Developmental pattern of cAMP, adenyl cyclase, and cAMP phosphodiesterase in the palate, lung, and liver of the fetal mouse: Alterations resulting from exposure to methylmercury at levels inhibiting palate closure

Exposure to methylmercury (MeHg: 10 mg Hg/kg maternal body weight) on 126 (dayshours) of gestation significantly delays palate closure in the Swiss Webster CFW mouse. The cAMP content and activity of adenyl cyclase and phosphodiesterase (PDE) were measured in the tissues of control and MeHg‐induced cleft palates between 136 and 176 of gestation. Lung and liver were investigated similarly to determine if MeHg affected the adenyl cyclase system of the palate in a unique manner. In control palatal tissue, cAMP levels increased sharply from 1322 (undetect‐able) to 146 (maximum). PDE activity increased similarly up to 142, but decreased 50% between 142 and 146. Since it has been reported that cAMP induces the synthesis of PDE, the difference in cAMP/PDE from 1322 to 142 and from 142 to 146 suggests the localization of relatively high levels of cAMP in at least two separate compartments. Between 146 and 1410, the adenyl cyclase activity of control palates decreased significantly. This rapid decrease suggests relatively high adenyl cyclase activity in the medial edge epithelial cells which undergo autolysis prior to shelf fusion (centered at 1415). Maternal MeHg administration at 126 delayed the median time of palatal shelf rotation (1413) by 5 hours, and significantly altered the developmental pattern of the adenyl cyclase system. Thus, the increase in cAMP between 142 and 146 was abolished and the decrease in adenyl cyclase activity between 146 and 1410 was delayed by almost 20 hours. These changes may be manifestions of a MeHg‐induced delay in medial edge epithelial cell differentiation. In a previous study, we observed that the fetal liver exhibits the highest MeHg concentration of all tissues. Since MeHg only slightly altered the adenyl cyclase system of the fetal liver compared to the lung and palate (in which MeHg uptake is considerably less), it may be that the effects of MeHg on palatal tissue are not due to a direct effect of MeHg on components of the adenyl cyclase system.