Acetylcholine increases intracellular sodium activity in sheep cardiac Purkinje fibers.

The action of acetylcholine (ACh) on intracellular sodium activity (alpha iNa) was studied in sheep Purkinje fibers by means of a Na+-selective microelectrode technique while transmembrane potentials and contractile force were simultaneously recorded. In quiescent fibers, 10(-4) to 10(-5) M ACh shifted the resting potential to less negative values and increased alpha iNa from 5.57 +/- 0.21 to 6.45 +/- 0.35 mM (+15.8%, P less than 0.005). In other experiments, ACh induced a depolarization that initiated spontaneous activity. In fibers driven at 60 beats/min, ACh prolonged the action potential, increased alpha iNa from 7.98 +/- 0.15 to 9.36 +/- 0.3 mM (+17.29%, P less than 0.005), and increased contractile force. Norepinephrine (10(-5) to 10(-6) M) increased contractile force and decreased alpha iNa, but in its presence ACh still increased force and alpha iNa and vice versa. Strophanthidin (10(-4) M) increased alpha iNa, and 3 x 10(-6) M propranolol and 10(-6) M atropine decreased alpha iNa. Both strophanthidin and atropine (but not propranolol) prevented the increase in alpha iNa by ACh. It is concluded that the ACh increases alpha iNa and contractile force through the inhibition of the sodium pump and that these actions are due to the activation of the muscarinic receptor and not to endogenously released norepinephrine.