INTERCONVERTING GATING MODES OF A NONSELECTIVE CATION CHANNEL FROM THE TAPEWORM ECHINOCOCCUS-GRANULOSUS RECONSTITUTED ON PLANAR LIPID BILAYERS

A 107-pS (symmetrical 150 mM KCl), nonselective cation channel was rec onstituted from a microsomal membrane fraction of the larval stage of the tapeworm Echinococcus granulosus. Most of the time, it displayed a high open probability (>0.95) irrespective of either the applied volt age, Ca2+, Ba2+, or tetraethylammonium concentration. Nevertheless, in contrast with this ''leaklike'' behavior, less frequently this ''all- the-time-open'' channel reversibly entered two different kinetic modes . One of them was characterized by lower P-o values and some voltage s ensitivity (V-1/2 congruent to 129 mV, and an equilibrium constant for channel closing changing e-fold per 63-mV change) the kinetic analysi s revealing that it resulted from the appearance of voltage-sensitivit y in the mean closed times and a sixfold increase in the equilibrium c onstant for channel closing at 0 mV. The other mode was characterized by a very fast open-close activity leading to poorly resolved current levels and a P-o around 0.6-0.7 which, occasionally and in a voltage-s ensitive manner, entered a long-lived nonconducting state. However, th e rare nature of these mode-shifting transitions precluded a more deta iled analysis of their kinetics. The conductive properties of the chan nel were not affected by these switches. Model gating alone does not s eem to ensure any physiological role of this channel and, instead, som e other channel changes must occur if this phenomenon were to be of re gulatory importance in vivo. Thus, mode-shifting might constitute an a lternative target for channel activity modulation also in tapeworms.