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In several species, lung maturation is accompanied by a decline in the phosphatidylinositol content of lung surfactant and a concomitant increase in its phosphatidylglycerol content. To examine the possibility that this developmental change is influenced by the availability of myo-inositol, potential sources of myo-inositol for the developing rabbit lung were investigated. On day 28 of gestation the myo-inositol content of foetal rabbit lung tissue (2.3+/-0.5mumol/g of tissue) was not significantly different from that of adult lung tissue but the activity of d-glucose 6-phosphate:1l-myo-inositol 1-phosphate cyclase (cyclase) in foetal lung tissue (81.0+/-9.0nmol.h(-1).g of tissue(-1)) was higher than that found in adult lung tissue (23.2+/-1.0nmol.h(-1).g of tissue(-1)). Day 28 foetal rabbit lung tissue was found also to take up myo-inositol by a specific, energy-dependent, Na(+)-requiring mechanism. Half-maximal uptake of myo-inositol by foetal rabbit lung slices was observed when the concentration of myo-inositol in the incubation medium was 85mum. When the myo-inositol concentration was 1mm (but not 100mum) the addition of glucose (5.5mm) stimulated myo-inositol uptake. myo-Inositol uptake was observed also in adult rabbit lung and was found to be sub-maximal at the concentration of myo-inositol found in adult rabbit serum. The concentration of myo-inositol in the serum of pregnant adult rabbits (47.5+/-5.5mum) was significantly lower than that of non-pregnant adult female rabbits (77.9+/-9.2mum). On day 28 of gestation the concentration of myo-inositol in foetal serum (175.1+/-12.0mum) was much less than on day 25, but more than that found on day 30. A transient post-partum increase in the concentration of myo-inositol in serum was followed by a rapid decline. Much of the myo-inositol in foetal rabbit serum probably originates from the placenta, where on day 28 of gestation a high cyclase activity (527+/-64nmol.h(-1).g of tissue(-1)) was measured. The gestational decline in serum myo-inositol concentration, together with the decreasing cyclase activity of the lungs, is consistent with the view that maturation of the lungs is accompanied by decreased availability of myo-inositol to this tissue.

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