Suppression of Ca2+ syntillas increases spontaneous exocytosis in mouse adrenal chromaffin cells

Abstract

A central concept in the physiology of neurosecretion is that a rise in cytosolic [Ca2+] in the vicinity of plasmalemmal Ca2+ channels due to Ca2+ influx elicits exocytosis. Here, we examine the effect on spontaneous exocytosis of a rise in focal cytosolic [Ca2+] in the vicinity of ryanodine receptors (RYRs) due to release from internal stores in the form of Ca2+ syntillas. Ca2+ syntillas are focal cytosolic transients mediated by RYRs, which we first found in hypothalamic magnocellular neuronal terminals. (scintilla, Latin for spark; found in nerve terminals, normally synaptic structures.) We have also observed Ca2+ syntillas in mouse adrenal chromaffin cells. Here, we examine the effect of Ca2+ syntillas on exocytosis in chromaffin cells. In such a study on elicited exocytosis, there are two sources of Ca2+: one due to influx from the cell exterior through voltage-gated Ca2+ channels, and that due to release from intracellular stores. To eliminate complications arising from Ca2+ influx, we have examined spontaneous exocytosis where influx is not activated. We report here that decreasing syntillas leads to an increase in spontaneous exocytosis measured amperometrically. Two independent lines of experimentation each lead to this conclusion. In one case, release from stores was blocked by ryanodine; in another, stores were partially emptied using thapsigargin plus caffeine, after which syntillas were decreased. We conclude that Ca2+ syntillas act to inhibit spontaneous exocytosis, and we propose a simple model to account quantitatively for this action of syntillas.

Type 1 ryanodine receptor knock-in mutation causing central core disease of skeletal muscle also displays a neuronal phenotype

“…These findings suggest that the Ca 2+ from syntillas elicited by either type 1 RyRs (in nerve terminals) or type 2 (in chromaffin cells) is released into a microdomain different from the one where the final exocytic step occurs. Moreover, in chromaffin cells, we have found that syntillas that arise from RyR2s suppress exocytosis monitored amperometrically (33). A study on the porcine stress syndrome indicates that a similar mechanism may be present in the parvocellular neurons.…”

“…Next, sparks were found in smooth muscle (39) (40-42). Thus, the microdomain in smooth muscle is the basis for Ca 2+ to exert multiple, even opposite, effects and provides a precedent for their unexpected action in chromaffin cells (33).…”

Cell biology of Ca2+-triggered exocytosis

“…The three phases of LDCV exocytosis have been attributed to different vesicle pools, but it is uncertain whether these pools represent physically distinct types of vesicles, or simply different functional states. In addition, spontaneous exocytosis of LDCVs has also been described in chromaffin cells, and also appears to be regulated by Ca 2+ [30]. …”

Ryanodine Receptors Selectively Interact with L Type Calcium Channels in Mouse Taste Cells

“…Calcium syntillas do not cause exocytosis in magnocellular neurons [30] or in chromaffin cells [31] but make significant contributions to initiating exocytosis in dorsal root ganglion neurons [32]. In chromaffin cells, the syntilla calcium is released into a microdomain that is different from the site of exocytosis [31] where it appears to suppress exocytosis [33]. Thus, in multiple neurons, RyRs physically couple to L type calcium channels to generate small transient calcium syntillas and contribute to depolarization evoked global calcium increases.…”