Aluminum fluoride induces a reversible Ca 2+ sensitization in α-toxin-permeabilized vascular smooth muscle

Abstract

The mechanism of aluminumfluoride (AlF)-induced Ca 2+ sensitization was explored in α-toxin-permeabilized rabbit mesenteric artery. In the presence of 0.18 μM Ca 2+ and deferoxamine, a strong chelator of aluminum (Al 3+), fluoride (F −; applied in the form of NaF) induced very slow tension development, while in the presence of tracer levels of Al 3+, tension developed rapidly possibly due to formation of Al−F complexes (especially AlF 4 −). As a result, AlF significantly shifted the relationship between tension development and free Ca 2+ concentration in the Ca 2+-t-EGTA buffer (pCa-tension curve) to the left. The rate of the tension development also dependend on the EGTA concentration: increasing the EGTA concentration from 0.5 to 10 mM markedly decreased the maximal rate of contraction ((dT/dt) max), probably due to chelation of Al 3+ by EGTA, without effect on the maximal tension (ΔT max). The AlF-induced Ca 2+ sensitization could be reversed by extensive washing with relaxing solution (pCa > 8), in contrast to the contractions induced by guanosine 5'-[γ-thio]triphosphate (GTPγs; a non-hydrolyzable GTP analogue) or phorbol 12,13-dibutyrate (PDBu) which were irreversible. However, the action of all the compounds appeared to be mediated through a H-7 (1-[5-isoquinolinesulfonyl]-2-methylpiperazine dihydrochloride)-sensitive pathway, and no additive effects among them were observed. In addition, GDP increased (dT/dt) max due to AiF without changing ΔT max, whereas guanosine 5'[β-thio]diphosphate (GDPβs; a non-hydolyzable GDP analogue) decreased both parameters. These findings suggest that AlF acts on G-proteins to enhance Ca 2+ sensitivity of contractile elements through a H-7-sensitive pathway.