On the evaluation of photoreceptor properties by micro-fluorimetric measurements of fluorochrome diffusion

By use of the microfluorimetric technique it is possible to study the diffusion of the fluorochrome di-dansylcystine (DDC) within isolated frog rod outer segments (ros) which are immobilysed in agarose gel. For this purpose, by a short hypotonic shock a leak is applied to one end of the ros. By this open end the DDC enters the rod and migrates through the whole outer segment. Following the propagation of the fluorescence boundary with time the cytoplasmatic diffusion constant can be determined if a chromatographic model is used to allow for the considerable binding of DDC to the inner membrane surface. With a binding constant K = 5 . 10(-4) cm the cytoplasmatic diffusion constant was found to be D = 1.3 . 10(-6) cm2/s whereas Dg = 2 . 10(-6 cm2/s and Dr = 3.5 . 10(-6) cm2/s were found in agarose gel or ringer solution, respectively. Using the mobility reduction factor given by D/Dr approximately equal to 0.4 to calculate the cytoplasmatic conductivity an inner resistance per length of 1.7 M omega/mu could be calculated for a frog rod which is in good agreement with corresponding data obtained from electrophysiological measurements.