KINETICS OF PULMONARY UPTAKE OF SEROTONIN DURING EXERCISE IN DOGS

The multiple indicator-dilution technique was employed in the exercisi ng dog to evaluate the effect of increasing activity on the pulmonary extraction and kinetics of removal of tracer H-3-labeled serotonin (5- HT) and on the measured central blood volume and tracer-accessible ext ravascular lung water C-51-labeled red blood cells, I-125-labeled albu min, and C-14-labeled 1,8-octanediol were injected with labeled 5-HT a t rest and at two increasing levels of exercise (lower and higher in 9 dogs). Blood flow approximately tripled at the highest levels of exer cise, and the central blood volume increased linearly with increasing blood flow. The tracer-accessible extravascular lung water increased i n the transition from rest to low-level exercise and stabilized at an average proportion of 0.85 of the gravimetric extravascular lung water at the higher values of blood flow. The average labeled 5-HT extracti on at rest was 42 +/- 11%, and this slowly decreased with increase in flow. The calculated permeability-surface area product for 5-HT increa sed approximately directly with increasing blood flow We conclude that exercise results in an increase in the central blood volume that is a ccompanied by an increase in the tracer-accessible extravascular lung water (lung tissue recruitment) over low exercise levels, with no chan ge at higher levels of exercise, and that the pulmonary capillary surf ace area subserving 5-HT uptake increases almost linearly with flow ov er the range of flows attained.