Photoreceptor cells with unusual functional properties on the ventral nerve of Limulus

Normal photoreceptor cells on the ventral nerve of Limulus respond to a mod erately intense flash with a large receptor potential or current. Occasiona lly, cells are found in which the same flash evokes only a small receptor p otential or current. Our investigations reveal physiological reasons for th e poor light sensitivity in these "unusual cells." In unusual cells prolong ed illumination with intense light evokes a step-like inward current with a n amplitude of some nanoamperes, but without a large transient peak. The cu rrent appears to be summed up of single photon responses with amplitudes sm aller than about 50 pA. Their time course is similar to that of small singl e photon responses forming the so-called macroscopic C-1 component in norma l cells. The macroscopic current evoked by an intense flash has slow activa tion and deactivation kinetics and reaches a saturated amplitude of about 4 -5 nanoamperes. The light-intensity dependence of the current evoked by fla shes or by prolonged illumination has a slope of about 1 in log-log plots. The decay kinetics of the current is similar to that of the C-1 component m easured in normal cells after the block of the C-2 component. Occasionally, the step-like current is superposed by large standard bumps. These bumps a re blocked by the Ca2+-ATPase inhibitor cyclopiazonic acid, while the susta ined inward current persists. We conclude that in unusual cells the light-a ctivated current is identical to the C-1 component of normal cells. The pho spholipase C pathway that in normal cells presumably gives rise to the C-2 component functions only with a low efficiency in unusual cells.