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 [Cal²⁺] 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. Cal²⁺ influx through voltage-sensitive Cal²⁺ channels can stimulate secretion 1000-fold over the basal levels measured biochemically. Unlike neurons, however, melanotrophs continue to secrete for seconds after a depolarizing pulse, while they extrude or sequester the Cat2+ that has entered through Cat+ channels. Following episodes of secretion, pituitary cells can retrieve membrane with half-times around 30 s at 32°C, even in the absence of cytosolic K⁺.