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To study the actions of Ca2+ on "early" stages of the transduction cascade, changes in cytoplasmic calcium concentration (Ca2+i) were opposed by manipulating Ca2+ fluxes across the rod outer segment membrane immediately following a bright flash. If the outer segment was exposed to 0 Ca2+/0 Na+ solution for a brief period immediately after the flash, then the period of response saturation was prolonged in comparison with that in Ringer solution. But if the exposure to 0 Ca2+/0Na+ solution instead came before or was delayed until 1 s after the flash then it had little effect. The degree of response prolongation increased with the duration of the exposure to 0 Ca2+/0 Na+ solution, revealing a time constant of 0.49 +/- 0.03 s. By the time the response begins to recover from saturation, Ca2+i seems likely to have fallen to a similar level in each case. Therefore the prolongation of the response when Ca2+i was prevented from changing immediately after the flash seems likely to reflect the abolition of actions of the usual dynamic fall in Ca2+i on an early stage in the transduction cascade at a site which is available for only a brief period after the flash. One possibility is that the observed time constant corresponds to the phosphorylation of photoisomerized rhodopsin.

Actions of Ca2+ on an Early Stage in Phototransduction Revealed by the Dynamic Fall in Ca2+ Concentration during the Bright Flash Response