Differential surface labeling and internalization of glucagon by peripheral leukocytes.

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Abstract
A colloidal Au-labeled glucagon-bovine serum albumin (GGA) reagent was prepared and employed for the ultrastructural visualization of glucagon binding sites on the surface of normal human peripheral leukocytes. This technique permitted the tracing of the pathway of intracellular glucagon translocation and sequestration in these different cell forms. Visual inspection and quantitative analysis showed that the extent of surface labeling, as well as the rate and degree of internalization of the glucagon complex, was directly related to cell type. The pathway of glucagon transport via round vesicles and elongated tubulo-vesicles or saccules and its subsequent fate in the leukocytic cells was related to cell variety. Monocytes, followed by neutrophils, bound the greatest amount of labeled glucagon. Although the majority of lymphocytes bound and internalized little or no GGA, .apprx. 10% of the lymphocytes did bind considerable quantities of the labeled hormone. Erythrocytes rarely bound the labeled glucagon complex, while platelets were noted to sequester modest quantities of the GGA within their extracellular canalicular system. GGA uptake by the various types of leukocytic cells appeared to occur primarily by micropinocytosis in noncoated surface invaginations. In monocytes, GGA-containing transport organelles clustered and fused in the peripheral cytoplasm and/or fused directly with lysosomes in these regions without the passing of the labeled glucagon to the Golgi complex. In neutrophils, the GGA complex-transport organelles passed directionally to the Golgi where they fused with small lysosomal-like granules. The degree of GGA labeling of neutrophils and monocytes was appreciably less than noted in similar experiments where a Au-labeled insulin-albumin complex was used at similar times and reagent concentrations. The relative binding of the various hemic cells proved to be similar with these 2 labeled hormones and the process of internalization and mode and pathways of intracellular transport and concentration also were identical.