The calcium feedback signal in the phototransduction cascade of vertebrate rods

Abstract

Intracellular free Ca (Ca i) was measured in functionally intact rod outer segments in darkness and during light responses using the fluorescent Ca indicator Indo-dextran. In darkness, Ca i was 554 ± 25 nM (n = 28) for −85 ± 2 pA of circulating dark current (I d) and declined in saturating light to a minimum value of ∼50 nM with a time course that paralleled the fall in Na:Ca,K exchange current. During a subsaturating flash response that reduced I d by 70%, Ca i fell to a minimum of ∼325 nM and recovered incompletely to a plateau of ∼450 nM that lasted ∼15 s after full recovery of I d. During a 60 s step that caused ∼7-fold reduction in sensitivity of superimposed flash responses, Ca i reached a steady-state level of ∼252 nM.