Evidence for P2-purinoceptor-mediated uptake of Ca2+ across a fish (Oreochromis mossambicus) intestinal brush border membrane

We have studied the effect of ATP on Ca²⁺ uptake in intestinal brush border membrane vesicles (BBMVs) of the teleost tilapia (Oreochromis mossambicus). ATP stimulated Ca²⁺ uptake 12-fold over the control, with a linear time course. Ionomycin and detergent treatment did not reduce BBMVs' Ca²⁺ content, indicating the binding of Ca²⁺ to a membrane component. A rank order of ATP > ADP > AMP was established for the stimulation of Ca²⁺ uptake. Adenosine, vanadate, adenosine 5ƀ-[α,β-methylene]triphosphate (a P_2X purinoceptor agonist) and adenosine 5ƀ-[γ-thio]triphosphate (a P-type ATPase inhibitor) were without effect. 2-Methylthioadenosine 5ƀ-triphosphate, a P_2Y purinoceptor agonist, mimicked the stimulation by ATP. As judged from a kinetic comparison, ATP hydrolysis and the stimulation by ATP of Ca²⁺ uptake were not compatible. The P₂ purinoceptor antagonist suramin and the P_2Y purinoceptor antagonist Reactive Blue-2 inhibited the Ca²⁺ uptake stimulated by 1 mM ATP (IC₅₀ 0.17 mM and 58 ƁM respectively). We conclude that ATP-stimulated Ca²⁺ uptake in tilapia intestine is dissociated from ATPase activity and is mediated through a P₂ purinoceptor.