Acetylcholine modulates I(f) and 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 (I(f)) 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 1 microM ACh, added to the bathing solution, were compared on I(f) and 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 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 IK(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 IK(ACh) fluctuations from 1.66 +/- 0.50 pA2 to 6.60 +/- 2.05 pA2 (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.
Renaudon
B
B
Laboratoire de Physiologie Générale, URA CNRS 1869, Faculté des Sciences, Poitiers, France.
Bois
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P
Bescond
J
J
Lenfant
J
J
D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D017609:Barium Compounds / Q000494:pharmacology; D002712:Chlorides / Q000494:pharmacology; D000242:Cyclic AMP / Q000502:physiology; D019204:GTP-Binding Proteins / Q000502:physiology*; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000502:physiology; D011817:Rabbits; D011976:Receptors, Muscarinic; D012849:Sinoatrial Node / Q000166:cytology / Q000187:drug effects* / Q000502:physiology*
D017609:Barium Compounds; D002712:Chlorides; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D011976:Receptors, Muscarinic; C024986:barium chloride; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D000109:Acetylcholine