Concurrent expression of erythroid and renal aquaporin CHIP and appearance of water channel activity in perinatal rats

Major phenotypic changes occur in red cell membranes during the perinatal period, but the underlying molecular explanations remain poorly defined. Aquaporin CHIP, the major erythroid and renal water channel, was studied in perinatal rats using affinity-purified anti-CHIP IgG for immunoblotting, flow cytometry, and immunofluorescence microscopy. CHIP was not detected in prenatal red cells but was first identified in circulating red cells on the third postnatal day. Most circulating red cells were positive for CHIP by the seventh postnatal day, and this proportion rose to nearly 100% by the 14th day. The ontogeny of red cell CHIP correlated directly with acquisition of osmotic water permeability and inversely with Arrhenius activation energy. Only minor alterations in the composition of red cell membrane lipids occurred at this time. Immunohistochemical analysis of perinatal kidneys demonstrated a major induction of CHIP in renal proximal tubules and descending thin limbs at birth, coincident with the development of renal concentration mechanisms. Therefore, water channels are unnecessary for oxygen delivery or survival in the prenatal circulation, however CHIP may confer red cells with the ability to rehydrate rapidly after traversing the renal medulla, which becomes hypertonic after birth.