PF4, a FMRFamide-related peptide, gates low-conductance Cl − channels in Ascaris suum

Here we describe the actions of the peptide Lys-Pro-Asn-Phe-Ile-Arg-Phe-NH 2, or PF4, on inside-out membrane patches ( n=164), recorded from vesicles derived from Ascaris suum somatic muscle cells. We observed numerous, small-amplitude Cl − channels in the membrane patches. The conductance of the Cl − channels ranged from 1.09 to 7.07 pS, the open probability ( P open) ranged from 0.047±0.015 (mean±S.E.M.) at 0 μM PF4 to 0.156±0.026 at 0.1 μM PF4. The channel mean open time was more variable and prolonged at negative potentials than when the membrane patch was clamped at positive potentials: at 0.03 μM PF4, the mean open time (±S.E.M) at −80 mV was 522±333 ms; at+80 mV, it was 25±7 ms. When patches were isolated from the parent vesicle, there were no changes in channel characteristics, suggesting that the channels function without the involvement of cytoplasmic components. Similarly, the channel characteristics were not affected by the G-protein inhibitor, guanosine-5′- O-(2-thiodiphosphate), indicating that the ion channels do not require a G-protein to function. These data indicate that the PF4-activated Cl − channels function independently of intracellular signal transducers and are, therefore, directly gated by PF4.