Comparative study of hyperpolarization-activated currents in pulmonary vein cardiomyocytes isolated from rat, guinea pig, and rabbit

Abstract

Pulmonary vein (PV) cardiomyocytes have the potential to generate spontaneous activity, in contrast to working myocytes of atria. Different electrophysiological properties underlie the potential automaticity of PV cardiomyocytes, one being the hyperpolarization-activated inward current (I_h), which facilitates the slow diastolic depolarization. In the present study, we examined pharmacological characteristics of the I_h of PV cardiomyocytes in rat, guinea pig and rabbit. The results showed that guinea pig and rat PV cardiomyocytes possessed sizeable amplitudes of the I_h, and the I_h of guinea pig was suppressed by Cs⁺, a blocker of the hyperpolarization-activated cation current. However, the I_h of rat was not suppressed by Cs⁺, but by Cd²⁺, a blocker of the Cl^- current. The current density of the I_h of rabbit PV cardiomyocytes was significantly smaller than those of other species. This suggests that the ion channels that carry the I_h of PV cardiomyocytes differ among the animal species.

Keywords: Atrial fibrillation; Automaticity; Hyperpolarization-activated Cl− current; Hyperpolarization-activated cation current; Pulmonary vein.

Fig. 1

Representative action potentials recorded in PV cardiomyocytes of rat (A), guinea pig (B), and rabbit (C). Spontaneous action potentials recorded in PV cardiomyocytes of guinea pig (D). The action potential indicated by the arrow is shown in an expanded time scale in the right panel. Typical tracings recorded from the PV cardiomyocytes of rabbit are an elicited action potential (Ea) and subsequent spontaneous electrical activity (Eb) after the train stimulation at a pacing cycle length of 2 s. The action potential indicated by the arrow is shown in an expanded time scale at the right panel. Dashed lines indicate 0 mV

Fig. 2

Comparison of membrane currents and I–V relationships of isolated PV cardiomyocytes from rat, guinea pig, and rabbit. A Current traces were obtained from PV cardiomyocytes of rat (a), guinea pig (b), and rabbit (c) in a normal Tyrode’s solution. Traces shown were obtained by applying 500-ms depolarizing or hyperpolarizing pulses from a holding potential of − 40 mV to a test potential from − 100 mV to + 60 mV. In PV cardiomyocytes of rabbit, two obviously different families of currents were detected, where some cells possessed transient outward current (left) and others did not (right). Dashed lines indicate the zero current level. BI–V relationships for the initial current (open circles) and the current near the end of the pulses (filled circles) in rat (a), guinea pig (b), and rabbit (c). A transient outward current was shown only in rabbit (open squares)

Fig. 3

I_h in rat, guinea pig, and rabbit PV cardiomyocytes. Recordings from PV cardiomyocytes of rat (left), guinea pig (middle), and rabbit (right). Recordings in each animal were obtained in the same cell with 2-s hyperpolarizing pulses from − 40 mV to − 140 mV in 10 mV steps. Representative current traces were obtained in Tyrode solution containing 0.3 μM nisoldipine (top row). Then, the same pulse protocol was applied after addition of 1 mM Ba²⁺ (2nd row), 5 mM Cs⁺ (3rd row) and 1 mM Cd²⁺ (bottom row). The dashed line indicates 0 current level

Fig. 4

Comparison of the I_h in PV cardiomyocytes from rat, guinea pig, and rabbit. A The amplitude of the I_h in rat (left), guinea pig (middle), and rabbit (right) PV cardiomyocytes was measured at − 140 mV in the control and in the presence of either 5 mM Cs⁺ or 1 mM Cd²⁺. B The current amplitudes were measured at the beginning and near the end of the test pulse of PV cardiomyocytes in rat (left), guinea pig (middle), and rabbit (right)

Fig. 5

Voltage-dependent kinetics of the I_h. A The steady-state activation curves constructed from rat and guinea pig. Continuous lines are the Boltzmann fits used to determine V_1/2 and slope factors. B Time constants obtained by fitting raw data with a double exponential function in rat, and with one exponential function in guinea pig

Fig. 6

Ba²⁺-sensitive component of the membrane currents in response to 2-s hyperpolarizing voltage pulses from − 40 mV. A Representative current traces recorded from PV cells of rat (a), guinea pig (b), and rabbit (c). The currents were obtained by subtracting the currents in the presence of 1 mM Ba²⁺ from those in the absence of Ba²⁺. The I–V relationships were obtained for the initial peak and the end of the Ba²⁺-sensitive current. B Time constants were obtained by the least squares fit of the Ba²⁺-sensitive current with a sum of two exponential functions. Open and filled symbols indicate fast and slow components, respectively, and circles, squares and triangles indicate rat, rabbit and guinea pig, respectively. C Relative amplitude of the fast component. Open circles, open triangles and filled squares indicate rat, guinea pig and rabbit, respectively