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Abstract

Ito cells (fat-storing cells) have been implicated in mechanisms of liver fibrosis, and transforming growth factor-β1 is a key factor that stimulates collagen production by Ito cells. Moreover, Ito cells are reported to possess contractile proteins and to contract with ligands. We recently reported the presence of L-type voltage-operated Ca2+ channels in Kupffer cells. In this study, we examined whether Ito cells contain Ca2+ channels and also evaluated the effect of transforming growth factor-β1 on Ca2+ channels. Cytosolic free calcium concentration was measured in individual cultured Ito cells with the fluorescent Ca2+ indicator dye fura-2. Partial replacement of extracellular Na+ with K+ caused an increase in cytosolic free calcium, presumably as a result of transmembrane Ca2+ influx. Basal cytosolic free calcium levels were around 40 to 50 nmol/L in both control and transforming growth factor-β1—treated cells. In transforming growth factor-β1—treated cells, cytosolic free calcium increased in response to K+ at values as low as 10 mmol/L, whereas untreated cells did not respond. Half-maximal increases in cytosolic free calcium in transforming growth factor-β1—treated cells were observed with 63 ± 6 mmol/L K+. With 100 mmol/L K+, intracellular free calcium increased around fourfold above basal values in transforming growth factor-β1—treated cells but was only increased about twofold in untreated controls. We conclude that this increase in cytosolic free calcium occurs by way of voltage-operated calcium channels; it did not occur in the absence of extracellular calcium and cannot be explained by Na+/Ca2+ exchange mechanisms. The influx of Ca2+ was only about one third as large on subsequent depolarizations and was not sensitive to the dihydropyridine-type calcium channel agonist BAY K 8644. In contrast, Ca2+ influx was inhibited by the dihydropyridine-type calcium channel blocker nitrendipine. These data indicate that Ito cells contain voltage-operated Ca2+ channels. Furthermore, transforming growth factor-β1 activates or up-regulates these Ca2+ channels.(HEPATOLOGY 1994;20:1009-1014)