Visual pigments and vitamins A in the adult bullfrog

Adult bullfrogs, Rana catesbeiana, have a pair of scotopic visual pigments: a rhodopsin and a porphyropsin. Partial bleaching experiments performed on rhodopsin and porphyropsin rich retinal extracts from the bullfrog revealed that the absorbance maxima (λ max) are 499 and 522 nm respectively. The rhodopsin absorbance maximum of 499 nm is slightly lower than the values (i.e. 500–503 nm) previously reported. After subjecting groups of bullfrogs to different light-temperature environments for 40 days, some frogs, when considering the retina as a whole, had predominately rhodopsin, whereas others possessed up to 50% porphyropsin because of the presence of porphyropsin in the dorsal part of the retina. The visual pigment composition in the dorsal (superior one third) part of the retina changed from predominately porphyropsin to predominately rhodopsin in response to specific light and temperature regimes. The ventral part of the retina of these frogs always remained rhodopsin rich. The vitamin A composition (i.e. the relative proportions of vitamin A 2 to vitamin A 1) of the associating pigment epithelium was, in most cases, similar to the visual pigment composition (i.e. the relative proportions of porphyropsin to rhodopsin) in both the dorsal and ventral parts of the retina. However, exceptions were found in dorsal retinas of frogs held in constant darkness (for 40 days) and in dorsal retinas which become rhodopsin rich in response to specific light and temperature treatments. In both instances, the vitamin A 2 proportions in the pigment epithelium were higher than the porphyropsin proportions in the retinas. The liver of all frogs had predominately vitamin A 1. This article provides the first report on a successful attempt to induce changes in the visual pigment and vitamin A compositions in the eyes of an adult amphibian using specific combinations of light and temperatures.