Binding or Oxytocin to Receptors in Lactating Mammary Gland

Abstract

<p>Oxytocin is a nonapeptide hormone which is synthesized in the hypothalamus and stored in the posterior pituitary gland (neurohypophysis). It has two known biological activities in mammals: 1) Oxytocin stimulates uterine contraction. Its precise role in the initiation of labour and function during parturition is not as yet well defined; 2) Oxytocin causes milk ejection from the mammary gland during lactation. This research has been involved with the latter aspect.</p> <p>In mammary gland, the target cells for oxytocin are myoepithelial cells. Interaction of oxytocin with specific receptor molecules, thought to be present in the outer, or plasma membrane of these cells causes their contraction and results in ejection of milk from the gland. The goal of this research has been to identify specific receptors for oxytocin in mammary gland from lactating rabbits, to examine some properties of the binding of oxytocin to its receptor and to solubilize the receptor with detergents to permit future examination of its properties in isolation from other plasma membrane components.</p> <p>Receptors for oxytocin can be identified in mammary gland from lactating rabbits by using a radioactive hormone, tritiated oxytocin (³[H]-oxytocin) to bind to the receptors. This receptor meets several criteria of specificity for oxytocin. The active oxytocin analog [1-deamino]-oxytocin competes with [³H]-oxytocin for binding; the almost inactive analog [4-proline]-oxytocin did not. The receptor was detectable in the target tissue, mammary gland, but was not detectable in a non-target tissue, rabbit liver. The receptor had a high affinity for oxytocin (Kd = 3.2 x 10ˉ⁹ M) and had a maximal binding capacity in the preparation used for these studies of 385 x 10ˉ¹⁵ moles per mg protein. A high affinity for the ligand is characteristic of hormone-receptor interactions. In view of the amount of oxytocin required for a biological response, the binding capacity found in this study indicates the presence of "spare" receptors for oxytocin.</p> <p>The binding of [³H]-oxytocin required the presence of divalent cations and was inhibited in the presence of EDTA. Re-addition of Mg²⁺ restored binding activity. Binding of [³H]-oxytocin was reversible. At 37°C, dissociation of approximately 90% of the bound oxytocin required 30 minutes.</p> <p>Partial purification of the particulate receptors was done by sucrose density gradient centrifugation. In one fraction, oxytocin binding activity was enriched 5-6 fold. 5'-Nucleotidase, a plasma membrane marker enzyme, was also enriched in this fraction to a similar extent. This provides evidence that the oxytocin receptor is present on the plasma membranes of its target cells.</p> <p>The particulate oxytocin receptor was treated with the detergents deoxycholic acid, Triton X-100 and Lubrol-PX in an attempt to obtain a soluble receptor. Such treatment prevented the binding of [³H]-oxytocin. The amount of binding ability destroyed was dependent upon the concentration of detergent employed. Removal of the detergent did not restore the ability to bind oxytocin. Incubation of the particulate receptor with [³H]-oxytocin prior to detergent treatment permitted the recovery of a hormone-receptor complex. Most of the complex remained in the particulate fraction. Approximately 25% was still present in the supernatant following centrifugation at 210,000 x gav. for 30 minutes. This portion of the hormone-receptor complex may be considered "solubilized", as judged by this single criterion.</p>