Ca2+-dependent proteolysis of junctophilin-1 and junctophilin-2 in skeletal and cardiac muscle

Excessive increases in intracellular [Ca(2+)] in skeletal muscle fibres cause failure of excitation-contraction coupling by disrupting communication between the dihydropyridine receptors in the transverse tubular system and the Ca(2+) release channels (RyRs) in the sarcoplasmic reticulum (SR), but the exact mechanism is unknown. Previous work suggested a possible role of Ca(2+)-dependent proteolysis in this uncoupling process but found no proteolysis of the dihydropyridine receptors, RyRs or triadin. Junctophilin-1 (JP1; ∼90 kDa) stabilizes close apposition of the transverse tubular system and SR membranes in adult skeletal muscle; its C-terminal end is embedded in the SR and its N-terminal associates with the transverse tubular system membrane. Exposure of skeletal muscle homogenates to precisely set [Ca(2+)] revealed that JP1 undergoes Ca(2+)-dependent proteolysis over the physiological [Ca(2+)] range in tandem with autolytic activation of endogenous μ-calpain. Cleavage of JP1 occurs close to the C-terminal, yielding a ∼75 kDa diffusible fragment and a fixed ∼15 kDa fragment. Depolarization-induced force responses in rat skinned fibres were abolished following 1 min exposure to 40 μm Ca(2+), with accompanying loss of full-length JP1. Supraphysiological stimulation of rat skeletal muscle in vitro by repeated tetanic stimulation in 30 mm caffeine also produced marked proteolysis of JP1 (and not RyR1). In dystrophic mdx mice, JP1 proteolysis is seen in limb muscles at 4 and not at 10 weeks of age. Junctophilin-2 in cardiac and skeletal muscle also undergoes Ca(2+)-dependent proteolysis, and junctophilin-2 levels are reduced following cardiac ischaemia-reperfusion. Junctophilin proteolysis may contribute to skeletal muscle weakness and cardiac dysfunction in a range of circumstances.

Threshold effects of glucose transporter-4 (GLUT4) deficiency on cardiac glucose uptake and development of hypertrophy

The aim of this study was to investigate the metabolic and structural consequences of a decrease in glucose transporter-4 (GLUT4) levels on the heart. The CreLoxP system was utilised to delete GLUT4 in muscle tIssue including heart. The presence of the PGK-neoR cassette in the GLUT4-Lox mice resulted in reduced expression in all tIssues to levels 15-30% of wild-type control mice. In mice expressing Cre recombinase, there was a further reduction of GLUT4 in cardiac tIssue to almost undetectable levels. Cardiac glucose uptake was measured basally and during a euglycaemic/hyperinsulinaemic clamp using 2-deoxy-[1-(14)C]glucose. Insulin-stimulated glucose uptake was normal in hearts expressing 15% of normal GLUT4 levels but markedly reduced in mice with more profound reduction in GLUT4. Cardiac enlargement occurred only when GLUT4 levels were less than 5% of normal values. In heart there is a threshold level of GLUT4 above which insulin-stimulated glucose uptake is maintained. As little as 5% of normal GLUT4 levels expressed in heart is sufficient to prevent the development of cardiac hypertrophy.

Effects of combined administration of ACE inhibitor and angiotensin II receptor antagonist are prevented by a high NaCl intake

BACKGROUND

To prevent the action of angiotensin II by blockade with either an angiotensin converting enzyme inhibitor (ACE I) or an angiotensin receptor antagonist (ARA) is difficult due to the physiological compensations. Combined therapy with both drugs may enable complete blockade, and in rats in high doses this has produced a syndrome that results in death.

OBJECTIVE

To determine the effect of combined blockade using losartan (10 mg/kg per day) and perindopril (6 mg/kg per day) on blood pressure, cardiac growth, renal function and behaviour, and to determine how this is influenced by different salt intakes in normotensive Sprague Dawley rats.

METHODS

Rats were fed an 0.2 or 4% NaCl diet and received the above drugs intraperitoneally. Blood pressure was measured by telemetry. Cardiac weight was measured after 10 days of therapy. Renal function was assessed by plasma creatinine and electrolytes, plasma renin and angiotensinogen concentrations were measured.

RESULTS

On 0.2% NaCl intake, combined blockade lowered blood pressure progressively; at day 7, rats on 0.2% NaCl developed a syndrome of listlessness and failure to eat which led to loss of weight and death. Cardiac size was dramatically reduced. Plasma creatinine was elevated to 50% above normal. There was a polyuria. The syndrome was reversed by adding NaCl to the drinking water or prevented in rats on a 4% NaCl intake. In rats on 0.2% NaCl plasma renin rose dramatically with medication and angiotensinogen became depleted. Haematocrit in all groups of rats did not differ.

CONCLUSION

Combined blockade of the renin-angiotensin system can cause death in rats on a reduced NaCl intake. This was prevented by a high salt intake. The syndrome may result from depletion of angiotensinogen and the failure to synthesize sufficient angiotensin II that may be critical for normal cardiac growth and function and critical for survival.

Regression of cardiac hypertrophy in the SHR by combined renin-angiotensin system blockade and dietary sodium restriction

Altered operation of the renin-angiotensin-aldosterone system (RAAS) and dietary sodium intake have been identified as independent risk factors for cardiac hypertrophy. The way in which sodium intake and the operation of the renin-angiotensin-aldosterone system interact in the pathogenesis of cardiac hypertrophy is poorly understood. The aims of this study were to investigate the cardiac effects of the renin-angiotensin system (RAS) blockade in the spontaneously hypertensive rat (SHR), using co-treatment with an angiotensin II receptor blocker (ARB) and an angiotensin-converting enzyme (ACE) inhibitor with different sodium intakes. Our experiments with SHR show that, at high levels of sodium intake (4.0%), aggressive RAS blockade treatment with candesartan (3 mg/kg) and perindopril (6 mg/kg) does not result in regression of cardiac hypertrophy. In contrast, RAS blockade coupled with reduced sodium diet (0.2%) significantly regresses cardiac hypertrophy, impairs animal growth and is associated with elevated plasma renin and dramatically suppressed plasma angiotensinogen levels. Histological analyses indicate that the differential effect of reduced sodium on heart growth during RAS blockade is not associated with any change in myocardial interstitial collagen, but reflects modification of cellular geometry. Dimensional measurements of enzymatically-isolated ventricular myocytes show that, in the RAS blocked, reduced sodium group, myocyte length and width were decreased by about 16—19% compared with myocytes from the high sodium treatment group. Our findings highlight the importance of `titrating' sodium intake with combined RAS blockade in the clinical setting to optimise therapeutic benefit.

Downregulation of the p75 neurotrophin receptor in tissue culture and in vivo, using beta-cyclodextrin-adamantane-oligonucleotide conjugates

Formation of complexes with beta-cyclodextrin derivatives via adamantyl groups was found to enhance the uptake and antisense efficacy of phosphorothioate oligos targeted to the p75 neurotrophin receptor in neuronally differentiated PC12 cells. After a 2-week course of systemic administration to mice (by intraperitoneal injection), there was evidence of a pronounced uptake of these oligos by the dorsal root ganglia (DRG), as well as by liver and kidney. There was no uptake by the brain. Consistent with uptake of antisense oligos by the DRG, systemic administration resulted in marked and consistent downregulation of p75 in DRG neurons. These results indicate that cyclodextrin-adamantane-oligo conjugates have great potential as agents to downregulate target genes in neurons, particularly in vivo in the peripheral nervous system.

Stimulus interval-dependent differences in Ca2+ transients and contractile responses of diabetic rat cardiomyocytes

OBJECTIVE

The aim of this study was to gain further insights into the consequences of insulin-dependent diabetes mellitus on cardiomyocyte calcium handling.

METHODS

The effects of steady state and transient changes in stimulus frequency on the intracellular Ca2+ transient and cell shortening were examined in left ventricular cardiomyocytes isolated from the hearts of control and streptozotocin-induced diabetic rats.

RESULTS

During steady state stimulation diabetic rat cardiomyocytes displayed a slower decay of the Ca2+ transient and longer times for maximum cell shortening and re-lengthening. At 1.5 mM extracellular [Ca2+], increasing stimulus frequency over the range 0.2-1.0 Hz led to an increase in resting and peak [Ca2+]i as well as the amplitude of the transient in both the control and diabetic groups. At frequencies greater than 0.4 Hz the amplitude of the transient was significantly depressed in diabetic rat cells and this was not normalized by increasing extracellular [Ca2+] to 2.5 mM. Recovery of sarcoplasmic reticulum (SR) Ca2+ release was measured from the time course of restitution of the intracellular Ca2+ transient. In both control and diabetic rat cardiomyocytes recovery of the transient occurred in two phases. In diabetic rat myocytes, the initial rapid phase of restitution at intervals <1 s was markedly slowed. The fraction of Ca2+ recirculating between the SR and the cytosol was estimated from the decline in amplitude of transients following post-rest potentiation. There was no difference in this fraction between control and diabetic rat cells either at 1.5 or 2.5 mM extracellular [Ca2+].

CONCLUSION

The blunted frequency response of diabetic rat cardiomyocytes at frequencies greater than 0.4 Hz is consistent with reduced SR Ca2+ uptake leading to reduced SR Ca2+ content and subsequent release. At stimulus intervals greater than 1 Hz this is likely to be exacerbated by slower recovery of SR Ca2+ release. Despite the evidence for depressed SR Ca2+ uptake, the relative amount of Ca2+ recirculating within diabetic rat cardiomyocytes remains unaltered. This is most likely due to an accompanying reduction in Ca2+ efflux from the cell due either to depressed Na+/Ca2+ exchanger activity, or an elevation in intracellular Na+ levels.

Ethanol effects on cardiomyocyte contractility

Little is known about the direct cardiac effects of socially common sub-intoxication levels of ethanol. Previous studies evaluating the responses of normal cardiomyocytes to short-term ethanol exposure have utilized ethanol concentrations equivalent to extreme intoxication or lethal levels in vivo. The purpose of the present study was to investigate the contractile responses of isolated rat ventricular cardiomyocytes during exposure to relatively low concentrations of ethanol in the range 0.05-0.5% (v/v) (8.6-86 mM) under physiological conditions (3 Hz stimulation; 36 degrees C; BSA vehicle). High-speed imaging techniques were used to study the kinetics of myocyte contraction, and shortening parameters were calculated for mechanistic evaluation. The concentration-response relationship was not linear and exhibited two plateau phases, suggesting at least two mechanisms of action of ethanol on cardiomyocyte contraction. At 0.05% (8.6 mM), ethanol treatment produced a 14.4% decrease in maximum myocyte shortening. The maximum rates of cell shortening and lengthening were similarly impaired, but there was no effect on contraction cycle timing at this low concentration. At 0.30% (51 mM), ethanol reduced maximum shortening by 40.2%, prolonged excitation-contraction coupling latency and abbreviated the contraction cycle time by 38%. The inotropic modulatory effect of ethanol was exaggerated in the absence of protein in the superfusion buffer. This is the first report which identifies ethanol at 0.05% (v/v) as a modulator of cardiac contractility. Kinetic analyses indicate that the mechanism of action involves disturbance of sarcoplasmic reticulum function, and this may contribute to arrhythmogenic vulnerability - especially in an in vivo context of heightened compensatory sympathetic drive.

Angiotensin blocking drugs and the heart beyond 2000

Cardiac hypertrophy can be caused in different ways, and the effect of hypertrophy on prognosis depends on whether it is concentric or eccentric in nature. It is simplistic to ascribe hypertrophy purely to workload, and it is a complex interaction between workload, wall stress, and the local and humoral environment. In rats, acute elevation of BP occurring during the rats' sleep cycle causes cardiac hypertrophy, and reduction of BP in hypertensive rats during the sleep cycle causes reversal of left ventricular hypertrophy. This may be due to secretion of growth hormone and renin during sleep. Experimental evidence indicates that angiotensin II possibly formed and acting locally may be implicated in the genesis of cardiac hypertrophy; however, angiotensin II by itself causes relatively minor hypertrophy, but this becomes intensified if there is a high sodium intake and a high angiotensin II level. Blockade of the angiotensin system with angiotensin-converting enzyme inhibitors causes reversal of cardiac hypertrophy and similar results are achieved with AT1 receptor blocking drugs, suggesting that bradykinin may be of relatively minor importance. Clinically, the AT1 receptor blocking drugs have few side effects and appear to have similar beneficial effects to angiotensin-converting enzyme inhibitors, making them suitable to treat many people with hypertension.

Cardiodepressant effects of interferon-gamma and endotoxin reversed by inhibition of NO synthase 2 in rat myocardium

Endogenous nitric oxide (NO) signalling pathways within the myocardium depress myocardial contractile function in septic shock and some cardiomyopathies. We have explored the role of NO synthases (NOSs) in mediating the cardiodepressant actions of interferon-gamma (IFN-gamma) and lipopolysaccaride (LPS) in rat papillary muscle. Muscles from the right ventricle were electrically stimulated (0.2 Hz) at 30 degrees C and isometric contraction monitored. Exposure to IFN-gamma and LPS for 15 h in vitro significantly decreased the peak tension (PT for IFN-gamma + LPS, from 0.13 +/- 0.03 to 0.07 +/- 0.02 g) and rate of tension development (dT/dt for IFN-gamma + LPS, from 1.78 +/- 0.36 to 1.17 +/- 0.28 g/s) compared to untreated controls, and this was prevented by dexamethasone (1 microM) and partly reversed by a non-specific NOS inhibitor, NG-nitro-L-arginine (NOLA, 30 microM). Likewise, the maximum inotropic response of the papillary muscles to isoprenaline (0.001-10 microM) decreased significantly after 15 h treatment with IFN-gamma and LPS (PT from 83 +/- 18 to 28 +/- 6%; +dT/dt from 83 +/- 12 to 31 +/- 7%; -dT/dt from 83 +/- 12 to 38 +/- 6%). Again, the depressant effects of IFN-gamma and LPS on inotropic responsiveness to isoprenaline were completely prevented by pretreatment with dexamethasone (1 microM), by a specific inhibitor of NOS2, mercaptoethylguanidine (MEG, 30 microM) and by NOLA. Whereas dexamethasone and NOLA protected against the attenuation of baseline contractions induced by LPS and IFN-gamma, MEG did not. Western blot analysis of cardiac myocytes showed that there was no constitutive expression of NOS2, but IFN-gamma and LPS induced expression of NOS2, and this was prevented by dexamethasone. Thus IFN-gamma, in the presence of LPS, reduced papillary muscle contraction and decreased responsiveness to beta-adrenoceptor stimulation through induction of NOS2 in the muscle. Increased NO production may contribute to the cardiac depression during septic shock and anti-cancer therapy with cytokines, and perhaps in heart failure.

Cardiac myocyte volume, Ca2+ fluxes, and sarcoplasmic reticulum loading in pressure-overload hypertrophy

Alterations in cellular Ca2+ transport and excitation-contraction coupling may contribute to dysfunction in cardiac hypertrophy. Left ventricular myocytes were isolated from rat hearts after 15-18 wk of suprarenal abdominal aortic banding to evaluate the hypothesis that hypertrophy alters the relationship between Ca2+ current (ICa) and sarcoplasmic reticulum (SR) Ca2+ load during steady-state voltage-clamp depolarization. Mean arterial pressure (MAP) and heart weight-to-body weight ratio of banded (B) animals were significantly higher than in control or sham-operated animals (C). Isolated myocyte dimensions and volume increased in parallel with whole heart hypertrophy and elevation in MAP. However, the relationship between membrane surface area (measured by capacitance) and cell volume (measured by laser scanning confocal microscopy) was unaltered (C: 8.9 +/- 0.3; B: 8.5 +/- 0.4 pF/pl). No differences in the voltage dependence of ICa activation, steady-state inactivation, or recovery from inactivation were detected between C and B myocytes. Maximal ICa density for the two groups was also not different either under basal conditions (C: 4.28 +/- 0.98; B: 4.57 +/- 0.60 pA/pF) or in the presence of 1 microM isoproterenol (C: 16.6 +/- 2.3; B: 16.5 +/- 2.3 pA/pF). The fraction of Ca2+ released from the SR by a single twitch was 55.4 +/- 9.4% in C and 37.1 +/- 6.9% in B (not significantly different). Steady-state Ca2+ influx during a twitch was calculated in units of micromoles per liter of nonmitochondrial volume from the integral of ICa (C: 13.4 +/- 0.7 microM; B: 13.3 +/- 0.8 microM). The SR Ca2+ load was similarly calculated by integration of Na+/Ca2+ exchange current induced by rapid caffeine application (C: 140 +/- 9 microM; B: 169 +/- 18 microM). We conclude that significant cellular hypertrophy is associated with proportional increases in sarcolemmal ICa influx, SR Ca2+ loading, and the amount of SR Ca2+ released in this model of pressure overload.

Optical methods to evaluate the contractile function of unloaded isolated cardiac myocytes

The enzymatically isolated cardiac myocyte has achieved prominence as an experimental model of myocardial excitation contraction coupling. Considerable effort has been directed to investigating the shortening behaviour of unattached and externally unloaded contracting cardiomyocytes - measured both at the level of the whole cell and at the level of the sarcomere. The purpose of this review is to provide a survey of these methodological approaches and to identify the crucial optical issues involved in striation imaging and whole-cell end-detection procedures. Factors limiting the precision with which measurements of cardiomyocyte shortening can be made, and sources of error attributable to unfavourable optical conditions and myocyte geometry are discussed. Original data are presented, examining the relationship between sarcomere and whole-cell shortening dynamics recorded simultaneously in the same cells. These results confirm that the measurement of cell length can provide an adequate index of sarcomere dynamics if the data are obtained under carefully controlled conditions where the entire cell is monitored to ensure minimal geometric or optical non-linearities. The role of the myocyte's internal load in modulating the observed contractile responses at differing levels of inotropic status is discussed. Finally, the validity of interpreting the shortening response of an untethered isolated cardiomyocyte as an index of myocardial contractility, reflecting the inotropic status, is considered.

Contractile function of cardiomyocytes from the spontaneously hypertensive rat

In essential hypertension it has been hypothesized that there is a generalized derangement in cellular Ca2+ homeostasis. The aim of this study was to assess whether there is functional evidence for increased vulnerability to Ca overload in cardiomyocytes derived from age-matched (11-week) normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) strains. Contraction cycles of isolated ventricular cardiomyocytes were recorded using a rapid digital imaging technique and were evaluated by computation of a range of normalized parameters. Significant differences in the basal contractile cycles of SHR and WKY ventricular cardiomyocytes recorded under control conditions (36 degrees C, 3 Hz, 1 mM Ca2+) were detected. SHR myocytes exhibited a reduction in maximum shortening attained (SHR/WKY = 0.79) and in maximum rates of shortening (SHR/WKY = 0.85) and lengthening (SHR/WKY = 0.87). In the SHR there was a small delay in excitation-contraction coupling latency and an abbreviation of the contractile cycle (SHR/WKY = 0.87). The time to peak contraction was not different in the two strains and was not altered by the inotropic interventions studied. No functional evidence for an increased susceptibility of the SHR myocytes to uncompensated "Ca2+ leakiness" was observed during repeated challenge of elevated extracellular Ca2+. In both strains, the relative increase in maximum shortening of myocytes to isoproterenol (10(-8) M) was similar, suggesting that differential sensitivity to adrenergic modulation is due to factors other than altered cardiomyocyte responsiveness. Diminished basal cardiomyocyte contractile function may represent an intrinsic feature of impaired cardiovascular function in this form of genetically determined hypertension.

Surface:volume relationship in cardiac myocytes studied with confocal microscopy and membrane capacitance measurements: species-dependence and developmental effects

The quantitative analysis of the contribution of ion fluxes through membrane channels to changes of intracellular ion concentrations would benefit from the exact knowledge of the cell volume. It would allow direct correlation of ionic current measurements with simultaneous measurements of ion concentrations in individual cells. Because of various limitations of conventional light microscopy a simple method for accurate cell volume determination is lacking. We have combined the optical sectioning capabilities of fluorescence laser scanning confocal microscopy and the whole-cell patch-clamp technique to study the correlation between cell volume and membrane capacitance. Single cardiac myocytes loaded with the fluorescent dye calcein were optically sectioned to produce a series of confocal images. The volume of cardiac myocytes of three different mammalian species was determined by three-dimensional volume rendering of the confocal images. The calculated cell volumes were 30.4 +/- 7.3 pl (mean +/- SD) in rabbits (n = 28), 30.9 +/- 9.0 pl in ferrets (n = 23), and 34.4 +/- 7.0 pl in rats (n = 21), respectively. There was a positive linear correlation between membrane capacitance and cell volume in each animal species. The capacitance-volume ratios were significantly different among species (4.58 +/- 0.45 pF/pl in rabbit, 5.39 +/- 0.57 pF/pl in ferret, and 8.44 +/- 1.35 pF/pl in rat). Furthermore, the capacitance-volume ratio was dependent on the developmental stage (8.88 +/- 1.14 pF/pl in 6-month-old rats versus 6.76 +/- 0.62 pF/pl in 3-month-old rats). The data suggest that the ratio of surface area:volume of cardiac myocytes undergoes significant developmental changes and differs among mammalian species. We further established that the easily measurable parameters of cell membrane capacitance or the product of cell length and width provide reliable but species-dependent estimates for the volume of individual cells.

Steady-state twitch Ca2+ fluxes and cytosolic Ca2+ buffering in rabbit ventricular myocytes

Intracellular Ca2+ ([Ca2+]i) transients and transsarcolemmal Ca2+ currents were measured in indo 1-loaded isolated rabbit ventricular myocytes during whole cell voltage clamp to quantitate the components of cytosolic Ca2+ influx and to describe the dynamic aspects of cytosolic Ca2+ buffering during steady-state contraction (0.5 Hz, 22 degrees C). Sarcolemmal Ca2+ influx was directly measured from the integrated Ca2+ current (Ica) recorded during the clamp (158 +/- 10 attomoles; amol). Sarcoplasmic reticulum (SR) Ca2+ content was determined from the integrated electrogenic Na+/Ca2+ exchange current (Ix) induced during rapid application and sustained exposure of cells to caffeine to elicit the release of the SR Ca2+ load (1,208 +/- 170 amol). The mean steady-state SR Ca2+ load was calculated to be 87 +/- 13 microM (mumol/l nonmitochondrial cytosolic volume). Ca2+ influx via Ica represented approximately 14% of the stored SR Ca2+ and 23% of the total cytosolic Ca2+ flux during a twitch (47 +/- 6 microM). Comparison of electrophysiologically measured Ca2+ fluxes with Ca2+ transients yields apparent buffering values of 60 for caffeine contractures and 110 for twitches (delta Ca2+ total/delta Ca2+ free). This is consistent with the occurrence of "active" buffering of cytosolic Ca2+ by SR Ca2+ uptake during the twitch.

Effects of endothelin-1 on the contractility of cardiomyocytes from the spontaneously hypertensive rat

1. Disturbances in cardiovascular responsiveness to endogenous endothelin-1 (ET-1) may play a significant role in the pathogenesis of essential hypertension. In this study the inotropic responses of cardiomyocytes derived from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rat (SHR) strains to ET-1 (10(-11)-10(-8) mol/L) were characterized. Isotonic contraction cycles of ventricular cardiomyocytes isolated from age-matched (11 week) WKY and SHR rats were recorded using a rapid digital imaging technique and evaluated by computation of a range of normalized parameters. 2. The maximum effect of ET-1, eliciting a 60-70% increase in myocyte shortening after 3 min, was observed at 10(-9) mol/L in both strains, and was associated with elevations in the rate of shortening and lengthening, abbreviated latency, contractile cycle prolongation and delayed time to peak shortening. 3. No evidence for a significant strain dependent difference in the relative responsiveness to ET-1 was detected. This finding indicates that altered sensitivity to ET-1 is unlikely to be a major factor underlying the development of hypertension in this model. 4. The distinct nature of the alterations in contractile parameters produced by ET-1 compared with angiotensin II (AII) suggests that the prevailing cellular mechanisms of action of these peptides are different and that ET-1 is not a paracrine or autocrine inotropic intermediate for AII.

Platelet-activating factor and WEB-2086 directly modulate rat cardiomyocyte contractility

In various isolated cardiac tissue preparations the phospholipid mediator platelet-activating factor (PAF, 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphorylcholine) may elicit a positive, negative or no inotropic response. In the multicellular preparation it is difficult to differentiate and characterize the direct actions of PAF on the myocardium from the effects due to the release of secondary mediators from non-myocyte cell types. Thus in the present study the inotropic effects of PAF (10(-11), 10(-9), 10(-7) M) and a specific PAF receptor antagonist, WEB-2086, PAF receptor antagonist. WEB-2086, on the contractility of enzymatically isolated adult rat cardiomyocytes have been investigated. PAF (10(-7) M) reduced maximum cell shortening by 23% compared to the initial level, delayed excitation contraction coupling, abbreviated the contraction cycle and reduced the maximum rate of cell lengthening during relaxation. Lower concentrations of PAF elicited smaller negative inotropic responses. There was no evidence of a positive inotropic effect of PAF at any concentration tested or at any time after the onset of treatment. Co-treatment with WEB-2086 (10(-5) M) prevented the negative inotropic response to PAF. WEB-2086 alone increased maximum shortening by 16% compared to initial performance, extended the contraction cycle and increased the maximum rates of shortening and lengthening. WEB-2086 had no effect on excitation contraction coupling latency. The selective alteration of contraction parameters induced by PAF and WEB-2086 indicates that, in addition to antagonizing the negative inotropic action of exogenous PAF, WEB-2086 alone enhances contractility. These results demonstrate that PAF has a direct, receptor-mediated negative inotropic effect on adult contractility. These results demonstrate that PAF has a direct, receptor-mediated negative inotropic effect on adult cardiomyocytes.

Spontaneous and propagated calcium release in isolated cardiac myocytes viewed by confocal microscopy

Laser scanning confocal microscopy of the Ca(2+)-sensitive fluorophore fluo-3 has been used to investigate spontaneous and propagated calcium release at high temporal and spatial resolution in enzymatically dispersed rat cardiomyocytes. Waves of fluorescence which propagated throughout the cytosol were evident in spontaneously contracting cardiac cells containing fluo-3, but not in cells containing Ca(2+)-insensitive fluorophores [2',7'-bis (carboxyethyl)-5,6-carboxyfluorescein, SNARF-1, rhodamine-123, or tetramethylrhodamine-labeled dextran]. These waves represent localized areas of elevated [Ca2+] [975 +/- 13 (SE) nM, range 800-1,500 nM; n = 16 cells]. Ca2+ waves were initiated by the spontaneous release of Ca2+ from the sarcoplasmic reticulum (SR) and propagated through cells at rates of 50-150 microns/s. Ca2+ waves were usually initiated at the cell ends, but multiple and variable initiation foci were observed in some cells. Where waves intersected within a single cell there was extinction of wave propagation, confirming the SR as the direct source of Ca2+ and revealing a refractory period in SR Ca2+ release. In some cells high-frequency Ca2+ waves lead to synchronized elevation of [Ca2+] throughout the entire cytosol and within the time period associated with cell depolarization. These observations support the hypothesis that some cardiac arrhythmias are initiated by spontaneous and propagated Ca2+ release and involve subsequent depolarization, global elevation of intracellular [Ca2+], and cell contraction.

A superfusion bath for single-cell recording with high-precision optical depth control, temperature regulation, and rapid solution switching

A superfusion system for isolated cell recording, which uses a novel optical technique for feedback control of solution depth (+/- 5 microns), is described. A retroreflective optosensor senses the distance between itself and a white hydrophobic plastic float, supported by surface tension on the surface of the solution. The combination of optical depth sensing with stepper-motor-driven pumps, pneumatic ("noise-free") solution switching and Peltier-effect solution heating/cooling provides a high degree of control over the cell superfusion environment. The solution flow rate to the bath is selected over the range 0.08-5.00 (+/- 0.01) ml min-1 with provision for solution recirculation. The temperature control range of the Peltier element is 4.0 degrees-70.0 degrees C. At temperatures selected over the range 16.0 degrees-36.0 degrees C, and at flow 3.00 ml min-1 the temperature gradient through the central working position of the bath is not more than 0.2 degrees C. At this flow, bath solution exchange on valve actuation is 90% complete within 0.7 s. This superfusion system is particularly suited to conventional and fluorescence imaging techniques where accurate control of the solution level ensures constant optical conditions. In electrophysiological experiments this apparatus will also provide stabilization of electrode capacitance with adjustment of solution depth. The robust and compact design of the system allows it to be carried between different applications.

Characterization of single heart cell contractility by rapid imaging

1. A rapid imaging technique with temporal resolution of about 1 ms was employed to describe cell-length changes during the isotonic contraction cycle of adult rat ventricular myocytes at 22 degrees C. 2. Parameters of cell contraction and relaxation were defined and values obtained under control conditions and after treatment with 5 mumol/L verapamil. 3. Over 15 min, verapamil dramatically reduced the maximum shortening attained. This was associated with delay in both excitation-contraction coupling latency and the time at which maximum rate of shortening occurred. However, peak shortening was recorded earlier in the contraction cycle. Total cell-cycle time was abbreviated under the influence of verapamil and maximum rates of shortening and lengthening were depressed in a similar manner. 4. The results highlight the value of improved temporal precision in describing myocyte contractility and validate the use of the parameters defined in the single-cell model for the study of the mechanisms of action of cardiotonic agents.

Heterogeneity of the action potential in isolated rat ventricular myocytes and tissue

The objectives of this study were to measure action potential parameters in enzyme-dissociated, adult rat ventricular myocytes stimulated at 1 Hz, to compare these measurements with those obtained from intact ventricular tissue, and to determine myocyte and tissue responses at stimulus frequencies between 0.1 and 5 Hz. Action potentials were characterized in terms of amplitude, overshoot, resting potential, duration at 25% and 75% repolarization (APD25, APD75), and Vmax. Based on statistical differences in APD25 and APD75, myocyte action potentials were classified as type I (3.1 +/- 1.0 and 21.5 +/- 3.6 msec), type II (7.4 +/- 1.1 and 38.2 +/- 6.7 msec), or type III (14.5 +/- 1.9 and 46.0 +/- 4.1 msec). Action potentials corresponding to type I were found in right ventricular endocardium and right papillary muscles, and those corresponding to types II and III in the left ventricular endocardium [apex, middle (II); base (III)] and left papillary muscles (II). Myocytes and papillary muscles responded to increases in driving rate with nearly identical lengthening of APD25 and shortening of APD75. The one exception was at 5 Hz where a lengthening of the APD75 occurred in some myocytes. We conclude that action potential configuration in rat ventricle is heterogeneous, and that this is reflected by the different types of action potentials in isolated myocytes. It is likely that the magnitude of a transient outward current is a determinant of action potential configuration, and that slow reactivation of this current is a significant factor underlying the stimulus frequency response.

An ultrastructural investigation into the size dependency of contractility of isolated cardiac muscle

The maximal stress (force per cross-sectional area), maximal rate of stress development, and cross-sectional area of 30 rabbit right ventricular papillary muscles were measured. Both mechanical parameters declined with increasing muscle size (the stress-area relation). Three large and three small muscles were fixed for electronmicroscopy. Stereological analyses of these six muscles were undertaken to test the hypothesis that the stress-area relation is caused by differences in the relative proportions of mitochondria and contractile matrix between muscles of large and small cross-sectional area. No difference in the proportion of these two cytoplasmic components was found. Other possible explanations underlying the stress-area relation are discussed.