Response univariance in bull-frog rods with two visual pigments.

Rods in the bull-frog retina contain varying proportions of rhodopsin (lambda max = 502 nm) and porphyropsin (lambda max = 527 nm) in a dorso-ventral gradient from the porphyropsin-rich dorsal rim to the virtually pure rhodopsin fields of the central and ventral retina. We investigated if quantal excitations in the same rod are different depending on whether they are initiated by isomerization of a rhodopsin or a porphyropsin molecule. Current photoresponses were recorded from dark-adapted rods by sucking the outer segment into a recording pipette. The relation between pigment composition and spectral sensitivity was established by comparison with microspectrophotometrically measured absorbance spectra of rods from the same neighbourhood. Rods with suitable porphyropsin: rhodopsin mixtures (ideally between 1:4 and 1:2) were stimulated with flashes of red (608 nm) and blue (465 nm) light, whereby the red light will isomerize porphyropsin much more often than rhodopsin, and the reverse will be true of the blue light. The amplitude and shape of the single-photon response were found to be identical for the "red" and "blue" flash series to within measurement error (ca 10%). This indicates that the quantal responses initiated by the two pigments are identical.