Time- and voltage-dependent ionic components of the rod response.

1. The electrical properties of individual rods, physically isolated from the rod network, were measured in terms of the time course of response and voltage-current relations derived from current steps. Properties were measured in normal and altered bathing media designed to reveal the ionic basis for the time and voltage dependent properties of the rod response. 2. In normal media the rod membrane was strongly outward-rectifying with slope resistance near 100 M omega when hyperpolarized, but near 10 M omega when depolarized from a typical ambient level near 35 mV. The membrane become inward rectifying for hyperpolarizations beyond -95 mV, with slope resistance near 70 M omega. 3. The normal hyperpolarizing overshoot associated with the rod response was strongly potential dependent: the overshoot in response to a current step disappeared when the membrane was first depolarized or hyperpolarized by more than about 10 mV from the -35 mV ambient potential level. The decay from overshoot elicited either by current or light, could be approximated with a first order time constant of about 150 msec. 4. In the absence of sodium the peak-plateau sequence remained intact. Membrane resistance increased during transition to the plateau. The plateau became more hyperpolarized than the early phase during responses beyond -75 mV. These results indicate a time- and voltage-dependent conductance other than sodium contributes to the peak-plateau response, probably potassium. 5. Outward rectification was greatly reduced in the presence of 15 mM-TEA, suggesting that it is mediated by potassium activation. 6. Inward rectification, and the associated transients near -95 mV were eliminated in the presence of 2 mM-caesium, suggesting that potassium conductance contributes to the time and voltage dependent inward rectification.