Development of motilin receptors and of motilin- and erythromycin-induced contractility in rabbits.

The development of the motilin receptor was studied through contraction and binding studies of groups of three rabbits aged between 2 and 289 days. The contractility of small intestinal smooth muscle strips was measured isotonically. The aborally decreasing gradient in response to motilin, known to exist in adult rabbits, was already present at day 8 (maximum contractile responses expressed as a percent of the maximal response to acetylcholine were 77% +/- 9%, 34% +/- 10%, and 25% +/- 8% for duodenal, jejunal, and ileal strips, respectively). Throughout the observation period, the doses of motilin and its agonist erythromycin that were required to induce 50% of the response remained constant and were not significantly different from doses required for adults (their negative logarithms were 8.55 +/- 0.35 and 5.70 +/- 0.25, respectively). The correlation between the maximum contractile response toward motilin and erythromycin was almost perfect (r2 = 0.82). Binding studies with iodinated norleucine13-porcine motilin were performed using antral smooth muscle tissue homogenates. The maximum number of binding sites increased rapidly after 8 days (3.3 +/- 0.4 fmol/mg protein) and reached a peak at 21 days (20.7 +/- 1.4 fmol/mg protein), but decreased at that point toward the adult value (40 days, 10.6 +/- 1.3; 289 days, 9.8 +/- 1.1 fmol/mg protein). The dissociation constant, however, remained unchanged. The peak value of receptor density occurred at about the time that the rate of increase of the length of the intestine and of the weight of the antrum were at maximum levels (at 18 and 27 days, respectively). Motilin receptors are expressed early postnatally, and the regional gradient in sensitivity towards motilin is also established soon after birth. If applicable to humans, an early response to erythromycin may have therapeutic value.