Surfactant Secretion by Type II Pneumocytes is Inhibited by High Glucose Concentrations

Abstract

Delayed fetal lung maturation is observed in poorly controlled diabetic pregnancies. To investigate whether elevated glucose levels inhibit basal surfactant secretion and synthesis in type II cells and whether inhibitory effects on secretion can be reversed by secretagogues, type II cells isolated from 20-day fetal rat lung explants were initially cultured in [H³]choline containing media with glucose concentrations of 5.5, 10, 25, 50, and 100 mM, or in equiosmolar mannitol controls. Further incubation in nonradioactive media containing matched glucose levels with and without 1 × 10⁻⁵ M terbutaline or 1 × 10⁻⁶ M and 1 × 10⁻⁸ M 12-O-tetradecanoylphorbol 13-acetate (TPA) allowed assessment of incorporation of choline into phosphatidylcholine (PC) and its subsequent secretion. PC secretion was inhibited by culture in high glucose conditions, resulting in an -30% reduction in secretion under 50 and 100 mM glucose conditions compared to culture at 5.5 or 10 mM glucose (p < .01); this decrease could not be explained by changes in osmolarity or in cell viability after culture in high glucose. Insulin (1 unit/mL) had no significant impact on secretion (92 ± 7% of control). Terbutaline-stimulated cells grown under 50 and 100 mM glucose conditions had significantly lower secretion rates than did terbutaline-stimulated cells cultured in 5.5 mM glucose (p < .05). Exposure to TPA resulted in significant increases in surfactant secretion by cells grown in both 5.5 and 100 mM glucose; however, the percentage increase (39.5 ± 6.8% and 94.8 ± 8.0% with 10⁻⁸ M and 10⁻⁶ M TPA, respectively) was significantly lower than in controls (87.8 ± 8.0% and 152.1 ± 18.8%, respectively) (p < .01). Choline incorporation into PC was also decreased by 100 mM glucose to 77 ± 9% of control (p < .01). These data indicate that high glucose levels inhibit both surfactant synthesis and baseline and secretagogue-stimulated surfactant secretion by type II cells. This inhibitory effect on surfactant secretion may further exacerbate the decrease in surfactant synthesis and the pulmonary maturational delay seen in infants of diabetic pregnancies.

Key Words:

• fetal lung maturation
• glucose
• secretogogues
• surfactant secretion
• type II cells