[ATP]i in Limulus photoreceptors: no correlation with responsiveness or discrete event rate

Firefly luciferin-luciferase was microinjected into single, intact photoreceptor cells of Limulus ventral eyes. In addition, total ATP content of whole end organs was measured. An attempt was made to correlate intracellular ATP levels with either the frequency of discrete events or responsiveness to light. Luciferin-luciferase luminescence emitted from a single photoreceptor cell increased after an injection of ATP or P[NH]P and decreased after injection of apyrase. We conclude that the luciferin-luciferase luminescence is monitoring increases as well as decreases in intracellular ATP concentration (ATPi). The ATPi was between 10(-3) and 10(-4) M. Bathing ventral eyes in vanadate or fluoride increased ATP content of whole end organs, increased ATPi in single photoreceptors, and increased the frequency of discrete events. However, apyrase decreased ATPi but did not affect the frequency of discrete events or responsiveness to light until ATPi was dramatically reduced. We conclude that changes in ATP levels are not strictly correlated with changes in responsiveness to light or to changes in the frequency of discrete events.