The Sensitivity of the Ventral Nerve Photoreceptor of Limulus Recovers after Light Adaptation in Two Phases of Dark Adaptation

T he tim e course of the recovery of the sensitivity of the Limulus ven tra l nerve pho to recep to r was m easured during dark adap tation following light ad ap tation by a bright 1 o r 5 s illum ination. The stimulus intensity / CR of a 300 îs light flash evoking a response o f c riterion am plitude (recep to r po ten tial or recep tor curren t u nder voltage clam p conditions) was used as m easure o f sensitivity. The tim e course of dark adap tation shows tw o phases w ith tim e constants in the range of 5 —9 s and 300—500 s (15 °C). O nly the first of the two phases is significantly changed w hen the ex tracel­ lular C a2+-concentration is varied. The pow er function / rR = a -I0■ tDA~b gives a good da ta fit for each o f the two phases of dark ad ap tation . In the first phase the factor a, and the exponent bx are decreased w hen the external calcium is low ered from 10 mmol/1 to 250 (jxnol/l. C onversely al and bx are increased w hen the C a2+-concentration is raised to 40 mmol/1. F o r the second phase ne ith e r a2 nor b2 is changed significantly upon the changes in calcium concen tra tion in the sam e experim ents. T he two phases of d ark adap tation reflect the behav iour o f the tw o com ponents Q and C 2 o f the electrical light response (recep to r po ten tial o r recep to r cu rren t). U n d er the conditions described here C, determ ines the size o f the light response during the first phase o f d ark ad ap tation w hereas C 2 m ainly influences the size of the response during the second phase. In terpre ta tion : T he fast first phase of dark ad ap tation is d e term ined by the change in in trace llu ­ lar C a 2 +-concentration. The slower second phase of dark ad ap tation is not prim arily calcium contro lled .