RESPONSES OF THE LONGITUDINAL MUSCLE AND THE MUSCULARIS MUCOSAE OF THE RAT DUODENUM TO ADENINE AND URACIL NUCLEOTIDES

1 Previous studies have shown that the rat duodenum contains P-1 and P -2Y purinoceptors via which it relaxes to adenosine and adenosine 5'-t riphosphate (ATP) respectively. It has also been shown to contract to uridine 5'-triphosphate (UTP) and adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S), and based on their differential inhibition by the P-2 a ntagonist suramin it has been suggested that they act via two separate receptors. In addition, the rat duodenum has been shown to dephosphor ylate ATP rapidly via ectonucleotidases and adenosine deaminase. In th is study the responses of two preparations from the rat duodenum, the longitudinal muscle and the muscularis mucosae, were investigated usin g a series of nucleotides and suramin. 2 2-Methylthioadenosine 5'-trip hosphate (2-MeSATP), ATP, ATP-gamma-S and adenosine 5'-alpha,beta-meth ylenetriphosphonate (AMPCPP) each relaxed the longitudinal muscle, wit h an agonist potency order of 2-MeSATP > ATP = ATP-gamma-S > AMPCPP, w hile UTP and uridine 5'-diphosphate (UDP) were not observed to elicit relaxation. This indicates the presence of a relaxant P-2Y-purinocepto r on the longitudinal muscle. The longitudinal muscle did not contract to any of the agonists at concentrations of 300 mu M, apart from ATP- gamma-S which caused very weak contractions. 3 ATP-gamma-S, adenosine 5'-methylenediphosphonate (AMPCP), AMPCPP, ATP, UTP, adenosine 5'-diph osphate (ADP), UDP and 2-MeSATP each contracted the muscularis mucosae with an agonist potency order of ATP-gamma-S greater than or equal to AMPCP greater than or equal to AMPCPP = ATP = UTP = ADP = UDP much gr eater than 2-MeSATP, although maximal responses were not obtained at c oncentrations of 300 mu M. The muscularis mucosae did not relax to any of the agonists at concentrations of 300 mu M. 4 Suramin (1 mM) inhib ited relaxations induced by ATP on the longitudinal muscle, shifting t he relaxation concentration-response curve to the right. This further supports the presence of a P-2Y-purinoceptor on this muscle layer. Sur amin (1 mM) inhibited contractions induced by AMPCPP, but not those in duced by ATP, UTP or ATP-gamma-S, in the muscularis mucosae. Desensiti zation of the muscularis mucosae was seen with AMPCPP, but not with UT P or ATP-gamma-S, and no cross-desensitization between AMPCPP and UTP or ATP-gamma-S was observed. This suggests there are two receptors whi ch mediate contraction on the rat duodenum muscularis mucosae, one sur amin-sensitive and the other suramin-insensitive. 5 ATP was rapidly de graded by the muscularis mucosae to ADP, adenosine 5'-monophosphate (A MP) and inosine, with no adenosine being detected. A similar rate of d egradation was seen for UTP with UDP, uridine 5'-monophosphate (UMP) a nd uridine being formed and for 2-MeSATP with 2-methylthioadenosine 5' -diphosphate (2-MeSADP), 2-methylthioadenosine 5'-monophosphate (2-MeS AMP) and 2-methylthioadenosine being formed. AMPCPP and ATP-gamma-S we re both degraded more slowly, AMPCPP being degraded to AMPCP, and ATP- gamma-S to ADP, AMP and inosine. Suramin (1 mM), did not significantly affect the rate and pattern of degradation of these nucleotides, apar t from AMPCPP which was degraded slightly more slowly in the presence of suramin. 6 These results show that there is a P-2Y-purinoceptor whi ch mediates relaxation in the rat duodenum longitudinal muscle. They a lso show that there is a contraction-mediating suramin-sensitive recep tor on the rat duodenum muscularis mucosae which is desensitized by AM PCPP, and thus is probably of the P-2X subtype. In addition, there is a contraction-mediating suramin-insensitive receptor on the rat duoden um muscularis mucosae which is not desensitized by UTP or ATP-gamma-S, and at which ATP and UTP show equal potency, and is thus probably of the P-2U subtype. In addition, the rat duodenum muscularis mucosae con tains ectonucleotidases and adenosine deaminase, which rapidly degrade nucleotides, although the inhibition by suramin of this degradation i s unlikely to explain the differential antagonism by suramin of the nu cleotides.