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

Background Previous 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 Ca2+ and contraction. Objective To characterize the effect of farnesol on cell membrane fluidity. Design The study was conducted using A7r5 cells, a rat aortic vascular smooth muscle cell line. Inhibition of Ca2+ 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 Ca2+ extrusion and reuptake processes by farnesol did not occur, Ca2+-ATPase activity was examined. Methods Membrane 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). Ca2+ uptake was determined by monitoring the changes in cytosolic Ca2+ cconcentration ([Ca2+](i)) in fura-2-loaded cells after addition of KCl. Ca2+-ATPase activity was measured in 100 000 x g cell fractions. Results Farnesol reduced KCl-induced [Ca2+](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 mu mol/l farnesol did not affect Ca2+-ATPase activity. Conclusion Farnesol inhibits KCl-dependent rise of [Ca2+](i) in A7r5 cells. This effect of farnesol is not related to a global change in plasma membrane lipid organization or to activation of Ca2+ pumps. Other mechanisms such as direct inhibition of voltage-dependent Ca2+ channels could therefore explain the biologic action of farnesol in the vascular tissue. (C) Rapid Science Publishers ISSN 0263-6352.