Myoblast fusion is not a prerequisite for the appearance of calcium current, calcium release, and contraction in rat skeletal muscle cells developing in culture

During in vitro development of rat skeletal muscle cells, contraction and calcium currents progressively appear after fusion of myoblasts. To investigate whether muscle-specific functions are expressed in the absence of myoblast fusion, rat neonatal muscle cells were cultured in a differentiation medium under conditions that are well known to inhibit fusion: prolonged culture in a low-calcium medium or treatment with cytochalasin B. We have demonstrated that the fusion-arrested cells expressed differentiative properties in L-type calcium current, transient release of calcium ions from internal stores in response to caffeine and depolarizing agents, and contraction elicited by depolarization. Properties and potential-dependence of L-type calcium currents were similar to that in control fused cells, but T-type calcium currents were not observed, while both types coexist in myotubes. Properties of calcium transients and voltage dependence of contraction suggested that the excitation-contraction mechanisms were well established. However, comparing to well-developed myotubes at the same time of culture, the characteristics of calcium transients and contraction of fusion-arrested cells were closer to those of younger myotubes, which can be interpreted in terms of a delay in maturation of excitation-contraction coupling and contractile machinery. All these observations demonstrate that myoblast fusion is not necessary for triggering the establishment of calcium transport and release and contractile functions of rat muscle cells developing in culture. The appearance of muscle-specific functions is consistent with previous results demonstrating that the fusion-arrested cells express muscle-specific proteins and structures.

Cultured rat skeletal muscle cells treated with cytochalasin exhibit normal dystrophin expression and intracellular free calcium control

Many studies performed to elucidate the molecular and cellular processes involved in muscular dystrophies have led to the working hypothesis of a key role for the cytoskeleton elements linking the extracellular matrix to myofibrils. It was recently suggested that cytochalasin B treatment of mouse soleus muscle promoted cell damage mediated by a cytosolic increase in free calcium concentration. Since intracellular calcium overload may be a primary event resulting from the alteration of cytoskeletal structure, this study was intended to evaluate whether or not the integrity of the F-actin microfilament network is necessary for calcium homeostasis. The developmental establishment of the normal cytoarchitecture was altered by treatment of myoblasts with the actin-disrupting agents cytochalasin B and D, and the effects were compared with those in myoblasts treated with colchicine. These drugs modified the morphogenesis in that they prevented the formation of elongated myotubes by myoblast fusion, but did not prevent the maturation of contractile myogenic cells. The subcellular organisation of actin filaments visualised by confocal fluorescence microscopy was modified by colchicine and cytochalasins, but appearance of contractile apparatus and mechanical activity were not precluded. Sarcolemmal addressing of dystrophin, the subsarcolemmal protein lacking in Duchenne muscular dystrophy, was not prevented by cytochalasin. The evaluation of the basal activity of cytosolic calcium measured with indo-1 suggested that the disruption of actin or microtubules did not prevent developing muscle cells to maintain a low basal calcium activity. We propose that the global integrity of the cytoskeleton network is not crucial for the maintenance of calcium homeostasis in muscle cells developing in vitro. These results are discussed with regard to current theories attempting to understand the functional consequences of an abnormal expression of the dystrophin-glycoprotein complex interacting with the extracellular matrix and the cytoskeleton.

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)

Surgical treatment of left main coronary aneurysm

Aneurysms of the coronary arteries occur in 0.3% to 4.9% of angiograms. Only 13 cases of left main artery (LMA) aneurysms have been reported. Of these, three were treated by ligation of the aneurysm and coronary revascularization; the others by revascularisation alone. We report the case of a 56 years old man presenting with unstable angina and previous history of myocardial infarction. Coronary angiography confirmed three vessels disease and the presence of an aneurysm of the LMA extending to the proximal part of the left anterior descending artery (LAD). The patient was treated with ligation of the aneurysm and total myocardial revascularization. The clinical and radiological features are reviewed, and the surgical options considered.

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.

A long-term follow-up study of asymptomatic hepatitis B surface antigen-positive carriers in Montreal

BACKGROUND/AIMS

Prospective studies from the Far East and Alaska have shown an increased mortality from cirrhosis and/or hepatocellular carcinoma in asymptomatic hepatitis B virus (HBV) carriers. The magnitude of this risk in apparently healthy North American carriers remains undefined.

METHODS

The outcomes of 317 asymptomatic hepatitis B surface antigen-positive carriers from the Montreal area were examined after 16 years of follow-up. A majority of carriers were of French Canadian origin, were positive for antibody to hepatitis B e antigen, and had normal serum transaminase levels; institutionalization in orphanages as infants or children was the most important epidemiological risk factor, suggesting horizontal transmission of HBV during childhood.

RESULTS

At follow-up, mean age was 46 +/- 8 years; 3 carriers had died of HBV-related cirrhosis, 1 of alcoholic cirrhosis, and 9 of causes unrelated to liver disease. No carrier died of hepatocellular carcinoma; had the risk of hepatocellular carcinoma been similar to that reported from the Far East and Alaska, 17 cases of hepatocellular carcinoma-related deaths would have been expected. During follow-up, the annual negativation rate for hepatitis B surface antigen was 0.7%.

CONCLUSIONS

In asymptomatic HBV carriers from Montreal, a majority are "healthy" carriers and remain asymptomatic after 16 years of follow-up and the risk of death from HBV-related cirrhosis and/or hepatocellular carcinoma is low.

Two sites of action for LCB29 (idrocilamide) in depressing mechanical tension of rat soleus muscle fibers?

The effects of 50 microM LCB29 (idrocilamide) were tested on depolarization-induced and caffeine contractures of rat soleus muscle fibers. When applied intracellularly by free diffusion in cut-end voltage-clamped fibers, LCB29 decreased tension amplitude by about 25%. The same amount of inhibition by LCB29 was observed on contractures induced by 6 mM caffeine. The drug did not affect the repriming of caffeine contractures, indicating that internal recycling of calcium was not affected. The voltage-dependent inactivation of tension was facilitated by external application of LCB29. This effect was calcium dependent, so that the greater the external calcium concentration, the greater the drug effectiveness. The spontaneous relaxation of K+ contractures was also accelerated by LCB29. It is concluded that LCB29 acts intracellularly by decreasing sarcoplasmic reticulum calcium release and externally by facilitating the voltage-dependent inactivation of the voltage sensor for excitation-contraction coupling.

Disorder of cerebellar foliation in Walker's lissencephaly and neu-laxova syndrome

A diffuse disorder of cerebellar foliation was found in eight infants and one fetus with Walker's lissencephaly. The cerebellar cortex consisted of fused and irregularly distorted folia. In the white matter, trilaminated rings of cortex were concentrically arranged around blood vessels and mesenchymal tissue. The normal relative position of the different classes of cortical nerve cells was preserved. Cells of the external granular layer invaded the meninges and migrated along penetrating blood vessels. We believe that this foliation disorder is caused by a defect in the external basal lamina that allows adjacent folia to be fused and sulci obliterated by intrameningeal ectopias of external granule layer cells. Physical forces applied during development probably contribute to the distortion of the gyral pattern. There was a volumetric reduction of the neocerebellum, which might also be a consequence of the basal lamina defect. The cerebellum of a fetus with the Neu-Laxova syndrome showed the same abnormalities as in Walker's lissencephaly. It is postulated that these two conditions belong to a class of prenatal developmental disorders that involves a defect of the extracellular matrix.

Cardio-facio-cutaneous (CFC) syndrome: neurological features in two children

Cardio-facio-cutaneous (CFC) syndrome consists of heart defects, a characteristic facial appearance, ectodermal abnormalities, growth retardation and developmental delay. The authors report two children with this condition, drawing particular attention to the neurological manifestations. The neurological features are neuro-opthalmological findings such as strabismus, ptosis and nystagmus, cortical atrophy, ventriculomegaly, mental retardation, seizures and hypotonia. These manifestations may allow differentiation of CFC from Noonan syndrome, which shares many of the physical features, but not these neurological features.

Changes in cytosolic resting ionized calcium level and in calcium transients during in vitro development of normal and Duchenne muscular dystrophy cultured skeletal muscle measured by laser cytofluorimetry using indo-1

Intracellular calcium activity was recorded during in vitro myogenesis of human normal and DMD muscle, using the calcium probe Indo-1 under laser illumination, at rest and during different kinds of stimulation (acetylcholine, high K+, caffeine). In myoblasts, the resting intracellular calcium level was significantly larger in DMD cells (89 +/- 9 nM; n = 40 vs 37 +/- 5 nM; n = 22) but there was no significant difference in myotubes, after fusion (44 +/- 4 nM; n = 34 vs 36 +/- 4 nM; n = 52). A similar evolution was observed in cells cultured from FSH biopsies. The amplitude of ACh- and high K(+)-induced calcium transients was significantly halved in DMD myotubes as compared to control ones and non-significantly decreased for caffeine responses. Some alterations in the kinetics of responses were observed in DMD muscle: the rising phases of ACh- and high K(+)-elicited transients and the decaying phase of the ACh-responses were significantly slowed down. It is concluded that: (i) in aneurally cultured human muscle, an increase in the basal level of internal calcium can occur at early stages of myogenesis before the expression of the dystrophin gene; and (ii) the changes in calcium transients induced by depolarization or direct stimulation of sarcoplasmic reticulum are not susceptible of inducing a calcium overload in DMD cells.

Activation of a slow outward current by the calcium released during contraction of cultured rat skeletal muscle cells

A slow outward current, activated during depolarization, which induced contraction in whole-cell patch-clamped rat skeletal muscle cells in primary culture [10], was extensively characterized in the present study. This current, Io, was simultaneously recorded with the contraction as a slow outward current during the test pulse, and a slow outward bell-shaped tail after repolarization. Io never appeared below the threshold potential for contraction, and the tail amplitude displayed a similar evolution with peak contraction amplitude as a function of membrane potential. This feature is consistent with the fact that Io was suppressed when contraction was blocked by 5 microM nifedipine [10], and it suggests that Io was dependent on calcium released during contraction. This was confirmed by the fact that the presence of 10 mM EGTA in the patch pipette prevented the development of both contraction and Io, and that Io could be activated during caffeine-induced contractures without applying depolarizations. Io could be carried by K+ or Cs+ ions, but not by Na+. The pharmacology of Io was different from that of Ca(2+)-dependent BK and SK channels, since it was resistant to tetraethylammonium (135 mM), charybdotoxin (25 nM) and apamin (50 nM). Io was also insensitive to 4-aminopyridine (1 mM) but blocked by 5 mM Ba2+ without change to contraction. It was concluded that rat cultured myoballs exhibit a Cs+ permeation through an atypical K+ channel type, which is activated by the calcium released during contraction.

Intracellular calcium transients induced by different kinds of stimulus during myogenesis of rat skeletal muscle cells studied by laser cytofluorimetry with Indo-1

Resting intracellular calcium levels and intracellular calcium transients induced by three types of stimulus (acetylcholine, high potassium and caffeine) were recorded, during in vitro myogenesis, by means of a ratiometric fluorescence method using the calcium probe Indo-1 under laser illumination. Resting levels seemed to decrease with the age of cultured cells and the depolarization-induced transients, through 100 mM K+ or Ach application, were progressively faster and larger as the muscle cells developed. An additive mechanism, likely due to calcium entry into the cell through nicotinic acetylcholine receptors, could explain the differences observed in Ach-induced responses as compared with the 100 mM K(+)-induced ones. In myoballs (the older cells) the calcium transients exhibited progressively a biphasic shape. From data obtained in different conditions (tetrodotoxin, nifedipine, strontium and free Ca EGTA) and those indicating the appearance of caffeine-releasable intracellular calcium stores only at 2-3 days stage, and from the previously reported developmental appearance of calcium currents and contraction, it was proposed that, in young myotubes, the calcium transients were more dependent on extracellular calcium than in older cells. These developmental data are discussed in the light of a known model of the in situ biogenesis of the structures involved in excitation-contraction coupling (ECC) like transverse tubules and triads.

Appearance and evolution of calcium currents and contraction during the early post-fusional stages of rat skeletal muscle cells developing in primary culture

Primary cultures from enzymatically dissociated satellite cells of newborn rat skeletal muscles enabled developmental in vitro studies of mechanical and electrical properties during the first steps of myogenesis. The present work focused on the appearance, evolution and roles of two types of calcium currents (ICa,T and ICa,L) and of depolarization-induced contractile activity during the early stages of muscle cell development in primary culture. Prefusional mononucleated cells (myoblasts), young myotubes of 1 day (with less than 10 nuclei) or 2–3 days (more than 9 nuclei) and myoballs from 4–6, 7–9, 10–12 and 13–16 days cultures were patch-clamped (whole-cell configuration), and calcium currents and contraction simultaneously recorded. Sodium but not calcium currents could be recorded at the myoblast stage. In young myotubes (1 day), ICa,L was present with high incidence as compared to ICa,T, which was poorly expressed. Contractile responses appeared at the next stage (2-3 days) while the occurrence of ICa,T progressively increased. This developmental evolution of the calcium currents and contraction expression was accompanied by some changes in their characteristics: the ICa,T/ICa,L amplitudes ratio progressively increased and the time-to-peak of contraction progressively decreased with the age of myoballs. Physiological functions for calcium currents in developing muscle are suggested and discussed: ICa,T, which is transiently expressed, could be involved in the pacemaker-like activity while ICa,L could serve as an early contraction triggering mechanism and/or initially to fill and then to maintain the intracellular calcium stores.

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.

Calcium current-dependent staircase in rat myotubes and myoballs developing in culture

Calcium current and contraction were simultaneously recorded in whole-cell patch-clamped rat skeletal muscle cells grown in primary culture. Repetitive depolarizations at low frequency, which elicited calcium currents, led to a staircase response, characterized by the progressive increase of both twitch amplitude and activation rate. It was sometimes possible to elicit a staircase response in 2 or 3 day old postfusion myotubes which did not or weakly contract initially. The staircase response was dependent on calcium entry through calcium channels, since it was reversed when calcium current was depressed by means of inorganic calcium blockers or depolarization to large positive potential. The entry of calcium was also necessary to allow the development of a staircase response following caffeine-induced contractures which partly emptied the intracellular stores of calcium. These features are consistent with the idea that calcium currents allow the initial loading of intracellular calcium stores and, in later stages, serve to replenish and maintain them constant.

The voltage-dependent effect of Agauria salicifolia leaf extract on the sodium current of isolated frog skeletal and cardiac muscle cells

An ethanolic extract of leaves of Agauria salicifolia was tested on voltage-clamped isolated skeletal and cardiac frog muscle cells using the double sucrose-gap (skeletal) and the whole-cell patch-clamp (cardiac) methods, at concentrations ranging from 10(-8) to 5 x 10(-5) g/l. At 10(-6) g/l a progressive and limited decrease was observed in resting membrane potential (approximately 5 mV). On normally polarized cells (-90 mV) the extract induced a dose- and use-dependent increase in the transient inward current which is accounted for by an increase in the sodium membrane conductance and an opposite shift of the activation and inactivation curves vs. potential. This agonistic effect on INa is fully reversed when the holding potential is maintained depolarized above -80 mV as a result of an actual decrease in the sodium conductance and a reversal of the shifts of activation and activation curves. The inhibitory effect observed on depolarized fibres can be partially relieved by a subsequent repolarization. It is concluded that the extract acts on the sodium channel regardless of its state (resting, activated and inactivated) on at least two different sites.

The effect of Agauria salicifolia leaf extract on the sodium current of tetrodotoxin-treated frog skeletal muscle fibres

An ethanolic extract of Agauria salicifolia leaf (AS) was tested on voltage-clamped single muscle fibres from frogs, using the double sucrose-gap technique, in the presence of tetrodotoxin (TTX) to partially block the transient inward sodium current. Below 5 x 10(-10) g/ml TTX, 10(-6) g/l AS reversed the blocking effect of TTX on INa and increased the current amplitude. For TTX concentrations ranging from 5 x 10(-10) to 5 x 10(-8) g/ml, which partially blocked INa and lengthened its time to peak, 10(-6) g/l AS reinforced the blocking effect on the current. The inhibitory effect of AS was then studied in the presence of 10(-9) g/ml TTX which was assumed to block all the sarcolemmal sodium channels and not the tubular ones. It appeared to be strongly voltage-dependent since it was fully relieved when the cell was hyperpolarized by 20 mV and was maximum at holding potential (h.p.) -70 mV (100% blockage of INa instead of 50% at h.p. -90 mV). Contrary to observations in depolarised fibres (in which the inhibitory effect of AS decreased as concentration rose) the INa blockage increased with concentration in TTX-treated fibres. It is concluded that AS extract exerts an antagonistic effect on tubular sodium channels by acting on a single site which is different from the high affinity inhibitory site demasked in normal fibres by depolarization of the resting membrane potential.

The effects of Agauria salicifolia leaf extract on calcium current and excitation-contraction coupling of isolated frog muscle cells

The effects of the ethanolic extract of Agauria salicifolia (AS) previously tested on sodium currents of normal and TTX-treated voltage-clamped skeletal and cardiac muscle cells was investigated on mechanisms involved in the excitation-contraction coupling of the same biological preparations. AS (10(-6) g/l) prolonged the action potential duration whereas the associated contraction was depressed. This is not due to the blocking action of AS on the tubular sodium current since similar results were obtained in the presence of high tetrodotoxin (TTX) concentrations. This is not due to some blocking action of AS on the L-type calcium current since this type of calcium current was enhanced by the extract. In contrast, the cardiac T-type calcium current was decreased by 10(-6) g/l AS. AS exerted a dose-dependent (tested for concentrations ranging from 5 x 10(-10) to 5 x 10(-5) g/l) inhibition of the two components of the contractile response elicited by durable depolarizations, with a prominent effect on the tonic phase. This effect was partially relieved by increasing the external divalent cation (Ca2+ or Cd2+) concentration. At the same concentration it shifted the inactivation/potential relationship for tension by 20 mV towards negative potentials. It is concluded that the inhibitory action of AS on excitation-contraction coupling is partly or completely due to the enhancement of the voltage- and Ca(2+)-dependent inactivation processes of the voltage-sensor.

Progressive predominance of 'skeletal' versus 'cardiac' types of excitation-contraction coupling during in vitro skeletal myogenesis

Simultaneous recordings, in the presence or absence of cadmium, of current and contraction of skeletal muscle cells in primary culture (myoballs) showed that the relative part of contraction depending on calcium current progressively decreases with the age of cells whereas the cadmium-insensitive component becomes predominant. The coexistence of "cardiac" and "skeletal" excitation-contraction coupling mechanisms in developing muscle and its developmental regulation are consistent with recent molecular data showing the expression, in newborn animals, of an abbreviated isoform of the alpha 1 subunit of the DHP-receptor of muscle cells.

[Endoscopic internal urethrotomy. Retrospective studies of 132 cases]

The authors report a series of 132 patients with urethral stricture all treated by the same surgical technique between 1979 and 1984: direct vision internal urethrotomy. 62% of good results were obtained after a single operation with a follow-up ranging between 18 months and 5 years. There was no mortality and the morbidity was considered to be 5%. The authors indicate that the results were more favourable when the operation was performed in a solitary, short (less than 2 cm) non-infected stricture of the proximal urethra. The duration of postoperative catheterization was 10 days; catheterization for a longer period did not provide any significant advantage. The poor results (38%) were reported in cases of extensive strictures situated in the distal urethra or in patients with a history of urethral surgery. These cases were treated by repeated internal urethrotomy; 32% were cured after a second urethrotomy, while the others required maintenance sessions of urethral dilatation or even a urethroplasty procedure.

Involvement of a pertussis toxin-sensitive G-protein in excitation-contraction coupling of intact and cut-end voltage-clamped skeletal muscle fibres

In voltage-clamped frog muscle fibres 10 ng/ml PTX induced a decrease (approximately 35%) of tension when applied externally. Internal application in cut-end fibres significantly depressed tension after 20 min. This effect increased with time to reach 65% after 60 min. PTX shifted the voltage-dependent inactivation curve of tension by 30 mV towards hyperpolarizations and this was counteracted by raising external calcium concentration. The toxin induced a parallel decrease in tension and voltage-sensitive charge movement (49 +/- 9% and 52 +/- 6% respectively; n = 6). This was not counteracted by prior impregnation with forskolin. Internally applied GTP gamma S (500 microM) induced a simultaneous increase in tension (57 +/- 5%) and charge amount displaced (40 +/- 7%). By contrast, GDP beta S decreased tension and charge movement by 35 +/- 5% and 36 +/- 6% respectively.

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.

A third type of calcium current in cultured human skeletal muscle cells

A third type of calcium current could be recorded on a non-negligible number of human skeletal muscle cells (normal and Duchenne dystrophic (DMD)) in primary culture. This transient current exhibited a maximum at 0 mV, a time-to-peak around 30 ms, an inactivation time constant around 70 ms and was insensitive to nifedipine. On these basis, it differentiates from T- and L-type previously described and looks like the neuronal N-type. However, this third type of current was not sensitive to omega-Cgtx, a specific N-type blocker. The occurrence and the possible role of this current are briefly discussed.