Glycosylated Prolactin in Porcine Plasma: Immunoblotic Measurement from Birth to One Year of Age*

As much as 40% of PRL in the pituitary gland of the pig is glycosylated. To help determine the physiological significance of this structural variant of PRL, we have measured glycosylated PRL (G-PRL) in the plasma of growing pigs from birth to 1 yr of age. An immunoblotting method developed originally for human plasma was used. With some modifications, the method could detect G-PRL in as little as 0.2 ml porcine plasma. Concanavalin-A affinity chromatography confirmed the glycoprotein nature of the plasma G-PRL band. Quantitative estimates of the immunoblotting results revealed marked differences with age in the secretion of the two monomeric forms of PRL. G-PRL concentrations averaged 138% higher than those of non-G-PRL between birth and 2 months of age, but lower thereafter. Chronologically, both forms displayed similar patterns between birth and 2 months, the concentration remaining unchanged or decreasing slightly. After 2 months, however, concentrations of non-G-PRL increased markedly; the increase was characterized by great fluctuations. G-PRL concentrations, on the other hand, increased only moderately, but the increase was consistent until the end of the study period. The results demonstrate the circulating nature of G-PRL in the pig and suggest that the variant may play a physiological role in the growth and development of the animal.