Ca2+-sequestering smooth endoplasmic reticulum in an invertebrate photoreceptor. II. Its properties as revealed by microphotometric measurements

Microphotometric measurements are used to investigate the functional properties of Ca2+-sequestering smooth endoplasmic reticulum (SER) in leech photoreceptors. 10-30 intact cells are mounted in a perfusion chamber, placed between crossed polarizers in a microphotometer, and permeabilized by saponin treatment. Subsequent perfusion with solutions containing Ca2+, MgATP, and oxalate leads to Ca uptake by SER. When the solubility product of Ca-oxalate is exceeded in the SER, birefringent Ca-oxalate precipitates form in the cisternae, leading to a large increase in the optical signal recorded from the preparation. The rate of increase in light intensity is used to measure the rate of Ca uptake. Ca uptake rate is linear with time over much of its course, can be switched on/off by the addition/withdrawal of Ca2+, ATP, or oxalate to/from the medium, and is inhibited by mersalyl and tetracaine. The Ca uptake mechanism has a high specificity for MgATP (KM,MgATP is approximately 0.8 mM). Uptake rates observed with dATP, GTP, UTP, ITP, and CTP are only 20-30% of the rate measured in ATP. The Ca pump has a high affinity for Ca2+ ions: the threshold for activation of the pump is approximately 5 x 10(-8) M, the apparent KM,Ca is approximately 4 x 10(-7) M. When Na+ or Li+ is substituted for K+, Ca uptake rate is decreased by 40-50%. The results show that the Ca2+-sequestering SER in leech photoreceptors shares some basic properties with skeletal muscle sarcoplasmic reticulum and supports the idea that certain subregions of the SER in invertebrate photoreceptors function as effective Ca2+ sinks/buffers close to the plasmalemma.