Role of Ca2+ in the secretion of milk caseins in lactating rabbit mammary epithelial cells

Prolactin and arachidonic acid increase milk casein secretion in mammary gland slices. These effects do not necessitate Ca2+ in the incubation medium. Prolactin does not modify the influx or the efflux of 45Ca2+. The Ca2+ channel blocking agent D600 (6 micrograms/ml) decreases the stimulatory effect of prolactin on casein secretion, but does not interfere in the stimulatory effect of arachidonic acid. The calmodulin inhibitor trifluoperazine (100 microM) inhibits stimulation of casein secretion by both prolactin and arachidonic acid. From these data, it is concluded that a flow of Ca2+ from the outside into the cell is not a requisite for the stimulation of casein secretion. However, stimulation by prolactin, but not stimulation by arachidonic acid, requires Ca2+ movement through calcium pathways. Intracellular transport of Ca2+ seems necessary for the stimulation of secretion.