were present between myocytes and fibroblasts or between fibroblasts. The amount of Cx43 protein, the proportion of phosphorylated forms to the nonphosphorylated one, and the number and total area of Cx43-positive spots increased with culture time. Gap-junctional intercellular communication measured by dye transfer assay was also increased with culture time and correlated well with the number and total area of Cx43-positive spots. Our systematic study suggests that a concerted action of the expression, phosphorylation, and localization of Cx43 and gap-junctional intercellular communication plays a major role in the reestablishment of synchronous beating of cultured neonatal rat cardiac myocytes.     

Fluctuations in the Concentration of Extracellular ATP Synchronized with Intracellular Ca2+ Oscillatory Rhythm in Cultured Cardiac Myocytes

Isolated and cultured neonatal cardiac myocytes contract spontaneously and cyclically. The intracellular concentration of free Ca²⁺ also changes rhythmically in association with the rhythmic contraction of myocytes (Ca²⁺ oscillation). Both the contraction and Ca²⁺ oscillatory rhythms are synchronized among myocytes, and intercellular communication via gap junctions has been considered primarily responsible for the synchronization. However, a recent study has demonstrated that intercellular communication via extracellular ATP‐purinoceptor signaling is also involved in the intercellular synchronization of intracellular Ca²⁺ oscillation. In this study, we aim to elucidate whether the concentration of extracellular ATP changes cyclically and contributes to the intercellular synchronization of Ca²⁺ oscillation among myocytes. In almost all the cultured cardiac myocytes at four days in vitro (4 DIV), intracellular Ca²⁺ oscillations were synchronized with each other. The simultaneous measurement of the concentration of extracellular ATP and intracellular Ca²⁺ revealed the extracellular concentration of ATP actually oscillated concurrently with the intracellular Ca²⁺ oscillation. In addition, power spectrum and cross‐correlation analyses suggested that the treatment of cultured cardiac myocytes with suramin, a blocker of P2 purinoceptors, resulted in the asynchronization of Ca²⁺ oscillatory rhythms among cardiac myocytes. Treatment with suramin also resulted in a significant decrease in the amplitudes of the cyclic changes in both intracellular Ca²⁺ and extracellular ATP. Taken together, the present study demonstrated the possibility that the concentration of extracellular ATP changes cyclically in association with intracellular Ca²⁺, contributing to the intercellular synchronization of Ca²⁺ oscillation among cultured cardiac myocytes.     

Fluctuations in the concentration of extracellular ATP synchronized with intracellular Ca(2+) oscillatory rhythm in cultured cardiac myocytes

Isolated and cultured neonatal cardiac myocytes contract spontaneously and cyclically. The intracellular concentration of free Ca²⁺ also changes rhythmically in association with the rhythmic contraction of myocytes (Ca²⁺ oscillation). Both the contraction and Ca²⁺ oscillatory rhythms are synchronized among myocytes, and intercellular communication via gap junctions has been considered primarily responsible for the synchronization. However, a recent study has demonstrated that intercellular communication via extracellular ATP-purinoceptor signaling is also involved in the intercellular synchronization of intracellular Ca²⁺ oscillation. In this study, we aim to elucidate whether the concentration of extracellular ATP changes cyclically and contributes to the intercellular synchronization of Ca²⁺ oscillation among myocytes. In almost all the cultured cardiac myocytes at four days in vitro (4 DIV), intracellular Ca²⁺ oscillations were synchronized with each other. The simultaneous measurement of the concentration of extracellular ATP and intracellular Ca²⁺ revealed the extracellular concentration of ATP actually oscillated concurrently with the intracellular Ca²⁺ oscillation. In addition, power spectrum and cross-correlation analyses suggested that the treatment of cultured cardiac myocytes with suramin, a blocker of P2 purinoceptors, resulted in the asynchronization of Ca²⁺ oscillatory rhythms among cardiac myocytes. Treatment with suramin also resulted in a significant decrease in the amplitudes of the cyclic changes in both intracellular Ca²⁺ and extracellular ATP. Taken together, the present study demonstrated the possibility that the concentration of extracellular ATP changes cyclically in association with intracellular Ca²⁺, contributing to the intercellular synchronization of Ca²⁺ oscillation among cultured cardiac myocytes.     

Role of the community effect of cardiomyocyte in the entrainment and reestablishment of stable beating rhythms

To investigate the roles that the community effect and entrainment function of cultured cardiomyocyte play in decreasing beating fluctuation and reestablishing synchronized beating, we developed a single-cell-based two-dimensional network culture assay to measure and compare the dynamics of beating rhythm synchronization of individual cells before and after they form networks. Studying the formation of two-cell networks, we found that their synchronized beating tended to be determined by the cardiomyocyte whose beat rate fluctuated less than that of the other cardiomyocyte. We further found that the strength of this tendency increased with the number of cells in the network. These results indicate that (1) beating fluctuation is one of the important factors influencing the reestablishment of a stable synchronous beating rhythm, (2) the larger networks reduce fluctuation, and (3) the formation of a spatial network can itself stabilize cardiomyocyte beat rates.     

Maintenance and characterization of spontaneous contraction rhythm in cultured cardiac myocytes fused with cardiac fibroblasts

Cardiomyocytes (CMs) fuse with various cells including endothelial cells, cardiac fibroblasts (CFs). In addition, recent studies have shown that stem cells fuse spontaneously with cells remaining in the damaged tissues, and restore tissue functions after myocardial infarction. In this study, we investigated whether cultured cardiomyocytes fused with proliferative cardiac fibroblasts maintained the phenotype of functional myocytes by analyzing the spontaneous contraction rhythm after fusion with CFs lacking a beating capability. CMs and CFs cultured for 4 days in vitro were used in this study. The fusion of cultured CMs and CFs was achieved with polyethylene glycol (PEG) and hemagglutinating virus of Japan (HVJ). Analyses of CMs fused with CFs by using either PEG or HVJ to imitate spontaneous fusion in vivo demonstrated that CMs and CFs actually fused together and fused cells expressed lineage marker proteins of both CMs and CFs. In addition, fused cells reentered the G2-M phase of the cell cycle. Furthermore, fused cells retained the spontaneous contraction activity. The present study demonstrated that CMs fused with proliferative CFs showed the phenotype of both CMs and CFs and spontaneous rhythmic contraction.     

Coupling between contraction rhythm and calcium oscillation in a cultured cardiac myocyte: Fluctuation behaviour and its modelling

Isolated and cultured neonatal cardiac myocytes show self-sustaining cyclic contraction, and have the properties of a nonlinear oscillator. We study the dynamics of mechanical contraction and cellular free Ca²⁺ in a single myocyte for the purposes of gaining an insight into the way in which excitation and contraction processes are inter-related. The concentration of intracellular Ca²⁺ in the myocyte is also found to vary periodically associated with its rhythmic contraction. The Ca²⁺ dynamics maintains its self-oscillatory nature when the spontaneous contraction is abolished by pharmacological treatment using 2,3-butanedione monoxime. However, fluctuation analysis of the Ca²⁺ oscillation intervals reveals that there occurs a characteristic change in the fluctuation behaviour due to the suppression of contraction; the mean value and fluctuation magnitude of the oscillation intervals and the persistency of the fluctuation correlations at short timescales all increase after pharmacological treatment. We develop a new nonlinear model based on Bonhoeffer - van der Pol oscillators to elucidate the mechanisms behind the observed effects of cardiac contraction on the Ca²⁺ oscillation. The model is composed of three coupled nonlinear differential equations that can describe the dynamics of both excitation (cellular free Ca²⁺) and contraction. Almost all the experimental findings are successfully reproduced by adjusting a parameter in the model responsible for excitation - contraction coupling.     

Exploring the implicit interlayer regulatory mechanism between cells and tissue: Stochastic mathematical analyses of the spontaneous ordering in beating synchronization

The present study focused on beating synchronization, and tried to elucidate the interlayer regulatory mechanisms between the cells and clump in beating synchronization with using the stochastic simulations which realize the beating synchronizations in beating cells with low cell–cell conductance. Firstly, the fluctuation in interbeat intervals (IBIs) of beating cells encouraged the process of beating synchronization, which was identified as the stochastic resonance. Secondly, fluctuation in the synchronized IBIs of a clump decreased as the number of beating cells increased. The decrease in IBI fluctuation due to clump formation implied both a decline of the electrophysiological plasticity of each beating cell and an enhancement of the electrophysiological stability of the clump. These findings were identified as the community effects. Because IBI fluctuation and the community effect facilitated the beating stability of the cell and clump, these factors contributed to the spontaneous ordering in beating synchronization. Thirdly, the cellular layouts in clump affected the synchronized beating rhythms. The synchronized beating rhythm in clump was implicitly regulated by a complicated synergistic effect among IBI fluctuation of each beating cell, the community effect and the cellular layout. This finding was indispensable for leading an elucidation of mechanism of emergence. The stochastic simulations showed the necessity of considering the synergistic effect, to elucidate the interlayer regulatory mechanisms in biological system.     

Reversible Inhibition of Gap Junctional Intercellular Communication, Synchronous Contraction, and Synchronism of Intracellular Ca Fluctuation in Cultured Neonatal Rat Cardiac Myocytes by Heptanol

We analyzed by Fotonic Sensor, a fiber-optic displacement measurement instrument, the effects of heptanol on synchronized contraction of primary neonatal rat cardiac myocytes cultured at confluent density. We also examined the effect of heptanol on the changes in gap junctional intercellular communication by using the microinjection dye transfer method, and on intercellular Ca²⁺ fluctuation by confocal laser scanning microscopy of myocytes loaded with the fluorescent Ca²⁺ indicator fluo 3. In addition, we studied expression, phosphorylation, and localization of the major cardiac gap junction protein connexin 43 (Cx43) using immunofluorescence and Western blotting. At Day 6 of culture, numerous myocytes exhibited spontaneous, synchronous contractions, excellent dye coupling, and synchronized intracellular Ca²⁺ fluctuations. We treated the cells with 1.5, 2.0, 2.5, and 3.0 mmol/liter heptanol. With 1.5 mmol/liter heptanol, we could not observe significant effects on spontaneous contraction of myocytes. At 3.0 mmol/liter, the highest concentration used in the current experiment, heptanol inhibited synchronous contractions and even after washing out of heptanol, synchronous contraction was not rapidly recovered. On the other hand, at the intermediate concentrations of 2.0 and 2.5 mmol/liter, heptanol reversely inhibited synchronized contraction, gap junctional intercellular communication, and synchronization of intracellular Ca²⁺ fluctuations in the myocytes without preventing contraction and changes of intracellular Ca²⁺ in individual cells. Brief exposure (5-20 min) to heptanol (2.0 mmol/liter) did not cause detectable changes in the expression, phosphorylation, or localization of Cx43, despite strong inhibition of gap junctional intercellular communication. These results suggest that gap junctional intercellular communication plays an important role in synchronous intracellular Ca²⁺ fluctuations, which facilitate synchronized contraction of cardiac myocytes.     

Beyond Bowditch: the convergence of cardiac chronotropy and inotropy

The ability of the heart to acutely beat faster and stronger is central to the vertebrate survival instinct. Released neurotransmitters, norepinephrine and epinephrine, bind to beta-adrenergic receptors (beta-AR) on pacemaker cells comprising the sinoatrial node, and to beta-AR on ventricular myocytes to modulate cellular mechanisms that govern the frequency and amplitude, respectively, of the duty cycles of these cells. While a role for sarcoplasmic reticulum Ca(2+) cycling via SERCA2 and ryanodine receptors (RyR) has long been appreciated with respect to cardiac inotropy, recent evidence also implicates Ca(2+) cycling with respect to chronotropy. In spontaneously beating primary sinoatrial nodal pacemaker cells, RyR Ca(2+) releases occurring during diastolic depolarization activate the Na(+)-Ca(2+) exchanger (NCX) to produce an inward current that enhances their diastolic depolarization rate, and thus increases their beating rate. beta-AR stimulation synchronizes RyR activation and Ca(2+) release to effect an increased beating rate in pacemaker cells and contraction amplitude in myocytes: in pacemaker cells, the beta-AR stimulation synchronization of RyR activation occurs during the diastolic depolarization, and augments the NCX inward current; in ventricular myocytes, beta-AR stimulation synchronizes the openings of unitary L-type Ca(2+) channel activation following the action potential, and also synchronizes RyR Ca(2+) releases following depolarization, and in the absence of depolarization, both leading to the generation of a global cytosolic Ca(i) transient of increased amplitude and accelerated kinetics. Thus, beta-AR stimulation induced synchronization of RyR activation (recruitment of additional RyRs to fire) and of the ensuing Ca(2+) release cause the heart to beat both stronger and faster, and is thus, a common mechanism that links both the maximum achievable cardiac inotropy and chronotropy.     

The effects of CGRP on calcium transients of dedifferentiating cultured adult rat cardiomyocytes compared to non-cultured adult cardiomyocytes: possible protective and deleterious results in cardiac function

CGRP has potent cardiovascular effects but its role in heart failure is unclear. Effects of CGRP on calcium concentrations in fresh adult rat cardiomyocytes, cultured adult cardiomyocytes and neonatal cardiomyocytes were determined by real time fluorescence spectrophotometry. Treatment of cultured adult cardiomyocytes with CGRP resulted in a rapid cessation of beating and a reduction in intracellular calcium. Similar results were obtained in cultured neonatal myocytes. However, rod-shaped adult cardiomyocytes revealed a number of responses; (a) non-beating cells began to beat with increased intracellular calcium; (b) spontaneously beating cells exhibited increased intracellular calcium content and a faster beating rate or (c), myocytes increased their beating rate and became arrhythmic, suggesting that CGRP action on cultured dedifferentiated adult and neonatal myocytes depletes intracellular calcium, whereas in the rod-shaped mature myocytes calcium is retained, pointing to a different mode of action for CGRP on developing and dedifferentiating cardiomyocytes, compared to fully developed cardiomyocytes.     

Gap junction formation and functional interaction between neonatal rat cardiocytes in culture: A correlative physiological and ultrastructural study

Summary The time course of gap junction formation and growth, following contraction synchronization of cardiac myocytes in culture, has been studied in a combined (electro)physiological and ultrastructural study. In cultures of collagenase-dissociated neonatal rat cardiocytes, pairs of spontaneously beating myocytes synchronized their contractions within one beat interval within 2–20 min after they apparently had grown into contact, 45 sec after the first synchronized beat an appreciable junctional region containing several small gap junctions was already present. In the following 30 min, neither the area of individual gap junctions nor their total area increased, 75 min after synchronization both the area of individual gap junctions and their total area had increased by a factor of 10–15 with respect to what was found in the first half hour. In the period between 75 and 300 min again no further increase in gap junctional area was found. In double voltage-clamp experiments, gap junctions between well-coupled cells behaved like ohmic conductors. In poorly coupled cells, in which the number of functional gap-junctional channels was greatly reduced, the remaining channels showed voltage-dependent gating. Their single-channel conductance was 40–50 pS. The electrophysiologically measured junctional conductance agreed well with the conductance calculated from the morphometrically determined gap-junctional area. It is concluded that a rapid initial gap junction formation occurs during the 2–20 min period prior to synchronization by assembly of functional channels from existing channel precursors already present in the cell membranes. It then takes at least another 30 min before the gap-junctional area increases possibly byde novo synthesis or by recruitment from intracellular stores or from nonjunctional membranes, a process completed in the next 45 min.     

A novel method of cultivating cardiac myocytes in agarose microchamber chips for studying cell synchronization

We have developed a new method that enables agar microstructures to be used to cultivate cardiac myocyte cells in a manner that allows their connection patterns to be controlled. Non-contact three-dimensional photo-thermal etching with a 1064-nm infrared focused laser beam was used to form the shapes of agar microstructures. This wavelength was selected as it is not absorbed by water or agar. Identical rat cardiac myocytes were cultured in adjacent microstructures connected by microchannels and the interactions of asynchronous beating cardiac myocyte cells observed. Two isolated and independently beating cardiac myocytes were shown to form contacts through the narrow microchannels and by 90 minutes had synchronized their oscillations. This occurred by one of the two cells stopping their oscillation and following the pattern of the other cell. In contrast, when two sets of synchronized beating cells came into contact, those two sets synchronized without any observable interruptions to their rhythms. The results indicate that the synchronization process of cardiac myocytes may be dependent on the community size and network pattern of these cells.     

The effect of angiotensin II on myosin heavy chain expression in cultured myocardial cells

Summary Angiotensin II (AII), the principal mediator of the renin-angiotensin system, is an important regulator of vascular and cardiac homeostasis. AII has also been shown to be a regulator of cardiac hypertrophy and of the corresponding changes in amount and composition of certain tissue proteins. We examined the trophic effects of AII on cultured myocytes derived from neonatal rat ventricles and followed, by Northern blot analysis and polyacrylamide gel electrophoresis, the expression of α- and β-myosin heavy chain iso-mRNAs and isoproteins. Our findings show that a single administration of AII is sufficient to induce a trophic response in cultured beating myocytes and to enhance the expression of β-myosin heavy chain iso-mRNA and isoprotein, having no effect on α-myosin heavy chain. Induction of α-myosin heavy chain expression by thyroid hormone before AII was administered showed that AII could not potentiate a shift from α- to β-myosin heavy chain predominance. We suggest that the potency of AII to regulate the expression of myosin heavy chain isogenes is restricted to the β isoform and is overridden by thyroid hormone.     

收缩活动促进新生大鼠培养心室肌细胞的^3H—亮氨酸...

To determine whether contraction could influence cell growth, the rate of protein synthesis (3H-leucine incorporation) and cell diameter and volume were measured in cultured neonatal rat cardiac myocytes beating spontaneously or arrested by high potassium. In medium supplemented with 10% calf serum, the 3H-leucine incorporation for 24 h in contracting myocytes (CMC) was significantly higher by 14.2% than that in quiescent myocytes (QMC), i.e. 1,229 +/- 29 cpm/10(5) cells vs. 1,076 +/- 60 cpm/10(5) cells (P < 0.01, n = 5 for each group). The cell diameter and cell volume in QMC group were respectively 15.14 +/- 0.42 microns and 1,842 +/- 123 microns3, while in the CMC group the corresponding figures reached to 16.82 +/- 0.64 microns3 and 2,495 +/- 210 microns3, increased by 11.1% and 35.5% respectively (P < 0.01, n = 6 for each group). With prolongation of culture time, the differences in these parameters between CMC and QMC became even more significant. In all these experiments, there was no significant difference in cell number between the two groups (P > 0.05). It is concluded that contraction per se can accelerate protein synthesis and cell growth in neonatal rat ventricular myocardium.