A gadolinium and pH-sensitive hyperpolarization-activated cation current in acutely isolated single neurones from Fasciola hepatica

Property of large conductance Ca(2+)-activated K+ channels from Fasciola hepatica incorporated into planar lipid bilayer

Fasciola hepatica causes biliary epithelial hyperplasia and obstructive jaundice in humans and animals. Using a planar lipid bilayer technique, we further characterized the single channel property of large conductance K(+)-permeable channels that were previously identified from F. hepatica. The single channel conductance was 254.7±17.9 pS under a symmetrical 200/200 mM (cis/trans) KCl gradient. Open state probability (P(o)) varied from channel to channel at a given membrane potential and Ca(2+) concentration, but increased with voltage (-60 to +40 mV) and cis Ca(2+) (1-200 μM). Under a near bi-ionic condition of 200 mM [K(+)](cis)/200 mM [Na(+)](trans), the permeability ratio of K(+) to Na(+) was 5.0. Charybdotoxin (1 μM) inhibited P(o), whereas tetraethylammonium reduced the conductance (K(D)=67.8mM). Taken together, the results show that the single channel properties of the large conductance K(+)-permeable channels in F. hepatica are similar to those of large conductance Ca(2+)-activated K(+) (BK) channels in general, but distinct from typical BK channels in the extent of voltage- and Ca(2+)-dependence, as well as permeability to Na(+). This study further reveals a variant BK channel in F. hepatica that could serve as a new drug target to treat fascioliasis.

Molecular and electrophysiological characterization of nucleotide-sensitive chloride current-inducing protein of Fasciola hepatica

Nucleotide-sensitive chloride current regulating proteins (ICln's) of the chloride channels have been characterized from man and animals. An ICln of Fasciola hepatica (ICln-Fh) consisting of 231 amino acids revealed high similarities to both consensus domain of ICln's and two acidic residue-abundant patches in its C-terminus. Native ICln-Fh protein was confirmed present in F. hepatica soluble extract by immunoblotting. The recombinant ICln-Fh protein expressed in collagenase-defolliculated Xenopus oocytes induced fast rising and outward rectifying Cl- currents (I(Cln-Fh)). The recombinant ICln-Fh protein, however, did not trigger cell swelling-induced Cl- currents (I(Cl-swell)). The I(Cln-Fh) currents were significantly reduced by substituting external Cl- with gluconic acid and by externally adding cAMP. Collectively, these results suggest that ICln-Fh protein is an inducer of Cl- currents in F. hepatica.

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

Fibres isolated from the ventral sucker ofFasciola hepaticawere 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 EKwas increased by 52 mV, confirming that this was a K+-current with electrophysiological characteristics similar to delayed rectifier and Ca2+-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 μM) 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 μM) and by 59±4% by nimodipine (30 μM). Superfusion with BAPTA-AM (50 μM), 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.

Ion channels of Fasciola hepatica incorporated into planar lipid bilayers

Ion channels are important target sites of anthelmintics, but little is known about those in Fasciola hepatica. In this work, we applied a planar lipid bilayer technique to characterize the properties of single ion channels in F. hepatica. Under a 200/40 mM KCl gradient, a large conductance channel of 251 pS was observed in 18% of the membranes studied. The channel was selective to K(+) over Cl(-) with a permeability ratio of K(+) to Cl(-) (PK/PCl) of 4.9. Open state probability (Po) of the channel was less than 0.5 and dependent on voltage (-60 to approximately +40 mV) and Ca(2+) (approximately 100 microM). The other two types of single channels observed in 11 and 5% of membranes, respectively, were a K(+)-permeable channel of 80 pS (PK/PCl=4.6) and a Cl(-)-permeable channel of 64 pS (PK/PCl=0.058). Open state probability of both channels showed little voltage dependence. The results indicate that distinct single channels of 60 to approximately 251 pS are present in relative abundance and, in addition, that the planar lipid bilayer technique can be a useful tool for the study of single ion channels in F. hepatica.