Stimulus–secretion coupling in neurohypophysial nerve endings: A role for intravesicular sodium?

Abstract

It is generally accepted that Ca is essentially involved in regulated secretion, but the role of this cation, as well as others such as Na, is not well understood. An illustrative example occurs in neurohypophysial secretion, where an experimentally induced increase in the cytosolic concentration of Na ؉ can induce continuous neuropeptide release. In contrast, an increase in cytosolic Ca 2؉ will have only a transient stimulatory effect. The secretion-promoting targets for Ca 2؉ are not known; they may be cytosolic, as is usually assumed, but they may also be intravesicular, especially in view of evidence that Ca-rich secretory vesicles are preferentially secreted. In the present work, we have investigated the movements of these cations into and out of secretory vesicles during stimulus-secretion coupling. Isolated rat neurohypophysial nerve endings were stimulated by potassium (55 mM) depolarization, and at 6 min (peak secretion) and 20 min after the onset of stimulation, the elemental content of individual secretory vesicles was measured by quantitative x-ray microanalysis. A depolarization-induced transient increase in intravesicular Na ؉ concentration was found to coincide with the onset of secretion. Moreover, only a predicted small fraction of peripheral vesicles-presumably the docked ones-were Na ؉ -loaded. The low sulfur concentration of Na ؉ -rich vesicles most likely resulted from vesicle swelling. The results suggest that high intravesicular Na ؉ concentrations in docked vesicles, occurring by Na ؉ ͞Ca 2؉ exchange or by transient fusion pore opening, is a proximal event in exocytosis. Ca 2ϩplays an essential role in several stimulatory, and perhaps also inhibitory, steps of regulated exocytosis but, despite much recent progress, its precise targets are still largely a matter of debate (1-3). In neurohypophysial secretion particularly, it is clear that Ca 2ϩ cannot be the ultimate and sufficient trigger for neurosecretion, because the obligatory depolarizationinduced increase in cytosolic Ca 2ϩ concentration stimulates release for only a few minutes (4, 5), even when depolarization is maintained. This behavior contrasts with the known but mechanistically obscure secretagogue effect of Na ϩ (6-8), wherein an artificially induced increase in cytosolic Na ϩ concentration can induce a sustained and continuous secretion (8). In previous work on neurohypophysial nerve endings, one of our laboratories reported that the Ca content of the secretory vesicles increased upon secretory stimulation, and that Ca-rich vesicles accumulated when secretion was blocked (9), suggesting a role of intravesicular ion concentration in the secretory mechanism. This hypothesis is further supported by observation that in insulin-secreting cells, Ca 2ϩ depletion from secretory granules inhibits exocytosis (10). Because the swelling of some granule matrices is inhibited by Ca 2ϩ and promoted by Na ϩ (11, 12), we have investigated in isolated and stimulated rat neurohypophysial nerve endings possible changes in the concentra...

Mechanisms of the Release of Anterogradely Transported Neurotrophin-3 from Axon Terminals

“…3B), indicating that TTX-sensitive sodium channels are not required for the release process when induced by high K ϩ . Together, these results may suggest that an unconventional sodium-dependence of neurotrophin release may be a general feature for neuropeptide release from axon terminals (Stuenkel and Nordmann, 1993;Thirion et al, 1999).…”

“…Initial studies on the release of neurotrophins concluded that extracellular sodium is required for regulated release of neurotrophins from dendritic and somal compartments (Blöchl and Thoenen, 1995), but this may have been attributable to an independent blocking effect of the sodium substitute, N-methyl-D-glucamine (NMG) (Hoener, 2000). Release of neuropeptides from terminals (rather than whole cells) does seem to require sodium influx (Stuenkel and Nordmann, 1993;Thirion et al, 1999). To determine whether low extracellular sodium may attenuate NT-3 release from axon terminals, 125 I-NT-3 loaded tecta were incubated in buffer containing high K ϩ , and NMG to replace Ͼ 90% of the extracellular sodium.…”

The Intraacrosomal Calcium Pool Plays a Direct Role in Acrosomal Exocytosis

“…A direct role for intravesicular calcium in membrane fusion has been proposed in several other transport events (32), including the secretion of exocytic granules (33)(34)(35). During the fusion process, membranes that are going to fuse are maintained in close proximity creating a partially isolated environment where the calcium concentration may be strongly altered by the local action of calcium pumps or the opening of calcium channels.…”

Calcium dynamics in catecholamine-containing secretory vesicles

“…Regarding alternative Ca 2+ transport systems in the granules, the presence of either Ca 2+ -ATPases, Na + /Ca 2+ or H + /Ca 2+ exchange systems have been proposed in different preparations [14][15][16][17][18]. Inhibition of sarcoendoplasmic reticulum Ca 2+ -ATPase with 10 M benzohydroquinone produced little effects on [Ca 2+ ] SG (data not shown), suggesting that this type of Ca 2+ -ATPase was not involved.…”