Regulation of exocytosis in neuroendocrine cells: spatial organization of channels and vesicles, stimulus-secretion coupling, calcium buffers and modulation.

Neuroendocrine cells display a similar calcium dependence of release as synapses but a strongly different organization of channels and vesicles. Biophysical and biochemical properties of large dense core vesicle release in neuroendocrine cells suggest that vesicles and channels are dissociated by a distance of 100-300 nm. This distinctive organization relates to the sensitivity of the release process to mobile calcium buffers, the resulting relationship between calcium influx and release and the modulatory mechanisms regulating the efficiency of excitation-release coupling. At distances of 100-300 nm, calcium buffers determine the calcium concentration close to the vesicle. Notably, the concentration and diffusion rate of mobile buffers affect the efficacy of release, but local saturation of buffers, possibly enhanced by diffusion barriers, may limit their effects. Buffer conditions may result in a linear relationship between calcium influx and exocytosis, in spite of the third or fourth power relation between intracellular calcium concentration and release. Modulation of excitation-secretion coupling not only concerns the calcium channels, but also the secretory process. Transmitter regulation mediated by cAMP and PKA, as well as use-dependent regulation involving calcium, primarily stimulates filling of the releasable pool. In addition, direct effects of cAMP on the probability of release have been reported. One mechanism to achieve increased release probability is to decrease the distance between channels and vesicles. GTP may stimulate release independently from calcium. Thus, while in most cases primary inputs triggering these pathways await identification, it is evident that large dense core vesicle release is a highly controlled and flexible process.