A patch-clamp study of effects of dihydroavermectin on Ascaris muscle.

1. Effects of bath-application of the anti-parasitic agent 22,23-dihydroavermectin B1a (DHAVM 20 fM to 0.2 microM) on outside-out and inside-out patches from collagenase-treated muscle membranes of Ascaris suum were examined. 2. DHAVM was tested as a gamma-aminobutyric acid (GABA) agonist by application to outside-out patches. DHAVM failed to open characteristic GABA channels even when high concentrations (20 nM to 0.2 microM) were used. These high concentrations of DHAVM also failed to potentiate opening of channels activated by application of 3-4 microM GABA; instead they acted to depress GABA-activated channel currents by reducing mean conductances and P open. 3. In outside-out patches, low concentrations of DHAVM (1 pM to 100 pM) produced opening of 0-13 channels, the number depending on the patch. The progressive opening of channels gave rise to a 'Staircase' effect. The conductance of these channels was 9-15 pS and open times were long (greater than 100 mS). Ion substitution experiments showed these channels to be permeable to Cl. The channels were not blocked by 20 microM picrotoxin. There was a long delay (greater than 15 s) between DHAVM application and channel opening; this delay and lipophilic nature of DHAVM suggested a site of action in the lipid phase of the membrane. 4. The effect of bath-application of low concentrations of DHAVM on inside-out patches was investigated under low-Ca conditions (to avoid Ca-activated Cl channels). DHAVM (20 fM to 1 nM) did not produce opening of Cl channels but produced opening after a long delay (mean 2.5 minutes) of noisy cation-selective channels which had conductances of 5-30 pS. 5. The actions of DHAVM are discussed.