Са2+-FUNCTIONAL UNITS CONCEPTION CONCERNING TO SECRETORY CELL OF CHIRONOMUS PLUMOSUS LARVAE SALIVARY GLAND

Abstract

On the basis of analysis of experimental data received during investigation of Ca²⁺-transporting systems of secretory cells of salivary glands of Chironomus plumosus larvae, a conception of Ca²⁺-functional units has been proposed. According to this conception, Ca²⁺-functional unit is an abstract model which consists of with active and passive Ca²⁺-transporting systems and cellular membrane providing Ca²⁺ cations compartmentalisation. There are type I (active and passive transporting systems located in the same membrane) and type II (active and passive transporting systems located in different membranes of the same cell) Ca²⁺-functional units. In both cases, Ca²⁺-functional unit is not a static structure, but a dynamic one, that ensure proper [Ca²⁺] support in cytosol. It may be in three states: 1) resting state the is characterised by a dynamic balance between the influx and efflux of Ca²⁺ flow; 2) active state of Ca²⁺ influx (regarding cytosol) prevailing over the efflux; 3) inactive state (oppositely, Ca²⁺ efflux prevailing over its influx). Factors which due to direct positive or negative feedback are supporting Ca²⁺-transporting systems transition between different states, are Ca²⁺ cations, since the activity of majority of Ca²⁺-transporting systems depends on cytosol concentration of Ca²⁺. Endoplasmic Ca²⁺-functional unit of investigated secretory cells combines Ca²⁺-pump of endoplasmic reticulum, lnsP₃Rs and RyRs. As a result of 1) ryanodine adding to incubatory medium of glands in submicromolar concentration causes Ca²⁺ content increasing in their tissue; 2) lnsP₃Rs stimulation prevents simultaneous RyRs stimulation or contrariwise; 3) heparin causes Ca²⁺ content increasing in gland tissue, treated with saponin, but only in ryanodine presence in medium in concentration activating RyRs. Ca²⁺-functional unit of plasma mambrane are formed by voltage-operated Ca²⁺-channels, Na⁺-Ca²⁺ exchanger and Ca²⁺-pump of plasma membrane. That assumption is based on the fact that between the voltage-operated Ca²⁺-channel and Na⁺-Ca²⁺ exchanger, from one side, and Na⁺-Ca²⁺ exchanger and Ca²⁺-pump of plasma membrane, from the other side, tight functional links are existing even in case of intracellular perfusion. To some extent, condition of this Ca²⁺-functional unit are defined by Na⁺-Ca²⁺ exchange dependence on Na⁺-K⁺ pump activity. Its peculiar feature is also a dependence on membrane potential level, not only on cytosolic [Ca²⁺]. Distinct Ca²⁺-transporting systems may be a part of several Ca²⁺-functional units. Endoplasmic-mitochondrial Ca²⁺-functional unit consists of Ca²⁺-release channels of endoplasmic reticulum and mitochondrial Ca²⁺-uniporter, so far as influence of ryanodine and ruthenium red, and also lnsP₃ and ruthenium red under the condition of their combination in incubation medium with Ca²⁺ content of salivary glands tissue is not additive. Postulated conception could be positioned as a working hypothesis for identification of the role of different Ca²⁺-transporting systems in coordination of Ca²⁺-signalling in different cells other than secretory cells of exocrine glands.