Effect of allyl sulfides from garlic essential oil on intracellular Ca ²⁺ levels in renal tubular cells

Diallyl sulfide (1), diallyl disulfide (2), and diallyl trisulfide (3), which are major organosulfur compounds of garlic (Allium sativum), are recognized as a group of potential chemopreventive compounds. In this study, the early signaling effects of 3 were examined on Madin-Darby canine kidney (MDCK) cells loaded with the Ca²⁺-sensitive dye fura-2. It was found that 3 caused an immediate and sustained increase of [Ca²⁺]_i in a concentration-dependent manner (EC₅₀ = 40 μM). Compound 3 also induced a [Ca²⁺]_i elevation when extracellular Ca ²⁺ was removed, but the magnitude was reduced by 45%. In Ca ²⁺-free medium, the 3-induced [Ca²⁺]_i level was abolished by depleting stored Ca²⁺ with 1 μM thapsigargin (an endoplasmic reticulum Ca²⁺ pump inhibitor). Elevation of [Ca ²⁺]_i caused by 3 in the Ca²⁺-containing medium was not affected by modulation of protein kinase C activity. The 3-induced Ca²⁺ influx was inhibited by nifedipine and nicardipine (1 μM). U73122, an inhibitor of phospholipase C, abolished ATP (but not the 3-induced [Ca²⁺]_i level). These findings suggest that 3 induced a significant [Ca²⁺]_i elevation in MDCK renal tubular cells by stimulating both extracellular Ca²⁺ influx and thapsigargin-sensitive intracellular Ca²⁺ release via as yet unidentified mechanisms. Furthermore, the order of the allyl sulfide-induced [Ca²⁺]_i elevation and cell viability was 1 < 2 < 3. The differential effect of allyl sulfides on Ca²⁺ signaling and cell death appears to correlate with the number of sulfur atoms in the structure of these allyl sulfides.