Increased Amplification and Phosphodiesterase Activity in Rods of Vitamin A–deprived Xenopus

Abstract

Abstract: : Purpose: To characterize the sensitivity, amplification, response recovery kinetics and phosphodiesterase activity of rods in Vitamin A–deprived Xenopus. Methods: Xenopus laevis tadpoles were raised on a custom diet lacking Vitamin A. Rod photocurrent sensitivity was measured with suction electrode techniques. Rod amplification was determined with the Pugh and Lamb model (1993). Rhodopsin content in individual rods was measured by microspectrophotometry. Sustained phosphodiesterase activity was inferred from membrane current changes during exposure to IBMX solutions (IBMX jump method). Results: In Vitamin A deficient rods, rhodopsin concentrations were about 10–fold less than in normal rods. Also, their photocurrent sensitivity was lower by more than 300–fold but the magnitude of the circulating currents were not significantly different. After factoring out the loss in quantum catch, we estimated that single photon response amplitudes of Vitamin A–deficient rods were about 20–fold smaller than in normal Xenopus rods. The photoresponses of these rods were faster than those of normal rods. The faster onset was consistent with a 10–fold increase in the amplification of the activation cascade and faster recovery kinetics is consistent with a 10–fold increase in sustained phosphodiesterase activity as measured using the IBMX jump method. Following reconstitution of the pigment content by incubating with 11–cis retinal in solution, rod sensitivity improved to normal levels and the recovery kinetics slowed down considerably, however, the surprisingly high amplification remained unaltered. Also, phosphodiesterase activity varied linearly with pigment content. Conclusions: Our results are consistent with constitutive activation of the phototransduction pathway by chromophore–free opsin. We find a linear relationship between phosphodiesterase activity and pigment content similar to that found for bleach–adapted rods (Fain and Cornwall, 1994). Compensatory mechanisms that increase amplification and circulating currents appear to counteract the desensitizing effects of a limited supply of 11–cis retinal.

Keywords: photoreceptors • electrophysiology: non-clinical • vitamin A deficiency