Human GH (hGH) extracted from pituitary glands consists of several molecular forms. Monomeric pituitary forms include the single chain 22,000-dalton polypeptide (22K; hGH-B), a 20,000-dalton variant with a 15-amino acid deletion (20K), 3 proteolytically cleaved 2-chain forms (hGH-C, -D, and -E), 2 deamidated forms, an acetylated form (fast hGH), and other, only partially characterized forms. It is not known which of these forms is secreted, nor what the precise nature of circulating hGH is. To answer these questions, we have extracted hGH from human plasma obtained by plasmapheresis from normal volunteers after L-dopa stimulation of hGH secretion and from acromegalic patients. We extracted and concentrated hGH by immunoadsorbent chromatography and examined its chemical nature by polyacrylamide gel electrophoresis under native and denaturing (sodium dodecyl sulfate and urea), nonreducing and reducing (dithiothreitol) conditions as well as by isoelectric focusing. In all cases, the predominant form of hGH present in plasma was 22K, which accounted for approximately 85% of all immunoreactive hGH. In addition, we found evidence for the presence of 20K as a minor form (approximately 7% of all hGH) and of one or more acidic forms (N-acetylated, deamidated, or cleaved hGH; 5-10% of all hGH). Exact identification of the acidic form(s) was not possible. However, the highly bioactive cleaved forms hGH-D and -E were judged to be undetectable (less than 5% of all hGH). We conclude that 1) several monomeric molecular forms of hGH circulate in normal and acromegalic man; 2) the pattern of circulating hGH forms reflects in part their relative prevalence in the pituitary gland; 3) proteolytically cleaved 2-chain hGH forms with enhanced bioactivity are not detectable in blood; and 4) monomeric hGH circulating in acromegaly is qualitatively indistinguishable from normal hGH.

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Insulin receptors are present in human erythrocytes and correlate negatively with cellular age. Little is known about the function of these receptors, about the precise kinetics of their decline during cell aging or about their fate after disappearance from the cells. To elucidate some of these questions, we have prepared red blood cell populations of widely varying cellular ages (ranging from the erythroblast stage to senescent mature erythrocytes) by isopycnic centrifugation on isosmolar density gradients. In addition, young red cells were cultured for 4 days in vitro to permit observation of short-term changes. In mature erythrocytes, insulin receptors decreased as an exponential function of cell age with an estimated half time of 40 days. A more rapid decline of insulin receptors occurred coincident with reticulocyte maturation. Loss of receptors from cultured cells was accompanied by appearance of a soluble insulin receptor in the medium. The effect of insulin on glucose utilization in erythroblast and reticulocyte preparations was negligible, as assessed by CO2 and lactate production. We conclude that 1) insulin receptors are progressively lost from the red blood cell after the erythroblast stage; 2) receptor loss is particularly rapid during reticulocyte maturation; 3) shedding of receptors into the extracellular environment is one reason for their depletion from cells; and 4) in basophilic erythroblasts and reticulocytes, insulin exhibits little metabolic action despite the relatively high receptor complement present in these cells.