Fetal overgrowth is commonly observed in pregnancies complicated by maternal diabetes. The exact causes of excessive fetal growth in these pregnancies are uncertain. However, where overgrowth occurs, net materno-fetal transfer of solutes by the placenta must, by definition, be increased. Changes in cellular hydration state have been demonstrated to regulate solute transport in epithelia. Many cells respond to hyposmotic extracellular media by rapid swelling followed by volume recovery (volume regulatory decrease; RVD). RVD is often achieved by activating membrane ion channels to promote the net efflux of K⁺, Cl^– and amino acids. We have investigated the effect of changes in cellular hydration state (induced by hyposmotic challenge) on the efflux of ⁸⁶Rb (a marker for K⁺) in placental fragments from insulin-dependent diabetic (D) and non-diabetic (C) women. Experiments were performed at 37 °C. Small fragments (~10 mg) of villous tissue were incubated in buffer containing 2 µCi ml^{-1 86}Rb for 1.5 h to load the tissue with radiolabel. The fragments were passed through a series of vials containing efflux buffer, and 13 sequential 5 min collections were made. Villous fragments were exposed to control buffer (290 mosmol (kg H₂O)^-1) for 15 min and then to either control buffer, isosmotic buffer (55 mM NaCl with mannitol; 290 mosmol (kg H₂O)^-1) or hyposmotic buffer (55 mM NaCl, 170 mosmol (kg H₂O)^-1) with or without the K⁺ channel inhibitor barium (Ba; 5 mM) for 50 min. At the end of the experiment the tissue was lysed in water to determine the remaining intracellular ⁸⁶Rb.

Figure 1 shows total percentage efflux during the 50 min experimental period. Data were analysed statistically using an ANOVA and t test with a Bonferroni correction. In both D and C group placentas, hyposmotic challenge stimulated a significant (P < 0.001) increase in ⁸⁶Rb efflux when compared with control and isosmotic buffers. This suggests that the increase in ⁸⁶Rb efflux observed during exposure to hyposmotic buffer is due to reduction in buffer osmolality rather than the reduction in NaCl concentration per se. The increase in isotope efflux stimulated during hyposmotic challenge is inhibited in the presence of Ba²⁺ (P < 0.001). We propose that Ba²⁺-sensitive K⁺ channels are activated in response to hyposmotic swelling and might play a role in RVD by the human placenta. The response to hyposmotic challenge was comparable in control and diabetic groups. This suggests that regulation of K⁺ efflux via hyposmotically induced changes in cellular hydration state is not compromised in pregnancies complicated by insulin-dependent maternal diabetes. The study was approved by the local ethics committee and informed patient consent obtained.

This work was supported by Lilly, Northern General Hospital Trust.