Photoreceptor sensitivity as a function of rhodopsin content in the isolated bullfrog retina.

Using isolated bullfrog retina treated with both aspartate and Ba2+, the relation between the threshold of fast PIII response and rhodopsin content was examined. The change caused by a decrease in rhodopsin in response threshold from the original dark-adapted level could be described by the Michaelis-Menten equation. However, the data points deviated slightly from the theoretical curve when the rhodopsin content was below 30%. After reducing Ca2+ concentration in the bathing solution from the normal (1.0 mM) to 0.01 mM, the threshold change of the response was more pronounced than that observed in the normal solution. In the presence of 0.1 mM papaverine, which is known as an inhibitor of phosphodiesterase, the threshold change increased in a similar way to that observed in 0.01 mM Ca2+ solution. However, isobutyl methylxanthine (IBMX), also an inhibitor of phosphodiesterase, showed no such effect. On the basis of the present findings, the transduction mechanism in the photoreceptors was discussed.