Alteration of the sodium current in cat cardiac ventricular myocytes during primary culture

Cultivation in rotating bioreactors promotes maintenance of cardiac myocyte electrophysiology and molecular properties

We tested the hypothesis that cardiomyocytes maintained their phenotype better if cultured as three-dimensional tissue constructs than if cultured as confluent monolayers. Neonatal rat cardiomyocytes were cultured on biomaterial scaffolds in rotating bioreactors for 1 week, and resulting tissue constructs were compared with confluent monolayers and slices of native ventricular tissue with respect to proteins involved in cell metabolism (creatine kinase isoform MM), contractile function (sarcomeric myosin heavy chain), and intercellular communication (connexin 43), as well as action potential characteristics (e.g., membrane resting potential, maximum depolarization slope, and action potential duration), and macroscopic electrophysiological properties (maximum capture rate). The molecular and electrophysiological properties of cardiomyocytes cultured in tissue constructs, although inferior to those of native neonatal ventricles, were superior to those of the same cells cultured as monolayers. Construct levels of creatine kinase, myosin heavy chain, and connexin 43 were 40-60% as high as ventricle levels, whereas monolayer levels of the same proteins were only 11-20% as high. Construct action potential durations were 1.8-fold higher than those in ventricles, whereas monolayer action potential durations were 2.4-fold higher. Pharmacological studies using 4-aminopyridine showed that prolonged action potential duration and reduced maximum capture rate in tissue constructs as compared with native ventricles could be explained by decreased transient outward potassium current.

Heterogeneity of sodium current in atrial vs epicardial ventricular myocytes of adult guinea pig hearts

The different sodium channel currents (I(Na)) were reported in myocardium, neuron, and skeletal muscles. To study whether I(Na) is homogeneous within the heart, we applied whole-cell voltage clamp technique to evaluate fast voltage-gated I(Na) in atrial and ventricular myocytes isolated from guinea pig heart. It was found that the density of inward I(Na) was 50% greater at -35 mV in atrial (-42.6+/-2.9 pA/pF) than in ventricular (-27.5+/-1.8 pA/pF, P<0.01) myocytes. The half activation and inactivation voltages (V(0.5)) of I(Na) in atrial myocytes were shifted 4.5+/-0.2 and 9.6+/-0.3 mV negative to those of ventricular myocytes. Time constants for I(Na) activation (tau(m)) and inactivation (tau(h)) were twice as rapid in atrial as in ventricular myocytes. The tau(m) and tau(h) were 0.34+/-0.03 and 1.36+/-0.07 ms for atrial myocytes, and 0.69+/-0.05 and 3.27+/-0.23 ms for ventricular myocytes, respectively. Recovery of I(Na) from inactivation was slower in atrial than in ventricular myocytes, whereas the development of resting state inactivation was more rapid in atrial (tau=67.5+/-4.3 ms) than in ventricular (152.8+/-7.5 ms, P<0.01) myocytes. The results reveal marked heterogeneity of I(Na) in the density and biophysical properties in atrial and ventricular myocytes, and the study suggests the potential possibility of tissue specific cardiac sodium channel isoforms.

Electrophysiological properties of rainbow trout cardiac myocytes in serum-free primary culture

A low-density primary culture of trout ventricular myocytes in serum-free growth medium was established and maintained for up to 10 days at 17 degrees C. The myocytes retained their normal rod shaped morphology, capacitive surface area of the sarcolemma (SL), and contractile quiescence. However, sarcolemmal cation currents changed significantly, some permanently, some transiently, after 8-10 days of culture. TTX-sensitive sodium current (I(Na)) and Ba(2+)-sensitive background inward rectifier potassium current (I(K1)) were permanently depressed to 24-28% of their control density measured in freshly isolated myocytes. In contrast, L-type calcium current (I(Ca)) was only transiently downregulated; after 2-3 days in culture, the density of the current was 32% of the control and recovered to the control value after 8-10 days in culture. The changes in membrane currents were reflected in the shape of the action potential (AP). After 2-3 days in culture, maximal overshoot potential and resting potential were significantly reduced, and the durations of the AP at 50 and 90% repolarization were significantly increased. These changes became significantly more pronounced after 8-10 days of culture, with the exception of AP duration at 50% repolarization level. The shortening of the early plateau phase may reflect an additional change to an outward current, presumably the rapid component of the delayed rectifier (I(Kr)). Although the present findings indicate that fish cardiac myocytes can be maintained in serum-free primary culture for at least 10 days at 17 degrees C, some but not all of the electrophysiological characteristics of the myocytes change markedly during culture. The changes in ion currents were not due to loss of sarcolemmal membrane and therefore are likely to represent altered expression of cation currents as an adaptive response to culture conditions.

Effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes

BDF 9198 (a congener of DPI 201 - 106 and BDF 9148) was found to be a positive inotrope on guinea-pig isolated ventricular muscle strips. The effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes were studied using the whole cell patch clamp method. In normal external solution, at 37 degrees C, action potential duration at 50% repolarization (APD(50)) was 167.4+/-8.36 ms (n=37). BDF 9198 produced a concentration-dependent increase in APD(50) (no significant increase at 1x10(-10) M; and APD(50) values of 273.03+/-35.8 ms at 1x10(-9) M; n=6, P<0.01 and 694.7+/-86.3 ms at 1x10(-7) M; P<0.001, n=7). At higher concentrations in the range tested, BDF 9198 also induced early and delayed and after-depolarizations. Qualitative measurements of I(Na) with physiological [Na](o) showed prolongation of the current by BDF 9198, and the appearance of transient oscillatory inward currents at high concentrations. Quantitative recording conditions for I(Na) were established using low external [Na] and by making measurements at room temperature. The current - voltage relation, activation parameters and time-course of I(Na) were similar before and after a partial blocking dose of Tetrodotoxin (TTX, 1 microM), despite a 2 fold difference in current amplitude. This suggests that voltage-clamp during flow of I(Na) was adequately maintained under our conditions. Selective measurements of I(Na) at room temperature showed that BDF 9198 induced a concentration-dependent, sustained component of I(Na) (I(Late)) and caused a slight left-ward shift in the current - voltage relation for peak current. The drug-induced I(Late) showed a similar voltage dependence to peak current in the presence of BDF 9198. Both peak current and I(Late) were abolished by 30 microM TTX and were sensitive to external [Na]. Inactivation of control I(Na) during a 200 ms test pulse to -30 mV followed a bi-exponential time-course. In addition to inducing a sustained current component, BDF 9198 left the magnitude of the fast inactivation time-constant unchanged, but increased the magnitude of the slow inactivation time-constant. Additional experiments with a longer pulse (1 s) raised the possibility that in the presence of BDF 9198, I(Na) inactivation may be comprised of more than two phases. No significant effects of 1x10(-6) M BDF 9198 were observed on the L-type calcium current, or delayed and inward rectifying potassium currents measured at 37 degrees C. It is concluded that the prolongation of APD(50) by BDF 9198 resulted from selective modulation of I(Na). Reduced current inactivation induced a persistent I(Na), increasing the net depolarizing current during the action potential. This action of the drug indicates a potential for 'QT prolongation' of the ECG. The observation of after-depolarizations suggests a potential for proarrhythmia at some drug concentrations.

Restoration of transient outward current by norepinephrine in cultured canine cardiac myocytes

The mechanism for the reduction of the transient outward K+ current (Ito) in diseased myocardium is unknown. To identify potential mechanisms, the reduction of Ito and its subsequent restoration by norepinephrine (NE) were studied in cultured canine epicardial myocytes. After myocytes were cultured for 9 days (day 9), Ito density was decreased compared with density on the day of isolation (day 0) (3.2 +/- 0.4 vs. 10.4 +/- 0.4 pA/pF; mean +/- SE). The time constant of current decay (taudecay) was increased, the time course of recovery from inactivation was prolonged, and the half-inactivation voltage (V1/2) was shifted to less negative potentials. Exposure of myocytes on day 8 to 1 microM NE or isoproterenol (Iso) for 1 h had no acute effect on Ito but restored Ito density to 7.6 +/- 1.2 or 9.7 +/- 2.3 pA/pF, respectively, on day 9. Recovery from inactivation and taudecay remained slowed, and V1/2 remained shifted to less negative potentials. The effects of NE and Iso were blocked by actinomycin D and were not mimicked by phenylephrine or phorbol ester. A-23187 (1 microM) also restored Ito. Thus beta-adrenergic agonists restored normal Ito density, but not normal Ito kinetics, in cultured epicardial myocytes, possibly via increased intracellular Ca2+ concentration.