LOCALIZATION OF RECEPTORS FOR PROLACTIN IN OVINE SKIN

Although prolactin (PRL) receptors are found in many mammalian tissues , specific PRL binding to mammalian skin has not been demonstrated. In view of the temporal relationships observed between photoperiod, circ ulating PRL and pelage replacement in seasonally responsive mammals, w e sought to provide evidence of PRL receptors in ovine skin. Cryosecti ons of skin from New Zealand Wiltshire ewes were incubated with radiol abelled human GH (I-125-hGH) and ovine PRL (I-125-oPRL) in the presenc e and absence of excess unlabelled hormones (hGH, oPRL or ovine GH (oG H)). Binding was inhibited by unlabelled oPRL and hGH but not by oGH. In microautoradiographs, both radioligands were localised most strongl y in the dermal papillae of wool follicles in the anagen (growth) phas e of the hair cycle and in apocrine sweat glands. Higher levels of spe cific binding to dermal papilla cells, compared with the follicle epit helial matrix and the surrounding dermis, were confirmed by measuremen t of microautoradiograph silver grain density (respectively, 34.1+/-3. 0, 11.4+/-1.0 and 5.5+/-0.5 grains per 100 mu m(2) (mean+/-S.E.M., n=1 0)). Total binding for I-125-hGH and I-125-oPRL radioligands to follic le dermal papilla was not significantly different (34.1+/-3.0 vs 43.6/-2.5 grains per 100 mu m(2) n=10) but the level of non-specific bindi ng of I-125-oPRL, was higher than for I-125-hGH (18.9+/-1.4 vs 6.1+/-0 .6 grains per 100 mu m(2), n=10; P<0.001). Binding assays of receptors in crude microsomal membranes extracted from ovine skin were used to ascertain binding capacity and specificity. Scatchard analysis yielded a dissociation constant (3.9 nM) and receptor concentration (2.22 pmo l/mg protein) similar to that reported for ovine mammary gland. These results provide evidence for the presence of PRL receptors in ovine sk in and are consistent with the hypothesis that PRL has a direct role i n mediating wool follicle activity.