Comparative Studies on the Ontogeny and Autonomic Responses of the Fetal Calf Bladder at Different Stages of Development: Involvement of Nitric Oxide on Field Stimulated Relaxation

Abstract

This initial study correlates the passive length-tension relationship, contractile and relaxant responses to field stimulation and contractile responses to specific autonomic agonists and antagonists with gestational age. Fetal bovine bladders were separated into three groups based on the head-rump length (FL): 30 to 45cm. (early gestation), 50 to 65cm. (middle gestation) and 70 to 85cm. (late gestation). Each bladder was separated into upper and lower bladder segments; longitudinal strips of smooth muscle were isolated and placed in individual muscle baths.Passive length-tension studies demonstrated that compliance was greatest in the bladder of late gestation and lowest in the bladder of early gestation period. Field stimulation (FS) elicited frequency-dependent contractile responses in all strips. In the upper bladder, the maximal response and maximal rate of tension generation to FS was lowest in the youngest fetuses and increased in proportion to the gestational age. In the lower bladders, there were no gestational age-related differences in the maximal response or maximal rate of tension generation in response to field stimulation.The maximal response of the upper bladder to bethanechol increased significantly from the youngest gestational age to mid-gestation, with no further changes between mid- and late gestation. The maximal response to field stimulation and bethanechol were equal between upper and lower bladder segments for the youngest gestational bladders, whereas for the oldest gestational ages, the maximal response of the upper bladder to FS and bethanechol were significantly greater than the responses of the lower bladder.In the presence of maximal precontraction with bethanechol, FS induced a rapid and marked decrease in tension. The magnitude of the relaxation was substantially greater for the strips of lower bladder than for the strips of upper bladder at late gestation. In lower bladders, the magnitude of the field stimulated relaxation was greater in the strips from the older fetuses than in the strips from younger fetuses. In all strips, field stimulated relaxations were completely inhibited by pretreatment with L-NAME (an inhibitor of nitric oxide synthesis), indicating that the FS-induced relaxation was due to nitric oxide. In addition to nitric oxide-induced relaxation, beta adrenergic stimulation also induced a significant relaxation of the isolated strips.In summary, these data suggest that, in the tubular shaped fetal bovine bladder, there were distinct differences in the autonomic responses between the upper bladder segment and the lower bladder segment in the late gestation period.