Intracellular Ca++ channel immunoreactivity in neuroendocrine axon terminals
The concentration of neuroendocrine terminals in the neurohypophysis facilitates the identification and localization of Ca2+ channel subtypes near neuroendocrine release sites. Immunoblots of rat neurohypophysial tissue identified the K11.3, K12.1, K12.2, and K12.3 Ca2+ channel subunits. Immunofluorescence staining of axon terminal plasma membranes was weak, suggesting that Ca2+ channels are dispersed. This contrasts with the highly punctate K12.2 immunoreactivity in bovine chromaffin cells; the neurohypophysial terminals may therefore lack the specialized release zones found in those cells. Immunofluorescence and immunogold labeling identify dense core granule-like structures in the terminal cytoplasm containing multiple Ca2+ channel types. Ca2+ channels in internal membranes may play an important role in channel targeting and distribution in neuroendocrine cells.