A Component of Platypus (Ornithorhynchus anatinus) Venom Forms Slow-Kinetic Cation Channels
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The lipid bilayer technique is used to examine the biophysical properties of anion and cation channels frequently formed by platypus (Ornithorhynchus anatinus) venom (OaV). The OaV-formed anion channel in 250/50 mm KCl cis/trans has a maximum conductance of 857 ± 23 pS (n= 5) in 250/50 mm KCl cis/trans. The current-voltage relationship of this channel shows strong inward rectification. The channel activity undergoes time-dependent inactivation that can be removed by depolarizing voltage steps more positive than the reversal potential for chloride, ECl, (+40 mV).
The reversal potential of the OaV-formed slow current activity in 250/50 mm KCl cis/trans is close to the potassium equilibrium potential (EK) of −40 mV. The conductance values for the slow channel are 22.5 ± 2.6 pS and 41.38 ± 4.2 pS in 250/50 and 750/50 mmcis/trans, respectively. The gating kinetics of the slow ion channels are voltage-dependent. The channel open probability (Po) is between 0.1 and 0.8 at potentials between 0 and +140 mV. The channel frequency (Fo) increases with depolarizing voltages between 0 and +140 mV, whereas mean open time (To) and mean closed time (Tc) decrease. Ion substitution experiments of the cis solution show that the channel has conductance values of 21.47 ± 2.3 and 0.53 ± 0.1 pS in 250 mm KCl and choline Cl, respectively. The amplitude of the single channel current is dependent on [K+]cis and the current reversal potential (Erev) responds to increases in [K+]cis by shifting to more negative voltages. The increase in current amplitude as a function of increasing [K+]cis can be best described by a third order polynomial fit. At +140 mV, the values of the maximal single channel conductance (γmax) and the concentration for half maximal γ (Ks) are 38.6 pS and 380 mm and decline to 15.76 pS and 250 mm at 0 mV, respectively. The ion selectivity of the channel to K+, Na+, Cs+ and choline+ was determined in ion substitution experiments. The permeability values for PK+:PNa+:PCs+:Pcholine+ were 1:1:0.63:0.089, respectively. On the other hand, the activity of the slow channel was eliminated (Fig. 7B). The slow channel was reversibly inhibited by [TEA+]trans and the half-maximal inhibitory concentration (Ki) was ∼48 mm.
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