UTP‐preferring P 2 receptor mediates inhibition of sodium transport in porcine thyroid epithelial cells

The effects of adenosine 5′‐triphosphate (ATP), uridine 5′‐triphosphate (UTP) and analogues on forskolin‐stimulated absorption of Na + by porcine thyroid epithelial cells were analysed in cultures grown as confluent monolayers on permeable supports in Transwell Ussing chambers. 85% of the forskolin (10 μ M )‐stimulated short‐circuit current was inhibited by phenamil (1 μ M ), which is a selective antagonist for epithelial type Na + channels. Phenamil‐sensitive current was inhibited in a dose dependent manner by nucleotides added to the apical compartment of Ussing chambers. In contrast, the phenamil‐resistant current, previously shown to represent anion secretion, was unaffected by nucleotides. The order of potency (with EC 50 values given in μ M ) was UTP (0.08)>>ATP (6.3)=uridine 5′‐diphosphate (UDP) (6.6)>2methyl‐thio‐adenosine‐5′‐triphosphate (2MeSATP) (84.5)>adenosine 5′‐diphosphate (ADP) (147.8)>α,β‐methylene ATP (>150)>>adenosine (>1000). P2 receptors mediating inhibition of sodium absorption were present on the apical membrane of the cells since addition of UTP (1–1000 μ M ) to the basal compartment of the Ussing chambers had little effect while subsequent addition to the apical compartment produced a normal response. Cibachron blue (Reactive blue 2) (1–100 μ M ), an antagonist at some P2 receptor subtypes, inhibited phenamil sensitive current in a dose dependent manner with half maximal inhibition occurring at 14.25 μ M . Suramin (100 μ M ), pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulphonic acid (PPADS) (100 μ M ) and pyridoxal 5′‐phosphate (P5P) (100 μ M ) showed only slight competitive antagonism against the response to UTP. These results indicate that a UTP‐preferring P 2 receptor located on the apical membrane of thyroid epithelial cells mediates inhibition of Na + absorption.British Journal of Pharmacology (1999) 127 , 1787–1792; doi: 10.1038/sj.bjp.0702733