Excitation-contraction coupling in cardiac muscle

Peroxisomes contribute to intracellular calcium dynamics in cardiomyocytes and non-excitable cells

Peroxisomes communicate with other cellular compartments by transfer of various metabolites. However, whether peroxisomes are sites for calcium handling and exchange has remained contentious. Here we generated sensors for assessment of peroxisomal calcium and applied them for single cell-based calcium imaging in HeLa cells and cardiomyocytes. We found that peroxisomes in HeLa cells take up calcium upon depletion of intracellular calcium stores and upon calcium influx across the plasma membrane. Furthermore, we show that peroxisomes of neonatal rat cardiomyocytes and human induced pluripotent stem cell-derived cardiomyocytes can take up calcium. Our results indicate that peroxisomal and cytosolic calcium signals are tightly interconnected both in HeLa cells and in cardiomyocytes. Cardiac peroxisomes take up calcium on beat-to-beat basis. Hence, peroxisomes may play an important role in shaping cellular calcium dynamics of cardiomyocytes.

Transcriptome analysis provides insights into the molecular mechanisms responsible for evisceration behavior in the sea cucumber Apostichopus japonicus

The sea cucumber Apostichopus japonicus (Selenka) is a valuable economic species in Southeast Asia. It has many fascinating behavioral characteristics, such as autolysis, aestivation, regeneration, and evisceration, thus it is a notable species for studies of special behaviors. Evisceration and autotomy are controlled by the neural network and involve a complicated physiological process. The occurrence of evisceration behavior in sea cucumbers is strongly related to their environment, and it negatively impacts their economic value. Evisceration behavior plays a pivotal role in the survival of A. japonicus, and when it is induced by dramatic changes in the coastal ecological environment and the aquaculture setting it can strongly affect the economic performance of this species. Although numerous studies have focused on intestinal regeneration of A. japonicus, less is known about evisceration behavior, especially its underlying molecular mechanisms. Thus, identification of genes that regulate evisceration in the sea cucumber likely will provide a scientific explanation for this significant specific behavior. In this study, Illumina sequencing (RNA-Seq) was performed on A. japonicus specimens in three states: normal (TCQ), eviscerating (TCZ), and 3 h after evisceration (TCH). In total, 129,905 unigenes were generated with an N50 length of 2651 base pairs, and 54,787 unigenes were annotated from seven functional databases (KEGG, KOG, GO, NR, NT, Interpro, and Swiss-Prot). Additionally, 190, 191, and 320 genes were identified as differentially expressed genes (DEGs) in the comparisons of TCQ vs. TCZ, TCZ vs. TCH, and TCQ vs. TCH, respectively. These DEGs mapped to 157, 113, and 190 signaling pathways in the KEGG database, respectively. KEGG analyses also revealed that potential DEGs enriched in the categories of "environmental information processing," "organismal system," "metabolism," and "cellular processes," and they were involved in evisceration behavior in A. japonicus. These DEGs are related to muscle contraction, hormone and neurotransmitter secretion, nerve and muscle damage, energy support, cellular stress, and apoptosis. In conclusion, through our comparative analysis of A. japonicus in different stages, we identified many candidate evisceration-related genes and signaling pathways that likely are involved in evisceration behavior. These results should help further elucidate the mechanisms underlying evisceration behavior in sea cucumbers.

Mechanisms underlying the cardio-protection of total ginsenosides against myocardial ischemia in rats in vivo and in vitro: Possible involvement of L-type Ca2+ channels, contractility and Ca2+ homeostasis

Here we aimed to observe the effects of total ginsenosides (TG) against isoproterenol (ISO) induced myocardial ischemia (MI) and to explore its underlying mechanisms based on L-type Ca²⁺ current (I_Ca-L), intracellular Ca²⁺ ([Ca²⁺]_i) and contraction in isolated rat myocytes. Rat model of MI was induced by subcutaneously injection of ISO (85 mg/kg) for 2 consecutive days. J-point elevation, heart rate, serum levels of creatine kinase (CK) and lactated dehydrogenase (LDH), and heart morphology changes were observed. Influences of TG on I_Ca-L, [Ca²⁺]_i and contraction in isolated rat myocytes were observed by the patch-clamp technique and IonOptix detection system. TG significantly reduced J-point elevation, heart rate, serum levels of CK and LDH, and improved heart pathologic morphology. TG decreased I_Ca-L in concentration-dependent manner with a half-maximal inhibitory concentration (IC₅₀) of 31.65 μg/mL. TG (300 μg/mL) decreased I_Ca-L of normal and ischemic ventricular myocytes by 64.33 ± 1.28% and 61.29 ± 1.38% respectively. At 30 μg/mL, TG reduced Ca²⁺ transient by 21.67 ± 0.94% and cell shortening by 38.43 ± 6.49%. This study showed that TG displayed cardioprotective effects on ISO-induced MI rats and the underlying mechanisms may be related to inhibition of I_Ca-L, damping of [Ca²⁺]_i and decrease of contractility.

Impact of detubulation on force and kinetics of cardiac muscle contraction

T-tubule uncoupling from the plasma membrane leads to myocardial contractile abnormalities.

Action potential–driven Ca²⁺ currents from the transverse tubules (t-tubules) trigger synchronous Ca²⁺ release from the sarcoplasmic reticulum of cardiomyocytes. Loss of t-tubules has been reported in cardiac diseases, including heart failure, but the effect of uncoupling t-tubules from the sarcolemma on cardiac muscle mechanics remains largely unknown. We dissected intact rat right ventricular trabeculae and compared force, sarcomere length, and intracellular Ca²⁺ in control trabeculae with trabeculae in which the t-tubules were uncoupled from the plasma membrane by formamide-induced osmotic shock (detubulation). We verified disconnection of a consistent fraction of t-tubules from the sarcolemma by two-photon fluorescence imaging of FM4-64–labeled membranes and by the absence of tubular action potential, which was recorded by random access multiphoton microscopy in combination with a voltage-sensitive dye (Di-4-AN(F)EPPTEA). Detubulation reduced the amplitude and prolonged the duration of Ca²⁺ transients, leading to slower kinetics of force generation and relaxation and reduced twitch tension (1 Hz, 30°C, 1.5 mM [Ca²⁺]_o). No mechanical changes were observed in rat left atrial trabeculae after formamide shock, consistent with the lack of t-tubules in rodent atrial myocytes. Detubulation diminished the rate-dependent increase of Ca²⁺-transient amplitude and twitch force. However, maximal twitch tension at high [Ca²⁺]_o or in post-rest potentiated beats was unaffected, although contraction kinetics were slower. The ryanodine receptor (RyR)2 Ca-sensitizing agent caffeine (200 µM), which increases the velocity of transverse Ca²⁺ release propagation in detubulated cardiomyocytes, rescued the depressed contractile force and the slower twitch kinetics of detubulated trabeculae, with negligible effects in controls. We conclude that partial loss of t-tubules leads to myocardial contractile abnormalities that can be rescued by enhancing and accelerating the propagation of Ca²⁺-induced Ca²⁺ release to orphan RyR2 clusters.

Evaluation of myocardial performance index to predict mild rejection in cardiac transplantation

BACKGROUND

Early diagnosis of heart transplant rejection is mandatory, since even mild rejection can rapidly progress to more severe rejection. Noninvasive diagnosis of heart transplant rejection still remains a challenge.

HYPOTHESIS

The purpose of the study was to determine a possible association between myocardial performance index (MPI) and biopsy score of the heart transplant.

METHODS

This is a retrospective cohort analysis of 99 complete Doppler echocardiographic studies from 24 consecutive patients (23 men) performed within 24 h of endomyocardial biopsy. Mean age of the cohort was 50 +/- 9 years and mean time from transplantation was 19 +/- 21 months (1-81). All patients were in sinus rhythm. Myocardial performance index was calculated as the ratio of isovolumic contraction time plus isovolumic relaxation time divided by ejection time. Left ventricular dimensions, left ventricular mass, ejection fraction, and a number of Doppler indices (E-point velocity, A-point velocity, deceleration time, and deceleration slope) were also measured. The International Society for Heart and Lung Transplantation (ISHLT) grading system was used for the classification of endomyocardial biopsies.

RESULTS

Myocardial performance index was significantly prolonged (0.60 +/- 0.13, 0.68 +/- 0.08, 0.75 +/- 0.20, in biopsy scores 0, IA, and IB, respectively; p<0.001). Isovolumic contraction time was significantly prolonged; isovolumic relaxation time was not significantly changed. Ejection time and deceleration time were significantly shortened. Multivariate stepwise regression analysis revealed that MPI and deceleration time were the only independent predictors of biopsy score (r=0.48, F=10.53, p<0.0001).

CONCLUSION

Myocardial performance index seems to be a useful adjunct in the follow-up of cardiac transplant patients. These preliminary data suggest that a larger study may be indicated to clarify the relevance of myocardial performance index.

Value of Doppler index combining systolic and diastolic myocardial performance in predicting cardiopulmonary exercise capacity in patients with congestive heart failure: effects of dobutamine

STUDY OBJECTIVE

Doppler-derived myocardial performance index (MPI), a measure of combined systolic and diastolic myocardial performance, was assessed at rest and after low-dose dobutamine administration in patients with idiopathic or ischemic dilated cardiomyopathy. MPI also was correlated with other conventional echocardiographic indexes of left ventricular (LV) function, and its ability to assess cardiopulmonary exercise capacity in those patients was investigated.

SETTINGS

A tertiary-care, university heart failure clinic.

PATIENTS

Forty-two consecutive patients (27 men; mean [+/- SD] age, 57 +/- 10 years) with heart failure (New York Heart Association [NYHA] class, II to IV) who had received echocardiographic diagnoses of dilated cardiomyopathy. Coronary angiography distinguished the cause of dilated cardiomyopathy.

INTERVENTIONS

Low-dose IV dobutamine was infused after patients underwent a baseline echocardiographic study. All patients also underwent a cardiopulmonary exercise test using a modified Naughton protocol.

RESULTS

Advanced NYHA class and restrictive LV filling pattern were associated with higher index values. A negative correlation was found between MPI and LV stroke volume, cardiac output, early filling/late filling velocity ratio, and late LV filling velocity, as well as oxygen uptake at peak exercise (r = -0.550; p < 0.001) and at the anaerobic threshold (r = -0.490; p = 0.002). Dobutamine administration produced an improvement in MPI, reducing its value and decreasing the isovolumic relaxation and contraction times. Stepwise regression analysis revealed that the rest index and the late LV filling velocity were the only independent predictors of cardiopulmonary exercise capacity.

CONCLUSION

MPI correlates inversely with LV performance, reflects disease severity, and is a useful complimentary variable in the assessment of cardiopulmonary exercise performance in patients with heart failure.

Prednisolone hastens recovery from histamine-induced bronchospasm in asthmatics

Short-term treatment with oral steroids is very effective in control ling symptoms and improving lung function in asthma but has not been shown unequivocally to reduce bronchial hyperresponsiveness. Recently it has been shown to increase the activities of sodium-potassium and calcium adenosine triphosphatases, enzymes that regulate intracellular calcium levels. This action may be expected to promote recovery of cells from an excitatory stimulus. The present study was carried out to determine how prednisolone modulates airway response to histamine, including recovery from induced bronchospasm in asthmatics. Spirometry and measurement of bronchial responsiveness (forced expiratory volume in 1 sec [FEV1] and concentration of histamine causing a 20% reduction FEV1 [PD20 FEV1]) to inhaled histamine were carried out in 10 clinically stable asthmatics. Subsequently, all of the patients were prescribed oral prednisolone, 0.6-0.75 mg/kg body weight for 1 week. At the end of 1 week, spirometry was repeated and bronchial reactivity was measured again. Comparison of PD20 FEV1 values before and after treatment (geometric means 0.66 and 0.81 mg/mL, respectively) for the whole group did not show any significant change. The mean +/- SD time for 95% recovery from histamine-induced bronchospasm was 33.00 +/- 19.47 min before treatment and decreased significantly to 18.00 +/- 7.88 min after treatment. It was concluded that short-term benefits from oral prednisolone do not result from changes in bronchial responsiveness. These benefits may be related to effects on mechanisms that lead to recovery from an excitatory stimulus.

The mechanisms of sarcoplasmic reticulum Ca2+ release in toad pacemaker cells

The mechanisms of sarcoplasmic reticulum (SR) Ca2+ release in pacemaker cells from the sinus venosus of the cane toad (Bufo marinus) were studied. Single, isolated cells were voltage clamped using a nystatin-perforated patch. Ionic currents and intracellular Ca2+ concentration ([Ca2+]i) were recorded simultaneously. Depolarizations of 300 ms duration from a holding potential of -55 mV produced an inward current which had a bell-shaped relationship with voltage. Inward current first appeared at about -45 mV, reached a maximum of -343 +/- 46 pA at -15 mV and reversed at +45 mV. In contrast the amplitude of the increase in [Ca2+]i caused by depolarization (Ca2+ transient) increased monotonically with the increasing depolarization. At -15 mV the amplitude of the Ca2+ transient was 243 +/- 33 nM and at +45 mV it was 411 +/- 43 nM. The inward current produced by depolarizations to -5 mV was largely eliminated by the L-type Ca2+ channel blocker nifedipine (10 microM) while 37 +/- 7 % of the Ca2+ transient persisted. A significantly larger proportion of the Ca2+ transient (56 +/- 5 %) remained at +85 mV in the presence of nifedipine. The SR Ca2+ pump inhibitor 2, 5-di(tert-butyl)-1,4-hydroquinone (10 microM), which causes depletion of the SR Ca2+, reduced the amplitude of the Ca2+ transient to 34 +/- 1 % of control, irrespective of the voltage. Brief exposure to extracellular Ca2+-free solution abolished the Ca2+ transients caused by depolarization while the caffeine-induced Ca2+ release persisted. Tetrodotoxin (1 microM) had no effect on the amplitude of the depolarization-induced Ca2+ transient, although it reduced the fast component of the inward current. In contrast, Ni2+ (5 mM) abolished the Ca2+ transients at any given voltage. Ni2+ also abolished spontaneous Ca2+ transients. In conclusion, in toad pacemaker cells Ca2+ release from SR contributes approximately 66 % of the Ca2+ involved in the Ca2+ transient and requires extracellular Ca2+ influx to trigger its release. The L-type Ca2+ channels and Na+-Ca2+ exchange are major sources of Ca2+ influx under physiological conditions.

Excitation-contraction coupling in rat ventricular myocytes after formamide-induced detubulation

Formamide-induced osmotic shock has been used to detubulate isolated adult rat ventricular myocytes (i.e., disrupt the surface membrane-T tubule junction). Cell volume, calculated from cell length and width, rapidly decreased and increased upon application and removal of formamide, respectively. After treatment with formamide, membrane capacitance decreased by 26.4% (from 199.4 +/- 18.7 pF in control cells to 146.7 +/- 6.4 pF in formamide-treated cells; n = 13, P < 0.05). However, the amplitude of the L-type Ca(2+) current (I(Ca)) decreased by a greater extent (from 0.75 +/- 0.14 to 0.18 +/- 0.03 nA; n = 5, P < 0.05) so that the density of I(Ca) decreased by 74.5%. Simultaneous measurements of I(Ca) and Ca(2+) transients (monitored using fura 2) showed that both decreased rapidly upon removal of formamide. However, the Ca(2+) content of the sarcoplasmic reticulum showed little change. Cross-striations, visualized with the fluorescent dye di-8-aminonaphthylethenylpyridinium, were sparse or absent in cells that had been treated with formamide, suggesting that formamide can successfully detubulate cardiac cells and that I(Ca) is concentrated in the T tubules, which therefore play an important role in excitation-contraction coupling.

How does beta-adrenergic stimulation increase the heart rate? The role of intracellular Ca2+ release in amphibian pacemaker cells

1. The mechanism by which sympathetic transmitters increase the firing rate of pacemaker cells was explored in isolated cells from the sinus venosus of the cane toad Bufo marinus. Intracellular calcium concentration ([Ca2+]i) was measured with indo-1 and membrane potential and currents were recorded with the nystatin perforated-patch technique. 2. Adrenaline or isoprenaline (2 microM) increased the transient rise in [Ca2+]i and increased the firing rate; these effects were blocked by propranolol (2 microM). 3. To determine whether the changes in [Ca2+]i might influence the firing rate we studied agents which affect either the loading or the release of Ca2+ from the sarcoplasmic reticulum (SR). Rapid application of caffeine (10 mM) to spontaneously firing cells caused a large Ca2+ release from the SR and the cells were then quiescent for 24 s. In the presence of beta-adrenergic stimulation the caffeine-induced [Ca2+]i was 14 % larger but the period of quiescence after application was reduced to 12 s. 4. Ryanodine, at either low (1 microM) or high (> 10 microM) concentration, stopped firing. However, when the SR store content of Ca2+ was tested with caffeine, at low ryanodine concentration the SR Ca2+ store was empty whereas at the high concentration the SR store was still loaded with Ca2+. beta-Adrenergic stimulation was not able to restore firing at the low concentration of ryanodine but did restore firing at the high ryanodine concentration. 5. An SR Ca2+ pump blocker, 2, 5-di(tert-butyl)-1,4-hydroquinone (TBQ) which depletes the SR store of Ca2+, also rapidly and reversibly stopped spontaneous firing. 6. The relation between the amplitude of the [Ca2+]i transient and firing rate established in the presence of ryanodine was similar when firing was restored by beta-stimulation. 7. In both spontaneously firing and voltage-clamped cells, depleting the SR store with either ryanodine or TBQ suggested that about half of the Ca2+ which contributes to the calcium transient is released from the SR. 8. These results show that the amplitude of the [Ca2+]i transient is an important factor in the firing rate of toad pacemaker cells and consequently agents which modify SR Ca2+ release influence firing rate. The effects of beta-stimulation on firing rate seem to be largely mediated by changes in amplitude of the [Ca2+]i transient.

Sodium withdrawal contractures in developing and regenerating rat extensor digitorum longus muscles

During postnatal development of extensor digitorum longus (EDL) muscle, sodium withdrawal contractures were observed during the first 6 days after birth, and not after this time. In regenerating EDL muscles, zero-Na contractures were demonstrated: (1) 7 days after bupivacaine injection, but not 14 or 90 days after this injection; (2) 7, 14, and 90 days after autotransplantation; and (3) 7, 14, and 90 days after the intervention in sliced muscles. The present findings emphasize the role of the denervation in the development of zero-Na contractures in the regenerating muscles and suggest that a calcium-sodium exchange across the sarcolemma may appear in these muscles.

Intracellular calcium and Na+-Ca2+ exchange current in isolated toad pacemaker cells

1. Single pacemaker cells were isolated from the sinus venosus of cane toad (Bufo marinus) in order to study the mechanisms involved in the spontaneous firing rate of action potentials. Intracellular calcium concentration ([Ca2+]i) was measured with indo-1 to determine whether [Ca2+]i influenced firing rate. A rapid transient rise of [Ca2+]i was recorded together with each spontaneous action potential. [Ca2+]i at the peak of systole was 655 +/- 64 nM and the minimum at the end of diastole was 195 +/- 15 nM. 2. Reduction of extracellular Ca2+ concentration from 2 to 0.5 mM caused a reduction in both systolic and diastolic [Ca2+]i and the spontaneous firing rate also gradually declined. 3. Application of the acetoxymethyl (AM) ester of BAPTA (10 microM), in order to increase intracellular calcium buffering, caused a decline in systolic and diastolic [Ca2+]i. The firing rate declined progressively until the cells stopped firing after 10-15 min. At the time that firing ceased, the diastolic [Ca2+]i had declined by 141 +/- 38 nM. 4. In the presence of ryanodine (2 microM), which interferes with Ca2+ release from the sarcoplasmic reticulum, the systolic and diastolic [Ca2+]i both declined and the firing rate decreased until the cells stopped firing. At quiescence diastolic [Ca2+]i had declined by 93 +/- 20 nM. 5. Exposure of the cells to Na+-free solution caused a rise in [Ca2+]i which exceeded the systolic level after 4.8 +/- 0.3 s. This rise is consistent with Ca2+ entry on a Na+-Ca2+ exchanger. 6. Rapid application of caffeine (10-20 mM) to cells clamped at -60 mV caused a rapid increase in [Ca2+]i which then spontaneously declined. An inward current with a similar time course to that of [Ca2+]i was also generated. Application of Ni2+ (5 mM) or 2,4-dichlorobenzamil (25 microM) reduced the amplitude of the inward current produced by caffeine by 96 +/- 1 % and 74 +/- 10 %, respectively. In a Na+-free solution the caffeine-induced current was reduced by 93 +/- 7 %. 7. Under a variety of circumstances the diastolic [Ca2+]i showed a close association with pacemaker firing rate. The existence of a Na+-Ca2+ exchanger and its estimated contribution to inward current during the pacemaker potential suggest that the Na+-Ca2+ exchange current makes a contribution to pacemaker activity.

Development of mechanisms regulating intracellular Ca2+ concentration in cardiac muscle cells of early chick embryos

The development of mechanisms for the regulation of intracellular-free calcium ion concentration ([Ca2+]i) was investigated in precardiac mesodermal cells (PMC) and cardiac muscle cells (CMC) from early chick embryos by microfluorometry using a Ca2+-sensitive fluorescent probe, fura-2, and transmission electron microscopy. Microfluorometry indicated that two types of regulatory mechanisms, involving the dihydropyridine receptor (DHPR) and the ryanodine receptor (RYR), are present in CMC when the heartbeat begins at the 8-9 somite stages. Nifedipine completely suppressed the beating of hearts isolated from embryos on Days 1.5 and 2. Ryanodine had no effect on the beating of hearts isolated from embryos on Day 1.5, though it completely suppressed beating in hearts from Embryonic Day 2. Microfluorometry revealed that a change occurred in the Ca2+-regulating mechanisms of CMC on Day 2. Transmission electron microscopy showed the appearance in CMC, also on Day 2, of peripheral couplings with feet structures, and SR adjacent to the Z-line of myofibrils. These findings suggest that the calcium-induced calcium-release (CICR) mechanism appears in the CMC of the chick on the second day of embryonic development.

Noninvasive Doppler-derived myocardial performance index: correlation with simultaneous measurements of cardiac catheterization measurements

A simple, reproducible, noninvasive Doppler index for the assessment of overall cardiac function has been described previously. The purpose of this study was to correlate the Doppler index with accepted indexes of cardiac catheterization of left ventricular performance. Thirty-four patients with ischemic heart disease or idiopathic dilated cardiomyopathy prospectively underwent a simultaneous cardiac catheterization and Doppler echocardiographic study. Invasive measurements of peak +dP/dt, peak -dP/dt, and tau were obtained from the high-fidelity left ventricular pressures. A Doppler index of myocardial performance was defined as the summation of isovolumetric contraction and relaxation time divided by ejection time. There was a correlation between Doppler measurement of isovolumetric contraction time and peak +dP/dt (r = 0.842; p < 0.0001) and Doppler measurement of isovolumetric relaxation time and peak -dP/dt (r = 0.638; p < 0.001). Left ventricular ejection time correlated with both peak +dP/dt (r = 0.539; p < 0.001) and peak -dP/dt (r = 0.582; p < 0.001). The Doppler index correlated with simultaneously recorded systolic peak +dP/dt (r = 0.821; p < 0.0001) and diastolic peak -dP/dt (r = 0.833; p < 0.001) and tau (r = 0.680; p < 0.0001). This study documents that a simple, easily recordable, noninvasive Doppler index of myocardial performance correlates with invasive measurement of left ventricular systolic and diastolic function and appears to be a promising noninvasive measurement of overall cardiac function.

Calcium entry attenuates adenylyl cyclase activity. A possible mechanism for calcium-induced catecholamine resistance

In experimental animals, coadministration of calcium (Ca) salts with beta-adrenergic receptor agonists reduces the increased blood pressure and cyclic AMP (cAMP) produced by beta-adrenergic receptor agonists alone. In patients, coadministration of these drugs reduces the increased cardiac output and blood glucose produced by selective administration of beta-adrenergic agonists. The mechanism by which Ca might produce catecholamine resistance remains unclear. Healthy volunteers donated venous blood from which lymphocytes were isolated. The cAMP production was measured by radioimmunoassay under control conditions and after incubation with epinephrine or colforsin (forskolin) in the presence and absence of inhibitors. Epinephrine and colforsin produced concentration-dependent increases in cAMP production. Extracellular Ca concentration over the range from 0 to 8 mM did not inhibit basal cAMP production or that stimulated by either colforsin or epinephrine. The calcium channel agonist Bay K 8644 (50 microM) combined with normal extracellular Ca concentration significantly attenuated colforsin-induced increases in cAMP production. When barium was substituted for Ca in the extracellular fluid, the cAMP response to colforsin was restored, despite Bay K 8644. Inhibition of Ca channel permeability with cadmium or cobalt ions partially restored colforsin-stimulated cAMP production, despite the presence of extracellular Ca and Bay K 8644. These results suggest that entry of Ca ions through Ca channels attenuates adenylyl cyclase. The inhibition appears specific for Ca ions over other permeant divalent cations, and favors a possible physiologic role for the recently cloned Ca-inhibited adenylyl cyclase.

Calcium-containing vacuolated mitochondria during early heart development in chick embryos as demonstrated by cytochemistry and X-ray microanalysis

The ultrastructure of mitochondria in the developing heart was examined in chick embryos from 2 to 7 days after fertilization. Vacuolated mitochondria were observed in the heart muscle cells of embryos at all stages examined. The number of vacuolated mitochondria as a percentage of total mitochondria in muscle cells was high in embryos at 3 and 4 days and was much higher in the ventricular cells than in the atrial cells. Examination by cytochemistry and X-ray microanalysis revealed the accumulation of calcium in the vacuoles of mitochondria. These results suggest important roles for vacuolated mitochondria in the regulation of the intracellular concentration of calcium during the early development of the chick heart.

Role of sodium-calcium exchange in activation of contraction in rat ventricle

1. The functional role of reverse Na(+)-Ca2+ exchange in the activation of contraction of rat ventricular myocytes has been studied. Mechanical activity of single cells, measured as unloaded cell shortening, was recorded simultaneously with membrane current and voltage using a single microelectrode voltage clamp and a video edge detection device. 2. The voltage dependence of contraction was studied by applying trains of depolarizations. At test potentials between +20 and +80 mV (under conditions where large outward currents were activated) a plateau on the shortening vs. voltage (S-V) relationship was observed. Significant cell shortening also occurred at test potentials between -70 and -40 mV; and these contractions were accompanied by large inward Na+ currents. We have investigated the ionic mechanisms for three components of the S-V relation in rat ventricle: (i) shortening which occurs between -70 and -40 mV and is thought to be dependent on the sodium current; (ii) phasic contractions in the voltage range -40 to +40 mV where the L-type Ca2+ current is present; (iii) the plateau of the S-V relation at strongly depolarized voltages where reverse Na(+)-Ca2+ exchange may occur. 3. Experiments in which two independent microelectrode impalements were made in a single myocyte showed that during activation of contraction at test potentials between -70 and -40 mV, and during very large depolarizations (+20 to +80 mV), there were significant deviations of the measured membrane potential from the applied voltages. Activation of cell shortening in these voltage ranges could be eliminated by electronic series resistance compensation, which significantly reduced these voltage errors. Consistent with these findings, when tetrodotoxin (TTX) and 4-aminopyridine (4-AP) were used to block inward Na+ and transient outward K+ currents, respectively, no significant voltage errors were present and a bell-shaped shortening-voltage (S-V) relationship was obtained. 4. When Na+ and K+ currents were blocked, depolarizations from holding potentials of either -80 or -50 mV demonstrated that the threshold for activation of contraction was about -30 mV, and that the voltage dependence of peak shortening was very similar to that of the L-type Ca2+ current (ICa,L). These contractions were suppressed completely by either Cd2+ or ryanodine, showing that activation of cell shortening was due to Ca2+ influx through L-type channels which induced release of Ca2+ from the sarcoplasmic reticulum (SR). No T-type calcium currents were observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Diminished myofibrillar sensitivity to calcium produced by simultaneous superfusion of cAMP and phosphodiesterases inhibitors in toad (Bufo arenarum Hensel) ventricle

Experiments were performed in EGTA-skinned trabeculae from toad ventricle to explore the effects of the cAMP on calcium sensitivity of the contractile system. 10(-3) M of the cAMP derivative dibutyryl cAMP (dcAMP) failed to affect calcium sensitivity of chemically skinned ventricular trabeculae. 10(-5) M of the phosphodiesterase inhibitor isobutylmethylxantine (IBMX) produced a significant shift to the left of the tension-pCa relationship. The computed half maximally activating pCa (pCa50) were 6.32 +/- 0.03 and 6.40 +/- 0.01 in the absence and presence of IBMX respectively, (P < 0.05). Simultaneous perfusion of 10(-5) M IBMX and 10(-3) M dcAMP suppressed the leftward shift of the tension-pCa curve induced by IBMX (pCa50: 6.33 +/- 0.04 and 6.33 +/- 0.05 for control and IBMX respectively). Perfusion with the phosphodiesterase inhibitor milrinone (10(-5) M), did not produce any significant changes in the tension-pCa relationship. Simultaneous perfusion of 10(-5) M milrinone and 10(-3) M dcAMP significantly shifted to the right the tension-pCa curve. The computed pCa50 were 6.32 +/- 0.02 and 6.23 +/- 0.03 under control conditions and in the presence of dcAMP plus milrinone respectively (P < 0.05). In agreement with what has been described in mammalian heart, the results indicate that in amphibian ventricle, cAMP produced a decrease in the calcium sensitivity of the contractile proteins that is only evident in the presence of phosphodiesterase inhibitors. It is suggested that this decrease in myofilament sensitivity to calcium may be one of the mechanisms by which beta-agonists enhance myocardial relaxation in the amphibian heart.

The effects of hypoxic stress on the fine structure of the flounder heart (Platichthys flesus)

1. Flounder hearts were examined by conventional transmission electron microscopy. Hearts showed clear evidence of a coronary circulation but no intrinsic conduction network or innervation was detected. 2. The box-like cells showed many surface inpocketings and many scattered glycogen granules and membrane bound liposomes. The nucleus position was variable and the cells contained numerous small ovoid mitochondria. 3. Dark staining intercalated discs with clear nexal and desmosomal areas separated individual cells. 4. No organized T-tubular system or sarcoplasmic reticulum was present but the cells displayed abundant surface vesicles which may perform the physiological role of the latter. 5. Hearts of flounder subjected to 3 weeks hypoxia showed striking changes to the mitochondria which were smaller than in controls and there was evidence of increased mitochondrial budding and also evidence of mitochondrial necrosis. 6. Hypoxically-stressed hearts exhibited normal liposome populations but increased glycogen granule deposits. These hearts also showed evidence of myofibril degeneration with torn Z discs and with a build up of fibrous material. 7. It is concluded that the cellular damage seen in hypoxic cells may be due to excessive calcium accumulation or increased catecholamine release.

Different effects of reoxygenation on the electrical activity of ventricular muscle

Response of a hypoxic and acidotic (HA, with exogenous lactate) ventricular muscle tissue to subsequent reoxygenation in the absence of substrate (0 mM dextrose) was different from that of a Purkinje fiber. The K+ concentration in this solution (4.6 mM) was slightly higher than that in Tyrode solution (2.7 mM). The observed effects of reoxygenation of such a ventricular tissue were also variable. The ventricular muscle tissue exhibited the following different responses on reoxygenation after hypoxia and acidosis: (1) arrhythmias, without much depolarization of the membrane potential, (2) oscillatory after-potentials (OAPs) during the late diastole, which lessened in amplitude as the time of reoxygenation increased, but no arrhythmias, or (3) a pronounced slowed phase of repolarization (hump), but no arrhythmias. These different effects of reoxygenation did not occur if concentration of K+ in HA was very much higher than 4.6 mM. Common to these three different responses was the prolongation of the action potential durations during reoxygenation at the 50% and 90% levels of repolarization (APD50 and APD90) and a slight increase in the resting tension after 30-40 minutes of reoxygenation. Some of the observed responses of ventricular muscles were well mimicked by increasing extracellular calcium, but the different and variable effects of arrhythmias, OAPs, and prolonged APD require further analysis.

Effects of aluminium on electrical and mechanical properties of frog atrial muscle

1. The effects of aluminium on membrane ionic currents were studied in single cardiac myocytes. Most of the work was done on frog atrial cells, but some experiments were also carried out on single cells isolated from rabbit ventricles and atria. 2. The effects of aluminium on the force of contraction of frog atrial trabeculae were also investigated. 3. Aluminium was prepared from AlCl3 as a stock 0.5 M solution which has a pH of 3.5. Before each experiment, this solution was added to the control solution, to give a final concentration of 20-100 micrograms ml-1 aluminium (0.75-3.75 mM AlCl3). The solutions were brought to a pH of 7.4 or 7.6. at which they consist of a mixture of amorphous aluminium hydroxides and a very small amount of soluble ionic aluminium complexes: free aluminium cations (less than 10 pM), aluminohydroxide anions (less than 8 microM). The addition of this suspension reduced the peak inward calcium currents in single rabbit atrial and ventricular cells and in frog atrial cells. In the latter, the peak current was reduced (at + 10 mV) to 45% of control (mean of 9 cells). This effect was reversible upon washout, and was obtained at all membrane potentials, with no shift of the calcium current voltage relationship along the voltage axis. 4. Aluminium also reduced the time-dependent potassium current IK. This reduction was observed at all membrane potentials. For example, at + 10 mV, the mean reduction of IK (n = 9) was to 69% of the control amplitude. This effect, which was very difficult to reverse, was not due to IK rundown. The fully activated current-voltage relationships (obtained by standard 'tail' analysis) showed that the effect of aluminium was due mainly to a decrease in conductance and not to a shift in the activation range of IK. The mean voltage of half activation was shifted by 8 mV in the depolarizing direction (n = 5). 5. The background potassium current IK1 was also slightly but consistently changed in a complex fashion, with an outward shift at membrane potentials positive to -60 mV. For example, at a membrane potential of -40mV, the mean shift was by 22 + 4pA. At more negative potentials, there was an inward shift in the current amplitudes. For example, for steps to -I00 mV the current elicited was larger (more inward) by 53 pA (mean value, n = 10). The reversal potential was slightly shifted (<10 mV) in the hyperpolarizing direction. 6. The force of contraction of frog atrial trabeculae was altered by aluminium in a complex manner, which showed marked seasonal variation. During most of the year, 50-100,ug ml-1 aluminium caused a biphasic change, with an early small and consistent decrease, followed by a large increase in twitch amplitude. For a short period corresponding to the (local) winter months the sensitivity to aluminium was greatly enhanced. Aluminium lOOupgml-1 totally abolished contraction (n = 5), while a lower concentration (20,ug ml- 1) produced a sustained reduction in the force of contraction. Similar biphasic and seasonal responses have been reported to be induced by lanthanum. 7. The biphasic changes in twitch amplitude were independent of the transmembrane sodium gradient. Aluminium produced the same effects when 90% of the extracellular sodium was replaced by lithium. Caffeine (5 mM) attenuated or even inverted the positive inotropic effect of aluminium. These results imply that aluminium alters the release of calcium from intracellular, caffeine-sensitive stores. This could be effected either by augmenting the amount released during each activation, and/or by increasing the loading of stores prior to release.

Maturational changes in excitation-contraction coupling in mammalian myocardium

The single sucrose gap voltage clamp technique has been used to study the relation between membrane electrical activity and tension generation in right ventricular papillary muscles from New Zealand White rabbits at various stages of development. In response to voltage clamp-controlled depolarization, muscles from newborn rabbits develop monotonically increasing tension that reaches a steady state level, whereas more mature myocardium responds to similar depolarization by developing an early peak of tension before relaxing to a steady state level. The ratio of early peak or phasic tension to steady state or tonic tension increases significantly with maturation. Calcium (Ca2+) loading of immature myocytes enhances phasic tension in a subsequent test depolarization. Although the voltage dependence of tonic tension increases monotonically in all age groups, phasic tension, seen only in the more mature myocardium, displays a "bell-shaped" dependence on voltage. Addition of ryanodine, known to interfere with Ca2+ release from the sarcoplasmic reticulum, markedly reduces the phasic component of tension in mature myocardium, and the voltage clamp-induced tension in the adult closely resembles that of the normal newborn. These results suggest that tension development in immature myocardium is supported largely by the influx of Ca2+ across the sarcolemma. As the myocardium matures, the sarcoplasmic reticulum plays an increasingly important role in tension generation. A developmental schema is presented to account for the observed maturational changes in excitation-contraction coupling. The clinical implications of these changes are discussed as they relate to the practice of pediatric cardiology.

Role of Ca2+ channel in cardiac excitation-contraction coupling in the rat: evidence from Ca2+ transients and contraction

1. Optical methods were used to measure simultaneously unloaded cell shortening and intracellular Ca2+ transients in whole-cell voltage clamped rat ventricular myocytes. Red light (greater than 670 nm) was used to measure cell shortening with a linear photodiode array. The dyes Fura-2 (Kd = 140 nM) and Mag-Fura-2 (Kd = 44 microM) were used as Ca2+ indicators with fluorescence excitation at 340 and 410 nm and emission at 510 nm. 2. Repeated measurements at 6 s intervals as 0.4 mM-Fura-2 diffused into the cell from the tip of the voltage clamp pipette showed no decrease in the rate of rise and peak value of the intracellular Ca2+ transient and only a small suppression of cell shortening, suggesting that the molecular mechanisms regulating the Ca2+ release were not significantly altered by the buffering capacity of the Fura-2. 3. Experiments in which the sarcoplasmic reticulum (SR) was depleted of Ca2+ either by exposure to caffeine or by repeated brief (20 ms) voltage clamp depolarizations confirm that the SR is the major source of activator Ca2+. 4. Mag-Fura-2 (1 or 5 mM) was used to register the initial rapid development of the [Ca2+]i transient but the later time course of the Ca2+ transients measured with this dye was obscured by motion artifacts resulting from cell shortening. 5. Both Fura-2 and Mag-Fura-2 showed that depolarization to 0 mV from a holding potential of -80 mV resulted in a [Ca2+]i transient which developed with a delay of 3-9 ms and approached its peak value in an additional 8-19 ms. Both Ca2+ indicators also showed that the Ca2+ transient approached its peak value more slowly as the clamped membrane potential was made increasingly more positive. 6. The voltage dependencies of the Ca2+ signal (Fura-2) and cell shortening were both bell-shaped and were qualitatively similar to the voltage dependence of Ca2+ current simultaneously measured. This was observed with holding potentials of both -40 and -80 mV. 7. Comparison of the temporal relation of the Ca2+ current, ICa, and intracellular Ca2+ transient (Fura-2) and cell shortening at different membrane potentials showed that Ca2+ transient measured 25 ms into the depolarization correlated closely to the integral of the Ca2+ current measured prior to this time. Cell shortening, on the other hand, peaked about 100 ms later and correlated with measurements of the Ca2+ activity at the later time.(ABSTRACT TRUNCATED AT 400 WORDS)

Why does halothane relax cardiac muscle but contract malignant hyperthermic skeletal muscle?

We have studied the question of the possible role of sarcoplasmic reticulum (SR) in the interaction of volatile anesthetics (such as halothane, enflurane and isoflurane) with muscle. We used two cardiac muscle models, i.e., isolated rat myocytes and Langendorff perfused rat hearts. We compared the results with those for skeletal muscle SR from rabbits, rats and pigs susceptible to malignant hyperthermia (MH). In both skeletal and cardiac muscle SR, volatile anesthetics enhanced the calcium release from the SR. In cardiac muscle, these agents are known to decrease contractility (negative inotropism). We found that caffeine, a well-known agent which releases calcium from the SR, also had a negative inotropic effect in cardiac muscle, raising the possibility of an unexpected link between the potentiation of calcium release and mechanism underlying the observed negative inotropism. Current understanding of anesthetic mechanisms does not include this possibility. We further found that both volatile anesthetics and caffeine decrease the content of calcium in the SR, suggesting that the increase of calcium permeability results in the decrease of calcium ions in the SR which are available for excitation-contraction (E-C) coupling. In MH-susceptible skeletal muscle, a similar increase in calcium permeability does not cause a decrease of contractility, but rather may contribute to a fatal syndrome of temperature increase provoked by abnormal contracture. This difference may be because in skeletal myoplasm calcium ions recycle internally, while in the cardiac muscle cell they are in dynamic equilibrium with extracellular calcium ions.

Negative inotropic effects of disopyramide on guinea-pig papillary muscles

1. The inhibitory effects of disopyramide on electromechanical responses were investigated in guinea-pig papillary muscles driven by electrical stimuli. Disopyramide up to 10(-5) M did not cause a negative inotropic effect, while the maximum upstroke velocity of the action potential (dV/dtmax) was significantly decreased. 2. At higher concentrations, this drug dose-dependently inhibited the contraction, and dV/dtmax was further decreased. This inhibition of contraction was accompanied by a depression of the slow action potential in partially depolarized preparations by increasing [K+]0 (26 mM). 3. In preparations pretreated with nifedipine (10(-6) M) and ryanodine (10(-6) M), the contraction was almost completely inhibited. In such preparations, ouabain (2 x 10(-6) M) markedly increased the contraction, probably through the Na(+)-Ca2+ exchange mechanism. This contraction was inhibited by disopyramide above 10(-8) M, and an almost complete inhibition was caused at 3 x 10(-5) M. 4. A similar inhibitory effect was observed on the contraction increased by the lowering of [Na+]0 (36 mM). 5. These results suggest that disopyramide at high concentrations inhibits Ca influx through slow Ca2+ channels and at low concentrations, it reduces the contraction increased through the Na(+)-Ca2+ exchange mechanism. Disopyramide had a greater effect on cardiac contractility mediated by the Na(+)-Ca2+ exchange mechanism.

Developmental changes in cardiac myocyte calcium regulation

Developmental changes in the contributions of transsarcolemmal Ca2+ influx and Ca2+ release from intracellular storage sites to myocardial contraction were evaluated in isolated ventricular myocytes from neonatal (aged 1-7 days) and adult (aged 8-10 weeks) New Zealand White rabbits. Contractions ceased in one beat when extracellular Ca2+ was decreased from 1mM to micromolar levels using a rapid perfusion technique. On reperfusion with 1 mM Ca2+, recovery of control contraction amplitude occurred after significantly fewer beats in neonatal myocytes compared with adult myocytes, and after 1 minute compared with 5 minutes of reduced Ca2+. After 15 minutes of perfusion with either 1 or 10 microM ryanodine, contraction amplitude decreased in both age groups, but the decrease was significantly greater in adults than in neonates. These experiments indicate that isolated ventricular myocytes may be used in the study of developmental changes in intracellular Ca2+ regulation. Results suggest that cardiac contraction in neonates is relatively more dependent on transsarcolemmal Ca2+ influx. Furthermore, although Ca2+ release from intracellular storage sites is present in both neonates and adults, its role in cardiac contraction is more significant in adults.

Effects of lactate, reoxygenation, and fast drive on hypoxic, hyperkalemic, and acidotic Purkinje fibers

The effects of hypoxia, hyperkalemia, and acidosis (HHA) and subsequent reoxygenation were studied in isolated canine Purkinje tissue. After 60-70 minutes of HHA, in the absence of exogenous lactate, reoxygenation reduced action potential duration (APD), increased amplitude, made maximum diastolic potential (Emax) more negative, increased diastolic depolarization rate (DD) of the action potential (AP), and increased developed force initially, compared to these values in the prehypoxic stage (control). In the presence of exogenous lactate (20-22 mmol) during HHA, reoxygenation after HHA leads to the development of afterdepolarization in the AP, prolongation of APD90, subsequent arrhythmia, cessation of developed force, and residual small potentials at a depolarized level. Higher stimulus voltage (by 70%) and higher driving rate (2 Hz) may restore normal APD and force sooner than when the rate of drive and the stimulus strength are kept unchanged.

Nonlinear charge movement in mammalian cardiac ventricular cells. Components from Na and Ca channel gating

Intramembrane charge movement was recorded in rat and rabbit ventricular cells using the whole-cell voltage clamp technique. Na and K currents were eliminated by using tetraethylammonium as the main cation internally and externally, and Ca channel current was blocked by Cd and La. With steps in the range of -110 to -150 used to define linear capacitance, extra charge moves during steps positive to approximately -70 mV. With holding potentials near -100 mV, the extra charge moving outward on depolarization (ON charge) is roughly equal to the extra charge moving inward on repolarization (OFF charge) after 50-100 ms. Both ON and OFF charge saturate above approximately +20 mV; saturating charge movement is approximately 1,100 fC (approximately 11 nC/muF of linear capacitance). When the holding potential is depolarized to -50 mV, ON charge is reduced by approximately 40%, with little change in OFF charge. The reduction of ON charge by holding potential in this range matches inactivation of Na current measured in the same cells, suggesting that this component might arise from Na channel gating. The ON charge remaining at a holding potential of -50 mV has properties expected of Ca channel gating current: it is greatly reduced by application of 10 muM D600 when accompanied by long depolarizations and it is reduced at more positive holding potentials with a voltage dependence similar to that of Ca channel inactivation. However, the D600-sensitive charge movement is much larger than the Ca channel gating current that would be expected if the movement of channel gating charge were always accompanied by complete opening of the channel.

Strontium-induced creep currents associated with tonic contractions in cardiac myocytes isolated from guinea-pigs

1. Strontium can replace calcium in a number of physiological and biochemical processes. The effects of Sr2+ were investigated in enzymatically isolated ventricular myocytes of the guinea-pig. Action potentials and membrane currents were measured with the patch-clamp technique used in the whole-cell recording configuration. Mechanical activity was assessed utilizing a laser-light diffraction system for sarcomere length measurements in single heart cells. 2. When experiments where carried out using 2 mM-Sr2+ to replace 2 mM-Ca2+ action potentials were found to be prolonged up to severalfold. Voltage-clamp experiments revealed that the slow inward current (Isi) inactivated more slowly. With Ca2+ replaced by Sr2+, the onset of the twitch was delayed, the maximum shortening was increased and a marked voltage-dependent tonic shortening developed. 3. Voltage-clamp pulses of 3.3 s duration were applied to investigate changes of the steady-state current-voltage relationship produced by replacing Ca2+ with Sr2+. Large slow changes of membrane currents produced by Sr2+ were observed. The identity and time course of these currents were investigated after blocking Isi and potassium currents pharmacologically. The remaining current had many of the characteristics of 'creep currents' (Eisner & Lederer, 1979; Hume & Uehara, 1986 a, b). The creep currents were found to be paralleled by changes of the intracellular Sr2+ concentration, as determined by tracking the sarcomere length during the accompanying tonic contractions. 4. The creep currents were suppressed by Ni2+ (2 mM), a finding that suggests that the Na+-Ca2+ exchanger may be responsible for producing these currents (Kimura, Miyamae & Noma, 1987). The question remains, however, whether the Na+-Ca2+ exchanger is responsible for generating the currents itself or whether it may influence another current source by changing the intracellular Sr2+ concentration. 5. To test the role of the Na+-Ca2+ exchanger in producing the creep currents, the reversal potential of the creep current was investigated. Simple voltage protocols were inadequate to distinguish between the two current sources. However, loading the cytosol with Sr2+ by means of a second pipette sealed to the same cell in the presence of Ni2+ as an inhibitor of the Na+-Ca2+ exchanger revealed difference currents compatible with a non-specific cationic channel activated by intracellular Sr2+ (Ehara, Noma & Ono, 1988). 6. In conclusion, the creep currents produced when Ca2+ is replaced by Sr2+ appear to arise from an increase of intracellular Sr2+ which activates a non-specific cation channel. A contribution from the Na+-Ca2+ exchanger can not be excluded.

Phosphodiesterase inhibition by new cardiotonic agents: mechanism of action and possible clinical relevance in the therapy of congestive heart failure

Cyclic AMP is known as a secondary messenger regulating the myocardial force of contraction. For the degradation of cAMP multiple forms of PDE within the cell are described, which vary according to substrate specificity, kinetic characterization, and cellular localization. One of these isoenzymes, the low Km cAMP-specific PDE (PDE III), which seems to be closely related to cardiotonic effects of PDE inhibitors, exists either in a particulate form (in dogs), probably associated with the sarcoplasmic reticulum, or in soluble form (in guinea pig). The existence of different forms of PDE III possibly reflects a different pooling or compartmentalization of cAMP. Many agents selectively inhibiting PDE III are described which potently increase the force of contraction and which exert vasodilatory effects. Besides PDE inhibition some of these agents possess additional cAMP-independent actions, e.g., sensitization of the contractile proteins to Ca2+, prolongation of the action potential, or prolongation of the open state of the Na+-channel. Since agents which nonselectively inhibit PDE are known as potent positive inotropic agents (e.g., IBMX), PDE III inhibition itself, but not a selectivity for PDE III inhibition, seems to be a prerequisite for this mechanism of action of cardiotonic drugs. Investigations with preparations from diseased human myocardium show that the beta-adrenoceptor agonist isoprenaline as well as the PDE inhibitor IBMX increase the force of contraction to only about one-third of the maximal effect of the cardiac glycoside dihydro-ouabain or Ca2+. In nonfailing human heart preparations all agents had equal activity. Possible reasons for these differences may be a decreased responsiveness to beta-adrenoceptor stimulation (beta-receptor down-regulation) or an inappropriate increase in cAMP levels due to increased activity of inhibitory Gi-proteins with resulting decrease of adenylate cyclase activity in the failing heart. Besides a short-term clinical and hemodynamic improvement of congestive heart failure, uncontrolled long-term administration of PDE III-inhibitor agents failed to produce sustained clinical benefit and had no effect on survival. Controlled long-term studies with new cardiotonic agents in patients with severe CHF are still lacking.

Pharmacology of calcium release from sarcoplasmic reticulum

Calcium release from sarcoplasmic reticulum (SR) has been elicited in response to additions of many different agents. Activators of Ca2+ release are here tentatively classified as activators of a Ca2+-induced Ca2+ release channel preferentially localized in SR terminal or as likely activators of other Ca2+ efflux pathways. Some of these pathways may be associated with several different mechanisms for SR Ca2+ release that have been postulated previously. Studies of various inhibitors of excitation-contraction coupling and of certain forms of SR Ca2+ release are summarized. The sensitivity of isolated SR to certain agents is unusually affected by experimental conditions. These effects can seriously undermine attempts to anticipate effects of the same pharmacological agents in situ. Finally, mention is made of a new preparation ("sarcoballs") designed to make the pharmacological study of SR Ca2+ release more accessible to electrophysiologists, and some concluding speculations on the future of SR pharmacology are offered.

Calcium-sensitive and insensitive transient outward current in rabbit ventricular myocytes

1. A suction pipette whole-cell voltage-clamp technique was used to record membrane currents and potentials of isolated ventricular myocytes from rabbit hearts. 2. Transient outward current (Ito) was activated by voltage steps positive to -20 mV, increasing in amplitude with further depolarization to reach a maximum around +70 mV. The current attained its peak within 10 ms and then it inactivated for 100-200 ms. 3. A large portion of Ito still remained after the calcium current (ICa) was blocked when depolarizing pulses were applied at a frequency of 0.1 Hz or less. Therefore, this current component is referred to as calcium-insensitive Ito or It. 4. It showed voltage- and time-dependent inactivation similar to that observed in Purkinje fibres and other cardiac preparations. 5. The reversal potential of It depended on external K+ concentration, [K+]o, with a slope of 32 mV per 10-fold change in the presence of a normal [Na+]o (143 mM), while the slope was 48 mV per 10-fold change in low [Na+]o (1.0 mM). 6. It was completely inhibited by 2-4 mM-4-aminopyridine. Ito in the presence of ICa was also partially blocked by 4-aminopyridine and the remainder was abolished by 5 mM-caffeine. 7. The calcium-insensitive and caffeine-sensitive Ito differed in their decay rates as well as in their recovery time courses. The former was predominantly available at a slow pulsing rate, while the latter increased its amplitude with high-frequency depolarization. 8. The caffeine-sensitive Ito was inhibited by a blockade of ICa, by replacing Ca2+ with Sr2+, by external application of ryanodine and by internal application of EGTA. This indicates that the current is calcium-sensitive and is dependent on increased myoplasmic Ca2+ through Ca2+ influx via the sarcolemma and Ca2+ release from the sarcoplasmic reticulum. The current is therefore designated as IK, Ca. 9. The physiological functions of IK, Ca and It are indicated by their contribution to ventricular repolarization at fast and slow heart rates, respectively.

Assessment of diastolic function of the heart: background and current applications of Doppler echocardiography. Part I. Physiologic and pathophysiologic features

In the past, evaluation of the myocardium has been limited to examining systolic function of the heart. Recently, however, investigators have demonstrated that abnormalities of diastolic function of the heart provide important contributions to the signs and symptoms experienced by patients with heart disease. In addition, abnormalities of diastolic function may precede abnormalities of systolic function in the early stages of disease. Diastolic filling of the heart, however, is a complex sequence of interrelated events. In order to understand diastolic function, each of these factors contributing to filling of the heart must be examined. They include relaxation, passive compliance, atrial contraction, erectile effect of the coronary arteries, viscoelastic properties, ventricular interaction, and pericardial restraint--all of which are interrelated. In addition, diastolic factors are affected by changes in loading conditions and contractility, and they demonstrate nonuniformity in time and space. This report provides an overview of these various factors from the clinical perspective, based on studies involving the isolated papillary muscle and the isolated heart as well as basic clinical studies.

Role of intracellular sodium in the regulation of intracellular calcium and contractility. Effects of DPI 201-106 on excitation-contraction coupling in human ventricular myocardium

Experiments were performed to investigate the mechanism of action of DPI 201-106 on human heart muscle. In both control and myopathic muscles, DPI produced concentration-dependent increases in action potential duration, resting muscle tension, peak isometric tension, and duration of isometric tension. These changes were associated with increases in resting intracellular calcium and peak calcium transients as measured by aequorin. At higher concentrations of DPI, a second delayed Ca2+ transient (L') appeared. L' was inhibited by tetrodotoxin and ryanodine, suggesting that DPI acts at both the sarcolemma and the sarcoplasmic reticulum. DPI toxicity was manifested by after-glimmers and after-contractions reflecting a Ca2+-overload state: DPI effects were mimicked by veratridine, a Na+ channel agonist, and reversed by tetrodotoxin, yohimbine, and cadmium, Na+ channel antagonists. These results suggest that DPI acts primarily as a Na+ channel agonist. DPI may produce an increase in intracellular Ca2+ by increasing intracellular Na+ and altering Na+-Ca2+ exchange across the sarcolemma. DPI may also increase intracellular Ca2+ by directly altering sarcoplasmic reticulum Ca2+ handling.

The negative inotropic effect of acetylcholine on ferret ventricular myocardium

1. The effects of acetylcholine (ACh) on developed tension and intracellular Ca2+ concentration (as measured with aequorin) were studied in ferret papillary muscles, and on twitch shortening, the action potential and membrane currents in ferret ventricular myocytes. 2. Addition of ACh to ferret papillary muscles resulted in decreases in developed tension and the intracellular Ca2+ transient, both of which then partially recovered in the continued presence of ACh ('fade' of the response). On wash-off of ACh both developed tension and the intracellular Ca2+ transient increased above control ('rebound') before returning to control values. 3. Addition of ACh to ferret ventricular myocytes resulted in a membrane hyperpolarization of 2 +/- 0.5 mV (mean +/- S.E.M.; n = 9), a decrease in action potential duration to 23 +/- 6% of control and a decrease in twitch shortening to 31 +/- 5% of control. In the continued presence of ACh these responses to ACh faded. Thirty seconds after the maximal effect of ACh, action potential duration had partially recovered to 34 +/- 6% of control and twitch shortening to 46 +/- 7% of control. 4. The effects of ACh on twitch shortening could be mimicked under voltage clamp by varying voltage clamp pulse duration to simulate the ACh-induced changes in action potential duration. 5. When ACh was applied during a train of voltage clamp pulses of constant duration, 81% of the cells showed less than a 20% decrease in Ca2+ current and twitch shortening. However in 19% of the cells twitch shortening and the apparent Ca2+ current decreased by more than 30%. 6. In the 81% of cells, the normal decrease in twitch shortening was wholly the result of the shortening of the action potential. This in turn was the result of an increase in an outward background current which increased the rate of repolarization during the action potential. The ACh-induced background current reversed at -89 +/- 2 mV and showed inward-going rectification; these properties suggest that it was carried by K+. 7. In the 19% of cells, the normal decrease in twitch shortening was only partly the result of the shortening of the action potential (due to both the increase in outward background current as well as the apparent decrease in Ca2+ current). In these cells the decrease in twitch shortening may also have been partly the direct result of the apparent decrease of Ca2+ current.

Inactivation of calcium current in bull-frog atrial myocytes

1. A single-microelectrode technique has been used to study the voltage dependence and the kinetics of inactivation and reactivation of a tetrodotoxin-resistant inward current (ICa) in single cells from bull-frog atrium. 2. In most cases the kinetics of both inactivation and reactivation can be well described as a single-exponential process. 3. Several different observations indicate that inactivation of ICa in these cells is controlled by both voltage-dependent and current-dependent processes, as has been demonstrated previously in heart (Kass & Sanguinetti, 1984; Lee, Marban & Tsien, 1985) and in other tissues (Hagiwara & Byerly, 1981; Tsien, 1983; Eckert & Chad, 1984). 4. Evidence in favour of a voltage-dependent inactivation mechanism included: (a) In paired-pulse measurements of steady-state inactivation ('f infinity') a 'conventional' steady-state f infinity vs. membrane potential (Vm) relationship was obtained in the range of membrane potentials from -60 to 0 mV. (b) Increasing [Ca2+]o from 2.5 to 7.5 mM, which resulted in a 2-3-fold increase in ICa, did not produce any significant increase in the amount of inactivation. (c) Using a 'gapped' double-pulse protocol non-monotonic U-shaped inactivation relationships were obtained, i.e. positive to approximately +20 mV some removal of inactivation occurred. However, f never approached a value near 1.00 at very depolarized potentials; it reached a maximum between 0.5 and 0.6. (d) In constant [Ca2+]o and at fixed Vm, the kinetics of ICa inactivation were independent of peak size of ICa. This was demonstrated by: (i) varying the holding potential (-90 to -30 mV), (ii) using paired-pulse 'recovery' protocols, and (iii) partial block by La3+ (1-10 microM) and Cd2+ (0.1 mM). (e) Influx of Ca2+ ions was not an obligatory prerequisite for development of inactivation. In all ionic conditions (Ca2+, Sr2+, Ba2+, Na+-free and Ca2+-free Ringer solutions) currents displayed inactivation phenomena, although the extent and kinetics of inactivation were dependent upon ionic conditions. Outward currents recorded above the reversal potential for ICa exhibited time- and voltage-dependent inactivation, and could be inactivated by brief depolarizing pre-pulses that produced no net inward current flow. Evidence against a possible role of the electrogenic Na+-Ca2+ exchanger in producing inactivation of these outward currents was obtained.(ABSTRACT TRUNCATED AT 400 WORDS)

Reversal potential of the calcium current in bull-frog atrial myocytes

1. Voltage clamp recordings of the calcium current (ICa) in single myocytes which were enzymatically isolated from bull-frog atrium show that a genuine reversal of the current flowing through Ca2+ channels can be recorded (ef. Reuter & Scholz, 1977; Lee & Tsien, 1982, 1984; Campbell, Giles & Shibata, 1988c). In normal 2.5 mM [Ca2+]0 Ringer solution this apparent reversal potential (Erev) is near +50 mV, a value well below the predicted thermodynamic Ca2+ equilibrium potential (ECa). 2. None the less, Erev shifts with variations in extracellular divalent ion concentrations (Ca2+, Sr2+ and Ba2+) according to the predictions of a Nernstian divalent cation electrode, i.e. approximately 29 mV per 10-fold change in the external concentration of divalent ion. 3. The existing theoretical analysis of this Erev has been extended in order to clarify its interpretation with regard to the selectivity characteristics of ICa. 4. The apparent reversal potential is analysed using a form of the constant field equation which has been modified to include (i) simultaneous monovalent and divalent cation movements and (ii) the presence of a surface potential (V'). This equation can be solved to yield an explicit expression for Erev. The effects of V' on apparent permeability ratios for the Ca2+ channel Erev are demonstrated. 5. In combination, our experimental results and calculations suggest that: (i) previous estimates of V' which were used to describe permeability (P) ratios of Ca2+ channels in various cardiac preparations may be in error, (ii) in normal [Ca2+]o the PNa/PCa ratio is very small, and (iii) PCa/PK must be greater than 1000. An analysis of the relative selectivity of the channel for divalent cations compared to K+ shows that PCa greater than PSr greater than PBa, assuming that PK remains the same after the divalent substitutions. 6. The Ca2+ channel in bull-frog atrial cells is thus much more selective for Ca2+ ions than had previously been estimated; in particular, inward flow of monovalent cations (e.g. Na+) through these channels does not contribute significantly to the observed ICa. The physiological implications of this high selectivity for Ca2+ ions are discussed.

Studies of the sodium-calcium exchanger in bull-frog atrial myocytes

1. Experimental measurements and computer simulations have been used in attempts to identify an exchanger current (Iex) generated by an electrogenic Na+-Ca2+ exchanger in single cells from bull-frog atrium. 2. Voltage clamp measurements of an inward 'slow tail' current observed upon repolarization after depolarizing clamp pulses that elicit net inward Ca2+ currents (ICa) (see Campbell, Giles & Shibata, 1988c), show that these slow tails have a cationic dependence different from ICa. Slow tails are large and prominent in normal [Na+]o solutions containing either Ca2+ or Sr2+, but they are markedly reduced or absent in Ba2+, Ca2+-free, and Na+-free solutions. 3. Kinetic measurements on the slow tails show that they are not generated by deactivation of ICa, and suggest that they may be due to activation of Iex at negative potentials (-70 to -100 mV). 4. Computer simulations of the influx, buffering, and extrusion of Ca2+ provide further indirect evidence that the slow tails correspond to Iex. In addition, these calculations give insights into one plausible mechanism of Ca2+ homeostasis in frog atrium. When the Na+-Ca2+ exchanger formalism of Mullins (1979, 1981), as modified by DiFrancesco & Nobel (1985), is combined with equations for intracellular Ca2+ buffering by myoplasmic proteins (cf. Robertson, Johnson & Potter, 1981), slow inward tails are produced which are qualitatively similar to those recorded experimentally. 5. Comparisons of the size and time course of ICa with those of Iex suggest that Iex does not generate a physiologically significant current (or membrane potential change) during the plateau of the action potential. However, at potentials near the resting potential the inward current due to Iex may be significant. 6. Our theoretical results suggest that in the intact single atrial cell myoplasmic Ca2+-binding proteins (e.g. calmodulin and troponin) could be physiologically important modulators of the amplitude, polarity and kinetics of Iex. Hence, the specificity, capacity and kinetics of intracellular Ca2+ binding are essential components of any quantitative treatment of Iex in excitable tissue.

Ion transfer characteristics of the calcium current in bull-frog atrial myocytes

1. Voltage clamp studies on single cells from bull-frog atrium have been carried out to study the ion transfer characteristics of the calcium current, ICa. In agreement with the preliminary results of Hume & Giles (1983), a TTX-resistant, 'second transient inward current' was recorded consistently. Its average peak size at 0 mV in 2.5 mM [Ca2+]o Ringer solution was approximately -200 pA, and it was blocked by Cd2+ and La3+ but not by tetrodotoxin (TTX, 3 x 10(-6) M). 2. The peak size of this current increases by approximately 4 times when [Ca2+]o is raised from 1.25 to 7.5 mM, indicating that Ca2+ is a major charge carrier. 3. A well-defined reversal potential, Erev, for ICa can be recorded in normal Ringer solution and also when Ba2+ or Sr2+ serve as the charge carriers. When [Ca2+]o is changed the shifts in Erev follow the predictions of a Nernstian Ca2+ electrode. However, all Erev values are well below those predicted from the thermodynamic Nernstian ECa values (see Campbell, Giles, Hume, Noble & Shibata, 1988a). 4. The Ca2+ current exhibits voltage-dependent inactivation, whether the direction of net current flow is inward or outward; however, the rate of inactivation is affected by the species of cation carrying the current. Inactivation is reduced substantially in Ba2+ Ringer solution. 5. Magnesium (5 mM) is not a significant carrier or blocker of ICa in normal [Ca2+]o Ringer solution; however, 5 mM [Mg2+]o can block the current carried by either Sr2+ or Ba2+. In the absence of Mg2+, equimolar substitutions of Sr2+ or Ba2+ for Ca2+ result in larger currents than those carried by Ca2+ in the normal Ringer solution. 6. Sodium appears not to be a significant charge carrier in the presence of normal [Ca2+]o. However, after free [Ca2+]o has been reduced to extremely low levels (less than 10(-6) M) Na+ can carry a significant fraction of 'ICa'. Thus, it appears that the high selectivity of ICa for Ca2+ ions depends upon the presence of Ca2+. 7. 'Slow tails' are frequently recorded after repolarizing clamp steps back to the holding potential. These 'slow tails' are prominent in normal [Na+]o, [Ca2+]o and [Sr2+]o Ringer solution; however, they are markedly reduced in [Ba2+]o, in Na+-free and Ca2+-free Ringer solutions. Experimental and theoretical work suggests these slow tails may be generated by an electrogenic Na+-Ca2+ exchanger (see Campbell, Giles, Robinson & Shibata, 1988b).(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular basis for inotropic changes in the heart

Cardiac contraction is a highly regulated process that involves nearly every aspect of the cardiac cell function. Many steps in the process are regulated by the cell to optimize contraction, and these can often be modified to benefit the patient with heart damage. These include the action potential, calcium channels, release and uptake of cellular calcium, sensitivity of contractile proteins to calcium, and energy utilization. A dramatic expansion of our understanding of these cellular contractile and regulatory processes gives us an unprecedented opportunity to devise new ways of modifying cardiac contraction to the benefit of our patients.

Monophasic action potentials at discontinuation of cardiopulmonary bypass: evidence for contraction-excitation feedback in man

Mechanical dysfunction is the strongest predictor of sudden cardiac death due to arrhythmia. Contraction-excitation feedback whereby changes in myocardial length/tension influence the time course of repolarization and excitability would provide a possible mechanism. Such a relationship has been shown in animals but has yet to be demonstrated in man. A useful model for studying this relationship is provided by the process of weaning off cardiopulmonary bypass after routine coronary artery surgery. During this weaning period of approximately 1 min, the heart is converted from being partially empty and flaccid (i.e., a "nonworking" state) to being filled and stretched to support the circulation (i.e., a "working" state). Monophasic action potentials (MAPs) were recorded from the left ventricular epicardium as a measure of repolarization time in 16 patients at discontinuation of cardiopulmonary bypass. Systolic pressure was recorded from the radial artery line. Measurements were made at three stages that related to different dynamic states of the heart: (1) starting to come off bypass ("minimally working"), defined as the time of first appearance of an inflection on the arterial pressure trace indicating the start of left ventricular ejection and valve opening, when arterial pressures represent left ventricular pressure, (2) half off bypass ("partially working"), and (3) off bypass ("wholly working"). During the process of discontinuing bypass MAP duration shortened, while systolic pressure increased. MAP duration at 90% and 60% repolarization (MAP D90, MAP D60) decreased from 288.0 +/- 29.5 msec (mean +/- SEM) and 235.0 +/- 27.9 msec in the minimally working heart to 274.5 +/- 30.2 msec and 224.2 +/- 27.3 msec in the partially working heart (p less than .001), with a subsequent decrease to 261.0 +/- 28.8 and 214.0 +/- 28.7 when the heart was wholly working (p less than .001). Systolic pressure increased from 54.1 +/- 9.3 mm Hg in the minimally working heart to 65.9 +/- 13.8 mm Hg in the partially working heart (p less than .001) and subsequently increased to 75.5 +/- 13.3 mm Hg when the heart was wholly working (p less than .001). Mean heart rates did not change significantly. A strong correlation was obtained between absolute MAP duration and systolic pressure. Regression analysis revealed: MAP D90 vs systolic pressure (p less than .001) and MAP D60 vs systolic pressure (p less than .01).(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of repetitive activity upon intracellular pH, sodium and contraction in sheep cardiac Purkinje fibres

1. The influence of repetitive activity upon intracellular pH (pHi), intracellular Na+ activity (aNA(i)) and contraction was examined in isolated sheep cardiac Purkinje fibres. Ion-selective microelectrodes were used to measure intracellular Na+ and H+ ion activity. Twitch tension was elicited by field stimulation or by depolarizing pulses applied using a two-microelectrode voltage clamp. Experiments were performed in HEPES-buffered solution equilibrated either with air or 100% O2. 2. An increase in action potential frequency from a basal rate of 0.1 to 1-4 Hz induced a reversible fall in pHi and a reversible rise in aNa(i). These effects reached a steady state 3-10 min following an increase in stimulation frequency, and showed a linear dependence on frequency with a mean slope of 0.023 pH units Hz-1 and 0.57 mmol l-1 Hz-1, respectively. The rise in total intracellular acid and aNa(i) associated with a single action potential was estimated as 5.3 mu equiv l-1 of acid and 3.5 mu equiv l-1 of Na+. 3. At action potential frequencies greater than 1 Hz, the rate-dependent rise in aNa(i) was usually accompanied by a positive force staircase. 4. The fall in pHi following a rate increase also occurred when fibres were bathed in Tyrode solution equilibrated with 23 mM-HCO3- plus nominally 5% CO2/95% O2. In these cases, however, the fall in pHi was halved in magnitude. 5. In fibres exposed to strophanthidin (0.5 microM), the rate-dependent fall in pHi was doubled in magnitude and its time course was more variable than under drug-free conditions. The rate-dependent rise in aiNa was also usually larger in strophanthidin. 6. In order to examine the influence of the rate-dependent acidosis on developed tension, the acidosis was reversed experimentally by adding 2 mmol l-1 NH4Cl to the bathing solution. This produced a rise in pHi accompanied by a large increase in twitch tension. Such an effect of pHi upon tension was quantitatively similar to that observed in previous work on Purkinje fibres (Vaughan-Jones, Eisner & Lederer, 1987). 7. It is concluded that the rate dependence of pHi will influence both the magnitude and the time course of an inotropic response to a change in heart rate.