Expression and regulation of M-type K⁺ channel in PC12 cells and rat adrenal medullary cells.

M-type K⁺ channels contribute to the resting membrane potential in the sympathetic ganglion neurons of various animals, whereas their expression in adrenal medullary (AM) cells has been controversial. The present experiment aims to explore the expression of M channels comprising the KCNQ2 subunit in the rat AM cell and its immortalized cell line PC12 cells at the protein level and how its expression in PC12 cells is regulated. The KCNQ2 isoform was recognized in homogenates of PC12 cells but not the rat adrenal medullae by immunoblotting and KCNQ2-like immunoreactivity (IR) was detected in PC12 cells but not in rat AM cells. When the PC12 cells were maintained in a dexamethasone-containing medium, KCNQ2-like IR in the cells was suppressed, whereas the removal of fetal bovine serum from the culture medium for 1 day resulted in an increase in KCNQ2-like IR. A similar enhancement occurred when PC12 cells were cultured under conditions where glucocorticoid receptor (GR) and/or mineralocorticoid receptor (MR) activities were suppressed. These morphological findings were confirmed in functional analysis. The cells cultured in the presence of an inhibitor of either GR or MR exhibited larger amplitudes of Ca²⁺ signal in response to an M channel inhibitor than did the cells in its absence, whereas the resting Ca²⁺ level in the former was lower than that in the latter. These results indicate that the M channel is not expressed in rat AM cells and this absence of expression may be ascribed to the suppression by glucocorticoid activity.

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