Canine Myocytes Represent a Good Model for Human Ventricular Cells Regarding Their Electrophysiological Properties

Abstract

Due to the limited availability of healthy human ventricular tissues, the most suitable animal model has to be applied for electrophysiological and pharmacological studies. This can be best identified by studying the properties of ion currents shaping the action potential in the frequently used laboratory animals, such as dogs, rabbits, guinea pigs, or rats, and comparing them to those of human cardiomyocytes. The authors of this article with the experience of three decades of electrophysiological studies, performed in mammalian and human ventricular tissues and isolated cardiomyocytes, summarize their results obtained regarding the major canine and human cardiac ion currents. Accordingly, L-type Ca²⁺ current (I_Ca), late Na⁺ current (I_Na-late), rapid and slow components of the delayed rectifier K⁺ current (I_Kr and I_Ks, respectively), inward rectifier K⁺ current (I_K1), transient outward K⁺ current (I_to1), and Na⁺/Ca²⁺ exchange current (I_NCX) were characterized and compared. Importantly, many of these measurements were performed using the action potential voltage clamp technique allowing for visualization of the actual current profiles flowing during the ventricular action potential. Densities and shapes of these ion currents, as well as the action potential configuration, were similar in human and canine ventricular cells, except for the density of I_K1 and the recovery kinetics of I_to. I_K1 displayed a largely four-fold larger density in canine than human myocytes, and I_to recovery from inactivation displayed a somewhat different time course in the two species. On the basis of these results, it is concluded that canine ventricular cells represent a reasonably good model for human myocytes for electrophysiological studies, however, it must be borne in mind that due to their stronger I_K1, the repolarization reserve is more pronounced in canine cells, and moderate differences in the frequency-dependent repolarization patterns can also be anticipated.

Keywords: action potential configuration; action potential voltage clamp; canine myocytes; cardiac ion currents; human ventricular cells.

Figure 1

Action potential configurations (A), steady-state rate-dependent action potential durations at 90% of repolarization (APD₉₀, (B)), and APD₉₀ restitution relations (C) measured in multicellular human, canine, guinea pig, rabbit, and rat ventricular preparations using sharp 3 M KCl-filled microelectrodes. The restitution curves were obtained by gradually increasing the diastolic interval following a train of action potential stimulated at a stable cycle length. Symbols and bars are mean ± SEM values; (n) denotes the number of preparations studied [57].

Figure 2

L-type Ca²⁺ currents in human (A) and canine (B) ventricular myocytes of epicardial (EPI) and endocardial (ENDO) origin. Command action potentials above and I_Ca recordings below. I_Ca was excised by 1 µM nifedipine. Note the double-peaked I_Ca records in EPI and the single-peaked ones in ENDO preparations in both species. (C) Comparison of peak current densities between pooled human and canine I_Ca obtained under conventional voltage clamp conditions. At test potentials of +5 mV or more negative, significant differences were observed (human I_Ca was greater), while no significant differences were found at +10 mV or more positive voltages. Columns and bars are mean ± SEM values, (n) denotes the number of myocytes studied, the asterisk (*) indicates significant differences between human and canine I_Ca data. (Data from references [21,22,24]).

Figure 3

Transient outward K⁺ currents in human and canine left ventricular myocytes. (A) Differences in action potential morphology in canine ventricular cells derived from the subepicardial (EPI), midmyocardial (MID), and subendocardial (ENDO) layers. (B) Command action potentials (top) and transient outward K⁺ current (I_to1) records (bottom) taken from midmyocardial human and canine myocytes under action potential voltage clamp conditions. The cycle length of stimulation was 700 ms in the canine and 1000 ms in the human cells. (C) Average I_to1 densities. I_to1 was defined as a 100 µM chromanol 293B-sensitive current. Columns and bars are mean ± SEM values, (n) denotes the number of myocytes studied. Unpublished data from Varró et al.

Figure 4

(A–C) Late Na⁺ current (I_Na-late) in human and canine ventricular myocytes. I_Na-late was recorded under conventional voltage clamp (A) and action potential voltage clamp conditions (B). The current in this latter case was excised by 10 µM TTX. Pulse protocols are shown above. (C) Peak densities of I_Na-late measured with action potential voltage clamp in human and canine myocytes. (D) Representative I_NCX, defined as a 10 mM Ni²⁺-sensitive current, recorded using a voltage ramp (−40 mV -> + 60mV -> −100 mV -> −40 mV). (E) Peak inward (measured at −80 mV) and outward (measured at +50 mV) NCX current densities. Columns and bars are mean ± SEM values, (n) denotes the number of myocytes studied. (Data from references [17,21]).

Figure 5

Comparison of peak I_Kr, I_Ks, and I_K1 current densities in human and canine ventricular myocytes under action potential voltage clamp conditions (A,B). I_Kr, I_Ks, and I_K1 were excised by 5 µM E-4031, 0.1 µM L-735,821, and 500 µM BaCl₂, respectively. (C) Expression of the main channel proteins in human and canine ventricular myocardium. Columns and bars are mean ± SEM values, (n) denotes the number of myocytes in (B), and myocardial samples in (C), the asterisks (*) indicate significant differences between human and canine data. (Data from reference [21]).

Figure 6

Phase-plane analysis of I_Kr (excised by 1 µM E-4031, (A)) and I_K1 (excised by 10 µM BaCl₂, (B)) obtained under action potential voltage clamp conditions in human and canine ventricular myocytes. Representative current–voltage relationships were compared at two different pacing cycle lengths (0.4 and 5 s). The currents showed no rate-dependent properties and displayed similar current–voltage relationships in the two species. Note that the peak amplitude of I_Kr was identical in the canine and human myocytes, while the peak amplitude of I_K1 was three-fold greater in dogs than in humans. (Data from reference [23]).

Figure 7

Regional inhomogeneity of the channel protein expression pattern in human and canine ventricular myocardium. (A,B) Epicardial versus midmyocardial distribution. (C,D) Apical versus basal distribution. Columns and bars are mean ± SEM values, (n) denotes the number of myocardial samples studied, the asterisks (*) indicate significant differences from the ratio of 1. (Data from references [19,20]).