Evidence for two distinct P sub(2)-purinoceptors subserving contraction of the rat anococcygeus smooth muscle

The effects of the P sub(2)-purinoceptor agonists, adenosine 5'-triphosphate (ATP), alpha , beta -methylene ATP ( alpha , beta -MeATP), beta , gamma -methylene ATP ( beta , gamma -MeATP), L- beta , gamma -methylene ATP (L- beta , gamma -MeATP), adenosine-5'-O-(2-thiodiphosphate) (ADP beta S), and 2-methylthio ATP (2-MeSATP) were investigated on the isometric tension of the rat anococcygeus muscle. Non-cumulative additions of ATP (100-1500 mu M), alpha , beta -MeATP (1-300 mu M), beta , gamma -MeATP (10-300 mu M), L- beta , gamma -MeATP (3-100 mu M) and ADP beta S (1-100 mu M) produced concentration-dependent contractions, whereas 2-MeSATP (1-100 mu m) had no effect. The rank order of potency was alpha , beta -MeATP > L- beta , gamma -MeATP greater than or equal to ADP beta S > beta , gamma -MeATP > > ATP > 2-MeSATP. Contractions to cumulative additions of ATP, alpha , beta -MeATP, beta , gamma -MeATP and L- beta , gamma -MeATP were subject to desensitization whilst those to ADP beta S were unaffected. Contractions to ATP, alpha , beta -MeATP, beta , gamma -MeATP and ADP beta S were abolished by the non-selective P sub(2X)/P sub(2Y)-purinoceptor antagonist, suramin (100 mu M). In contrast, contractions to ATP, alpha , beta -MeATP and beta , gamma -MeATP were not affected by the non-selective P sub(1)-purinoceptor antagonist 8-(p-sulphophenyl)-theophylline (8SPT, 30 mu M). Blockade of P sub(2X)-purinoceptors with the selective P sub(2X)-purinoceptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 10 mu M) or desensitization with L- beta , gamma -MeATP (10 mu M) abolished contractions to alpha , beta -MeATP, but enhanced those to ADP beta S. The P sub(2Y)-purinoceptor antagonist, reactive blue 2 (RB2, 100 mu M) enhanced contractions to ATP and alpha , beta -MeATP but abolished those to ADP beta S. Simultaneous addition of alpha , beta -MeATP and ADP beta S produced in additive contraction. The findings suggest that in the rat anococcygeus, smooth muscle cells are endowed with two distinct P sub(2)-purinoceptors which subserve contractions: a P sub(2X)-purinoceptor activated by ATP and its analogues, and another type of P sub(2)-purinoceptor activated by ADP beta S.