Ontogeny of P2-purinoceptors in the longitudinal muscle and muscularis mucosae of the rat isolated duodenum

1 The ontogeny of P2-purinoceptors in the longitudinal muscle and the muscularis mucosae of the rat isolated duodenum was investigated by use of functional assays in tissues from neonatal animals. The degradation of purinoceptor agonists by the rat duodenum muscularis mucosae was also investigated. 2 In the rat duodenum muscularis mucosae adenosine 5'-(alpha,beta-methylene)triphosphonate (AMPCPP), adenosine 5'-triphosphate (ATP), uridine 5'-triphosphate (UTP) and 2-methylthioadenosine 5'-triphosphate (2-Me-S-ATP) all caused a contraction from day 10 to day 40, day 10 being the earliest age it could be tested. The potency order of agonists above day 25 was AMPCPP>ATP=UTP>2-Me-S-ATP and this is similar to the potency order previously obtained for the adult tissue. However, in the neonatal tissues below day 20, 2-Me-S-ATP was the most potent agonist and at days 10 and 15 the order was 2-Me-S-ATP>AMPCPP>ATP=UTP. 3 In the rat duodenum muscularis mucosae desensitization was observed with AMPCPP at day 30 but not at day 15. At day 30, cross-desensitization was also observed between AMPCPP and 2-Me-S-ATP but not between AMPCPP and ATP or UTP, whereas no cross-desensitization was observed at day 15 with AMPCPP and any of the agonists. At day 15 and below AMPCPP and 2-Me-S-ATP may therefore both activate P2Y-receptors (2-Me-S-ATP>AMPCPP, no desensitization with AMPCPP) whereas above day 20 the agonists activate P2X-receptors (AMPCPP>2-Me-S-ATP, desensitization with AMPCPP) which is similar to the adult tissue. Since ATP and UTP were equipotent in the muscularis mucosae and as no cross-desensitization was observed with AMPCPP and UTP or ATP at days 15 or 30, it is likely that ATP and UTP both activate P2U-receptors throughout the ages, as in the adult. 4 The potency of all the agonists in causing contraction in the rat duodenum muscularis mucosae decreased with age. The potency of AMPCPP and 2-Me-S-ATP in causing contractions was highest in the neonates before day 25, and reached values not significantly different from adult by day 30, and the potency of ATP and UTP causing contractions in this tissue was also highest in the neonates at days 10 and 15, and reached values not significantly different from adult by day 20. This suggests either that the receptor populations mediating contraction are highest in the neonates below day 20 or that the agonists are degraded by the muscularis mucosae to a greater extent after day 20. 5 In the rat duodenum muscularis mucosae the degradation of ATP, UTP, 2-Me-S-ATP and AMPCPP was followed by high pressure liquid chromatography at days 15 and 30. ATP was degraded to adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and inosine with no adenosine being detected, 2-Me-S-ATP was degraded to 2-methylthioadenosine 5'-diphosphate (2-Me-S-ADP), 2-methylthioadenosine 5'-monophosphate (2-Me-S-AMP) and 2-methylthioadenosine (2-Me-S-adenosine), and UTP was degraded to uridine 5'-diphosphate (UDP), uridine 5'-monophosphate (UMP) and uridine. The rate of degradation of these agonists was much faster at day 30 than at day 15, probably due to the increase in the size of the tissue. AMPCPP was also degraded with adenosine 5'-(alpha,beta-methylene)diphosphonate (AMPCP) being detected at both ages. However, at day 30 the rate of degradation of AMPCPP was much slower than for ATP, UTP or 2-Me-S-ATP. 6 In the rat duodenum longitudinal muscle 2-Me-S-ATP and AMPCPP both caused a relaxation with a potency order of 2-Me-S-ATP>AMPCPP, suggesting the activation of P2Y-receptors, as previously found for the adult tissue. Weak relaxations were observed to both the agonists at day 15 (the earliest age it could be studied), and the potency of the agonists reached values not significantly different from adult tissues by day 25. 7 Overall, these results suggest that in the neonatal rat duodenum longitudinal muscle there are P2Y-receptors mediating relaxation and that the receptor population is fully developed by day 25. In the neonatal rat duodenum muscularis mucosae before day 20 there are P2Y-receptors mediating contraction, while after day 20 P2X-receptors mediate this effect. P2U-receptors also mediate contraction throughout the ages. The results also indicate that the ectonucleotidase activity in the rat duodenum muscularis mucosae increases with age, and the potency of agonists in the rat duodenum may therefore reflect both the number and nature of the receptors involved and the activity of the ectonucleotidases in the tissue.