SINGLE-CHANNEL PROPERTIES OF CLONED CGMP-ACTIVATED CHANNELS FROM RETINAL RODS

Single-channel properties of a cloned channel activated by cyclic GMP have been analysed. The mRNA encoding for the channel was injected int o oocytes of Xenopus laevis and the current flowing through a single i onic channel activated by cGMP was studied in excised patches under vo ltage-clamp conditions. The ionic channel activated by cGMP had a sing le-channel conductance of 32 +/- 2 pS at + 120 mV and 25 +/- 4 pS at - 120 mV, and its conductance was not significantly affected by increas ing the cGMP concentration from 20 mu M to 200 mu M. The single-channe l currents in the presence of NH4+, Na+, K+, Li+ and Rb+ in the medium bathing the cytoplasmic side of the membrane at + 140 mV were 5.3, 4. 7, 3.8, 1.3 and 0.8 pA, respectively. The single-channel current in th e presence of Cs+ was less than 0.5 pA. Ca2+ and Mg2+ (both 0.5 mM) in the presence of 100 mu M cGMP did not appreciably affect the channel activity at membrane potentials more negative than -80 mV, whereas at + 100 mV they reduced the single-channel conductance by about threefol d. The ionic selectivity and the blockage by divalent cations of the n ative channel found in amphibian rods and in the cloned channel from b ovine rods are quite similar. However, the cloned channel has well-res olved openings, especially at positive membrane voltages, whereas the native channel is characterized by a continuous flickering between the open and closed state.