Prolactin and Growth Hormone Binding in Mammary and Liver Tissue of Lactating Cows

A radioligand/receptor binding assay was developed using homologous hormones to distinguish between bovine growth hormone (bGH) and bovine prolactin (bPRL) receptors in liver and mammary tissue of lactating cows. Mammary and liver tissues were homogenized in 0.3 M sucrose and centrifuged at 100,000 x g over a 1.3 M sucrose density gradient. Membranes from the 0.3 - 1.3 M sucrose interface were incubated with 1 ng of iodinated bGH or bPRL for 20 h at 22 degrees C in the presence of increasing concentrations of native bGH or bPRL. High affinity receptor binding sites were found for bPRL in liver and mammary tissue membranes (Ka = 3.2 and 1.3 x 10(8) l/mol with 34 and 63 fmol receptors/mg liver and mammary membrane protein, respectively) and for bGH only in liver tissue (Ka = 1.8 x 10(9) l/mol, 18 fmol receptors/mg membrane protein). Receptor number estimates were 3 and 11 times higher in mammary and liver tissue using a heterologous hGH system indicating that heterologous systems may overestimate the number of receptors in bovine tissue. The absence of demonstratable bGH receptors in lactating bovine mammary tissue supports in vitro results of others with isolated mammary tissue indicating that the positive effect of bGH on milk production in intact cows is via an indirect mechanism.