Ontogeny of adenosine receptors in the longitudinal muscle and muscularis mucosae of the rat distal colon

The development of adenosine A(1) and A(2B) receptors on the longitudinal m uscle and muscularis mucosae of the neonatal rat distal colon has been inve stigated using homogenate binding, quantitative autoradiography and functio nal studies. In homogenate binding studies 1,3-[H-3]-dipropyl-8-cyclopentylxanthine ([H- 3]DPCPX) bound with high affinity to A(1) receptors in the muscularis mucos ae and intact colon from rats aged 10, 15, 20, 25 and 30 days. The affinity of [H-3]DPCPX was similar to that in the adult at all ages, but the densit y of binding sites was higher in the neonatal tissues. Quantitative autorad iography showed a higher density of [H-3]DPCPX binding sites in the longitu dinal muscle than in the muscularis mucosae at all ages studied (day 10 to adult), and this binding was concentration-dependently displaced by N-6-cyc lopentyladenosine (CPA). In functional studies the longitudinal muscle rela xed in response to 5'-N-ethylcarboxamidoadenosine (NECA) and CPA at all age s studied (15-30 days), NECA being more potent than CPA. The potency of NEC A remained constant and it was antagonised by 1 mu M DPCPX at all ages with pA(2)-values consistent with activation of A(2) receptors. The inactivity of 2-[p-(carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CG S 21680) indicated that the A(2) receptors were of the A(2B) subtype. The m uscularis mucosae contracted in response to CPA at all ages studied (day 15 to adult) and the antagonism by DPCPX (10 nM) were consistent with activat ion of A(1) receptors. In conclusion, binding, autoradiographic and functional studies identified A(1) receptors on the rat colon muscularis mucosae at all ages studied. How ever, while binding and autoradiographic localisation showed the presence o f A(1) receptors in the longitudinal muscle at all ages studied, functional data only revealed the presence of A(2B) receptors.