Calcium extrusion from mammalian photoreceptor terminals

Ribbon synapses of vertebrate photoreceptors constantly release glutamate in darkness. Transmitter release is maintained by a steady influx of calcium through voltage-dependent calcium channels, implying the presence of a mechanism that is able to extrude calcium at an equal rate. The two predominant mechanisms of intracellular calcium extrusion are the plasma membrane calcium ATPase (PMCA) and the Na⁺/Ca²⁺-exchanger. Immunohistochemical staining of retina sections revealed strong immunoreactivity for the PMCA in rod and cone terminals, whereas staining for the Na⁺/Ca²⁺-exchanger was very weak. The PMCA was localized to the plasma membrane along the sides of the photoreceptor terminals and was excluded from the base of the terminals where the active zones are located. The amplitude of a calcium-activated chloride current was used to monitor the intracellular calcium concentration. An upper limit for the time required to remove intracellular free calcium is obtained from two time constants measured for the calcium-activated chloride current tail currents: one of 50 msec and a second of 190 msec. Calcium extrusion was inhibited in the absence of intracellular ATP or in the presence of the PMCA inhibitor orthovanadate, but was unaffected by replacement of external Na⁺ with Li⁺. The data indicate that the PMCA, rather than the Na⁺/Ca²⁺-exchanger, is the predominant mechanism for calcium extrusion from photoreceptor synaptic terminals.