Spectral and polarization sensitivity of photocurrents of amphibian rods in the visible and ultraviolet

Abstract

Photocurrents from isolated rods of adults and sub-adults of three species of amphibians, Rana pipiens, Ambystoma tigrinum, and Xenopus laevis, were measured with suction pipette electrodes. The intensity for a half-maximal response was 0.91 ± 0.48 photons μm⁻² flash⁻¹ (mean ± s.d., 10-ms flashes) for Rana, 0.92 ± 0.44 for Ambystoma, and 6.14 ± 1.33 for Xenopus. The mean number of photoisomerizations at half-saturation was 22 ± 12 for Rana, 50 ± 24 for Ambystoma, and 221 ± 48 for Xenopus. The photocurrent per photoisomerization is several times smaller in Xenopus rods than in the other two species. Spectral sensitivity was measured from 277–737 nm with light polarized both parallel and perpendicular to the planes of the membrane disks. Dichroism fell in the near UV and was absent in the region of absorption by tryptophan and tyrosine. Maximum sensitivity of Rana was at 503.9 ± 2.6 nm (n = 86), and of Ambystoma, 505.8 ± 1.8 nm (n = 24). Animals from these same batches that were sampled by HPLC had no 3-dehydroretinal (retinal₂). Xenopus containing about 94% retinal₂ and 6% retinal₁ had λ_max at 519.3 ± 2.7 nm (n = 11). Spectral position of the β-band, estimated by the method of Stavenga et al. (1993), appears to be at longer wavelengths in amphibian photoreceptors than in other vertebrates. Fits of log sensitivity to a normalized-frequency template that tracks the long-wavelength tail of the α-band (Lamb, 1995) show that the rod pigments of Rana and Ambystoma are slightly narrower than those found in the photoreceptors of fish and mammals.