PERSISTENT ACTIVATION OF TRANSDUCIN BY BLEACHED RHODOPSIN IN SALAMANDER RODS

The hydrolysis-resistant GTP analogue GTP-gamma-S was introduced into rods isolated from the retina of the salamander Ambystoma tigrinum to study the origin of the persistent excitation induced by intense bleac hing illumination. Dialysis of a dark-adapted rod with a whole-cell pa tch pipette containing 2 mM GTP-gamma-S resulted in a gradual decrease in circulating current. If the rod was first bleached and its sensiti vity allowed to stabilize for at least 30 min, then dialysis with GTP- gamma-S produced a much faster current decay. The circulating current could be restored by superfusion with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, suggesting that the decay in current orig inated from persistent excitation of the phosphodiesterase by transduc in bound to GTP-gamma-S. We conclude that the persistent excitation wh ich follows bleaching is likely to involve the GTP-binding protein tra nsducin, which mediates the normal photoresponse. This observation sug gests that a form of rhodopsin which persists long after bleaching can activate transducin much as does photoisomerized rhodopsin, although with considerably lower gain.