The pacemaker current I(f) in single human atrial myocytes and the effect of beta-adrenoceptor and A1-adenosine receptor stimulation

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 microm, width 17.9+/-0.42 microm, 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 beta-adrenoceptor agonist isoprenaline (Iso, 1 microM), 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 microM) 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 microM 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 beta-adrenoceptors and A1-adenosine receptors.