Alteration in tissue electrolyte and water balance in pregnant mice: the importance of potassium.

Edema is a common manifestation of pregnancy and is often regarded as a pathological condition, particularly when associated with preeclampsia. However, the physiological characteristics and roles of tissue-level electrolyte balance in gestational edema remain unclear. Sodium, a major extracellular osmole, has long been emphasized, while the contribution of potassium, a key intracellular osmole essential for cell proliferation, has not been fully explored. In this study, we examined sodium, potassium, and water content at the tissue level in pregnant mice. Compared to non-pregnant controls, pregnant mice exhibited significantly increased relative potassium and water content in the skin, without altering relative sodium levels. In muscle, pregnancy increased potassium and water in proportion to muscle mass, resulting in no change in relative content. A low-potassium diet reduced fetal growth. Under potassium restriction, pregnancy did not induce the skin potassium and water alterations, and even decreased muscle potassium. These findings suggest that potassium contributes to maternal water retention at the tissue level and that maternal tissues function as reservoirs to store potassium and water for fetal growth. This adaptive response may underlie physiological skin edema during pregnancy. Furthermore, mobilization of maternal muscle observed under potassium deficiency may represent a compensatory mechanism to support fetal needs, potentially contributing to the pathophysiology of preeclampsia. Our results highlight the importance of potassium in maternal adaptations during pregnancy and underscore the need for further investigation into tissue-level electrolyte dynamics in both normal pregnancy and hypertensive disorders of pregnancy.