From the vasodilator and hypotensive effects of an extract fraction from Agelanthus dodoneifolius (DC) Danser (Loranthaceae) to the active compound dodoneine.

AIM OF THE STUDY: Effects of the different fractions obtained by partition of ethanolic extract (EE) of Agelanthus dodoneifolius through column chromatography were investigated on rat blood pressure and aortic relaxation and compared to those observed in the presence of crude EE. MATERIALS AND METHODS: The acute hypotensive activity of EE, fractions and dodoneine, administrated intravenously, was evaluated in anaesthetized rats using the invasive method of blood pressure recording. Bioassay-guided fractionation using rat aorta pre-contracted by norepinephrine to monitor the relaxant activity led to the isolation of dodoneine. RESULTS: In normotensive rats, injection of EE (0.01-10 mg/kg) produced a dose-dependent decrease in both systolic and diastolic blood pressure without any significant change in heart rate. In a similar way, the EE (0.001-3 mg/mL) caused relaxation of rat pre-contracted aorta in a concentration-dependent manner. Fractionation of the EE afforded 14 fractions, F1-F14, that were tested on rat precontracted aortic rings. At the concentration level of 1 mg/mL, a maximum relaxation effect was observed for fractions F2-F5. F4 was the most effective to elicit a concentration-dependent relaxation effect with an ED(50)=160±1.1 µg/mL (n=5) and to decreased systolic and diastolic control pressure by 56.9% and 81.6% respectively. F4 contains most of the dihydropyranone dodoneine, with 93% of the sample mass. Dodoneine separated from this fraction was also able to decrease both systolic and diastolic arterial pressure by 32.5% and 38.7% at 100 µg/kg, respectively. CONCLUSION: For the first time, this study demonstrates the hypotensive property of the dodoneine present in Agelanthus dodoneifolius.

[Modulation of skeletal muscle contraction by the non-toxic fraction of Buthus occitanus tunetanus venom via the cholinergic receptors]. / Modulation de la contraction du muscle squelettique par la fraction non toxique du venin de Buthus occitanus tunetanus via les récepteurs cholinergiques.

Cholinergic receptors have an essential physiological role in the central nervous system because of their implication in higher functions in the neuromuscular junction within the brain and also in the peripheral nervous system by activating nicotinic (nAChRs) or muscarinic (mAChRs) receptors. Moreover, cholinergic receptors could be recognized by animal toxins isolated from snake venoms or alkaloids having animal or vegetal origin. In this context, we aim to find such molecules in a non toxic venom fraction of Buthus occitanus tunetanus scorpion, M1, which could therefore constitute promising medical tool. We present here a physiological study in skeletal muscle cells that regroups data that have been recently published and some new results reinforcing the last ones. The global effect of M1, was firstly studied on isolated nerve-muscle preparation. In cultured myotubes, we have found that the intracellular calcium increase, induced by M1 was blocked when ryanodine or inositol 1,4,5-triphosphate receptors are inhibited. Moreover, we have shown that M1 application on myotubes, induced a membrane depolarization as seen with acetylcholine. The treatment of myotubes with alpha-bungarotoxin blocked in most parts the depolarization amplitude. Thus, these results confirm the presence of at least one component in M1 active in nAChRs.

Interleukine-6 (IL-6) synthesis and gp130 expression by human pericardium.

Growing evidence shows that cytokines of the IL-6 family play an important regulatory role in heart physiology such as inducing cardiomyocyte hypertrophy. The purpose of this study was to see if IL-6 and its soluble receptors (sIL-6R and sgp130) could be detected in pericardial fluids, and to see if they are produced by the pericardium. We report that human pericardial fluid from patients with coronary pathologies contained IL-6, sIL-6R, and sgp130. However, the levels present in sera and pericardial fluid did not correlate, which suggests local production. This observation was confirmed by in vitro studies demonstrating massive IL-6 production by cultured pericardial samples, which could be strongly inhibited by methylprednisolone. RT-PCR studies revealed that IL-6 was weakly expressed in fresh tissues and strongly induced after culture. In situ hybridisation and immunohistochemical analysis showed that IL-6 and gp130 were mainly present in mesothelial cells. sIL-6R and sgp130 were also produced by pericardium in vitro, and their synthesis was upregulated by methylprednisolone. Taken together, these results demonstrate that IL-6 is present in pericardial fluid and that its presence could be due to synthesis by pericardial tissue. In vitro studies suggest that IL-6 production by this tissue could be strongly induced and regulated. A potential paracrine role of these factors in cardiomyocyte functions in normal or pathological conditions is discussed.

Characterization of nifedipine-resistant calcium current in neonatal rat ventricular cardiomyocytes.

Calcium current was recorded from ventricular cardiomyocytes of rats at various stages of postnatal development using the whole cell patch-clamp technique. In cultured 3-day-old neonatal cells, the current carried by Ca(2+) or Ba(2+) (5 mM) was not completely inhibited by 2 microM nifedipine. A residual current was activated in the same voltage range as the L-type, nifedipine-sensitive Ca(2+) current, but its steady-state inactivation was negatively shifted by 16 mV. This nifedipine-resistant calcium current was not further inhibited by other organic calcium current antagonists such as PN200-110, verapamil, and diltiazem nor by nickel, omega-conotoxin, or tetrodotoxin. It was completely blocked by cadmium and increased by isoproterenol and forskolin. This current was >20% of total calcium current in ventricular myocytes freshly isolated from neonatal rats, and it decreased during postnatal maturation, disappearing at the adult stage. This suggests that this current could be caused by an isoform of the L-type calcium channel expressed in a way that reflects the developmental stage of the rat heart.

Circulating soluble gp130, soluble IL-6R, and IL-6 in patients undergoing cardiac surgery, with or without extracorporeal circulation.

OBJECTIVE: Soluble forms of interleukin-6 (IL-6) receptors are known to modulate biological activities of IL-6. The purpose of the study was to measure circulating levels of IL-6, sIL-6R and sgp130 in patients undergoing coronary artery bypass grafting with cardiopulmonary bypass (CPB group) or without CPB (non-CPB group). METHODS: The CPB group included 19 patients and the non-CPB group 12 patients. Sera levels of IL-6, sIL-6R and sgp130 were measured by specific ELISA at the beginning of the operation (T0, 15 min before skin incision) and 6 h later (T1). RESULTS: IL-6 sera levels were respectively 9+/-20 pg/ml (mean+/-SD) and 13+/-19 pg/ml at T0 and reached 340+/-250 pg/ml and 965+/-1060 pg/ml at T1 in CPB and non-CPB groups, indicating a significant increase from T0 to T1, but no differences between the two groups. When compared to T0 values, sgp130 levels decreased in both groups (respectively 105+/-37 and 115+/-35 ng/ml at T0 for CPB and non-CPB groups, and 72+/-25 and 84+/-29 ng/ml at T1) while we are not able to detect differences between the groups. Whatever the group or the time, sIL-6R concentrations remained unchanged. CONCLUSIONS: We showed that the increase of IL-6 after artery bypass grafting was similar between patients operated with CPB or without CPB. We conclude that the main inductor of IL-6 release is linked to surgical trauma rather than a reaction to CPB. Since it is known that gp130 inhibits IL-6-biological activities, we suggest that the decrease of sgp130 sera levels could further enhance the inflammatory effects of IL-6 in cardiac surgery.

Alpha-adrenoceptor stimulation induces hypertrophy and increases L-type calcium current density in neonatal rat ventricular cardiomyocytes in culture.

Effects of chronic alpha-adrenoceptor stimulation on hypertrophy and L-type calcium current (I(Ca-L) ) were investigated in neonatal rat ventricular cardiomyocytes in culture using whole-cell patch-clamp technique and measurement of protein- and RNA-to-DNA ratios. Chronic exposure to norepinephrine (2 microM) plus propranolol (2 microM) of cardiomyocytes during 1 and 3 days in culture increased cell membrane capacitance, protein- and RNA-to DNA ratios and was accompanied by an increase in I(Ca-L) density. These effects were not observed in the presence of prazosin (2 microM) suggesting that they could be due to alpha(1) -adrenoceptor stimulation. They were also prevented by cycloheximide (5 microM) and actinomycin D (1 microM). These effects were not observed in 1 and 3 day-cultured cells pre-treated for only 1 hour with norepinephrine. They were potentiated when calcium concentration was increased in the culture medium and, in contrast they were abolished in the presence of the L-type calcium current inhibitor, nifedipine (2 microM). The present study demonstrates that hypertrophy induced by long-term stimulation of alpha(1) -adrenoceptors is accompanied by an increase in the expression of functional calcium channels in neonatal rat cardiomyocytes. These results reveal the existence of a novel alpha(1) -mediated positive regulation of L-type calcium current different from that due to acute stimulation of alpha(1) -adrenoceptors in neonatal ventricular cardiomyocytes.

Characterization of a hyperpolarization-activated current in dedifferentiated adult rat ventricular cells in primary culture.

1. The presence of a hyperpolarization-activated pacemaker (I(f)-like current was tested in dedifferentiated adult rat ventricular myocytes up to 12 days in primary culture with the whole-cell patch clamp technique. 2. An I(f)i-like current was found and characterized on freshly isolated and cultured ventricular cells. Both activation and density of the current varied in relation to the stage of dedifferentiation. The current was activated from -92.0 +/- 2.5 and -63 +/- 1.0 mV at the beginning (4-day-cultured cells) and end of the dedifferentiation process (12 days), respectively. Its density measured at 170 mV progressively increased from -2.34 +/- 0.36 to -6.12 +/- 0.64 pA pF-1 between the two farthest stages of cellular remodeling. In freshly isolated cells the current was activated at -108.0 +/- 1.5 mV and its current density measured at -170 mV was -1.97 +/- 0.56 pA pF-1. 3. The current was blocked by extracellular CsCl (3mM) in a voltage-dependent manner. Modification of reversal potentials obtained at various values of [K+]o ( 5.4 or 25 mM) and [Na+]o (140 or 30 mM) suggests that the current was carried by both K+ and Na+ ions. 4. It is concluded that the hyperpolarization-activated inward current, recorded in freshly isolated and in cultured ventricular cells has characteristics similar to those of I(f). In adult rat ventricular cells it is activated in a non-physiological potential range, but can be elicited in a more physiological range when the cells are remodelled through a dedifferentiated way. It is suggested that such a current could be implicated in ventricular arrhythmias developed in pathological events.

Effects of adrenergic stimulation on postnatal development and calcium current in newborn rat cardiomyocytes in primary culture.

With a primary culture of ventricular cardiomyocytes from newborn rats as an in vitro model, the long-term effects of norepinephrine (NE) on hypertrophic postnatal development and the I/V properties of L-type calcium currents were investigated with the whole-cell configuration of patch-clamp technique. These effects of NE also were tested in the presence of propranolol (P). Compared with mean values obtained in control conditions, the measurement of cell membrane capacitance (Cm) as an index of cell growth demonstrated that Cm was increased by 12, 35, and 42% after 1, 3, and 6 days, respectively, of treatment with 2 microM NE. Similar increases were observed when propranolol (2 microM) was added to the NE treatment, suggesting that growth potentiation could be attributed to the alpha-adrenergic effect of NE. Under control conditions, the L-type calcium current (ICa-L) density did not alter with the age of the culture. However, in the presence of NE, ICa-L density increased significantly compared with control conditions at the same stage of culture and was also significantly increased after 3 and 6 days of NE treatment when compared with ICa-L density after 1 day of NE treatment. Similar results were obtained in the presence of propranolol. These results show that the growth and functional properties of neonatal cardiomyocytes in primary culture can be regulated by catecholamines and demonstrate that these regulatory effects were achieved through activation of alpha-adrenoceptors.

Endothelin-1 inhibits L-type Ca2+ current enhanced by isoprenaline in rat atrial myocytes.

Endothelin-1 (ET-1) was shown to exert direct cardiac effects by complex signaling pathways and to interact with neurotransmitter regulation of cardiac activity. The effect of ET-1 was investigated on the beta-adrenergic stimulation of cardiac L-type Ca2+ current (ICaL) on isolated rat atrial myocytes by using the patch-clamp technique. ET-1 (5 x 10(-8) M) reversed the increase in ICaL induced by isoprenaline (10(-6) M) but had no effect on basal ICaL and on (-) Bay K 8644-increased ICaL (10(-6) M); so ET-1 might exert an effect only when the Ca2+ channels are phosphorylated. The antiadrenergic action of ET-1, blocked by BQ-123 (10(-6) M) and unaffected by IRL 1038 (3.5 x 10(-8) M) should be mediated by ET-A receptors. The inhibitory action of ET-1 was still observed when ICaL was previously increased by forskolin (3 x 10(-6) M), 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP; 200 microM), or cAMP (100 microM) in presence of isobutyl methyl xanthine (IBMX; 10(-6) M), suggesting that the antiadrenergic action of ET-1 on ICaL was exerted independent of the cAMP-dependent phosphorylation pathway. ET-1 is known to be an activator of phosphoinositide hydrolysis, resulting in an increased production of IP3 and diacylglycerol (DAG). A Ca(2+)-dependent inhibition of ICaL consequently to an elevation of the intracellular Ca2+ pool via IP3 might be excluded in the action of ET-1, because of the presence of EGTA in the intrapipette medium. ET-1 reversed the isoprenaline-induced increase in ICaL in the presence of protein kinase C inhibitor [PKC(19-31); 100 microM), making unlikely the involvement of a DAG-dependent activation of PKC. Therefore the antiadrenergic action of ET-1 might also be independent on the phosphoinositide pathway.

Effects of Bay K 8644 on L-type calcium current from newborn rat cardiomyocytes in primary culture.

We examined the effects of the dihydropyridine agonist Bay K 8644 on L-type calcium channels in newborn rat ventricular cardiomyocytes during their development in primary culture. Experiments were performed at day 2 and 7 of the culture which constituted the early postnatal and maximally developed stages, respectively, of isolated cells in our experimental conditions. In the presence of racemic Bay K 8644 (10(-6) M), L-type calcium current (ICa-L) density recorded by perforated patch-clamp technique was increased by 127 +/- 4% (n = 8) in 2-day-old cells. The increase was only 103 +/- 5% (n = 10) in 7-day-old cells, resulting in a current density similar to that observed in freshly-dissociated adult cells (90 +/- 7%; n = 10). At every stage of the culture Bay K 8644 increased ICa-L with a 10-mV shift of the peak current towards hyperpolarized levels but without change in activation threshold and reversal potential. This shift can be explained by the corresponding change in steady-state activation and inactivation relationships towards negative potentials. The potentiating effect of Bay K 8644 was further studied as a function of phosphorylation levels of calcium channels. When calcium channels were phosphorylated by dibutyryl cyclic AMP (2 x 10(-4) M), increase of ICa-L density by Bay K 8644 was comparable at every stage of cell culture. However, direct activation of beta-receptors by isoproterenol did not increase ICa-L density in 2-day-old cultured cells as it did in 7-day-old cells, while direct activation of adenylate cyclase by forskolin similarly increased ICa-L, at both stages of culture. From these results, it can be suggested that the higher increase of ICa-L density by Bay K 8644 in 2-day- than in 7-day-old cultured cells could be interpreted as the result of a difference in the phosphorylation level of calcium channels for each stage of development. The possible increase in the basal phosphorylation level of calcium channels during culture of neonatal cardiac cells is discussed with respect to changes in functional properties of calcium channels during postnatal maturation of these cardiac cells.

T-type calcium current is expressed in dedifferentiated adult rat ventricular cells in primary culture.

Ventricular myocytes isolated from the heart of adult rats were able do dedifferentiate within 7 days in primary culture using a medium containing 10% fetal calf serum. Calcium current was recorded in these cells by means of the whole cell patch clamp technique and compared to the calcium current obtained in freshly isolated myocytes and in non dedifferentiated cultured myocytes. In dedifferentiated cardiomyocytes, both T-type (ICa-T) and L-type (ICa-L) calcium current components were recorded while only L-type was observed in freshly isolated and in non dedifferentiated cultured myocytes. ICa-T was separated from ICa-L through its voltage dependence and its pharmacology. These results demonstrate that ICa-T which has been shown to be present at the neonatal stage but absent in adult ventricular cells can be reexpressed when adult cells were dedifferentiated in culture. Its possible involvement in development of cardiac cells and in electrophysiological properties leading to spontaneous activity is discussed.

Effects of thapsigargin and cyclopiazonic acid on intracellular calcium activity in newborn rat cardiomyocytes during their development in primary culture.

The effects of specific inhibitors of sarcoplasmic reticulum (SR) calcium ATPase, thapsigargin (TG), and cyclopiazonic acid (CPA) were investigated on the resting and transient levels of intracellular free calcium concentrations recorded in Indo-1-loaded ventricular myocytes of newborn rat heart in primary culture. The calcium transients were induced by caffeine (10 mM) or high potassium (100 mM) solutions. In 2 day- as in 7-day-old cultured cells, the calcium transients induced by 10 mM caffeine were blocked dose dependently by TG and CPA. The dose-response curves suggest that TG was more efficient than CPA and that both drugs were more efficient in 7-day- than in 2-day-old cells. The calcium transients induced by 100 mM K+ were also strongly inhibited by these agents. The lack of effect on sarcolemmal calcium currents, as shown by whole-cell patch-clamp experiments, suggests that these drugs affect only SR function. In cells exhibiting spontaneous activity, the associated calcium transients were not affected by TG or CPA at the beginning of the culture (2-day-old cells) but were fully blocked at the end (7-day-old cells). These results confirm that TG and CPA specifically inhibit the cardiac SR Ca2+ pump without affecting the sarcolemmal calcium current. Their blocking effect of the calcium transients as a function of the developmental stage of neonatal cardiac cells in culture suggests an increasing role of the SR in the regulation of intracellular calcium. This argues for developmental changes of the SR through the differentiation and maturation of newborn cardiomyocytes at the early stage of the postnatal life, leading to a predominant role of the SR in excitation-contraction coupling mechanisms in adult cells.

Antagonism of beta-adrenergic stimulation of L-type Ca2+ current by endothelin in guinea-pig atrial cells.

Experiments carried out with isolated guinea pig atrial cells, using the patch clamp technique, demonstrated that endothelin-1 reversed the increase in L-type Ca2+ current (ICaL) induced by isoprenaline. Similar effects of endothelin-1 were observed when ICaL was previously increased by forskolin or 8-bromo-cAMP. These results suggested that the endothelin antagonism of beta-adrenergic stimulation of ICaL was exerted independently of the cAMP-dependent phosphorylation pathway.

Depressed transient outward current in single hypertrophied cardiomyocytes isolated from the right ventricle of ferret heart.

OBJECTIVE: The aim of this study was to investigate transient outward potassium current (Ito) changes as a basis for the prolongation of the action potential associated with cardiac hypertrophy. METHODS: Right ventricular hypertrophy was induced by chronic pulmonary artery constriction in adult male ferrets. After 4-6 weeks, hearts were excised and single myocytes were isolated from the right ventricles of banded and sham-operated (control) animals by enzymatic dissociation. Ito was recorded by means of the whole cell patch clamp technique. RESULTS: Heart weight:body weight ratio and cell membrane capacitance, as indications of hypertrophy, were increased by 17% (P < 0.05) and 32% (P < 0.01) respectively in the banded group. Analysis of Ito showed that in hypertrophied myocytes compared to normal controls: (1) the density of current was significantly reduced; (2) both the time to peak and the time constant of inactivation were increased; (3) the voltage-dependent steady-state availability was not changed, with similar potentials for half activation (30.4 +/- 6.8 mV in control and 33.9 +/- 8.5 mV in hypertrophied cells) and half inactivation (-12.3 +/- 3.3 mV in control and -11.4 +/- 2.7 mV in hypertrophied cells); (4) the time constant for recovery from inactivation was significantly increased regardless of the holding potentials (-50 mV, -70 mV or -90 mV). CONCLUSIONS: Alterations of the transient outward potassium current with respect to its density, kinetics and recovery from inactivation can explain the prolongation of the action potential in myocytes isolated from pressure-overload hypertrophied heart and may thus be an important step in such cardiac adaptation.

Developmental changes in Ca2+ currents from newborn rat cardiomyocytes in primary culture.

Electrophysiological characteristics of neonatal rat ventricular cardiomyocytes in primary culture were studied using the whole-cell patch-clamp recording technique. Cell size, estimated by measurement of membrane capacitance, was significantly increased throughout the culture from 22.4 +/- 5.4 pF at day 2 to 55.0 +/- 16.1 pF at day 7, reflecting the hypertrophic process which characterises postnatal cell development. The Ca2+ current was investigated at day 2 and 7 of the culture which constituted the early postnatal and maximally developed stages, respectively, of isolated cells in our experimental conditions. At 2 days of culture, two types of Ca2+ current could be distinguished, as also observed in freshly dissociated newborn ventricular cells. From their potential dependence and pharmacological characteristics, they could be attributed to the T- (ICa-T) and L-type (ICa-L) Ca2+ current components. After 7 days of culture, only the latter ICa-L was present and its density was significantly increased when compared to the density in 2-day-old cells, but lower than that obtained in freshly dissociated adult cells. As the age of the culture progressed, the steady-state inactivation curve was shifted toward negative potentials, in the direction of the inactivation curve obtained for adult cells. Compared to the serum-free control conditions, the density of ICa-L was significantly increased in the presence of fetal calf serum throughout the culture. Consequently, the density of ICa-L obtained in 7-day-old cells was similar to the density of ICa-L obtained in freshly dissociated adult cardiac cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular calcium transients from newborn rat cardiomyocytes in primary culture.

Resting and transient levels of intracellular free calcium concentrations were recorded in indo-1 loaded neonatal rat ventricular cardiomyocytes in primary culture by means of an interactive laser cytometer. The calcium transients were induced by high potassium and caffeine applications. The resting level of intracellular calcium remained constant (about 140 nM) throughout the culture (up to 7 days). The calcium transients induced by 100 mM K+ changed during culture from a low, cobalt sensitive response at 2 days, to a strong biphasic response at 7 days. At 2 days the response was fully blocked by cobalt. At 7 days the transient phase was abolished by cobalt and ryanodine, whereas the second sustained phase was only partially blocked. The calcium transient induced by caffeine was present as early as the first days, and increased with the age of the culture. This transient was blocked by ryanodine. The calcium influx through sarcolemmal calcium channels could be responsible for intracellular calcium transients in 2 day-old cells, whereas in 7 day-old cells, they seem to be only the trigger for sarcoplasmic reticulum calcium release via a mechanism such as 'calcium-induced calcium-release'. Other mechanisms, such as the sodium-calcium exchange mechanism activated by sarcolemmal depolarisation, seem to be implicated too and therefore could explain the sustained level of intracellular calcium during 100 mM K+ stimulation. The developmental changes through differentiation and maturation of myocytes in culture could account for the age dependent evolution of the responses obtained. From these results it is possible to conclude that calcium movements implicated in the excitation-contraction coupling mechanism in the development of rat neonatal cardiomyocytes are similar in primary culture and in the postnatal period in vivo.

Kinetic analysis of the L-type calcium current in enzymatically dissociated ferret ventricular myocytes.

The L-type calcium current (ICa-L) was studied in single ferret ventricular myocytes using whole-cell recording with single patch pipettes. Voltage-clamp experiments were performed at room temperature with internal and external Na(+)- and K(+)-free Tyrode solutions in order to isolate ICa-L. For depolarizing steps eliciting small ICa-L the decay of the current is best described by one exponential. For depolarizing steps eliciting large ICa-L (i.e. between -10 and +30 mV), the decay of the current is best described by the sum of two exponentials with a calcium-dependent fast (Tf) time constant and a voltage-dependent slow (Ts) time constant. Experiments conducted with different external concentration of Ca2+ and Ba2+ suggested that the inactivation and the time course of reactivation of the current after a depolarizing pulse are dependent on calcium ions. This confirms previous observations in heart muscle and reveals the existence of a calcium-dependent regulation process of the L-type calcium current in enzymatically dissociated ventricular myocytes from ferret heart.

The L type calcium current in single hypertrophied cardiomyocytes isolated from the right ventricle of ferret heart.

OBJECTIVE: The aim was to study L type calcium current alterations in relation to the action potential lengthening induced by hypertrophy in isolated cardiomyocytes from the right ventricle of ferret. METHODS: Chronic pulmonary artery constriction was established in adult male ferrets under anaesthesia. Sham operated animals were used as controls. Four to six weeks later the heart was excised and treated with a mixed collagenase-elastase solution to isolate the right ventricular myocytes. The calcium current was investigated in control and hypertrophied cells with the whole cell configuration of the patch clamp technique. The validity of the model was tested by analysis of the structural and passive electrical characteristics of the cells, which were enzymatically isolated from right ventricles previously overloaded (4 to 6 weeks) by clipping the pulmonary artery. RESULTS: Isolated cells from right ventricles submitted to a chronic pressure overload had well preserved cellular integrity suggesting the absence of myocardial failure. This compensated form of hypertrophy was characterised by a dilated transverse tubular system, which could explain the increased membrane capacity. Such cells developed a prolonged action potential with a less pronounced fast repolarisation phase inducing a higher plateau phase. When studied in physiological Tyrode solution the density and kinetics of the L type calcium current were not apparently modified, but a significant decrease in density was unmasked when sodium and potassium currents were suppressed by external and internal substitution of sodium and potassium by tetraethyl ammonium. CONCLUSIONS: The decrease in L type calcium current cannot be involved in the lengthening of action potential observed on hypertrophied myocytes, but it could account for the depressed contractile activity. A noticeable decrease of the transient outward current is suggested to explain the action potential alterations.

Possible involvement of a chloride conductance in the transient outward current of whole-cell voltage-clamped ferret ventricular myocytes.

The transient outward current was studied, using the whole-cell patch-clamp technique, in isolated ventricular cells from the ferret heart. In the presence of 4-aminopyridine and cadmium chloride which respectively blocked the Ca-insensitive and the Ca-dependent outward currents, a residual transient outward current was observed in about 30% of the cells tested. This current was suppressed in external hypochloride solution, completely inhibited by SITS (3 mM) and reversed at the equilibrium potential for chloride ions. This suggests the presence of a chloride permeability which could contribute to the repolarization phase of the cardiac action potential.

The correlation between the increase in slow outward current and in contraction induced by caffeine, ryanodine, and rapid cooling in voltage-clamped frog muscle fibers.

The effects of caffeine, ryanodine, and rapid cooling were tested on the depolarization-induced contraction and the apamin-insensitive slow outward current (Iso) of voltage-clamped (double mannitol gap) single frog muscle fibers. Subthreshold caffeine concentrations (0.5-2 mM) induced a monotonic increase in contractile and Iso amplitude. Whatever the concentration, the increase in contraction was roughly twice the one in current. Similar results were obtained upon rapid cooling (20-4 degrees C) in the presence of 0.5 mM caffeine. In the absence of external Na+ (choline-substituted) 10(-5) M ryanodine induced a delayed increase (approximately 30 min) in contraction and in current, shortly before the development of a drastic and irreversible contracture. Here again, the increase in contraction was twice that in current. In the presence of 5 mM tetraethylammonium (TEA) and (or) 25 nM charybdotoxin, 2 mM caffeine still induced a strong facilitating effect on contraction but the parallel increase in current was strongly reduced. The linear relationship between the increase in current and contractile amplitude has a slope approximately 0.5 (whatever the drug used to increase contractility); it is approximately 0.1 in the presence of TEA and (or) charybdotoxin. In conclusion, provided the changes in contractile amplitude are caused by parallel changes in depolarization-induced sarcoplasmic reticulum Ca2+ release, about 50% of the apamin-insensitive Iso is controlled by internal Ca2+ release. The main part of this current corresponds to the TEA- and charybdotoxin-sensitive component of Iso.