Injection of RNA from carp retina induces the formation of a membrane potassium channel in Xenopus oocytes

Abstract

Total RNA was purified from freshly isolated retinas of adult carp and injected into oocytes of Xenopus laevis (stages 5-6). Two to six days after injection, depolarizing voltage-clamp steps evoked a slowly activated outward current as large as 3 microA. This current inactivated slowly with a single time constant (tau = 3.1 +/- 0.24 S.E.M., for Vm = +30 mV). The current was inhibited by tetraethylammonium (3.8 mM for half-maximal inhibition). In the presence of Co2+ (1 mM) or barium methanesulfonate (40 mM), the current-voltage relationship shifted to slightly more depolarized values (5-10 mV); the maximal value of the current that was sensitive to Co2+ or Ba2+ treatments was only a small fraction (about 10%) of the TEA-sensitive current, and its current-voltage relationship was similar to that for uninjected oocytes. The reversal potential of the membrane current was studied with [K+]0 of 1-77 mM. For [K+]0 greater than 20 mM, the reversal potential changed with a slope of 63 mV (+/- 2 mV S.E.M.) per 10-fold change in [K+]0. The conductance was induced half-maximally at 17 mV (+/- 0.9 mV S.E.M.). The depolarization required for an e-fold increase in conductance was 13 mV (+/- 0.6 mV S.E.M.). From these results, we conclude that the injection of total RNA from carp retinas induces the formation of a membrane K+ channel in Xenopus oocytes. The channel formed has many of the properties reported for the maintained outward current of goldfish horizontal and bipolar cells.