Calcium-dependent inactivation and depletion of synaptic cleft calcium ions combine to regulate rod calcium currents under physiological conditions

L-type Ca2+ currents (I (Ca) ) in rod photoreceptors exhibit Ca2+ -dependent inactivation. Perforated-patch whole-cell recordings were obtained from isolated rods of the tiger salamander using 1.8 mm Ca2+ in the bathing medium to determine the extent of Ca2+ -dependent inactivation of I (Ca) with physiological [Ca2+ ] and endogenous buffering. I (Ca) was measured with voltage ramps applied before and after 5-s steps to -40, -30, -20, or -10 mV. Long depolarizing steps in isolated rods produced inactivation of I (Ca) ranging from 15% at -40 mV to > 80% at -10 mV. Because, in addition to Ca2+ -dependent inactivation, depletion of synaptic cleft Ca2+ accompanying activation of I (Ca) can reduce presynaptic I (Ca) at calycal synapses, we investigated whether a similar mechanism worked at the invaginating rod synapse. Rods from retinal slices with intact synapses were compared with isolated rods in which synaptic cleft depletion is absent. I (Ca) was more strongly depressed by depolarization of rods in retinal slices, with I (Ca) reduced by 47% following voltage steps to -40 mV. The depression of currents by depolarization was also greater for rods from retinal slices than isolated rods when Ca2+ was replaced with Ba2+ to reduce Ca2+ -dependent inactivation. The stronger depolarization-evoked inhibition of I (Ca) in retinal slices compared to isolated rods probably reflects depletion of synaptic cleft Ca2+ arising from sustained Ca2+ influx. Inactivation of I (Ca) exhibited slow onset and recovery. These findings suggest that Ca2+ -dependent inactivation and depletion of synaptic cleft Ca2+ may combine to regulate I (Ca) in response to light-evoked changes in rod membrane potential.