Péter P. Nánási

Ionic currents and action potentials in rabbit, rat, and guinea pig ventricular myocytes

SummaryDistinct differences exist in action potentials and ionic currents between rabbit, rat, and guinea pig ventricular myocytes. Data obtained at room temperature indicate that about half of the rabbit myocytes show prominent phase 1 repolarization and transient outward current. Action potentials in guinea pig ventricular myocytes resemble those from rabbit myocytes not exhibiting phase 1 repolarization; and guinea pig myocytes do not develop transient outward current. Rat ventricular action potentials are significantly shorter than those from rabbit and guinea pig ventricular myocytes. Unlike rabbit and guinea pig myocytes, rat ventricular myocytes also exhibit a prominent phase 1 and lack a well defined plateau phase during repolarization. All rat ventricular myocytes exhibit a transient outward current which can be best fitted by a double exponential relation. There are no significant differences between the amplitude, voltage dependence and inactivation kinetics of the inward calcium currents observed in rabbit, rat and guinea pig. The steady-state current-voltage relations between −120 mV and −20 mV, which mostly represent the inward rectifier potassium current are similar in rabbit and guinea pig. The amplitude of this current is significantly less in rat ventricular myocytes. The outward currents activated upon depolarization to between −10 and +50 mV are different in the three species. Only a negligible, or absent, delayed rectifier outward current has been observed in rabbit and rat; however, a relatively large delayed rectifier current has been found in guinea pig. These large interspecies variations in outward membrane currents help explain the differences in action potential configurations observed in rabbit, rat, and guinea pig.

Selective inhibition of sodium-calcium exchanger by SEA-0400 decreases early and delayed afterdepolarization in canine heart

The sodium–calcium exchanger (NCX) was considered to play an important role in arrhythmogenesis under certain conditions such as heart failure or calcium overload. In the present study, the effect of SEA‐0400, a selective inhibitor of the NCX, was investigated on early and delayed afterdepolarizations in canine ventricular papillary muscles and Purkinje fibres by applying conventional microelectrode techniques at 37°C. The amplitude of both early and delayed afterdepolarizations was markedly decreased by 1 μM SEA‐0400 from 26.6±2.5 to 14.8±1.8 mV (n=9, P<0.05) and from 12.5±1.7 to 5.9±1.4 mV (n=3, P<0.05), respectively. In enzymatically isolated canine ventricular myocytes, SEA‐0400 did not change significantly the L‐type calcium current and the intracellular calcium transient, studied using the whole‐cell configuration of the patch‐clamp technique and Fura‐2 ratiometric fluorometry. It is concluded that, through the reduction of calcium overload, specific inhibition of the NCX current by SEA‐0400 may abolish triggered arrhythmias.

Endocardial versus epicardial differences in L-type calcium current in canine ventricular myocytes studied by action potential voltage clamp

OBJECTIVES The aim of the present study was to assess and compare the dynamics of L-type Ca(2+) current (I(Ca,L)) during physiologic action potential (AP) in canine ventricular cardiomyocytes of epicardial (EPI) and endocardial (ENDO) origin. METHODS I(Ca,L) was recorded on cells derived from the two regions of the heart using both AP voltage clamp and conventional whole cell voltage clamp techniques. RESULTS AP voltage clamp experiments revealed that the decay of I(Ca,L) is monotonic during endocardial AP, whereas the current is double-peaked (displaying a second rise) during epicardial AP. The amplitude of the first peak was significantly greater in ENDO (-4.6+/-0.8 pA/pF) than in EPI cells (-2.8+/-0.3 pA/pF). Application of epicardial APs as command pulses to endocardial cells yielded double-peaked I(Ca,L) profiles, and increased the net charge entry carried by I(Ca,L) during the AP from 0.187+/-0.059 to 0.262+/-0.056 pC/pF (n=5, P<0.05). No differences were observed in current densities and inactivation kinetics of I(Ca,L) between EPI and ENDO cells when studied under conventional voltage clamp conditions. Nisoldipine shortened action potentials and eliminated the dome of the epicardial AP. CONCLUSION I(Ca,L) was shown to partially inactivate before and deactivate during phase-1 repolarization and reopening of these channels is responsible for the formation of the dome in canine EPI cells. The transmural differences in the profile of I(Ca,L) could be well explained with differences in AP configuration.

A Multiscale Investigation of Repolarization Variability and Its Role in Cardiac Arrhythmogenesis

Enhanced temporal and spatial variability in cardiac repolarization has been related to increased arrhythmic risk both clinically and experimentally. Causes and modulators of variability in repolarization and their implications in arrhythmogenesis are however not well understood. At the ionic level, the slow component of the delayed rectifier potassium current (I(Ks)) is an important determinant of ventricular repolarization. In this study, a combination of experimental and computational multiscale studies is used to investigate the role of intrinsic and extrinsic noise in I(Ks) in modulating temporal and spatial variability in ventricular repolarization in human and guinea pig. Results show that under physiological conditions: i), stochastic fluctuations in I(Ks) gating properties (i.e., intrinsic noise) cause significant beat-to-beat variability in action potential duration (APD) in isolated cells, whereas cell-to-cell differences in channel numbers (i.e., extrinsic noise) also contribute to cell-to-cell APD differences; ii), in tissue, electrotonic interactions mask the effect of I(Ks) noise, resulting in a significant decrease in APD temporal and spatial variability compared to isolated cells. Pathological conditions resulting in gap junctional uncoupling or a decrease in repolarization reserve uncover the manifestation of I(Ks) noise at cellular and tissue level, resulting in enhanced ventricular variability and abnormalities in repolarization such as afterdepolarizations and alternans.

Effects of sex hormones on ECG parameters and expression of cardiac ion channels in dogs

Aim:  The aim of the study was to examine the effects of testosterone and oestrogen on the ECG parameters and expression of cardiac ion channels in male and female dogs, and to compare the dofetilide‐induced lengthening of QTc interval in control, castrated and hormone‐treated animals.

Ionic mechanisms limiting cardiac repolarization reserve in humans compared to dogs.

•  Cardiac repolarization, through which heart‐cells return to their resting state after having fired, is a delicate process, susceptible to disruption by common drugs and clinical conditions. •  Animal models, particularly the dog, are often used to study repolarization properties and responses to drugs, with the assumption that such findings are relevant to humans. However, little is known about the applicability of findings in animals to man. •  Here, we studied the contribution of various ion‐currents to cardiac repolarization in canine and human ventricle. •  Humans showed much greater repolarization‐impairing effects of drugs blocking the rapid delayed‐rectifier current IKr than dogs, because of lower repolarization‐reserve contributions from two other important repolarizing currents (the inward‐rectifier IK1 and slow delayed‐rectifier IKs). •  Our findings clarify differences in cardiac repolarization‐processes among species, highlighting the importance of caution when extrapolating results from animal models to man.

Activation of transient receptor potential vanilloid-3 inhibits human hair growth.

In the current study, we aimed at identifying the functional role of transient receptor potential vanilloid-3 (TRPV3) ion channel in the regulation of human hair growth. Using human organ-cultured hair follicles (HFs) and cultures of human outer root sheath (ORS) keratinocytes, we provide the first evidence that activation of TRPV3 inhibits human hair growth. TRPV3 immunoreactivity was confined to epithelial compartments of the human HF, mainly to the ORS. In organ culture, TRPV3 activation by plant-derived (e.g., eugenol, 10-1,000 μM) or synthetic (e.g., 2-aminoethoxydiphenyl borate, 1-300 μM) agonists resulted in a dose-dependent inhibition of hair shaft elongation, suppression of proliferation, and induction of apoptosis and premature HF regression (catagen). Human ORS keratinocytes also expressed functional TRPV3, whose stimulation induced membrane currents, elevated intracellular calcium concentration, inhibited proliferation, and induced apoptosis. Of great importance, these effects on ORS keratinocytes were all mediated by TRPV3, as small interfering RNA-mediated silencing of TRPV3 effectively abrogated the cellular actions of the above agonists. These findings collectively support the concept that TRPV3 signaling is a significant player in human hair growth control. Therefore, TRPV3 and the related intracellular signaling mechanism might function as a promising target for pharmacological manipulations of clinically relevant hair growth disorders.

Rate and concentration-dependent effects of UK-68,798, a potent new class III antiarrhythmic, on canine Purkinje fibre action potential duration and Vmax.

1 The frequency‐dependent electrophysiological effects of UK‐68,798 in concentrations of 1, 3, 10 and 30 nm were examined in isolated cardiac Purkinje fibres of the dog at both a number of constant rates of stimulation and following abrupt changes in pacing cycle length. 2 In all concentrations evaluated, UK‐68,798 lengthened action potential duration in a concentration‐and rate‐dependent manner (e.g., at a cycle length = 500 ms, control APD90 = 234.0 ± 3.3 ms, while after 10 nm UK‐68,798, APD90 = 315.0 ± 5.9 ms). 3 The duration of action potentials evoked following abrupt changes in pacing rate were also increased in a concentration‐dependent manner at all diastolic intervals tested. 4 The fast and slow time constants for restitution of APD were not altered by UK‐68,798. However, the amplitude terms for this relation were increased. 5 In addition, the maximum upstroke velocity (V̇max) was not significantly affected by exposure to UK‐68,798 at any concentration or diastolic interval. The kinetics for recovery of V̇max were thus unaffected. 6 These findings are similar to those previously reported for recognized class III antiarrhythmic agents (e.g., bretylium, clofilium, and sotalol); however, UK‐68,798 was 1,000 to 10,000 times more potent. 7 The combined potency and selectivity of this agent seem to make it an ideal tool for the investigation of cardiac potassium channels believed responsible for controlling the duration of the action potential. 8 This potent and highly selective compound may prove extremely useful in the control of cardiac arrhythmias.

Reverse rate dependency is an intrinsic property of canine cardiac preparations

AIMS Class III antiarrhythmic agents exhibit reverse rate-dependent lengthening of the action potential duration (APD). In spite of the several theories developed so far to explain this reverse rate dependency (RRD), its mechanism has not yet been clarified. The aim of the present work was to further elucidate the mechanisms responsible for reverse rate-dependent drug effects. METHODS AND RESULTS Action potentials were recorded from multicellular canine ventricular preparations and isolated cardiomyocytes, at cycle lengths (CLs) varying from 0.3 to 5 s, using conventional sharp microelectrodes. APD was either modified by applying inward and outward current pulses, or by superfusion of agents known to lengthen and shorten APD. Net membrane current (I(m)) was calculated from action potential waveforms. The hypothesis that RRD may be implicit in the relationship between I(m) and APD was tested by numerical modelling. Both drug-induced lengthening (by veratrine, BAY-K 8644, dofetilide, and BaCl(2)) and shortening (by lidocaine and nicorandil) of action potentials displayed RRD, i.e. changes in APD were greater at longer than at shorter CL. A similar dependency of effect on CL was found when repolarization was modified by injection of inward or outward current pulses. I(m) measured at various points during repolarization was inversely proportional to APD and to CL. Model simulations showed that RRD is expected as a consequence of the non-linearity of the relationship between I(m) and APD. CONCLUSION RRD of APD modulation is shared, although with differences in magnitude, by interventions of very different nature. RRD can be interpreted as a consequence of the relationship between I(m) and APD and, as such, is expected in all species having positive APD-CL relationship. This implies that the development of agents prolonging APD with direct rate dependency, or even completely devoid of RRD, may be difficult to achieve.

Effects of thymol on calcium and potassium currents in canine and human ventricular cardiomyocytes

Concentration‐dependent effects of thymol (1–1000 μM) was studied on action potential configuration and ionic currents in isolated canine ventricular cardiomyocytes using conventional microelectrode and patch clamp techniques. Low concentration of thymol (10 μM) removed the notch of the action potential, whereas high concentrations (100 μM or higher) caused an additional shortening of action potential duration accompanied by progressive depression of plateau and reduction of Vmax. In the canine cells L‐type Ca current (ICa) was decreased by thymol in a concentration‐dependent manner (EC50: 158±7 μM, Hill coeff.: 2.96±0.43). In addition, thymol (50–250 μM) accelerated the inactivation of ICa, increased the time constant of recovery from inactivation, shifted the steady‐state inactivation curve of ICa leftwards, but voltage dependence of activation remained unaltered. Qualitatively similar results were obtained with thymol in ventricular myocytes isolated from healthy human hearts. Thymol displayed concentration‐dependent suppressive effects on potassium currents: the transient outward current, Ito (EC50: 60.6±11.4 μM, Hill coeff.: 1.03±0.11), the rapid component of the delayed rectifier, IKr (EC50: 63.4±6.1 μM, Hill coeff.: 1.29±0.15), and the slow component of the delayed rectifier, IKs (EC50: 202±11 μM, Hill coeff.: 0.72±0.14), however, K channel kinetics were not much altered by thymol. These effects on Ca and K currents developed rapidly (within 0.5 min) and were readily reversible. In conclusion, thymol suppressed cardiac ionic channels in a concentration‐dependent manner, however, both drug‐sensitivities as well as the mechanism of action seems to be different when blocking calcium and potassium channels.