Cytosolic Ca2+, Exocytosis, and Endocytosis in Single Melanotrophs of the Rat Pituitary

We have monitored cytosolic [Ca2+] with fura-2 and exocytosis by measuring the membrane capacitance, and we have studied the influence of cytosolic [Ca2+] on secretion in single endocrine cells. As in neurons, cytosolic Ca2+ is sufficient to trigger exocytosis. The rate of secretion grows with the fourth or fifth power of cytosolic [Ca2+], and paired stimuli reveal facilitation. Ca2+ influx through voltage-sensitive Ca2+ channels can stimulate secretion 1000-fold over the basal levels measured biochemically. Unlike neurons, however, melanotrophs continue to secrete for seconds afer a depolarizing pulse, while they extrude or sequester the Ca2+ that has entered through Ca2+ channels. Following episodes of secretion, pituitary cells can retrieve membrane with half-times around 30 s at 32 degrees C, even in the absence of cytosolic K+.