Mws

milk let-down

FIGURE 12 Control of oxytocin secretion during lactation.

minutes relative to start of suckling

FIGURE 13 Relation of blood oxytocin concentrations to suckling. Note that the initial rise in oxytocin preceded the initial period of suckling. (From McNeilly AS, Robinson ICA, Houston MJ et al. Br Med. J 1983; 286:257. With permission.)

minutes relative to start of suckling

FIGURE 13 Relation of blood oxytocin concentrations to suckling. Note that the initial rise in oxytocin preceded the initial period of suckling. (From McNeilly AS, Robinson ICA, Houston MJ et al. Br Med. J 1983; 286:257. With permission.)

sufficient to produce milk letdown (Fig. 13). Conversely, stressful conditions may inhibit oxytocin secretion, preventing the suckling infant from obtaining milk even though the breast is full.

Cellular Actions of Oxytocin

The oxytocin receptor is expressed principally in uterine smooth muscle and myoepithelial cells that surround mammary alveoli. It is a typical heptahelical receptor that is coupled through Gq to phospholipase Cp. When activated, it signals through inositol trisphosphate (IP3) and diacylglycerol (DAG) released from phosphatidyli-nositol in the membrane. Inositol trisphosphate increases intracellular calcium by signaling its release from intra-cellular stores. Diacylglycerol activates protein kinase C, which may phosphorylate membrane calcium channels and further increase intracellular calcium. Increased intracellular calcium binds to calmodulin and activates myosin light-chain kinase, which initiates contraction of myoepithelial or myometrial cells.

Control of Prolactin Secretion

Suckling is also an important stimulus for secretion of prolactin. During suckling, the prolactin concentration in blood may increase by tenfold or more within just a few minutes (Fig. 14). Although suckling evokes secretion of both oxytocin and prolactin, the two secretory reflexes are processed separately in the central nervous system, and the two hormones are secreted independently.

Lactation

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