MUSCARINIC RECEPTORS IN DEVELOPING RAT COLON

Muscarinic receptor subtypes were characterized in fetal (21 day), new born (3 day), and adult (3 month) rat colon smooth muscle. Saturation binding of the nonselective muscarinic antagonist radioligand [H-3]qui nuclidinyl benzilate revealed a single class of binding sites in all t hree age groups. The binding affinities of [H-3]quinuclidinyl benzilat e were not significantly different among three age groups (K-D: 0.19 - -> 0.27 nM). In contrast, the receptor densities (B-max, fmol/mg prote in) showed a significant age-related decrease with fetus (518.9 +/- 7. 4) > newborn (480.3 +/- 45.6) --> adult (192.4 +/- 32.8). In both newb orn and adult tissues, the muscarinic agonist carbachol bound to two s ites with high and low affinities. Although the agonist binding affini ties in the newborn tissue were not significantly different from those in the adult tissue, the high-affinity binding sites for carbachol we re significantly increased in the later (41% --> 61%). Addition of gua nosine-5'-O-(3-thio)triphosphate (100 mu M) abolished apparent high-af finity binding sites in both newborn and adult tissues. Antagonist com petition binding in the newborn tissue indicated a homogeneous populat ion of muscarinic M(2) receptors. Unlike in newborn tissues, the heter ogeneous binding of pirenzepine and 4-diphenylacetoxy-N-methylpiperidi ne methobromide in adult tissues revealed coexistence of muscarinic M( 3) (45%) and M(2) (55%) receptors. In accordance, activation of muscar inic receptors in the adult tissue stimulated synthesis of inositol 1, 4,5-trisphosphate. These results suggest maturational changes of musca rinic receptor subtypes and their coupling to G proteins in rat coloni c smooth muscle. These changes may account, at least in part, for deve lopmental alterations of functional responses in colonic smooth muscle .