E. Coraboeuf

Effect of 2,3-butanedione 2-monoxime on slow inward and transient outward currents in rat ventricular myocytes

The effect of 2,3-butanedione 2-monoxime (BDM), a substance possessing phosphatase-like activity, was studied on action potentials of isolated rat heart and on the slow inward calcium current and outward current (including the 4-aminopyridine (4-AP)-sensitive transient outward component), in rat ventricular myocytes. In contrast to what was observed by other authors in different species and cardiac tissues, BDM increased markedly the amplitude and duration of the rat ventricular action potential plateau. On the other hand, in the presence of 4-AP and ryanodine BDM shortened action potential duration. BDM decreased in a dose dependent manner the amplitude of both the slow inward calcium current and the transient outward current, accelerated their inactivation and shifted their steady-state inactivation-voltage relationships towards negative potentials. BDM also depressed other components of outward current. It is suggested that the lengthening effect of BDM on action potential duration results mainly from the simultaneous reduction of both the slow inward calcium current and the transient outward current, two antagonistic currents with unequal influences on action potential plateau development. The similarity of effect of BDM on these two currents also suggests that ionic channels generating them might require similar phosphorylation for their functioning.

Large-conductance chloride channels of new-born rat cardiac myocytes are activated by hypotonic media

Large-conductance chloride (LC-type Cl−) channel activity was studied in rat ventricular myocyte membrane during development. In contrast with results previously obtained in cultured ventricular myocytes of the new-born rat, we failed to record single-channel activity in freshly isolated myocytes whatever the age of the animal (from 2 days old to adult) and the recording patch configuration used. However, spontaneous single-channel activity of LC-type Cl− channels was recorded in bleb membranes of myocytes of rats younger than 12 days, with a higher frequency in excised inside-out membrane patches than in cell-attached membrane patches. In intact neonatal myocytes, application of hypotonic media (150 mOsm) also initiated the channel activity, after variable delays (25–200 s). The channel could not be activated by suction applied through the pipette and was not observable in cells from rats older than 15 days. The LC-type Cl− channels showed properties similar to those reported in other preparations and previously observed in cardiac cultured cells: they had a large single-channel conductance of 400 pS in symmetrical 150 mM NaCl, showed multiple subconductance states, a relatively high selectivity to Cl− ions over Na+ ions (PCl/PNa=24.6), were blocked by 10 μM 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulphonic acid (SITS) and showed voltage-dependent inactivation. They were not activated by 10 μM colchicine or 3 μM cytochalasin D. The fact that these channels exhibit spontaneous activity when cells are swollen and membranes distended suggests that they play a role in the volume regulation of young cardiac cells, possibly through tension exerted at the channel level by cytoskeletal attachments. This channel activity is absent in adult cells.

Effects of atrionatriuretic factor on Ca2+current and Cai-independent transient outward K+ current in human atrial cells

The effect of 10 nM atrial natriuretic peptide (ANF) on macroscopic L-type calcium current, ICa, and calcium-independent outward potassium current, Ilo, were studied in myocytes isolated from human atrial trabeculae using the whole-cell-recording patch-clamp technique. When cells were dialysed with pipette media containing 0.2 mM GTP, ANF reduced ICa by 37.81%±5.4% at +20mV and Ilo by 21.72%±3.68% at +60 mV in a reversible manner. When ICa was increased by β-adrenoreceptor stimulation (0.1 μM isoproterenol) or by the phosphodiesterase inhibitor isobutylmethylxanthine (10 μM) ANF reduced ICa by 24.99±3.4% and by 39.9±6.3% respectively. In cells dialysed with GTP-free pipette media, ANF increased ICa markedly (39.8%±7%) and reversibly, whereas it still depressed Ilo (18.92%±2%). Addition of 0.2 mM GTP[γS] to the pipette solution in the absence of GTP increased ICa, decreased Ilo and suppressed the effect of ANF on both ICa and Ilo. It is suggested that activation of the ANF receptor in human atrial cells reduces ICa via guanylate-cyclase-dependent cGMP production, increases ICa via Gs protein activation and decreases Ilo via Gi protein activation.

The calcium antagonist D600 inhibits calcium-independent transient outward current in isolated rat ventricular myocytes.

1. The whole‐cell voltage‐clamp technique was applied to isolated rat ventricular myocytes to investigate the effects of D600 (10(‐9)‐10(‐3) M) on the intracellular calcium‐independent component of transient outward current. I(lo), recorded in a sodium‐free medium containing 0.5 x 10(‐3) M‐cadmium and 10(‐6) M‐ryanodine. 2. Externally applied D600 reduced Ilo in a dose‐dependent, reversible manner, and accelerated the decay of the current. 3. Current‐voltage relationships and corresponding activation curves (determined assuming I(lo) to be a pure potassium current) were shifted towards positive potentials in the presence of 10(‐3) M but not 10(‐5) M‐D600. Steady‐state inactivation curves were not affected by either low or high concentrations of D600. 4. Under control conditions, the inactivation of I(lo) is composed of a fast and a slow component. The amplitude of the slow component was more strongly reduced by D600 than that of the fast one. In the presence of 10(‐3) M‐D600, the slow component was entirely suppressed. 5. Both the time to peak Ilo and the time constant of the fast component of inactivation were markedly reduced at all potentials by D600. The time constant of the slow component was less sensitive to the drug. 6. When the relative quantity of charge carried by each kinetic component of Ilo was plotted versus the concentration of D600, the data could be fitted by two distinctly separate dose‐response curves with an almost 100‐fold difference between the two apparent dissociation constants, of which the values were 2.88 x 10(‐6) M for the slow phase of inactivation and 2.07 x 10(‐4) M for the fast one, with Hill coefficients of 0.68 and 0.73 respectively. 7. The inhibition of I(lo) by D600 displayed little or no use dependence, one of the major characteristics of the effects of phenylalkylamines on the cardiac calcium current ICa. 8. Our results show that at least part of I(lo) is sensitive to D600 in the same range of concentrations as ICa. Although the effects of D600 on the two currents differ in several points, this observation raises the possibility that, besides clear differences, certain similarities may exist between the channels responsible for I(lo) and ICa.

Reduction of calcium-independent transient outward potassium current density in DOCA salt hypertrophied rat ventricular myocytes

Saline-drinking, left-nephrectomized rats made hypertensive by deoxycorticosterone acetate (DOCA) pellet implantation at the time of surgery develop a cardiac hypertrophy, which becomes maximal after 6–7 weeks. The hypertrophy results in a marked increase in the amplitude and duration of both the early and the late component of the ventricular action potential plateau recorded in the isolated perfused rat heart. The 4-aminopyridine(4-AP)-sensitive calcium-independent transient outward potassium current was markedly depressed in hypertrophied ventricular myocytes resulting in a highly significant decrease in current density (from 19.9±3.5 to 6.4±3.1 pA/pF at +60 mV). Activation/ voltage and steady-state inactivation/voltage relationships were moderately although non-significantly shifted towards negative potentials. The steady-state outward current measured at the end of 1-s depolarizing pulses was not significantly changed in hypertrophied myocytes. 4-AP induced a smaller increase in plateau amplitude and duration in hypertrophied rather than in control hearts, a point that is well explained by the depression of the transient outward current resulting from hypertrophy. We also demonstrated that a complete recovery of both cell capacitance and transient outward current amplitude occurs in myocytes from saline-drinking rats studied 13 weeks after DOCA pellet implantation, showing that hypertrophy regresses as a result of pellet elimination. Several mechanisms can be involved in the observed phenomena, including the possibility that the expression of potassium channels responsible for the transient outward current is not enhanced by hypertrophy in contrast with what occurs in the case of calcium channels. We conclude that the depression of the calcium-independent transient outward potassium current appears responsible for the major part of the hypertrophy-induced action potential lengthening in rat ventricular myocytes.

Role of adrenocortical hormones on the ontogenesis of ventricular action potential of rat myocardium

Abstract The ontogenesis of the rat epiventricular action potential was studied using hearts isolated from fetuses aged 13.5 to 21.5 days and from newly born animals. The resting membrane potential (RP), the overshoot (OS) and the maximal rate of depolarization of the action potential ( V max ) were slightly but significantly increased during the studied period of gestation through to the second day following birth. From the 16th day of gestation, the duration of action potential (APD) began to decrease gradually to attain a characteristic triangular shape at birth. Under these in vitro conditions the intrinsic cardiac frequency did not vary during the period investigated. The duration of the cardiac action potential recorded at the end of the gestation was greater for fetuses which had been deprived of hypophyseal hormonal influence by decapitation on day 15.5 or 16.5 than for control fetuses of the same litter. Maternal adrenalectomy performed on day 13.5 of gestation did not affect cardiac electrogenesis in normal fetuses but led to the absence of the normal shortening of the action potential in decapitated fetuses. This effect was prevented by cortisol as well as d -aldosterone administered to the fetus on day 19.5 of gestation. In conclusion, corticoids of fetal and/or maternal origin appear to play an important role in the modifications of the repolarization phase of the epicardial action potential during the final days of gestation.