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 muscle and muscularis mucosae of the neonatal rat distal colon has been investigated using homogenate binding, quantitative autoradiography and functional 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 mucosae 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 density of binding sites was higher in the neonatal tissues. Quantitative autoradiography showed a higher density of [H-3]DPCPX binding sites in the longitudinal muscle than in the muscularis mucosae at all ages studied (day 10 to adult), and this binding was concentration-dependently displaced by N-6-cyclopentyladenosine (CPA). In functional studies the longitudinal muscle relaxed in response to 5'-N-ethylcarboxamidoadenosine (NECA) and CPA at all ages studied (15-30 days), NECA being more potent than CPA. The potency of NECA 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 (CGS 21680) indicated that the A(2) receptors were of the A(2B) subtype. The muscularis mucosae contracted in response to CPA at all ages studied (day 15 to adult) and the antagonism by DPCPX (10 nM) were consistent with activation 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. However, while binding and autoradiographic localisation showed the presence of A(1) receptors in the longitudinal muscle at all ages studied, functional data only revealed the presence of A(2B) receptors.