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Properties of Exocytotic Response in Vertebrate Photoreceptors

Properties of Exocytotic Response in Vertebrate Photoreceptors,10.1152/jn.01025.2002,Journal of Neurophysiology,M. Kreft,D. Kri,S. Grilc,R. Zorec

Properties of Exocytotic Response in Vertebrate Photoreceptors   (Citations: 13)
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intracellular calcium concentration ((Ca 2 )i). We here examined the physiological properties of exocytosis in tiger salamander ( Ambys- toma tigrinum) retinal rods and cones. Patch-clamp capacitance mea- surements were used to monitor exocytosis elicited by a rapid and uniform increase in (Ca 2 )i by photolysis of the caged Ca 2 com- pound NP-EGTA. The amplitude of flash-induced increases in mem- brane capacitance (Cm) varied monotonically with (Ca 2 )i beyond approximately 15 M. The following two types of kinetic responses in Cm were recorded in both rods and cones: 1) a single exponential rise (39% of cells) or 2) a double-exponential rise (61%). Average rate constants of rapid and slow exocytotic responses were 420 168 and 7.85 5.02 s 1 , respectively. The rate constant for the single expo- nential exocytotic response was 17.5 12.4 s 1 , not significantly different from that of the slow exocytotic response. Beyond the threshold (Ca 2 )i of approximately 15 M, the average amplitude of rapid, slow, and single Cm response were 0.84 0.35, 0.82 0.20, and 0.70 0.23 pF, respectively. Antibodies against synaptotagmin I, a vesicle protein associated with fast exocytosis, strongly stained the synaptic terminal of isolated photoreceptors, suggesting the presence of fusion-competent vesicles. Our results confirm that photoreceptors possess a large rapidly releasable pool activated by a low-affinity Ca 2 sensor whose kinetic and calcium-dependent properties are similar to those reported in retinal bipolar cells and cochlear hair cells.
Journal: Journal of Neurophysiology - J NEUROPHYSIOL , vol. 90, no. 1, pp. 218-225, 2003
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