Effects of acetylcholine on a slow voltage-activated non-selective cation current mediated by non-nicotinic receptors on isolated Ascaris muscle bags.

Isolated muscle bags from the parasitic nematode Ascaris suum were prepared by collagenase treatment and dissection. Single bags were mounted in a V-shaped plastic pipette for voltage clamp application and intra- and extracellular perfusion. With 'physiological' intra- and extracellular solutions, depolarizing voltage steps from near the normal resting membrane potential, -40 mV, produced in leak-corrected currents, a slowly activating outward current at potentials more positive than -20mV. At the end of the depolarizing pulse there was a slow inward tail current with a reversal potential near -20mV. Hyperpolarizing voltage steps produced an outward current relaxation and an outward tail current with the same reversal potential. The observations can be explained by the presence in the bag of a non-selective cation channel current, Ibcat, that activates spontaneously at the holding potential; depolarization increases opening of the channel and hyperpolarization decreases opening. Bath-applied acetylcholine in concentrations greater than 10(-7) M produced an increase in the amplitude of Ibcat. The effect of acetylcholine was not antagonized or prevented by 100 microM tubocurarine, suggesting the presence of a non-nicotinic acetylcholine receptor. Atropine (100 microM) had no detectable influence on the effect of acetylcholine but the FMRFamide peptide, PF1, in concentrations > 1 microM, reduced the amplitude of Ibcat. Ibcat was maintained when Cs+ was used to replace intra- and extracellular cations, showing that the channels were permeable to Cs+. It is concluded that the bag membrane possesses a slow voltage-activated non-selective cation channel current, Ibcat. The effect of acetylcholine in the presence of nicotinic antagonist indicates the presence of non-nicotinic acetylcholine receptors on the bag membrane. The effect of PF1 indicated the presence of PF1 receptors on the bag membrane.