RESPONSE UNIVARIANCE IN BULLFROG RODS WITH 2 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 do rso-ventral gradient from the porphyropsin-rich dorsal rim to the virt ually pure rhodopsin fields of the central and ventral retina. We inve stigated if quantal excitations in the same rod are different dependin g on whether they are initiated by isomerization of a rhodopsin or a p orphyropsin molecule. Current photoresponses were recorded from dark-a dapted rods by sucking the outer segment into a recording pipette. The relation between pigment composition and spectral sensitivity was est ablished by comparison with microspectrophotometrically measured absor bance spectra of rods from the same neighbourhood. Rods with suitable porphyropsin: rhodopsin mixtures (ideally between 1:4 and 1:2) were st imulated with flashes of red (608 nm) and blue (465 nm) light, whereby the red light will isomerize porphyropsin much more often than rhodop sin, 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 th e ''red'' and ''blue'' flash series to within measurement error (ca 10 %). This indicates that the quantal responses initiated by the two pig ments are identical.