Electrophysiological measurement of the number of rhodopsin molecules in single Limulus photoreceptors.

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Abstract
Two partly independent electrophysiological methods are described for measuring the number of rhodopsin molecules (R) in single ventral photoreceptors. Method I is based on measurements of the relative intensity required to elicit a quantal response and the relative intensity required to half-saturate the early receptor potential (ERP). Method 2 is based on measurementsof the absolute intensity required to elicit a quantal response. Both methods give values of R .simeq. 109. From these and other measurements, estimates are derived for the surface density of rhodopsin (8000/.mu.m2), the charge movement during the ERP per isomerized rhodopsin (20 .times. 10-21 C), and the half-time for thermal isomerization of rhodopsin (36 yr).

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