Lanthanum reduces the excitation efficiency in fly photoreceptors

Lanthanum (La3+), a known inhibitor of Ca2+ binding proteins, was applied to the extracellular space of fly retina. Shot noise analysis indicated that a combination of intense light and La3+ caused a large (down to zero) reduction in the rate of occurrence of the quantal responses to single photons (quantum bumps) which sum to produce the photoreceptor potential. Light in the presence of La3+ also increased the effective bump duration. These effects are very similar to the effects of the mutations trp of Drosophila and nss of Lucilia flies on the quantum bump rate and duration. La3+ applied to the nss mutant caused only a small reduction in the bump rate, suggesting that La3+ may affect the nss gene product which is deficient in the mutant. The close similarity in the properties of the receptor potential of the La(3+)-treated photoreceptor of the wild type and of the nss mutant together with existing evidence for the highly reduced intracellular Ca2+ ([Ca2+]i) level in nss photoreceptors suggest that both La3+ and the mutation cause a severe reduction in [Ca2+]i. This effect may arise from an inhibition of a Ca2+ transporter protein located in the surface membrane that normally replenishes Ca2+ pools in the photoreceptors, a process essential for light excitation.