Extracellular ATP and UTP trigger calcium entry in mouse cortical thick ascending limbs

The effects of extracellular nucleotides on the cytosolic free Ca2+ concentration ([Ca2+]i) of mouse cortical thick ascending limb (CTAL) segments were investigated using the Ca(2+)-sensitive fluorescent probe fura 2. ATP (50% effective dose, ED50, 40 microM) transiently increased [Ca2+]i, while adenosine (a P1 purinoceptor agonist), N6-cyclohexyladenosine (an A1 agonist), AMP, ADP (a P2t agonist), beta, gamma-methyleneadenosine 5'-triphosphate (a P2x agonist), or 2-methylthioadenosine 5'-triphosphate (a P2y agonist) all had little or no effect. CTAL tubules were also sensitive to UTP. The responses to 100 microM ATP and UTP were similar but not additive. Both [Ca2+]i responses were strongly inhibited by 300 microM suramin (a P2 purinoceptor antagonist). Adenosine 5'-O-(3- thiotriphosphate) and ITP were slightly less potent than ATP, while GTP and CTP had no effect. The absence of external Ca2+ or the presence of 50 microM nifedipine similarly and markedly reduced the ATP- and UTP-evoked [Ca2+]i transients. We conclude that mouse CTAL tubules possess nucleotide receptors that are equally sensitive to ATP and UTP and that transiently elevate [Ca2+]i by triggering Ca2+ entry via a nifedipine-sensitive pathway.