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The secretion of milk lipids was investigated by incubating mammary tissue fragments from lactating ewes and rabbits. After the lipids were pulse-labelled (3 min with 14C-sodium acetate or 14C-sodium butyrate), the intracellular and extracellular lipid globules, localized in the acinar lumen, were labelled (as shown by electron microscopy), and the labelled lipids were released into the medium. Colchicine, a microtubule modifier inhibiting protein secretion, did not modify lipid secretion. NH4Cl, a lysosomotropic agent, did not decrease lipid secretion. When prolactin was added immediately after the pulse, the amount of labelled lipids secreted for two hours increases significantly. These results show that prolactin stimulates lipid secretion in the same way as protein secretion. However, microtubular integrity, necessary for protein secretion, is not required for lipid secretion. |
