Dual role for potassium in Balanus photoreceptor: antagonist of calcium and suppression of light‐induced current.

Publisher Website
Google Scholar
Abstract
The mechanism of reduction and final abolition of the depolarizing receptor potential of B. eburneus photoreceptors in K+-free saline was examined with electro-physiological techniques including voltage-clamp and ion specific electrodes. An extended exposure to K+-free saline reduces the transient peak and the steady phases of the depolarizing receptor potential by approximately equal amounts. The process can be reversed in normal saline although the wave form of the response is often more rectangular upon recovery. Restoration of K+ induces a transient hyperpolarization of the resting membrane for several minutes. The depolarizing receptor potential can also be restored in K+-free solution by reducing Ca2+ concentration. This saline depolarizes the resting membrane, and the wave form of the depolarizing receptor potential assumes a rectangular configuration. Voltage-clamp experiments revealed that an extended exposure to K+-free saline produced an extreme reduction of the inward light-induced current (LIC), but no detectable change in the membrane potential at which the current reverses sign. Membrane conductance in darkness showed little change. Reduction of the Ca2+ concentration from 20 to 0.2 mM in K+-free restored the current and produced a negative 8-10 mV shift in the zero current potential. There was also a significant decrease in membrane conductance in darkness. Current-voltage relations of the membrane in K+-free, low Ca2+, or K+-free low Ca2+ salines were somewhat dependent upon the order the salines were presented. Low Ca2+ saline (0.2 mM) by itself produced a -5 mV shift in the zero-current potential. Removing K+ in low Ca2+ produced an additional shift (-5 mV) in the zero-current potential. Intracellular Na+ and K+ activities .**GRAPHIC**. were monitored with Na+-sensitive glass and K+-sensitive liquid ion-exchanger electrodes in these salines. Removal of K+ from the saline was associated with a large increase in .**GRAPHIC**. and a decrease in .**GRAPHIC**. over 30 min. Reduction of Ca2+ had little effect on .**GRAPHIC**. or .**GRAPHIC**. over the same time span, but reduction of Ca2+ in K+-free facilitated the changes observed in K+-free solutions. Increasing the K+ in low Ca2+ saline can suppress the inward light induced current [LIC] but is without effect when a normal amount of Ca2+ is present. The relation between LIC and K+ concentration appears to be linear and the relations are parallel in different Ca2+ concentrations. Some reduction of the depolarizing receptor potential in K+-free saline is attributable to a reduced driving force for the light-induced current due to an increase in .**GRAPHIC**. The major effect of K+-free saline on this current appears to originate at a membrane site associated with Ca2+. In the absence of K+, Ca2+ can almost totally suppress the LIC, while a normal amount of K+ antagonizes Ca2+ suppression at this site. At low Ca2+ concentrations and high K+ concentration, K+ ions can also suppress the LIC. Thus, both K+ and Ca2+ exert control on the Na+ current in this receptor and thereby can significantly modify the depolarizing receptor potential. This mechanism is probably involved in adaptation of the photoreceptor during light stimulation.

This publication has 2 references indexed in Scilit: