Titration of sodium channel sites for hydrogen ion block and sensitized photochemical modification of lobster axons.

The pH dependence for sensitized photochemical block of sodium channels in lobster giant axons was determined and compared with direct channel block by protons. Isolated axons were studied in a double sucrose gap voltage clamp arrangement and the pH of the external bath was varied over the range 4.1--11.0. Irreversible photochemical block was achieved by illumination with visible light in the presence of eosin Y or acriding orange. The rate constant for photochemical block of sodium channels was depressed at both high and low pH relative to that at neutral pH, revealing the existence of two receptors involved in the process with pK values of 4.8 and 10.4. A direct reversible channel-blocking receptor titrates with a pK of 4.8, the same as one of the receptors involved in the photochemical block, and senses about 9% of the electric field as determined by a Woodhull analysis. Lowering the pH from 8.2 to 4.6 shifted the sodium conductance versus voltage relation in the depolarizing direction. It is proposed as a hypothesis that the low and high pK receptors are histidine imidazole and primary amino groups, photooxidation of which leads to channel block via cross-linking of channel proteins.