[Effects of monensin on the secretion of caseins by mammary epithelial cells from lactating rabbits].

The effects in vitro of monensin on the secretory pathway of caseins in lactating rabbit mammary gland fragments were investigated. Addition of monensin (0.1 microM or 1 microM) to the incubation medium induced a dilatation of Golgi saccules and vesicles and a decrease of the relative volume of microvesicles. Dilated vesicles did not contain acid phosphatase. Neosynthesized proteins were localized by electron microscope autoradiography near the membranes of dilated vesicles. 1 microM monensin did not inhibit basal secretion of neosynthesized caseins (radioactive caseins, labelled during a 3 min pulse, released into the medium), but increased basal secretion of total beta-caseins (measured by radioimmunoassay). Monensin abolished all the effects of prolactin (increase in the relative volume of Golgi microvesicles and stimulation of secretion of total and neosynthesized caseins). These results show that monensin provokes simultaneously modifications in the Golgi apparatus morphology and inhibition of the prolactin stimulating effect. Moreover, secretion of intracellular caseins is differently modified, depending probably on whether caseins are neosynthesized or stored.