Heterogeneity of human growth hormone and prolactin secreted in vitro: Immunoassay and radioreceptor assay correlations

Abstract

Gel filtration of sera and of in vitro pituitary incubation media from a normal subject and from subjects with pituitary tumors or physiologically elevated growth hormone (hGH) and/or prolactin (hPRL) levels, has been performed. Heterogeneous immunoreactive forms of both hormones with comparable elution profiles in sera and in in vitro incubation media were identified. In each instance the most retarded component ('little hPRL' and 'little hGH') migrated identically with the respective radioiodinated pituitary standard and constituted the major component identified. Several components of intermediate mobility were identified and characterized as 'big hPRL', 'big hGH,' and 'big big hGH'. In addition, a void volume immunoreactive peak was often identified. No interconversion was demonstrated on refiltration of prolactin tumor sera or normal pituitary incubation media. Radioreceptor activity utilizing pregnant rabbit liver membranes for hGH and rabbit mammary membranes for hPRL indicates comparable immunologic and receptor activity in all forms identified following gel filtration of prolactin tumor sera (for hPRL) and normal pituitary incubation media (for hGH). Only for the larger species of HGH, identified by gel filtration of clinical grade hGH, was diminished receptor activity demonstrated. These data suggest that the human pituitary synthesizes and secretes protein hormones of different molecular size according to gel filtration elution volume profiles but with comparable receptor and immunologic assay reactivity. These results support but do not establish the proposal that pituitary hormones are also secreted as prohormones.