ELECTROPHYSIOLOGICAL MEASUREMENT OF PHOTORECEPTOR MEMBRANE DICHROISM AND POLARIZATION SENSITIVITY IN A GRAPSID CRAB

Discrete depolarizations (bumps) were recorded from retinula cells of the shore crab Leptograpsus in response to appropriate low light intensities. The statistical analysis of bumps is consistent with the hypothesis that each bump represents the photoreceptor''s response to an absorbed photon. The photoreceptive membrane was estimated to have a quantum capture efficiency of 0.46 (.+-. 0.12 SD; n = 6). The modal polarization sensitivity value measured by bump frequency (BPS) in 11 cells was 5.0 with a range from 1.9-5.63. There is no statistically significant difference between BPS at wavelengths 621 nm and 499 nm. Polarization sensitivity (PS) measured by graded potentials is equal to the PS measured in the same cells by bumps. PS values ranged from 1.8-7.5 (n = 28) with a modal value of 5.0. Bump rates represent the direct electrophysiological measurement of the visual pigment''s selective absorption to single photons having different E-vector orientations, and therefore PS measured from bumps provides an accurate estimate of the physiological dichroic properties of the rhabdomere in vivo. The measurements of BPS show that the rhabdomere''s dichroism accounts for the observed PS values recorded in Leptograpsus.