Acetylcholine Modulates Ifand IK(ACh)Via Different Pathways in Rabbit Sino-atrial Node Cells

The aim of the present study was to examine whether a direct G-protein pathway was necessary to explain the depression of the cardiac pacemaker current (If) by acetylcholine (ACh). Pacemaker current and spontaneous fluctuations of muscarinic K+current (IK(ACh)) were simultaneously measured in sino-atrial (SA) node cells isolated from rabbit hearts, using cell-attached and outside-out macro-patches. The effects of 1μ ACh, added to the bathing solution, were compared on Ifand on IK(ACh), known to be activated via a direct G-protein mechanism. In a cell-attached configuration where cytoplasmic substrates were present, ACh depressed Ifby a negatively directed shift of the open probability curve of 7.6±0.3 mV. ACh never induced modifications in spontaneous openings of muscarinic K+channels. These results indicate that ACh added to the external solution is unable to modulate IK(ACh)recorded in the membrane-delimited mode, via a G-protein pathway. The ACh depressing effect on Ifis related to changes in second messenger activity. In outside-out conditions, with guanosine triphosphate (GTP) added to the pipette solution, ACh increased the variance of IK(ACh)fluctuations from 1.66±0.50 pA2to 6.60±2.05 pA2(at −120 mV), indicating direct G-protein action. ACh had no effect on If. It is concluded that in SA node cells, the regulation of Ifby muscarinic receptors does not involve a direct G-protein pathway. ACh depresses Ifby a mechanism that probably implicates reduction of intracellular cAMP production.