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 5'-[alpha, beta-methylene]triphosphate (a P2x purinoceptor agonist) and adenosine 5'-[gamma-thio]triphosphate (a P-type ATPase inhibitor) were without effect. 2-Methylthioadenosine 5'-triphosphate, a P2y 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 P2y purinoceptor antagonist Reactive Blue-2 inhibited the Ca2+ uptake stimulated by 1 mM ATP (IC50 0.17 mM and 58 microM respectively). We conclude that ATP-stimulated Ca2+ uptake in tilapia intestine is dissociated from ATPase activity and is mediated through a P2 purinoceptor.