Extracellular calcium sensitivity and voltage-dependent calcium channels in C cells

An essential function of C cells is to monitor [Ca2+]e and to increase CT secretion in response to small increments in [Ca2+]e. CT, in turn, decreases [Ca2+]e via its effects on bone and kidney. [Ca2+]e-dependent CT secretion is known to be mediated by corresponding changes in [Ca2+]i. The [Ca2+]e-sensing of C cells is mediated by DHP-sensitive, voltage-dependent Ca2+ channels, which allow Ca2+ influx even at the resting membrane potential. An increase of [Ca2+]e stimulates transmembrane Ca2+ influx via DHP-sensitive Ca2+ channels, thereby increasing [Ca2+]i and consequently CT secretion. Moreover, [Ca2+]e and cAMP-dependent oscillations of [Ca2+]i are observed in C cells. Various neuropeptides and hormones involved in the control of CT secretion act by regulating Ca2+ channel activity via G proteins. One of these endogenous modulators is SS, which is produced by the C cells themselves and tonically inhibits CT secretion by inhibiting voltage-dependent Ca2+ channels.