ABSTRACT
Measurement Techniques .......................................................................................... 376 19.2.3 Hardware Requirements ............................................................................................ 377 19.2.4 Loading of Fluorescence Dyes into Cells ................................................................ 379 19.2.5 Calibration of the Calcium Signal ............................................................................ 380
19.3 Calcium Imaging in Moving Flagellates .............................................................................. 382 19.3.1 Experimental Procedure ............................................................................................ 382
19.3.1.1 Cells and Loading Procedure ..................................................................... 382 19.3.1.2 Space Experiment ...................................................................................... 382
References ...................................................................................................................................... 384
Calcium ions play an eminent role in the regulation of many cellular processes in eukaryotic as well as in prokaryotic cells. Various mechanisms have evolved to keep the cytosolic calcium concentration at a very low level of about 10-7 M, which is about 4 orders of magnitude lower than the calcium concentration in the surrounding medium. This tight control of the calcium concentration is achieved by means of calcium pumps (Ca2+-ATPases and Ca2+JH+ antiporters) located in the plasma membrane and in the membrane of organelles.1·4
Changes of the cytosolic calcium concentration mediated by activated calcium channels in the plasma membrane or in the membrane of internal calcium stores lead to a defined response of the cell. 5 Various mechanisms trigger the opening or closing of the different calcium channels. Voltagegated channels are triggered by changes in the membrane potential and can be located in the plasma membrane, the T-tubule membrane of muscle cells, or the cilia membrane. Activated voltagesensitive calcium channels in the T-tubules trigger the opening of RyR/Ca2+ release channels (ryanodine receptors), probably via mechanical coupling. Many calcium channels are activated due to binding of specific ligands such as hormones, ions, or cyclic nucleotides at binding sites at or
near the channels. In these cases, calcium acts as an intracellular second messenger. Some types of calcium channels are triggered by mechanical stimulation;6·7 for example, stretch-activated channels in membranes of various ciliates and flagellates8·10 and transduction channels in the hair bundle of vertebrates. 11 Stretch-activated channels are also found in plant cells 12 and retinal pigment epithelial cells.U The pharmacology of mechanogated channels is reviewed in Refs. 14 and 15. A general overview of membrane channels can be found in Ref. 16.
