It is very important to identify Ca²⁺-transporting systems and to investigate their structure, properties and pharmacology, because many diseases are caused by an alteration in their functioning (Missiaen et al. 2000). Such studies could result in the development of new drugs that by an affect on Ca²⁺-transporting proteins could restore proper intracellular Ca²⁺ signalling and cell functioning. It has been shown that SH groups play an important role in Ca²⁺-transport system functioning (Marshall et al. 1993). Moreover, some drugs (NO, for example) could influence ion-transporting system functioning by affecting SH groups (Manko et al. 2002). That is why it is very interesting to determine the role of SH groups in the functioning of Ca²⁺-transporting systems, in particular, IP₃-sensitive Ca²⁺ channels (IP₃-channels).

Experiments were performed on isolated salivary glands of Chironomus plumosus L. larvae. Ca²⁺ content in gland tissue was measured using arsenazo III. Saponin (0.1 mg ml^-1) was used for cell permeabilization. p-Chlormercuribenzoate (PCMB; 10 µM) was used as an SH-group blocker, eosin Y (20 µM) as a Ca²⁺ pump blocker and inositol-1,4,5-trisphosphate (IP₃; 10 µM) as an IP₃-channel activator. Data are presented as means ± S.E.M. and were compared with Student’s paired t test.

We found the following. (1) IP₃ evoked a Ca²⁺ content decrease in gland tissue by 41.14 ± 11.75 % (P < 0.05, n = 11). This allows us to suggest that IP₃ channels are present in the endoplasmic reticulum (ER) membrane of investigated cells. (2) Incubation of glands with PCMB (1, 5 and 10 µM) did not cause a statistically significant change in the Ca²⁺ content of gland tissue. We assume the reason for this is the simultaneous effect of PCMB on different Ca²⁺-transporting systems. (3) PCMB evoked a statistically significant (P < 0.05, n = 6) Ca²⁺ content decrease in gland tissue by 12.60 ± 11.61 % with eosin Y present in the incubation medium. This allows us to suggest that PCMB influences the IP₃ channels of exocrine secretory cell ER membrane. (4) IP₃ caused an increase in the Ca²⁺ content of gland tissue by 25.36 ± 7.42 % (P < 0.05, n = 6) with PCMB present in the incubation medium.

So, we suppose that SH groups play an important role in IP₃-channel functioning. However, because it is not known how IP₃ influences the functioning of different Ca²⁺-transporting systems, the explanation of the PCMB effect needs additional investigation.

We conclude that SH-groups play an important role in the functioning of IP₃-sensitive Ca²⁺-channels of salivary gland secretory cell endoplasmic reticulum membrane of Chironomus plumosus L. larvae.

This work was supported by The West-Ukrainian BioMedical Research Center.