THE ROLE OF ENDOPLASMIC Са2+-FUNCTIONAL UNIT IN P2Y-RECEPTORS SIGNAL TRANSDUCTION PROCESSES IN CHIRONOMUS PLUMOSUS LARVAE SECRETORY CELLS OF SALIVARY GLANDS

Abstract

ІP₃-dependent Са²⁺-channels inhibitor 2-АPB in low concentrations (1–10 µmol/l) causes increase of stored Са²⁺ content in salivary glands of Chironomus plumosus larvae, both incubated as in nominally calcium-free medium and in the medium with physiological Са²⁺ concentration. This effect is caused by inhibition of ІP₃-dependent Са²⁺-channels, decreasing of Ca²⁺-release from endoplasmic reticulum which results in shift of endoplasmic Са²⁺-functional unit equilibrium in the direction of Са²⁺ accumulation in the intracellular stores. After incubation of glands with 2-APB in hypercalcium medium decrease of Са²⁺ content in secretory cells was observed. Presence of high concentration of 2-APB (20–100 µmol/l) in incubation medium leads to decrease of Са²⁺ content in secretory cells independently of Са²⁺ concentration. This effect most probably is caused by the inhibition of SERCA Са²⁺-pump, which is a part of endoplasmic Са²⁺-functional unit, decrease of Ca²⁺ uptake resulting in shift of equilibrium in direction of release of stored Са²⁺. Application of ATP or ADP in the nominal Ca²⁺-free medium induces a decrease of Са²⁺ content in the secretory cells as a result of P2Y-receptors activation. At the same time, presence of 2-APB (10 µmol/l) in nominally Ca²⁺-free medium abolishes the ATP-induced Са²⁺ release of Са²⁺, and did not influence ADP-induced decrease of Са²⁺ concentration in the secretory cells. According to the obtained data, we assume the presence of two subtypes of Р2Y-receptors in secretory cells of Chironomus plumosus larvae salivary glands. Receptors of the first subtype (P2Y_ATP) have higher affinity to ATP and are coupled with IP₃-dependent Са²⁺-channels. Activation of endoplasmic Са²⁺-functional unit is a basic mechanism of the signal transduction following activation of this subtype of receptors. Receptors of the second subtype (P2Y_ADP) have higher affinity to ADP and exert their functions through other mechanism. Addition of 2-APB to hipercalcium media abolishes the ATP- and ADP-induced changes in Са²⁺ content in glands' tissue due to changes in the integrity of endoplasmic Са²⁺-functional unit.