TURNOVER OF CHOLESTEROL AND DIPALMITOYL PHOSPHATIDYLCHOLINE IN SURFACTANT OF ADULT-RAT LUNG

In order to compare the turnover of two major surfactant components, [1alpha,2alpha(n)-H-3]cholesterol and [methyl C-14 choline] dipalmitoyl phosphatidylcholine (DPPC) were introduced as lamellar bodies via the trachea into lightly anesthetized rats which were then allowed to recover. The radiotracers were assumed to have entered the alveolar surfactant pool and to have subsequently recycled in part into the lamellar bodies of alveolar type II cells. For DPPC, the specific activity vs. time curves of tubular myelin rich (alv-1) and tubular myelin poor (alv-2) alveolar lavage fractions were similar, and there was a plausible precursor-product relationship between lamellar bodies and either (or both) of these compartments. In contrast, however, the specific activities of alv-1 and alv-2 for cholesterol were quite different, allowing us to reject the hypothesis of a precursor-product relationship between classical lamellar bodies and alv-2. The estimated turnover time for DPPC in alv-1 was 240 or 206 min, depending on which subfraction of lamellar bodies one takes to be the precursor. For cholesterol it was 583 or 624 min. These longer turnover times for cholesterol should lead to a greater than twofold increase in the relative concentration of cholesterol in the putative product compartment. Such an increase was not found. We interpret this as reflecting either noncompartmental behavior of the alveolar surfactant pool, or multiple pools of lamellar bodies with different turnover times. We conclude that two major components of pulmonary surfactant, cholesterol and DPPC, are handled differently, and that for at least one of these substances, the widely accepted scenario of a compartmental precursor-product relationship between lamellar bodies and alveolar surfactant must be rejected.