Diversity of calcium ion channels in cellular membranes.

Successful introduction of techniques for separation of different ionic currents and recording of single channel activity has demonstrated the diversity of membrane structures responsible for generation of calcium signal during various forms of cellular activity. In excitable cells the electrically-operated calcium channels have been separated into two types functioning in different membrane potential ranges (low- and high-threshold ones). The low-threshold channels are ontogenetically primary and may play a role in regulation of cell development and differentiation. A similar function may also be characteristic of chemically-operated channels in some highly specialized cells (lymphocytes). The high-threshold channels in excitable cells generate an intracellular signal coupling membrane excitation and intracellular metabolic processes responsible for specific cellular reactions (among them retention of traces of previous activity in neurons--"learning"--being especially important). Chemically-operated N-methyl-D-aspartate-channels also participate in this function. The calcium signal can be potentiated by activation of calcium-operated channels in the membranes of intracellular structures, resulting in the liberation of calcium ions from the intracellular stores. Although different types of calcium channels have some common features in their structure which may indicate their genetic similarity, their specific properties make them well suited for participation in a wide range of cellular mechanisms.