Insulin Secretion : A High - affinity Ca 2 Sensor After All ?

Insulin is secreted from the -cells of the pancreatic islets by Ca 2 -dependent exocytosis of large dense core vesicles (LDCVs) (Ämmälä et al., 1993), a process that is triggered by glucose-induced electrical activity (Henquin and Meissner, 1984). In response to a step elevation of glucose, insulin secretion follows a characteristic biphasic time course (Curry et al., 1968): an initial transient first phase of secretion, which is completed within 10–15 min, is followed by a slowly developing and sustained second phase. It has been estimated that the first phase of secretion is due to the rapid release of a total of 40–80 LDCVs per -cell, after which secretion proceeds at a rate of five vesicles per -cell per minute (Rorsman and Renström, 2003). The Ca 2 channel density in -cell plasma membranes is very low, only about one-twentieth of that in chromaffin cells (Barg et al., 2001). Yet, the -cell is capable of remarkably high rates of exocytosis. Capacitance measurements have suggested that secretion transiently may proceed at rates as high as 500 LDCV per second (Barg et al., 2001). It therefore has been proposed that the -cell exocytosis is efficiently coupled to Ca 2 entry via the assembly of a functional complex consisting of Ca 2 channels and exocytotic proteins (Wiser et al., 1999) so that exocytosis is triggered by the large increases in the cytoplasmic Ca 2 concentration ([Ca 2 ] i ) occurring at the inner mouth of the Ca 2 channels (Fig. 1 A). In accordance with such a scenario, the rates of exocytosis that can be elicited by voltage-clamp depolarizations require elevation of [Ca 2 ] i by several tens of micromolar, as estimated from experiments using photolytic release of caged Ca 2 (Takahashi et al., 1997; Barg et al., 2001). The latter experiments suggested that exocytosis is sigmoidally related to [Ca 2 ] i with a K d of 20 M and a Hill coefficient (n) as high as 5. By contrast, measurements of insulin secretion from permeabilized cells have indicated that insulin secretion is activated already at submicromolar [Ca 2 ] i (Yaseen et al., 1982; Wollheim et al., 1987; Okazaki et al., 1994) and capacitance increases have also been observed at such low Ca 2 concentrations (Proks et al., 1996). Indeed, the latter type of measurements indicate that exocytosis at Ca 2