Effects of calcium and guanosine-3',5'-cyclic-monophosphoric acid on receptor potentials of toad rods.

Membrane potential was recorded intracellularly from rod outer segments in the isolated retina of Bufo marinus. The composition of the solution flowing over the exposed receptor side of the retina was changed rapidly and ionophoretic injections were made into single rod outer segments. Lowering the external Ca2+ concentration, adding 3-isobutyl-1-methylxanthine (IBMX) to the bath, or ionophoretic injections of guanosine-3':5'-cyclic monophosphoric acid (cGMP) lead to depolarization of the rod membrane and an increase in the maximum peak amplitude of the receptor potential. Lowering the Ca2+ concentration or adding IBMX to the bath shifted the curve of response amplitude vs. long intensity of stimulus and sigma (intensity at half saturation) towards dimmer intensities. The ionophoresis of cGMP did not shift the position of the curve along the log stimulus intensity axis. Changing the external concentration of Ca or IBMX changes the form of the response amplitude vs. log intensity curve. This is most obvious at dim intensities. The amplitude of receptor potentials elicited by dim light was decreased by lowering Ca but increased by IBMX. The kinetics of receptor potentials were affected differently by solutions with a low Ca2+ concentration and those containing IBMX. The recovery phase of the receptor potential was slightly accelerated in the low Ca solution, whereas it was greatly slowed in the IBMX-containing solution. The ionophoretic injection of cGMP also slowed the recovery of the receptor potential. The data presented do not support the proposal that cGMP alone maintains membrane voltage in the dark and that the light-induced hydrolysis of cGMP results in the light-induced decrease in membrane conductance underlying the receptor potential. It is more likely that both Ca2+ and cGMP interact in the modulation of membrane voltage, perhaps as interrelated messengers.