Development of the muscarinic receptor in rabbit gastric smooth muscle

We used binding of [N-methyl-3H]scopolamine ( [3H]-NMS) to tissue homogenates and isometric contraction of muscle strips to characterize perinatal changes in the muscarinic receptor on rabbit gastric smooth muscle. In homogenates from fetal (28 days of gestation), 1-, 3-, and 7-day, 4- and 11-wk-old rabbits, specific binding was saturable and temperature dependent, achieved equilibrium by 10 min at 30 degrees C, and was linearly related to tissue concentration. Specific binding was 80 +/- 2% of total binding at 0.2 nM [3H]NMS. The number of binding sites was 120,000 receptors/cell, maximal during the first week of life compared with the fetus or older animals. Affinity of [3H]NMS was highest in the first week of life (Kd = 345 +/- 24 pM, 1 day old). Age did not affect Hill coefficients or Ki values; secoverine and 4-diphenylacetoxy-N-methylpiperidine methiodide were 50-fold more potent than pirenzepine. In muscle strips, bethanechol stimulated dose-dependent atropine-inhibitable isometric contraction. The doses required for half-maximal contraction were similar in both age groups (5-6 microM), but maximal contraction was fivefold greater in weanlings compared with neonates. Increasing extracellular potassium concentration resulted in similar differences, suggesting that the differences were not receptor related. These results suggest that well-differentiated M2-muscarinic receptors are functional on rabbit gastric smooth muscle during the perinatal period.