Pressure injection of 3',5'-cyclic GMP into solitary rod photoreceptors of the tiger salamander.
The outer segments of solitary rods of large neotenic Ambystoma tigrinum were impaled with double-barrel micropipettes. One barrel was used to record membrane voltage and the other barrel was used both to pass voltage-clamp current and to pressure-inject cyclic GMP. Injections of cyclic GMP induced inward currents in normal cells that were bathed in Ringer's solution and studied at the dark, resting voltage. These cyclic GMP-induced currents reversed sign for voltages 25-40 mV more positive than the dark, resting voltage; however, light-induced currents did not reverse sign for voltages as depolarized as 55 mV more positive than the dark, resting voltage. The responses to light became slow and very prolonged in cells that had been injected with large amounts of cyclic GMP. In these over-injected cells, both light-induced and cyclic GMP-induced currents increased in amplitude but did not reverse sign, for voltages 35-45 mV more positive than the dark, resting voltage. We conclude that light causes the simultaneous change in concentration of an intracellular messenger for excitation in addition to cyclic GMP, or that a change in the cytosolic concentration of cyclic GMP is not a step in the excitatory process.