Phylogenetic considerations of neurosecretory granule contents: role of nucleotides and basic hormone/transmitter packaging mechanisms.

The characteristics of neurosecretory granules include the presence of an acidic interior, a hyperosmolar concentration of granule solutes, the presence of chromogranin (CG) or CG-like soluble acidic proteins and a high content of nucleotides, predominantly ATP. The identification of "nucleotides" within the neuroendocrine "stem cells" of coelenterates (e.g. Hydra) has raised some interesting evolutionary questions as to the function of intragranular nucleotides. The chromaffin granules of adrenal medullary cells have been studied extensively, and are representative of the neurohormone/neurotransmitter packaging problems encountered in neurosecretory granules, in general. At the acid pH (5.7) of the interior of the chromaffin granule, ATP has three negative charges based on the pK value of the gamma-phosphate group. ATP can therefore interact with positively charged amines, acetylcholine and divalent cations, forming binary and ternary complexes. The results of nuclear magnetic resonance (NMR) spectroscopy indicate that the hyperosmolar solutes within the chromaffin granule exist in a viscous, but fluid state; one function of ATP could be to help lower the osmotic pressure of the granule contents through extensive, but weak, intermolecular bonding. In addition, ATP is an excellent buffer to help maintain a pH of 5.7 within the interior of the chromaffin granule. An acidic milieu contributes to neurohormone/neurotransmitter packaging and granule stability. The presence of nucleotides within neurosecretory granules cannot, however, be explained on the basis of the ability of ATP to simply reduce osmotic pressure, since insulin molecules exist in a crystalline phase, a condition which, by itself, could substantially reduce osmotic pressure; nucleotides, nevertheless, co-exist in these insulin cores. ATP and ATP metabolites such as ADP, AMP and adenosine, formed as a result of the action of ectonucleotidases, can have extensive extracellular trophic and feedback effects after secretion. Extracellular nucleotides and adenosine can function as neuromodulators, agonists and antagonists to inflammatory cells, and regulators of blood flow, etc. It is possible that intragranular nucleotides were retained through a billion or more years of evolution because of the importance of these trophic and feedback effects. Parts of the neuroscretory granule, such as the F1 subunit of the proton-translocating ATPase, can be traced back to the aerobic bacteria, vacuolar amine transport to yeast and a CG-like acidic protein to protozoan secretory granules (i.e., the trichocysts of Paramecia).