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Pyrimidine nucleotide-sensitive phosphoinositidase C activity (PLC), previously identified in frog semicircular canal ampulla, was pharmacologically characterized. Binding of [(3)H]UTP and abilities of unlabeled nucleotide analogs to inhibit binding and to stimulate PLC in myo-[(3)H]inositol-loaded ampullas were determined. Specific [(3)H]UTP binding was competitively inhibited by UTP [apparent dissociation binding constant = 0.8 microM; Hill coefficient = 0.7]. Scatchard analysis revealed a minor class of high-affinity binding sites [45 fmol UTP bound/microgram protein; dissociation constant (K(D1)) = 0.4 microM] and a major class of moderate-affinity binding sites (365 fmol UTP bound/microgram protein; K(D2) = 10 microM). The stereospecificity pattern for UTP analog recognition was UMP &gt; UDP &gt;/= ADP = UTP = dTTP &gt; adenosine 5'-O-(3-thiotriphosphate) = ATP = CTP = 2'-and 3'-O-4-(benzoylbenzoyl)-ATP (Bz-ATP) &gt;/= AMP &gt;/= 2-methylthio-ATP = alpha,beta-methylene-ATP &gt; uridine = diadenosine tetraphosphate (Ap(4)A); cAMP and adenosine were inactive. Antagonist recognition pattern was DIDS = pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) = reactive blue 2 &gt; suramin. The rank order of potencies for agonist-induced PLC activation was UDP &gt;/= UTP &gt;/= Ap(4)A &gt;/= UMP = Bz-ATP; uridine was inactive. UTP-stimulated PLC activity was inhibited by DIDS = reactive blue 2 = PPADS &gt; suramin. These results suggest that the population of [(3)H]UTP-labeled binding sites is heterogeneous, with a low number of high-affinity UTP receptors whose function(s) need to be determined and a large number of moderate-affinity receptors triggering PLC activation.

american journal of physiology. regulatory, integrative and comparative physiology/american journal of physiology. regulatory, integrative, and comparative physiology

UTP binding and phosphoinositidase C activation in ampulla from frog semicircular canal