Arginine vasopressin (AVP) potentiates corticotrope responses to CRH by increasing the percentage of target cells that secrete in a reverse hemolytic plaque assay for ACTH. The present studies were designed to test more specific effects of AVP and its second messengers on CRH binding to individual corticotropes. Spectrophotometric analyses of 560 corticotropes from fractions enriched to 90% by counterflow centrifugation showed a 30% increase in the average area of the dark blue label for biotinylated CRH after a 1-h exposure to 10 nM AVP or after activation of protein kinase-C [by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or L calcium channels (by Bay K 8644). In addition, computer analysis of the color of the label (wavelength 476-483) showed a 13% increase in saturation (intensity of the blue) and a 23% decrease in brightness (amount of white) after stimulation. The gray level readings of the blue color were also 18% lower after stimulation, which indicates an increase in density (less light transmitted). Taken together, the increases in label area and intensity indicated that activation of L calcium channels or protein kinase-C enhanced CRH binding by individual corticotropes. When mixed pituitary cell populations were analyzed for percentages of labeled cells, exposure to Bay K 8644, TPA, angiotensin II, or AVP resulted in 30-40% increases in the percentage of CRH-bound cells. Dual reactions for biotinylated CRH and ACTH showed that most of the added CRH-bound cells stored ACTH. The effect of exposure to two of the activators was not additive, however. If L calcium channels were blocked with nimodipine, the protein kinase-C-mediated enhancement in CRH binding and ACTH release was blocked, indicating that these actions are dependent on extracellular calcium. In contrast, nimodipine did not block the TPA-mediated enhancement of ACTH storage. These studies show that the potentiation of CRH-mediated ACTH release by AVP or angiotensin II may occur by the enhancement of CRH binding to individual corticotropes. This appears to promote the cytochemical detection of additional CRH-bound corticotropes which may stem from a reserve cell population that normally has levels of CRH receptors or ACTH stores below thresholds needed for detection. The source of these cells (from stem cells or multipotential cells) remains to be determined.