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Modulation of store-operated Ca²⁺ Channels in rat liver cells by arachidonic acid

Vasopressin and other phospholipase C-coupled hormones induce oscillations (waves) of cytoplasmic Ca²⁺ concentration ([Ca²⁺]_cyt) in rat hepatocytes. Maintenance of these oscillations requires replenishment of Ca²⁺ in intracellular stores through Ca²⁺ inflow across the plasma membrane. While this may be achieved by store-operated Ca²⁺ channels (SOCs), some studies in other cell types indicate that it is dependent on arachidonic acid (AA)-activated Ca²⁺ channels. We have investigated the contribution of AA to Ca²⁺ entry in the rat liver cell line, H4-IIE and in primary cultures of rat hepatocytes. For the preparation of isolated hepatocytes, fed male hooded Wistar rats were anaesthetised with an intraperitoneal injection of Nembutal (60 mg/kg body mass) and surgically prepared for liver perfusion. The liver was perfused in situ for 15 min in the presence of collagenase and hepatocytes isolated as described previously⁽¹⁾. After Nembutal injection, the animals do not recover consciousness, and are killed by cannulation of the liver. Using whole-cell patch clamping to measure the effects of AA on membrane conductance, we found no evidence that concentrations of AA in the physiological range could activate Ca²⁺-permeable channels. However, AA (1-10 µM) did inhibit (IC₅₀ = 2.4 ± 0.1 µM) Ca²⁺ inflow through SOCs (I_SOC) initiated by intracellular application of inositol 1,4,5-trisphosphate. Pre-incubation with AA did not inhibit I_SOC development, but decreased maximal amplitude of the current. Iso-tetrandrine, widely used to inhibit PLA₂, and therefore AA release, directly inhibited I_SOC in H4IIE cells. It is concluded that, in rat liver cells, AA-activated Ca²⁺ permeable channels do not contribute to hormone-induced increases or oscillations in [Ca²⁺]_cyt, but that AA does inhibit SOCs. Arachidonic acid may be a physiological modulator of Ca²⁺ inflow in liver cells.

(1) M.N. Berry, A.M. Edwards, G.J. Barritt, In: R.H. Burdon, P.H. van Knippenberg (Eds.), Laboratory Techniques in Biochemistry and Molecular Biology, Vol. 21, Elsevier, Amsterdam, 1991, pp. 15-81.