Changes in biliary lipid secretion during normal development and diurnal cycling in the rat

Biliary lipid secretion in conjunction with hepatic cholesterol synthesis was determined in normal male rats at 4, 5, 7, and 9 weeks of age, during a period of linear growth and a near fourfold increase in liver size. Studies were performed both at periods of low (mid-light cycle) and high (mid-dark cycle) hepatic cholesterol synthesis. Biliary bile salt, phospholipid, and cholesterol secretion (per g of liver) markedly decreased with an increase in liver size. Whereas the secretion of bile salts and phospholipid was not significantly different in mid-dark and mid-light periods for animals of the same age, cholesterol secretion was greater in the mid-dark than in the mid-light period at 5, 7, and 9 weeks of age. Relationships between biliary cholesterol secretion and bile salt and phospholipid secretion differed at mid-dark and mid-light periods, as follows: cholesterol secretion was not significantly related to bile salt secretion at mid-dark (r = 0.49, P > 0.05) but was related at mid-light (r = 0.73, P = 0.003); and although cholesterol secretion was significantly related to phospholipid secretion at mid-dark, this relationship was not nearly as strong as at mid-light (P < 0.005, comparing r = 0.95 at mid-light with r = 0.53 at mid-dark). In contrast, at mid-dark, biliary cholesterol secretion was strongly related to hepatic cholesterol synthesis (r = 0.84, P < 0.0001) whereas at mid-light the two were not significantly related (r = 0.13, P > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)