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REGULATION OF PULMONARY LECITHIN SYNTHESIS IN THE FETAL RAT

Regulation of pulmonary lecithin (the principal surfactant phospholipid) synthesis is of crucial importance to the fetus and neonate. In our studies metabolic control mechanisms have been investigated in lung samples from over 700 rat fetuses of varying gestational ages. It has been found by incubation of lung slices with isotopic precursors that the choline incorporation pathway accounts for >97% of lecithin synthesis. The rate of this pathway is significantly (p<.001) enhanced after 20 days gestation (term = 22 days). Specific activities of two pathway enzymes, choline kinase (CK) and choline phosphotransferase (CPT), have been measured in lung homogenates from rat fetuses of 16-22 days gestation (DG). CK rises from 740 at 16.5 DG to 1440 pmole/min/mg protein at 19.6 DG (p<.001) and falls from this peak value to 760 pmole/min/mg at 21.3 DG. CPT, although rising from 38 pmole/min/mg at 16.5 DG to 54 at 19.6 DG (p<.02), does not peak until 20.5 DG (60 pmole/min/mg, p<.001). The enzyme then declines in activity as the fetus approaches term. Kinetic properties (Km values, pH profiles, etc.) have also been determined for CK and CPT.These findings suggest that CK and CPT may play a key role in accelerating pulmonary lecithin synthesis in the developing fetus. However, since the rate of lecithin production continues to increase after 21 DG when CK and CPT activities are declining, other regulatory mechanisms may be operative in the term fetus and neonate.