The pacemaker current I_f in single human atrial myocytes and the effect of β-adrenoceptor and A₁-adenosine receptor stimulation

Abstract

1. We used single human atrial myocytes to study I_f occurrence, properties and pharmacological modulation. Cells were obtained by chunk enzymatic digestion from samples of right atrial appendages of patients undergoing corrective cardiac surgery.

2. Patch-clamped cells in the whole-cell configuration were superfused with a modified Tyrode solution to reduce contamination by interfering currents and to amplify I_f. The average cell membrane capacitance was 85.06±2.41 pF (n=531). Data were consistent with the geometrical dimensions of the cells (length 94.2±1.89 μm, width 17.9±0.42 μm, n=126).

3. When hyperpolarizing to −120 mV from a holding potential of −40 mV, 252 of 306 tested cells (82%) expressed a hyperpolarization-activated inward current (I_f density =3.77±0.25 pA pF⁻¹); the current was considered to be present in a given cell if its density at −120 mV was larger than 0.5 pA pF⁻¹.

4. Current activation was sigmoidal and fitted a Boltzmann model; the average activation curve (n=25) showed a maximum current amplitude of 205.97±19.94 pA, corresponding to 3.87±0.63 pA pF⁻¹, voltage of half-maximal activation (V_1/2) at −86.68±2.19 mV and a slope of −11.39±0.69 mV. The reversal potential of I_f measured by tail-current analysis was −13.07±1.92 mV (n=6). The addition of CsCl (5 mM) fully and reversibly blocked the current.

5. In the presence of the β-adrenoceptor agonist isoprenaline (Iso, 1 μM), V_1/2 was significantly shifted toward less negative potentials by 6.06±1.96 mV (n=16, P=0.0039). The selective A₁-adenosine receptor agonist cyclopentyladenosine (CPA, 1 μM) caused a statistically significant shift of V_1/2 toward more negative potentials with respect to the control curve, both in the absence (−7.37±1.83 mV, P=0.0005, n=11) and in the presence of 1 μM Iso (−4.97±1.78, P=0.031, n=6).

6. These results demonstrate that a current with the properties of I_f described in cardiac primary and secondary pacemakers occurs in the majority of human atrial cells. While the pathophysiological relevance of I_f in human atrial tissue remains to be defined, our data clearly show that it is modulated through stimulation of β-adrenoceptors and A₁-adenosine receptors.

Full Text

The Full Text of this article is available as a PDF (381.6 KB).