EXTRACELLULAR ATP AND UTP TRIGGER CALCIUM-ENTRY IN MOUSE CORTICAL THICK ASCENDING LIMBS

The effects of extracellular nucleotides on the cytosolic free Ca2+ co ncentration ([Ca2+](i)) of mouse cortical thick ascending limb (CTAL) segments were investigated using the Ca2+-sensitive fluorescent probe fura 2. ATP (50% effective dose, ED(50), 40 mu M) transiently increase d [Ca2+](i), while adenosine (a P-1 purinoceptor agonist), N-6-cyclohe xyladenosine (an A(1) agonist), AMP, ADP (a P-2t agonist), beta,gamma- methyleneadenosine 5'-triphosphate (a P-2x agonist), or 2-methylthioad enosine 5'-triphosphate (a P-2y agonist) all had little or no effect. CTAL tubules were also sensitive to UTP. The responses to 100 mu M ATP and UTP were similar but not additive. Both [Ca2+](i) responses were strongly inhibited by 300 mu M suramin (a P-2 purinoceptor antagonist) . Adenosine 5'-O-(3-thiotriphosphate) and ITP were slightly less poten t than ATP, while GTP and CTP had no effect. The absence of external C a2+ or the presence of 50 mu M nifedipine similarly and markedly reduc ed the ATP- and UTP-evoked [Ca2+](i) transients. We conclude that mous e CTAL tubules possess nucleotide receptors that are equally sensitive to ATP and UTP and that transiently elevate [Ca2+](i) by triggering C a2+ entry via a nifedipine-sensitive pathway.