Kinetic aspects of rabbit blastocoele fluid accumulation: an application of electron probe microanalysis.

X-ray spectrometry by electron probe excitation was used to analyze the content of Na, Cl, K, Ca, Mg, S, and P in picoliter samples of blastocoele fluid obtained from single rabbit blastocysts. There are significant changes in the concentrations of these elements between stages of development of blastocysts in vivo. The concentration of K increases approx 40% between 110–135 hr postcoitum (p.c.) and does not increase further by 159 hr. The concentrations of Cl at 110 and 135 hr p.c. are significantly lower than the Cl level at 159 hr p.c. The concentrations of magnesium are very low (less than 0.4 mM) at each stage of development. Ca and S concentrations do not change during development, whereas P decreases from 110–135 hr p.c.The rates of accumulation of the solutes in the rabbit blastocoele in vivo have been estimated from the measured concentrations and from previously published rates of fluid accumulation. These analyses indicate that each trophoblast cell transports an ever increasing quantity of certain solutes which is closely correlated with the inward movement of water.Rabbit blastocysts raised in vitro from 128–140 hr p.c. can accumulate fluid in the presence of increasing concentrations of sucrose (40, 80, 120 mM). Electron probe microanalysis of the blastocoele fluid under these conditions shows that the blastocoele concentrations of both Na and Cl (but not K, Ca, Mg, S, and P) increase 1 mM in response to every 2 mM of sucrose gradient across the trophectoderm. These results directly indicate that the accumulation of fluid in the blastocoele is secondary to the active transport of NaCl.