Modulation of the hyperpolarization-activated current (If) by calcium and calmodulin in the guinea-pig sino-atrial node

The aim of this study was to investigate possible regulation of the hyperpolarization-activated current (I-f) by cytosolic calcium in guinea-pig sino-atrial (SA) node cells. Isolated SA node cells were superfused with physiological saline solution (36degreesC) and the perforated patch voltage-clamp technique used to record I-f activated by hyperpolarizing voltage steps. A 10-min loading of SA node cells with the calcium chelator BAPTA (using 10 muM BAPTA-AM) significantly reduced the amplitude of I, at all potentials studied (69+/-8% at -80 mV, n=6). BAPTA loading also shifted the voltage of half-activation (V,) of the conductance from -83+/-2 mV in control to -93+/-2 mV in BAPTA (n=6) without significantly altering the slope of activation. The calmodulin antagonists W-7 (10 muM), calmidazolium (25 muM) and ophiobolin A (20 muM) caused similar reductions in I-f amplitude (73+/-4, 86 9 and 59+/-6% at -80 mV, n=6, 5 and 4, respectively) and shifts in V-h (11+/-3, 14+/-3 and 8+/-2 mV). In cells pre-treated with W-7, exposure to BAPTA caused no further reduction in current amplitude (n=6). I-f current amplitude was unaffected by the calmodulin dependent kinase (CaMKII) inhibitor KN-93 (1 muM) although this CaMKII inhibition did reduce L-type calcium by 48+/-19% at 0 mV (n=3). These results are consistent with a role for calcium and calmodulin in the regulation of I-f, via a mechanism that is independent of CaMKII. Alterations in intracellular calcium during the cardiac cycle may be involved in fine tuning the voltage-dependent properties of I-f and may thus determine its relative contribution to pacemaking in the SA node. (C) 2003 European Society of Cardiology. Published by Elsevier Science B.V. All rights reserved.