The morphology of release of vitamin A-containing lipid droplets by hepatocytes in rat liver.

Vitamin A-containing lipid droplets in the hepatocytes of rat liver were found to be exocytotically released from the cells in the form of a "lipid droplet--retinol-binding protein (RBP)--immunoreactive complex" following intraportal injection of retinol (17, 33, 67, or 100 micrograms). Evidence that the lipid droplets contain vitamin A was obtained by fluorescence microscopy of vitamin A. Intraportal injection of retinol produced varied numbers and sizes of vacuoles in the hepatocytes. The substance within the vacuoles exhibited a meshwork-like configuration in sections from slices incubated in a medium for revealing acid phosphatase activity or the corresponding control medium and was RBP-immunoreactive and proteinaceous in nature. The occurrence and number of the vacuoles depended on the dosage of injected retinol, being greatest at a dosage of 100 micrograms of retinol and becoming progressively less at dosages of 67, 33, and 17 micrograms. The vacuoles were formed by vacuolization of cisternae of the rough endoplasmic reticulum. The formation of vacuoles reached a maximum 30 min after intraportal injection of 100 micrograms retinol, and the vacuoles and lipid droplets had almost disappeared from the hepatocytes after 90 min. Little or no esterase activity was found in lipid droplets in the hepatocytes before intraportal injection of retinol, but after the injection, lipid droplets that had fused with the vacuoles become strongly positive for this enzyme activity. This suggests that hydrolysis of retinyl esters may occur in the process of complex formation in rat hepatocytes.