Increase in intracellular Ca2+ level by phenylsulfamide fungicides, tolylfluanid and dichlofluanid, in rat thymic lymphocytes

• Tolylfluanid and dichlofluanid are frequently detected pesticides in crops.
• Both pesticides increased intracellular Ca2+ levels in murine lymphocytes.
• Store-operated Ca2+ influx was involved in the Ca2+ increases.
• The pesticides activated the process of apoptotic cell death.

Tolylfluanid, a phenylsulfamide fungicide, is one of the many pesticides that are frequently detected in crops. Therefore, its health risk is a concern. Micromolar concentrations of tolylfluanid induce chromosomal aberrations and micronuclei in mammalian lymphocytes. The findings prompted us to study the cellular actions of tolylfluanid and another frequently detected pesticide, dichlofluanid, at submicromolar and micromolar concentrations. Of the cellular actions of chemicals, the action on cellular Ca2+ homeostasis is important since Ca2+ is involved in cell signaling and death. Consequently, in this study, the effects of phenylsulfamide fungicides were examined on rat thymocytes by using fluorescent probes in order to further characterize the cellular actions of phenylsulfamide fungicides. Both phenylsulfamide fungicides exhibited biphasic, early and late, increase in intracellular Ca2+ levels. The early phase was dependent on intracellular Ca2+ release and increased membrane Ca2+ permeability. The late phase was owing to Ca2+ influx via activation of store-operated Ca2+ channels and the further increase of membrane ionic permeability. Voltage-dependent Ca2+ channels were not involved. The increases in intracellular Ca2+ levels by phenylsulfamide fungicides were observed at drug concentrations of 0.1 μM or more (up to 10 μM). Thus, it is plausible that micromolar concentrations of phenylsulfamide fungicides deregulate intracellular Ca2+ homeostasis in rat thymocytes. Both phenylsulfamide fungicides at 10 μM promoted the transition from intact living cells to living cells with phosphatidylserine-exposed membranes. This was not the case for phenylsulfamide fungicides at 3 μM. The potency of tolylfluanid was similar to that of dichlofluanid. Although the information on residual concentrations of tolylfluanid and dichlofluanid is very limited, their residual concentrations do not reach micromolar levels. It is unlikely that humans will develop adverse effects on exposure to phenylsulfamide fungicides under present environmental conditions.