Abstract
1. The lipid bilayer technique is used to characterize the biophysical and pharmacological properties of a novel, fast, cation-selective channel formed by incorporating platypus (Ornithorhynchus anatinus) venom (OaV) into lipid membranes. 2. A synthetic C-type natriuretic peptide OaCNP-39, which is identical to that present in platypus venom, mimics the conductance, kinetics, selectivity and pharmacological properties of the OaV-formed fast cation-selective channel. The N-terminal fragment containing residues 1-17, i.e. OaCNP-39(1-17), induces the channel activity. 3. The current amplitude of the TEACl-insensitive fast cation-selective channel is dependent on cytoplasmic K+, [K+]cis. The increase in the current amplitude, as a function of increasing [K+]cis, is non-linear and can be described by the Michaelis-Menten equation. At +140 mV, the values of gammamax and KS are 63.1 pS and 169 mM, respectively, whereas at 0 mV the values of gammamax and KS are 21.1 pS and 307 mM, respectively. gammamax and KS are maximal single channel conductance and concentration for half-maximal gamma, respectively. The calculated permeability ratios, PK:PRb:PNa:PCs:PLi, were 1:0.76:0.21:0.09:0.03, respectively. 4. The probability of the fast channel being open, Po, increases from 0.15 at 0 mV to 0.75 at +140 mV. In contrast, the channel frequency, Fo, decreases from 400 to 180 events per second for voltages between 0 mV and +140. The mean open time, To, increases as the bilayer is made more positive, between 0 and +140 mV. The mean values of the voltage-dependent kinetic parameters, Po, Fo, To and mean closed time (Tc), are independent of [KCl]cis between 50 and 750 mM (P > 0. 05). 5. It is proposed that some of the symptoms of envenomation by platypus venom may be caused partly by changes in cellular functions mediated via the OaCNP-39-formed fast cation-selective channel, which affects signal transduction.
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