Fatty acid-induced Ca2 sequestration in intracellular Ca2+ pools

A specific and sensitive GTP-activated translocation process mediates transfer of Ca²⁺ between InsPj-insensitive and lnsP3-releasable Ca21 pools. In the presence of oxalate, GTP-activated Ca²⁺ transfer results in a substantial increase in Ca2' accumulation through the entry of oxalate into the InsPjreleasable pool and formation of insoluble Ca ²⁺-oxalate complexes. Measuring "5Ca²⁺ uptake into permeabilized DDT,MF-2 smooth muscle cells, palmitic acid (C16:0) in the 10-100 \MI range had a major effect on GTP-mediated Ca²⁺ accumulation. This effect was similar to the action of oxalate but occurred at 20-fold lower levels. The fatty acid effect was highly chain lengthand saturation-specific. Only pentadecanoic acid (C15:0) gave a similar effect; myristic acid (C14:0) was slightly effective and other saturated fatty acids had no effect. Unsaturated fatty acids including palmitoleic acid (C16:1) were also ineffective. Palmitate had no effect on mitochondria Ca²⁺ uptake or on the Ca²⁺ leak rate induced by the addition of 3 j.iM thapsigargin. Both palmitate- and oxalate- activated Ca²⁺ accumulation in the presence of GTP was inhibited by the anion transport inhibitor 4,4-diisothiocyanatostiibene-2, 2disulfonic acid (DIDS) suggesting their entry is mediated by a similar nonselective anion channel To examine the nature of the palmitate- and oxalate-induced Ca²⁺ complexes, the releasabililty of Ca²⁺ in response to InsP, and the ionophore A23187 were compared. The oxalate/GTP-induced Ca accumulation was slowly releasable with lnsP3 or A23187. In contrast, the Ca2' accumulated in the presence of palmitate and GTP was was rapidly and completely released by inophore and only very slowly released with lnsP? This indicates that the state and/or possible location of Ca²⁺ complexed with palmitate is distinct from that compiexed with oxalate. It appears that the palmitate-Ca²⁺ complex lies close to the ER membrane and hence highly accessible to the action of A23187. Such fatty acid-Ca2' complexes may play an important physiological role in Ca²⁺ sequestration within specific Ca* pools. (NIH grant NS19304; NSF grant MCB 9307746; AHA Md. Affiliate).