Distribution and function of the adrenergic and cholinergic receptors in the fetal calf bladder during mid‐gestational age

Previous studies on the developing fetal bovine bladder demonstrate that compliance is low during early stages of fetal development and increases with fetal age. In addition, the pharmacological response of isolated fetal bovine bladder smooth muscle strips to field stimulation and bethanechol increased in proportion with the gestational age. In the adult bladder (rabbit), the contractile response to autonomic receptor stimulation and the autonomic receptor density showed a parallel gradation in the the bladder between bladder dome and the urethra. The present studies were designed to determine the distribution of the cholinergic and adrenergic receptors in the urinary bladder of the fetal calf at mid‐gestational age, and to correlate the receptor density with the magnitude of the response to receptor stimulation. Each bladder body was separated into upper, middle, and lower segments. For the functional studies, circular and longitudinal strips were cut from each bladder segment and stimulated with field stimulation (FS), bethanechol, methoxamine, isoproterenol, and KCl. Autonomic receptor assays were performed using ³H‐QNB (muscarinic cholinergic), ³H‐DHE (alpha adrenergic), and ³H‐DHA (beta adrenergic) as ligands. In general, there were no significant differences in receptor density among the three bladder segments. However, the density of muscarinic receptors was significantly greater than the density of alpha‐ or beta‐adrenergic receptors for all bladder segments. The receptor densities correlated very well with the functional response of isolated bladder strips to the specific autonomic agonists. The contractile responses of the strips to FS, bethanechol, and methoxamine were not significantly different among the three different bladder segments. The magnitude of the contractile response to methoxamine was significantly smaller than the response to FS, bethanechol, and KC1 for all bladder segments. Isoproterenol (10 μM) relaxed precontracted bladder strips of all segments to a substantial degree (65–85%). In conclusion, at mid‐gestation, there were no significant differences in either the autonomic receptor densities or in the functional responses to receptor stimulation among three different segments of the fetal bovine bladder.