Acetylcholine modulates I-f and I-K(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 (I-f) by acetylcholine (ACh). Pacemaker current and spontaneous fluctuations of muscarinic K+ current (I-K(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 1 mu M ACh, added to the bathing solution, were compared on I-f and on I-K(ACh), known to be activated via a direct G-protein mechanism. In a cell-attached configuration where cytoplasmic substrates were present, ACh depressed I-f by 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 I-K(ACh) recorded in the membrane-delimited mode, via a G-protein pathway. The ACh depressing effect on I-f is 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 I-K(ACh) fluctuations from 1.66 +/- 0.50 pA(2) to 6.60 +/- 2.05 pA(2) (at -120 mV), indicating direct G-protein action. ACh had no effect on I-f. It is concluded that in SA node cells, the regulation of I-f by muscarinic receptors does not involve a direct G-protein pathway. ACh depresses I-f by a mechanism that probably implicates reduction of intracellular cAMP production. (C) 1997 Academic Press Limited.