Growth hormone (GH) cells in the rat anterior pituitary have been morphologically classified into three subtypes: type I (mature) containing large secretory granules about 350 nm in diameter
type II (intermediate) containing a mixture of large and small granules
and type III (immature) containing small granules about 150 nm in diameter. However
the functional implications of morphological heterogeneity
especially the different sensitivities to growth hormone-releasing hormone (GRH) under different corticosteroid conditions have not been elucidated to date.In the present study
by application of microwave irradiation (MWI) for fixation and immunocytochemistry
new findings of the exocytotic response have been revealed among the subsets of GH cells following adrenalectomy (ADX)
corticosterone treatment and/or GRH treatment.The MWI gave effective results for fixation
especially for the permeability of the fixative
and showed good results for immunoelectron microscopy using the protein-A gold method. Moreover
the use of MWI greatly shortened the fixation
processing and immunolabeling times without compromising the quality of ultrastructural preservation and the specificity of labeling.The number of exocytotic figures was low in all subtypes of GH cells in the sham-operated control rats. GRH treatment induced a significant increase in exocytosis in each subtype of GH cells
particularly in type I (mature) and type II (intermediate) GH cells in the control rats. GRH injection to rats for 4 days after ADX also showed an increase in exocytosis
but the degree was significantly less in comparison with the GRH injection in the control group. Corticosterone replacement given to ADX rats induced a clear recovery of the exocytotic response to GRH to the control level. Serum GH content measured by radioimmunoassay correlated with these morphological results.These results suggest that the secretion of GH stimulated by GRH is closely related to corticosteroids
and that the sensitivity to GRH differs among the three subtypes of GH cells.
