Influence of calcium on the Limulus photoreceptor potential

Abstract

Evidence is presented demonstrating that the magnitude of the initial component (C 1) of the Limulus photoreceptor potential elicited by moderately intense light flashes is both sodium and calcium dependent. The evidence is: (1) the reversal potential of C 1 of the lateral eye retinular cell and ventral eye cell receptor potential is appreciably more positive than the reversal potential of C 2; (2) exposure of both types of photoreceptors to environments deficient in Na + rapidly reduces and abolishes C 2 of the receptor potential whereas C 1 persists with reduced magnitude and rate of rise; (3) exposure of ventral photoreceptors to environments deficient in Ca 2+ reduces the magnitude and rate of rise of C 1 and enhances both the magnitude and duration of C 2; (4) exposure of ventral photoreceptors to calcium enriched environments rapidly increases the magnitude of C 1 and curtails the duration of C 2; (5) the increase in magnitude of C 1 for a ten-fold increase in the external CaCl 2 concentration (10 to 100 mM) is 21 mV; and (6) verapamil, an organic calcuim current blocking agent, reduces the magnitude of C 1 of the ventral photoreceptor potential and enhances the magnitude and duration of C 2. This evidence, although indirect, strongly suggests that Ca 2+ entry contributes to the production of C 1 of the Limulus ventral photoreceptor potential as well as C 1 of the Limulus lateral eye retinular cell receptor potential.