EFFECTS OF ACETYLCHOLINE ON THE NA-K+ PUMP CURRENT IN GUINEA-PIG VENTRICULAR MYOCYTES()

1. The whole-cell patch clamp technique was used to study the effects of acetylcholine (ACh) on Na+-K+ pump current (I-p) in acutely isolate d guinea-pig ventricular myocytes. Studies were performed in the absen ce and presence of the beta-agonist isoprenaline (Iso). 2. ACh had no effect on I-p at low or high [Ca2+](i) at any voltage in the absence o f Iso. Iso alone inhibited I-p at low [Ca2+](i) and shifted the I-p-V relationship at high [Ca2+](i) in a negative direction. Addition of 1 mu M ACh reversed these effects of Iso. K-0.5 for the effects of ACh w as about 16 nM, regardless of [Ca2+](i). 3. The actions of ACh on the heart are usually mediated via muscarinic receptors. Atropine, a musca rinic antagonist, blocked the effects of ACh on I-p in the presence of Iso, suggesting that these effects are also mediated by muscarinic re ceptors. 4. Muscarinic receptors are usually coupled to a G(i) protein , leading to inhibition of adenylyl cyclase and a reduction of cAMP le vels. We have shown previously that basal levels of cAMP are very low in guinea-pig ventricular myocytes, and that a membrane-permeant cAMP analogue, chlorophenylthio-cAMP (CPTcAMP), mimics the effects of Iso. ACh did not reverse the effects of CPTcAMP, supporting the hypothesis that the effects of ACh on I-p are also mediated via inhibition of ade nylyl cyclase. 5. The present results suggest that it high level of pa rasympathetic tone alone does not affect the activity of ventricular N a+-K+ pumps. However, if sympathetic tone is high, then muscarinic sti mulation can reciprocally modulate Na+-K+ pump activity.