Electrophysiological and pharmacological characterization of K+-currents in muscle fibres isolated from the ventral sucker of Fasciola hepatica

Fibres isolated from the ventral sucker of Fasciola hepatica were identified as muscle on the basis of their contractility, and their actin and myosin staining. They were voltage-clamped at a holding potential of -40 mV and depolarization-activated outward currents were characterized both electrophysiologically and pharmacologically. Activation was well fitted by a Boltzmann equation with a half-maximal potential of + 9 mV and a slope factor of -14.3 mV, and the kinetics of activation and deactivation were voltage-sensitive. Tail current analysis showed that the reversal potential was shifted by + 46 +/- 3 mV when E-K was increased by 52 mV, confirming that this was a K+-current with electrophysiological characteristics similar to delayed rectifier and Ca-2+-activated K+-currents in other tissues. The peak current at + 60 mV was inhibited by 76 +/- 6% by tetrapentylammonium chloride (1 mm) and by 84 +/- 7% by Ba2+ (3 mm), but was completely resistant to block by tetraethylammonium (30 mm), 3,4-diaminopyridine (100 mum) and 4-aminopyridine (10 mm). Penitrem A, a blocker of high-conductance Ca2+-activated K+-channels reduced the current at + 60 mV by 23 +/- 5%. When the effects of Ca2+- channel blocking agents were tested, the peak outward current at + 60 mV was reduced by 71 +/- 7%, by verapamil (30 mum) and by 59 +/- 4% by nimodipine (30 mum). Superfusion with BAPTA-AM (50 muM), which is hydrolysed intracellularly to release the Ca2+-buffer BAPTA, also decreased the current by 44 +/- 16%. We conclude that voltage-and Ca2+-sensitive K+-channels are expressed in this tissue, but that their pharmacology differs considerably from equivalent channels in other phyla.