Voltage-Activated Currents in Somatic Muscle of the Nematode Parasite Ascaris Suum Available to Purchase

Voltage-activated currents in cell bodies of the somatic muscle cells of Ascaris suum were studied using a two-microelectrode voltage-clamp technique. Cells recorded from had resting membrane potentials around −35 mV and had input conductances in the range 1–10 μS.

In cells bathed in artificial perienteric fluid, depolarizing steps from a holding potential of −35 mV elicited outward currents at a threshold of − 15 mV. These currents had inwardly directed inflections on the rising phase, suggesting the presence of more than one current. Hyperpolarizing steps did not activate current.

Tetraethylammonium (TEA⁺, 69 mmol l⁻¹) blocked the outward currents and allowed a voltage-dependent inactivating Ca²⁺ current to be observed. The peak current-voltage relationship was U-shaped with a threshold around −15 mV and peak at +5 mV. The reversal potential of the Ca²⁺ current was estimated by extrapolation to be +45 mV.

The permeability of the voltage-activated outward currents was studied by examining reversal potentials of tail currents. The reversal potentials were linearly dependent on the logarithm of the extracellular potassium concentration if extracellular [K⁺] was greater than 10 mmol 1⁻¹. The Na⁺/K⁺ permeability ratio of the currents was 0.04.

Inactivation, seen as a decline following the peak of the K⁺ current, was produced by maintained depolarization. The recovery from inactivation was complex and could be described by the sum of two exponentials with time constants of 0.67 s and 20.1 s. Steadystate inactivation of the K⁺ currents was observed at a range of holding potentials. Only a proportion (34%) of the total K⁺ current was inactivated by holding potentials more positive than −20 mV.

Extracellular application of 5 mmol l⁻¹ 4-aminopyridine (4-AP) selectively abolished an early fast component of the K⁺ current (the peak). The 4-AP-sensitive current decayed quickly with a time constant of around 10 ms; a Boltzmann fit to its activation curve had a half-maximal activation voltage of +14 mV and a ‘slope’ of 10.5mV. The 4-AP-resistant current decayed with a time constant of around Is; a Boltzmann fit to its activation curve had a half-maximal activation voltage of +29 mV and a ‘slope’ of 12 mV.

Depolarization activates a Ca²⁺ current and two K⁺ currents: the K⁺ currents were separated into lower-threshold, fast-inactivating (I_a-like) and higher-threshold, slowly inactivating (I_k-like) currents.