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′‐triphosphate (ATP) respectively. It has also been shown to contract to uridine 5′‐triphosphate (UTP) and adenosine 5′‐O‐(3‐thiotriphosphate) (ATP‐γ‐S), and based on their differential inhibition by the P 2 antagonist suramin it has been suggested that they act via two separate receptors. In addition, the rat duodenum has been shown to dephosphorylate ATP rapidly via ectonucleotidases and adenosine deaminase. In this study the responses of two preparations from the rat duodenum, the longitudinal muscle and the muscularis mucosae, were investigated using a series of nucleotides and suramin. 2 2‐Methylthioadenosine 5′‐triphosphate (2‐MeSATP), ATP, ATP‐γ‐S and adenosine 5′‐α,β‐methylene‐triphosphonate (AMPCPP) each relaxed the longitudinal muscle, with an agonist potency order of 2‐MeSATP>ATP=ATP‐γ‐S>AMPCPP, while UTP and uridine 5′‐diphosphate (UDP) were not observed to elicit relaxation. This indicates the presence of a relaxant P 2Y ‐purinoceptor on the longitudinal muscle. The longitudinal muscle did not contract to any of the agonists at concentrations of 300 μ m , apart from ATP‐γ‐S which caused very weak contractions. 3 ATP‐γ‐S, adenosine 5′‐methylenediphosphonate (AMPCP), AMPCPP, ATP, UTP, adenosine 5′‐diphosphate (ADP), UDP and 2‐MeSATP each contracted the muscularis mucosae with an agonist potency order of ATP‐γ‐S≥AMPCP≥AMPCPP=ATP=UTP=ADP=UDP> > 2‐MeSATP, although maximal responses were not obtained at concentrations of 300 μ m . The muscularis mucosae did not relax to any of the agonists at concentrations of 300 μ m . 4 Suramin (1 mM) inhibited relaxations induced by ATP on the longitudinal muscle, shifting the relaxation concentration‐response curve to the right. This further supports the presence of a P 2Y ‐purinoceptor on this muscle layer. Suramin (1 mM) inhibited contractions induced by AMPCPP, but not those induced by ATP, UTP or ATP‐γ‐S, in the muscularis mucosae. Desensitization of the muscularis mucosae was seen with AMPCPP, but not with UTP or ATP‐γ‐S, and no cross‐desensitization between AMPCPP and UTP or ATP‐γ‐S was observed. This suggests there are two receptors which mediate contraction on the rat duodenum muscularis mucosae, one suramin‐sensitive and the other suramin‐insensitive. 5 ATP was rapidly degraded by the muscularis mucosae to ADP, adenosine 5′‐monophosphate (AMP) and inosine, with no adenosine being detected. A similar rate of degradation was seen for UTP with UDP, uridine 5′‐monophosphate (UMP) and uridine being formed and for 2‐MeSATP with 2‐methylthioadenosine 5′‐diphosphate (2‐MeSADP), 2‐methylthioadenosine 5′‐monophosphate (2‐MeSAMP) and 2‐methylthioadenosine being formed. AMPCPP and ATP‐γ‐S were both degraded more slowly, AMPCPP being degraded to AMPCP, and ATP‐γ‐S to ADP, AMP and inosine. Suramin (1 mM), did not significantly affect the rate and pattern of degradation of these nucleotides, apart from AMPCPP which was degraded slightly more slowly in the presence of suramin. 6 These results show that there is a P 2Y ‐purinoceptor which mediates relaxation in the rat duodenum longitudinal muscle. They also show that there is a contraction‐mediating suramin‐sensitive receptor on the rat duodenum muscularis mucosae which is desensitized by AMPCPP, and thus is probably of the P 2X subtype. In addition, there is a contraction‐mediating suramin‐insensitive receptor on the rat duodenum muscularis mucosae which is not desensitized by UTP or ATP‐γ‐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 contains ectonucleotidases and adenosine deaminase, which rapidly degrade nucleotides, although the inhibition by suramin of this degradation is unlikely to explain the differential antagonism by suramin of the nucleotides.