Effects of acidosis on ventricular myocyte shortening and intracellular Ca2+ in streptozotocin-induced diabetic rats

We have investigated the effects of acute acidosis on ventricular myocyte shortening and intracellular Ca2+ in streptozotocin (STZ)-induced diabetic rat. Shortening and intracellular Ca2+ were measured in electrically stimulated myocytes superfused with either normal Tyrode solution pH adjusted to either 7.4 (control solution) or 6.4 (acid solution). Experiments were performed at 35-36 degrees C. At 8-12 weeks after treatment, the rats that received STZ had lower body and heart weights compared to controls, and blood glucose was characteristically increased. Contractile defects in myocytes from diabetic rat were characterized by prolonged time to peak shortening. Superfusion of myocytes from control and diabetic rats with acid solution caused a significant reduction in the amplitude of shortening; however, the magnitude of the response was not altered by STZ treatment. Acid solution also caused significant and quantitatively similar reductions in the amplitude of Ca2+ transients in myocytes from control and diabetic rats. Effects of acute acidosis on amplitude of myocyte contraction and Ca2+ transient were not significantly altered by STZ treatment. Altered myofilament sensitivity to Ca2+ and altered mechanisms of sarcoplasmic reticulum Ca2+ transport might partly underlie the acidosis-evoked reduction in amplitude of shortening in myocytes from control and STZ-induced diabetic rat.     

Distribution of atrial natriuretic peptide and its effects on contraction and intracellular calcium in ventricular myocytes from streptozotocin-induced diabetic rat

The distribution of atrial natriuretic peptide (ANP) in blood plasma and cardiac muscle and its effects on ventricular myocyte contraction and intracellular free calcium concentration [Ca2+]i in the streptozotocin (STZ)-induced diabetic rat have been investigated. Blood plasma concentration and heart atrial and ventricular contents of ANP were significantly increased in STZ-treated rats compared to age-matched controls. STZ treatment increased the number of ventricular myocytes immunolabeled with antibodies against ANP. In control myocytes the percentage of cells that labeled positively and negatively were 17% versus 83%, respectively. However, in myocytes from STZ-treated rat the percentages were 52% versus 53%. Time to peak (TPK) shortening was significantly and characteristically prolonged in myocytes from STZ-treated rats (360+/-5 ms) compared to controls (305+/-5 ms). Amplitude of the Ca2+ transient was significantly increased in myocytes from STZ-treated rats compared to controls (0.39+/-0.02 versus 0.29+/-0.02 fura-2 RU in controls) and treatment with ANP reduced the amplitude of the Ca2+ transient to control levels. ANP may have a protective role in STZ-induced diabetic rat heart.     

Measurement of cardiac mechanical function in isolated ventricular myocytes from rats and mice by computerized video-based imaging

Isolated adult cardiac ventricular myocytes have been a useful model for cardiovascular research for more than 20 years. With the recent advances in cellular physiology and transgenic techniques, direct measurement of isolated ventricular myocyte mechanics is becoming an increasingly important technique in cardiac physiology that provides fundamental information on excitation-contraction coupling of the heart, either in drug intervention or pathological states. The goal of this article is to describe the isolation of ventricular myocytes from both rats and mice, and the use of real-time beat-to-beat simultaneous recording of both myocyte contraction and intracellular Ca²⁺ transients. Published: December 11, 2001     

Effects of acidosis on ventricular myocyte shortening and intracellular Ca2+ in streptozotocin-induced diabetic rats

We have investigated the effects of acute acidosis on ventricular myocyte shortening and intracellular Ca²⁺ in streptozotocin (STZ)-induced diabetic rat. Shortening and intracellular Ca²⁺ were measured in electrically stimulated myocytes superfused with either normal Tyrode solution pH adjusted to either 7.4 (control solution) or 6.4 (acid solution). Experiments were performed at 35–36°C. At 8–12 weeks after treatment, the rats that received STZ had lower body and heart weights compared to controls, and blood glucose was characteristically increased. Contractile defects in myocytes from diabetic rat were characterized by prolonged time to peak shortening. Superfusion of myocytes from control and diabetic rats with acid solution caused a significant reduction in the amplitude of shortening; however, the magnitude of the response was not altered by STZ treatment. Acid solution also caused significant and quantitatively similar reductions in the amplitude of Ca²⁺ transients in myocytes from control and diabetic rats. Effects of acute acidosis on amplitude of myocyte contraction and Ca²⁺ transient were not significantly altered by STZ treatment. Altered myofilament sensitivity to Ca²⁺ and altered mechanisms of sarcoplasmic reticulum Ca²⁺ transport might partly underlie the acidosis-evoked reduction in amplitude of shortening in myocytes from control and STZ-induced diabetic rat. (Mol Cell Biochem 261: 227–233, 2004)     

Characterisation of ventricular myocyte shortening after administration of streptozotocin (STZ) to neonatal rats

Human and animal studies have shown that diabetes mellitus can be associated with altered cardiac function that is independent of vascular complications. Streptozotocin (STZ) is a diabetogenic agent and when administered to rats causes selective beta-cell necrosis which is accompanied by a drastic reduction in plasma insulin and hyperglycaemia. We have investigated the characteristics of shortening in ventricular myocytes isolated from rat heart at 10 months after administration of STZ to neonatal rats at 2 days of age. The characteristics of shortening in myocytes from the STZ-treated neonatal rat compared to shortening in myocytes from the STZ-treated young adult rat are discussed. STZ-treated rats gained significantly less weight compared to age-matched controls. Although non-fasting blood glucose was not significantly different in STZ-treated rats they were found to be markedly glucose intolerant when given an intraperitoneal challenge of glucose (2 g/kg) after an overnight fast. During electrical stimulation (1 or 2 Hz) ventricular myocyte resting length, time to peak shortening, time to half relaxation and amplitude of shortening were not altered after STZ treatment. The imposition of rest periods (2-60 s) after trains of electrically stimulated (1 Hz) steady-state contractions resulted in a potentiation of the contraction, which immediately followed the rest period (post rest potentiation). Post rest potentiation was larger, following rest periods between 20 and 60 s, in myocytes from STZ-treated rats compared to controls. The absence of major alterations to the amplitude and kinetics of contraction in myocytes from STZ-treated neonatal rats at 10 months after treatment might be explained by a partial recovery of the beta-cells in the growing animal.     

Sprint training restores normal contractility in postinfarction rat myocytes.

The significance of 6-8 wk of high-intensity sprint training (HIST) on contractile abnormalities of myocytes isolated from rat hearts with prior myocardial infarction (MI) was investigated. Compared with the sedentary (Sed) condition, HIST attenuated myocyte hypertrophy observed post-MI primarily by reducing cell lengths but not cell widths. At high extracellular Ca(2+) concentration (5 mM) and low pacing frequency (0.1 Hz), conditions that preferentially favored Ca(2+) influx over efflux, MI-Sed myocytes shortened less than Sham-Sed myocytes did. HIST significantly improved contraction amplitudes in MI myocytes. Under conditions that favored Ca(2+) efflux, i.e., low extracellular Ca(2+) concentration (0.6 mM) and high pacing frequency (2 Hz), MI-Sed myocytes contracted more than Sham-Sed myocytes. HIST did not appreciably affect contraction amplitudes of MI myocytes under these conditions. Compared with MI-Sed myocytes, HIST myocytes showed significant improvement in time required to reach one-half maximal contraction amplitude shortening, maximal myocyte shortening and relengthening velocities, and half time of relaxation. Our results indicate that HIST instituted shortly after MI improved cellular contraction in surviving myocytes. Because our previous studies demonstrated that, in post-MI myocytes, HIST improved intracellular Ca(2+) dynamics, enhanced sarcoplasmic reticulum Ca(2+) uptake and Ca(2+) content, and restored Na(+)/Ca(2+) exchange current toward normal, we hypothesized that improvement in MI myocyte contractile function by HIST was likely mediated by normalization of cellular Ca(2+) homeostatic mechanisms.     

Differential integration of Ca2+-calmodulin signal in intact ventricular myocytes at low and high affinity Ca2+-calmodulin targets

Cardiac myocyte intracellular calcium varies beat-to-beat and calmodulin (CaM) transduces Ca2+ signals to regulate many cellular processes (e.g. via CaM targets such as CaM-dependent kinase and calcineurin). However, little is known about the dynamics of how CaM targets process the Ca2+ signals to generate appropriate biological responses in the heart. We hypothesized that the different affinities of CaM targets for the Ca2+-bound CaM (Ca2+-CaM) shape their actions through dynamic and tonic interactions in response to the repetitive Ca2+ signals in myocytes. To test our hypothesis, we used two fluorescence resonance energy transfer-based biosensors, BsCaM-45 (Kd = approximately 45 nm) and BsCaM-2 (Kd = approximately 2 nm), to monitor the real time Ca2+-CaM dynamics at low and high affinity CaM targets in paced adult ventricular myocytes. Compared with BsCaM-2, BsCaM-45 tracks the beat-to-beat Ca2+-CaM alterations more closely following the Ca2+ oscillations at each myocyte contraction. When pacing frequency is raised from 0.1 to 1.0 Hz, the higher affinity BsCaM-2 demonstrates significant elevation of diastolic Ca2+-CaM binding compared with the lower affinity BsCaM-45. Biochemically detailed computational models of Ca2+-CaM biosensors in beating cardiac myocytes revealed that the different Ca2+-CaM binding affinities of BsCaM-2 and BsCaM-45 are sufficient to predict their differing kinetics and diastolic integration. Thus, data from both experiments and computational modeling suggest that CaM targets with low versus high Ca2+-CaM affinities (like CaM-dependent kinase versus calcineurin) respond differentially to the same Ca2+ signal (phasic versus integrating), presumably tuned appropriately for their respective and distinct Ca2+ signaling pathways.     

Occurrence and distribution of atrial natriuretic peptide-containing cells in the left ventricle of hypertensive rats

In the ventricles of adult mammalian hearts, production of atrial natriuretic peptide (ANP) is negligible, restricted to the impulse-conducting cells, the papillary muscles, and a minority of subendocardial myocytes. ANP expression is reinduced in the ventricles of pressure-overloaded and failing hearts and is frequently used as a marker for myocyte hypertrophy. Using an immunohistochemical approach, we have characterized the size distribution of ANP-containing myocytes in the left ventricle of the spontaneously hypertensive rat (SHR) before and after chronic antihypertensive therapy and compared the results to age-matched normotensive Wistar rats (WR). Our findings show that in SHR the frequency of cells presenting ANP granularity is positively correlated with myocyte size (r=0.746, P<0.02). The highest proportion of ANP-positive myocytes (55-57%) was measured among cells of diameters 30-34 microm. In any corresponding cell size, the proportion of ANP-presenting myocytes was five- to tenfold higher in SHR than in the normotensive WR. We studied the effects of the antihypertensive drugs captopril, hydralazine, and nifedipine and found that, regardless of their effect on blood pressure or hypertrophy, all three eliminated ANP immunoproducts from the majority of the left ventricular myocytes and reduced the level of ANP mRNA, captopril being the most effective. The positive correlation between myocyte size and ANP expression was not maintained in the hearts of drug-treated SHR. Myocytes on the border of fibrotic areas or in regions of ANP presentation within the normal heart resisted the suppressive effect of the antihypertensive therapy, indicating that blood pressure or hypertrophy are not the sole correlates for ANP expression.     

Effects of single high-dose and multiple low-dose streptozotocin on contraction and intracellular Ca2+ in ventricular myocytes from diabetes resistant and susceptible rats

Administration of a single high-dose (SHD) of streptozotocin (STZ) to young adult rats causes a diabetic cardiomyopathy. Albino Oxford (AO) and Dark Agouti (DA) inbred strains of rats are susceptible to developing diabetes when administered a SHD of STZ but differ in susceptibility to multiple low-dose (MLD) STZ. We have investigated the effects of SHD and MLD-STZ on contraction and intracellular Ca2+, measured with fura-2, in ventricular myocytes from AO and DA rats at 18-20 weeks after treatment. Time to peak shortening was significantly prolonged in myocytes from DA rats after SHD-STZ but was not altered in DA rats after MLD-STZ or in AO rats by either MLD or SHZ-STZ treatment. Time to peak shortening in myocytes from DA control and DA rats after SHD-STZ were 88+/-2 ms and 107+/-4 ms, respectively. Time to half relaxation and the amplitude of myocyte shortening were not altered in AO or DA rats by either MLD or SHD-STZ treatment. Amplitude, time to peak fura-2 transient and time to half relaxation of the fura-2 transient were not significantly altered in AO or DA rats by either MLD or SHD-STZ treatment. Contractile defects reported in myocytes from SHD-STZ treated DA rats may be a consequence of altered myofilament sensitivity to Ca2+. The hyperglycaemic effects of MLD-STZ and SHD-STZ induced diabetes was much greater in DA compared to AO rats and the effects of the hyperglycaemia on the time-course of ventricular myocyte contraction was most profound in DA rats after SHD-STZ.     

Voltage-dependence of contraction in streptozotocin-induced diabetic myocytes

Cardiac contractile dysfunction is frequently reported in human patients and experimental animals with type-1 diabetes mellitus. The aim of this study was to investigate the voltage-dependence of contraction in ventricular myocytes from the streptozotocin (STZ)-induced diabetic rat. STZ-induced diabetes was characterised by hyperglycaemia and hypoinsulinaemia. Other characteristics included reduced body and heart weight and raised blood osmolarity. Isolated ventricular myocytes were patched in whole cell, voltage-clamp mode after correcting for membrane capacitance and series resistance. From a holding membrane potential of -40 mV, test pulses were applied at potentials between -30 and +50 mV in 10 mV increments. L-type Ca2+ current (I Ca,L) density and contraction were measured simultaneously using a video-edge detection system. Membrane capacitance was not significantly altered between control and STZ-induced diabetic myocytes. The I Ca,L density was significantly (p < 0.05) reduced throughout voltage ranges (-10 mV to +10 mV) in myocytes from STZ-treated rats compared to age-matched controls. Moreover, the amplitude of contraction was significantly reduced (p < 0.05) in myocytes from STZ-treated rats at all test potentials between -20 mV and +30 mV. However, in electrically field-stimulated (1 Hz) myocytes, the amplitude of contraction was not altered by STZ-treatment. It is suggested that in field-stimulated myocytes taken from STZ-induced diabetic hearts, prolonged action potential duration may promote increased Ca2+ influx via the sodium-calcium exchanger (NCX), which may compensate for a reduction in Ca2+ trigger through L-type-Ca2+-channels and lead to normalised contraction.     

A primary culture system for sustained expression of a calcium sensor in preserved adult rat ventricular myocytes

For studying heart pathologies on the cellular level, cultured adult cardiac myocytes represent an important approach. We aimed to explore a novel adult rat ventricular myocyte culture system with minimised dedifferentiation allowing extended experimental manipulation of the cells such as expression of exogenous proteins. Various culture conditions were investigated including medium supplement, substrate coating and electrical pacing for one week. Adult myocytes were probed for (i) viability, (ii) morphology, (iii) frequency dependence of contractions, (iv) Ca(2+) transients, and (v) their tolerance towards adenovirus-mediated expression of the Ca(2+) sensor "inverse pericam". Conventionally, in either serum supplemented or serum-free medium, myocytes dedifferentiated into flat cells within 3 days or cell physiology and morphology were impaired, respectively. In contrast, myocytes cultured in medium supplemented with an insulin-transferrin-selenite mixture on substrates coated with extracellular matrix proteins showed an increased cell attachment and a conserved cross-striation. Moreover, these myocytes displayed optimised preservation of their contractile behaviour and Ca(2+) signalling even under conditions of continuous electrical pacing. Sustained expression of inverse pericam did not alter myocyte function and allowed long lasting high speed Ca(2+) imaging of electrically driven adult myocytes. Our single-cell model thus provides a new advance for high-content screening of these highly specialised cells.     

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.     

A novel method to study contraction characteristics of a single cardiac myocyte using carbon fibers

To facilitate cardiac muscle research, we developed a novel method by which the force and length of a single ventricular myocyte can be recorded with a pair of carbon graphite fibers attached firmly to both ends. One fiber was stiff, whereas the other fiber was compliant to allow the recording of force and shortening during twitch contractions. The image of the compliant carbon fiber was projected onto a pair of photodiodes, and their output was fed to a piezoelectric transducer after variable amplifications to alter the effective compliance of the carbon fiber. Thus contraction of the myocyte was induced under virtually isometric conditions as well as under auxotonic conditions. We obtained a bell-shaped relation between the compliance under an auxotonic load and the work output of the myocyte, which was directly related to myocyte performance in the heart. Because it is easy to attach myocytes to the experimental apparatus, the present method would allow us to study cardiac muscle mechanics at the cellular and molecular levels.     

Stimulation of single isolated adult ventricular myocytes within a low volume using a planar microelectrode array

Microchannels (40- microm wide, 10- microm high, 10-mm long, 70- microm pitch) were patterned in the silicone elastomer, polydimethylsiloxane on a microscope coverslip base. Integrated within each microchamber were individually addressable stimulation electrodes (40- microm wide, 20- microm long, 100-nm thick) and a common central pseudo-reference electrode (60- microm wide, 500- microm long, 100-nm thick). Isolated rabbit ventricular myocytes were introduced into the chamber by micropipetting and subsequently capped with a layer of mineral oil, thus creating limited volumes of saline around individual myocytes that could be varied from 5 nL to 100 pL. Excitation contraction coupling was studied by monitoring myocyte shortening and intracellular Ca(2+) transients (using Fluo-3 fluorescence). The amplitude of stimulated myocyte shortening and Ca(2+) transients remained constant for 90 min in the larger volume (5 nL) configuration, although the shortening (but not the Ca(2+) transient) amplitude gradually decreased to 20% of control within 60 min in the low volume (100 pL) arrangement. These studies indicate a lower limit for the extracellular volume required to stimulate isolated adult cardiac myocytes. Whereas this arrangement could be used to create a screening assay for drugs, individual microchannels (100 pL) can also be used to study the effects of limited extracellular volume on the contractility of single cardiac myocytes.     

Effects of single high-dose and multiple low-dose streptozotocin on contraction and intracellular Ca<Superscript>2+</Superscript> in ventricular myocytes from diabetes resistant and susceptible rats

Administration of a single high-dose (SHD) of streptozotocin (STZ) to young adult rats causes a diabetic cardiomyopathy. Albino Oxford (AO) and Dark Agouti (DA) inbred strains of rats are susceptible to developing diabetes when administered a SHD of STZ but differ in susceptibility to multiple low-dose (MLD) STZ. We have investigated the effects of SHD and MLD-STZ on contraction and intracellular Ca²⁺, measured with fura-2, in ventricular myocytes from AO and DA rats at 18–20 weeks after treatment. Time to peak shortening was significantly prolonged in myocytes from DA rats after SHD-STZ but was not altered in DA rats after MLD-STZ or in AO rats by either MLD or SHZ-STZ treatment. Time to peak shortening in myocytes from DA control and DA rats after SHD-STZ were 88 ± 2 ms and 107 ± 4 ms, respectively. Time to half relaxation and the amplitude of myocyte shortening were not altered in AO or DA rats by either MLD or SHD-STZ treatment. Amplitude, time to peak fura-2 transient and time to half relaxation of the fura-2 transient were not significantly altered in AO or DA rats by either MLD or SHD-STZ treatment. Contractile defects reported in myocytes from SHD-STZ treated DA rats may be a consequence of altered myofilament sensitivity to Ca²⁺. The hyperglycaemic effects of MLD-STZ and SHD-STZ induced diabetes was much greater in DA compared to AO rats and the effects of the hyperglycaemia on the time-course of ventricular myocyte contraction was most profound in DA rats after SHD-STZ. (Mol Cell Biochem 269: 103–108, 2005)     

Mechanism of enhanced cardiac function in mice with hypertrophy induced by overexpressed Akt

Transgenic mice with cardiac-specific overexpression of active Akt (TG) not only exhibit hypertrophy but also show enhanced left ventricular (LV) function. In 3-4-month-old TG, heart/body weight was increased by 60% and LV ejection fraction was elevated (84 +/- 2%, p < 0.01) compared with nontransgenic littermates (wild type (WT)) (73 +/- 1%). An increase in isolated ventricular myocyte contractile function (% contraction) in TG compared with WT (6.1 +/- 0.2 versus 3.5 +/- 0.2%, p < 0.01) was associated with increased Fura-2 Ca2+ transients (396 +/- 50 versus 250 +/- 24 nmol/liter, p < 0.05). The rate of relaxation (+dL/dt) was also enhanced in TG (214 +/- 15 versus 98 +/- 18 microm/s, p < 0.01). L-type Ca2+ current (ICa) density was increased in TG compared with WT (-9.0 +/- 0.3 versus 7.2 +/- 0.3 pA/pF, p < 0.01). Sarcoplasmic reticulum Ca2+ ATPase 2a (SERCA2a) protein levels were increased (p < 0.05) by 6.6-fold in TG, which could be recapitulated in vitro by adenovirus-mediated overexpression of Akt in cultured adult ventricular myocytes. Conversely, inhibiting SERCA with either ryanodine or thapsigargin affected myocyte contraction and relaxation and Ca2+ channel kinetics more in TG than in WT. Thus, myocytes from mice with overexpressed Akt demonstrated enhanced contractility and relaxation, Fura-2 Ca2+ transients, and Ca2+ channel currents. Furthermore, increased protein expression of SERCA2a plays an important role in mediating enhanced LV function by Akt. Up-regulation of SERCA2a expression and enhanced LV myocyte contraction and relaxation in Akt-induced hypertrophy is opposite to the down-regulation of SERCA2a and reduced contractile function observed in many other forms of LV hypertrophy.     

Mitogen-activated protein kinase (MAPK) in cardiac tissues

Mitogen-activated protein kinase (MAPK) has recently emerged as a prominent role player in intracellular signalling in the ventricular myocyte with attention being focussed on its possible role in the development of ventricular hypertrophy. It is becoming clear that MAPK is also active in other cells of cardiac origin such as cardiac fibroblasts and possible functions of this signalling pathway in the heart have yet to be explored. In this report the mammalian MAPK pathway is briefly outlined, before reviewing current knowledge of the MAPK pathway in cardiac tissue (ventricular myocytes, vascular smooth muscle cells and cardiac fibroblasts). New data is also presented on the presence and activity of MAPK in two additional cardiac celltypes namely atrial myocytes and vascular endothelial cells from the coronary microcirculation. (Mol Cell Biochem 157: 49–57, 1996)     

The capacity for reactive DNA synthesis of the myocytes in the heart conduction system in experimental and clinical myocardial pathology]

The DNA synthesis has been studied in the conductive system (CS) myocytes, compared to that in atrial and ventricular myocytes: 1) in the left ventricular myocardial infarction induced in two- and three-week-old and adult rats, 2) after isoproterenol injections to adult rats and mice, and 3) in the hypertrophied human heart. The extent of DNA synthesis reactivation was evaluated by the cumulative labeling indices in experiments with multiple 3HTdR injections to rats and mice. In the human cardiac myocyte nuclei, the DNA content was determined by the Feulgen-cytophotometry. The difference between the control and experimental mean values of the labeling indices for CS myocyte nuclei was statistically significant only for atrioventricular part of the CS in the infarcted hearts of adult rats. In the human heart CS the ability of myocytes to polyploidization varies from one cell type to another, the lowest being in nodal cells.     

Mechanisms of impaired calcium handling underlying subclinical diastolic dysfunction in diabetes

Isolated diastolic dysfunction is found in almost half of asymptomatic patients with well-controlled diabetes and may precede diastolic heart failure. However, mechanisms that underlie diastolic dysfunction during diabetes are not well understood. We tested the hypothesis that isolated diastolic dysfunction is associated with impaired myocardial Ca(2+) handling during type 1 diabetes. Streptozotocin-induced diabetic rats were compared with age-matched placebo-treated rats. Global left ventricular myocardial performance and systolic function were preserved in diabetic animals. Diabetes-induced diastolic dysfunction was evident on Doppler flow imaging, based on the altered patterns of mitral inflow and pulmonary venous flows. In isolated ventricular myocytes, diabetes resulted in significant prolongation of action potential duration compared with controls, with afterdepolarizations occurring in diabetic myocytes (P < 0.05). Sustained outward K(+) current and peak outward component of the inward rectifier were reduced in diabetic myocytes, while transient outward current was increased. There was no significant change in L-type Ca(2+) current; however, Ca(2+) transient amplitude was reduced and transient decay was prolonged by 38% in diabetic compared with control myocytes (P < 0.05). Sarcoplasmic reticulum Ca(2+) load (estimated by measuring the integral of caffeine-evoked Na(+)-Ca(2+) exchanger current and Ca(2+) transient amplitudes) was reduced by approximately 50% in diabetic myocytes (P < 0.05). In permeabilized myocytes, Ca(2+) spark amplitude and frequency were reduced by 34 and 20%, respectively, in diabetic compared with control myocytes (P < 0.05). Sarco(endo)plasmic reticulum Ca(2+)-ATPase-2a protein levels were decreased during diabetes. These data suggest that in vitro impairment of Ca(2+) reuptake during myocyte relaxation contributes to in vivo diastolic dysfunction, with preserved global systolic function, during diabetes.     

Power output is linearly related to MyHC content in rat skinned myocytes and isolated working hearts

The amount of work the heart can perform during ejection is governed by the inherent contractile properties of individual myocytes. One way to alter contractile properties is to alter contractile proteins such as myosin heavy chain (MyHC), which is known to demonstrate isoform plasticity in response to disease states. The purpose of this study was to examine myocyte functionality over the complete range of MyHC expression in heart, from 100% alpha-MyHC to 100% beta-MyHC, using euthyroid and hypothyroid rats. Peak power output in skinned cardiac myocytes decreased as a nearly linear function of beta-MyHC expression during maximal (r2 = 0.85, n = 44 myocyte preparations) and submaximal (r2 = 0.82, n = 31 myocyte preparations) Ca2+ activation. To determine whether single myocyte function translated to the level of the whole heart, power output was measured in working heart preparations expressing varied ratios of MyHC. Left ventricular power output of isolated working heart preparations also decreased as a linear function of increasing beta-MyHC expression (r2 = 0.82, n = 34 myocyte preparations). These results demonstrate that power output is highly dependent on MyHC expression in single myocytes, and this translates to the performance of working left ventricles.