The effect of colchicine on the diurnal variation of phagocytosis in mouse retinal pigment epithelium.

The effect of colchicine on the fine structure of the retinal pigment epithelium (RPE) was investigated morphometrically in ICR mice bred in a condition of 12-hr light: 12-hr dark cycle since birth. In untreated mice, the number of phagosomes in RPE, containing packets of rod outer segment (ROS) discs, increased maximally at 1.5 hr after light onset, and progressively declined to the bottom value at around the time of lights off. Intraperitoneal injection of mice with colchicine (1 mg per 100 g body weight) prior to a peak of phagocytosis (4 hr before light onset) resulted in a marked increase in the number of phagosomes and a clustering of lysosomes in the apical cytoplasm of RPE and a decrease in the number of microtubules. The colchicine treatment also induced the dispersion of mitochondria from a basal cytoplasmic portion to the whole cytoplasmic area. These changes became dominant at 8 hr after the treatment and continued thereafter for a period of 12 hr. On the other hand, when colchicine was given during the period in which phagocytosis is inactive (8 hr after light onset), it induced only a slight increase in the number of phagosomes which recovered to the normal level earlier than the above. Clustering of lysosomes and dispersion of mitochondria continued to the same extent as in mice treated with colchicine at 4 hr before light onset. It was suggested that the effect of colchicine on the phagocytosis of ROS by RPE might depend on the functional state of RPE during the lighting cycle.