Circadian variability of kinetics in a transplantable islet-cell tumor of the golden hamster.

The kinetics of a transplantable islet-cell tumor known to secrete paranormal hormonal compounds without any influence of usual regulatory mechanisms was studied in the golden hamster. It was found that during the 42nd diurnal period of growth, labelling and mitotic indices (peaks at midday), mean grain counts (lower at 4 a.m. and 8 a.m.), and PLM data (shift between G1 and S phase durations) varied significantly. The results are discussed in view of the discrepancy between hormonal release (unaffected by normal triggering factors) and circadian kinetics indicating the persistence of tumoral sensitivity to some control mechanisms. The theoretical importance of the diurnal variability for the correct interpretation of the internal kinetics of tumors is recalled.