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

Total RNA was purified from freshly isolated retinas of adult carp and injected into oocytes of Xenopus laevis (stage 5–6). Two to six days after injection, depolarizing voltage-clamp steps evoked a slowly activated outward currents as large as 3 μA. This current inactivated slowly with a single time constant (τ= 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 > 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 properties reported for the maintained outward current of goldfish horizontal and bipolar cells.