Inhibition of Ca2+ uptake into A7r5 vascular smooth muscle cells by farnesol: lack of effect on membrane fluidity and Ca2+-ATPase activities

BACKGROUNDPrevious studies have shown that farnesol, a 15-carbon nonsterol derivative of mevalonic acid, inhibits vasoconstriction. Because of its lipophilic properties, we hypothesized that farnesol increased membrane dynamics, thus reducing uptake of Ca and contraction. OBJECTIVETo characterize the effect of farnesol on cell membrane fluidity. DESIGNThe study was conducted using A7r5 cells, a rat aortic vascular smooth muscle cell line. Inhibition of Ca uptake by farnesol was first established in these cells. Then, the effect of farnesol on membrane dynamics was determined. Finally, to ascertain that activation of Ca extrusion and reuptake processes by farnesol did not occur, Ca -ATPase activity was examined. METHODSMembrane fluidity in cell homogenates was estimated using two fluorescent dyes (1,6-diphenyl-1,3,5-hexatriene) and (1-[-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene). Ca uptake was determined by monitoring the changes in cytosolic Ca concentration ([Ca]i) in fura-2-loaded cells after addition of KCl. Ca -ATPase activity was measured in 100 000 ×g cell fractions. RESULTSFarnesol reduced KCl-induced [Ca]i transients significantly (P < 0.001), but did not modify membrane dynamic properties [0.214 ± 0.007 versus 0.218 ± 0.007 (n = 10) and 0.142 ± 0.002 versus 0.146 ± 0.003 (n = 5) for 1-[-(trimethylamino)-phenyl]-6-phenyl-1,3,5-hexatriene and 1,6-diphenyl-1,3,5-hexatriene anisotropies, respectively; NS]. Administration of up to 30 μmol/l farnesol did not affect Ca -ATPase activity. CONCLUSIONFarnesol inhibits KCl-dependent rise of [Ca]i in A7r5 cells. This effect of farnesol is not related to a global change in plasma membrane lipid organization or to activation of Ca pumps. Other mechanisms such as direct inhibition of voltage-dependent Ca channels could therefore explain the biologic action of farnesol in the vascular tissue.