Ionophore mediated catecholamine release from chromaffin granules. Comparison of X-537 A and X-14547 A effects

Pharmacological evidence for the involvement of calcium/calmodulin in serotonin 5-HT3 receptor-mediated cation permeability in NG 108-15 cells

In NG 108-15 clonal cells, extracellular application of micromolar concentrations of serotonin [5-hydroxytryptamine (5-HT)] and substance P induces the opening of a cation permeability monitored by the influx of [14C]-guanidinium. The serotoninergic component of this cation permeability is linked to 5-HT3 receptor activation, whereas the substance P component probably involves an "N-terminal-dependent substance P receptor." In this study, [14C]guanidinium influx triggered by 1 microM 5-HT plus 10 microM substance P was shown to be insensitive to tetrodotoxin, verapamil, diltiazem, nimodipine, and omega-conotoxin, as expected from a process independent of voltage-sensitive sodium and calcium channels. In contrast, [14C]guanidinium influx was inhibited by millimolar concentrations of extracellular calcium and by the chelation of intracellular calcium by bis-O-aminophenoxyethanetetraacetic acid. The inhibition by extracellular calcium apparently involved a competition between the divalent cation and [14C]guanidinium for the same channel. When NG 108-15 cells were exposed to X537A, an ionophore that specifically induces release of calcium from intracellular stores, [14C]guanidinium uptake was markedly increased even in the absence of 5-HT and/or substance P. Conversely, [14C]guanidinium influx due to the latter substances could be reversibly and dose-dependently blocked by various drugs that possess calmodulin-antagonizing properties. These results strongly suggest that the cation permeability opened by 5-HT and substance P in NG 108-15 cells involves a calcium/calmodulin-dependent process.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of endogenous brain epinephrine by the calcium ionophores X537A and A23187

The Ca2+ ionophores X537A and A23187 produced dose-dependent release of endogenous epinephrine (Epi) as well as norepinephrine (NE) and dopamine (DA) from chopped rat hypothalamus and brainstem. The X537A-induced release of these catecholamines (CAs) was found to be Ca2+-independent, whereas the A23187-induced release was Ca2+-dependent. X537A and A23187 were approximately equipotent in causing the release of the hypothalamic Epi, NE and DA, but X537A was much more effective than A23187 in causing the maximal release. X537A, but not A23187, reduced the total endogenous CA content and increased the total 3,4-dihydroxyphenylacetic acid (DOPAC) content in the chopped hypothalamus. Ca2+-independent release of Epi, NE and DA in the chopped hypothalamus was also observed with indirectly acting sympathomimetic amines (e.g. tyramine and amphetamine). Tyramine and amphetamine did not affect the total endogenous CA contents nor the total DOPAC content. These results suggest that X537A caused release of endogenous brain Epi, NE and DA by transporting the biogenic amines across vesicular or intracellular storage sites. However, A23187 caused release of these CAs by exocytosis via transport of Ca2+ into the neurons.

Characterization of the monoamine uptake system in catecholamine storage vesicles isolated from a pheochromocytoma taken from a child

The catecholamine storage vesicles of a pheochromocytoma taken from a child have been isolated and characterized. The tumor contained almost exclusively noradrenaline and a large proportion of this amine was vesicle-bound. The noradrenaline-containing vesicles showed great resemblance to bovine chromaffin granules. Their catecholamine and dopamine beta-hydroxylase contents were that of chromaffin granules; their morphology and density were similar to those of the subpopulation of these granules that contain noradrenaline. The pheochromocytoma vesicles contained in their membranes an abundant polypeptide of mol. wt 110,000, which was not apparent in bovine adrenal medulla vesicle membranes. Monoamine uptake by pheochromocytoma noradrenaline vesicles did not differ significantly from that observed in bovine chromaffin granules. The time-course, plateau level and KM for noradrenaline were similar for both types of organelles. Both had an oligomycin-resistant ATPase with similar properties. Investigations using the tetrabenazine derivative [2-3H]dihydrotetrabenazine (2-hydroxy-3-isobutyl-9,10-dimethoxy-1,2,3,4,6, 7-hexahydro-11b-H-benzo[a]quinolizine), which binds specially to the bovine chromaffin granule monoamine carrier indicated that granule membranes from the tumor have a 10-fold increased number of [2-3H]dihydrotetrabenazine binding sites, with no change in dissociation constant. As in the case of bovine chromaffin granules, [2-3H]dihydrotetrabenazine can be totally displaced by noradrenaline and serotonin. To account for the discrepancy observed between the uptake data (which indicated no difference with bovine chromaffin granules) and the [2-3H]dihydrotetrabenazine binding studies (which showed a large excess of binding sites in the tumor membranes), we propose that granules in the investigated tumor contained a large amount of inactive monoamine carrier.

Characteristics and determinants of osmotic lysis in chromaffin granules

(1) Using isolated bovine chromaffin granules, we demonstrate that osmotic lysis is not a random process and establish the osmotic pressure dependence of osmotic lysis in chromaffin granules, the so-called osmotic fragility curve. (2) We show by measuring the release of constituents of the granule core and correlating these with changes in spectroscopic parameters (turbidity and endogenous catecholamine fluorescence), that the latter can be safely used to measure lysis. (3) Within a particular granule population, noradrenaline granules lyse at higher osmolarities than adrenaline granules, suggesting a higher core osmolarity of the noradrenaline granules. (4) The size distribution of chromaffin granules as a function of lysis was determined by the use of whole mount electron microscopy. It is shown that the mean size of chromaffin granules decreases as a function of lysis. (5) On the basis of theoretical considerations three alternative models of the sequence of osmotic lysis in chromaffin granules are proposed. The experimental results best support a model which postulates that during partial osmotic lysis, granule membranes reseal into smaller vesicles after graded release of contents. The osmotic fragility would represent several cycles of lysis and resealing and would not be a reflection of the distribution of osmotic pressures in the granule population.