Properties of hepatic binding sites for prolactin in ring doves ( Streptopelia risoria)

Abstract

Specific binding of 125I-labeled ovine prolactin ( 125I-oPRL) was detected in crude membrane fractions prepared from ring dove ( Streptopelia risoria) liver homogenates. In characterization studies, specific binding was found to depend upon pH, incubation time, incubation temperature, and membrane protein concentration. Competitive inhibition of specifically bound 125I-oPRL was observed with human growth hormone, human and rat prolactin, and dove pituitary extract but not with turkey prolactin, human placental lactogen, and several nonlactogenic hormone preparations. Dove liver membranes showed high affinity ( K d = 3 × 10 −10 M) for binding to oPRL but had relatively low binding capacity ( B max < 20 fmol/mg protein). PRL binding activity in pooled liver fractions from breeding doves during early stages of incubation prior to crop sac growth did not differ markedly from that observed in doves sampled at the end of incubation when crop sac weight and serum PRL were elevated. However, binding activity was higher in pooled male liver fractions than in pooled female liver fractions at both reproductive stages. A two- to threefold increase in binding capacity was observed in pooled liver fractions from late-incubating doves following MgCl 2-induced binding site desaturation. The MgCl 2 treatment did not eliminate the differences in specific binding observed between male and female liver fraction pools, thus suggesting the possibility of sex-specific mechanisms of hepatic PRL binding site regulation in this species. It is concluded that the dove liver possesses specific binding sites for PRL with properties similar, but not identical, to those found in other vertebrate target tissues.