THE CALCIUM FEEDBACK SIGNAL IN THE PHOTOTRANSDUCTION CASCADE OF VERTEBRATE RODS

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 similar to 50 nM with a time course that paralleled the fall in Na:Ca,K exchange current. During a subsaturating flash response that reduced id by 70%, Ca-i fell to a minimum of similar to 325 nM and recovered incompletely to a plateau of similar to 450 nM that lasted similar to 15 s after full recovery of I-d. During a 60 s step that caused similar to 7-fold reduction in sensitivity of superimposed flash responses, Cai reached a steady-state level of similar to 252 nM.