Ca(2+)-dependent unidirectional vesicular release detected with a carbon-fibre electrode in rat pancreatic acinar cell triplets

Controlling the electrochemically active area of carbon fiber microelectrodes by the electrodeposition and selective removal of an insulating photoresist

A new and simple method permits control of the electrochemically active area of a carbon fiber microelectrode. An electrophoretic photoresist insulates the 10-microm-diameter carbon fiber microelectrodes. Photolysis of the photoresist followed by immersion of the exposed area into a developing solution reveals electroactive carbon fiber surface. The electroactive surface area exposed can be controlled with a good degree of reproducibility.

Spatiotemporal analysis of exocytosis in mouse parotid acinar cells

Exocrine cells of the digestive system are specialized to secrete protein and fluid in response to neuronal and/or hormonal input. Although morphologically similar, parotid and pancreatic acinar cells exhibit important functional divergence in Ca(2+) signaling properties. To address whether there are fundamental differences in exocytotic release of digestive enzyme from exocrine cells of salivary gland versus pancreas, we applied electrophysiological and optical methods to investigate spatial and temporal characteristics of zymogen-containing secretory granule fusion at the single-acinar cell level by direct or agonist-induced Ca(2+) and cAMP elevation. Temporally resolved membrane capacitance measurements revealed that two apparent phases of exocytosis were induced by Ca(2+) elevation: a rapidly activated initial phase that could not be resolved as individual fusion events and a second phase that was activated after a delay, increased in a staircaselike fashion, was augmented by cAMP elevation, and likely reflected both sequential compound and multivesicular fusion of zymogen-containing granules. Optical measurements of exocytosis with time-differential imaging analysis revealed that zymogen granule fusion was induced after a minimum delay of approximately 200 ms, occurred initially at apical and basolateral borders of acinar cells, and under strong stimulation proceeded from apical pole to deeper regions of the cell interior. Zymogen granule fusions appeared to coordinate subsequent fusions and produced persistent structures that generally lasted several minutes. In addition, parotid gland slices were used to assess secretory dynamics in a more physiological context. Parotid acinar cells were shown to exhibit both similar and divergent properties compared with the better-studied pancreatic acinar cell regarding spatial organization and kinetics of exocytotic fusion of zymogen granules.

Exocytosis and movement of zymogen granules observed by VEC-DIC microscopy in the pancreatic tissue en bloc

The dynamic aspects of exocytosis, especially in the normal acinar tissue en bloc, have remained unclear. We visualized exocytosis directly in the tissue of the exocrine pancreas of rodents by video-enhanced contrast-differential interference contrast (VEC-DIC) microscopy to investigate various exocytosis-related rates and the relationship between the movement of granules and exocytotic responses. Stimulation of the tissue with bethanechol or cholecystokinin caused many of the zymogen granules in the apical pole to disappear abruptly. The exocytotic transients of individual granules were completed in 0.48-0.65 s. Granules destined to participate in the exocytotic response moved randomly at velocities of approximately 0.06 microm/s or less during stimulation. In the tissue preparation, granules located far from the apical pole frequently moved back and forth for 1-7 microm without showing exocytosis. Colchicine suppressed this movement and the late phase of the secretory response. Real-time (VEC-DIC) observation of granule dynamics revealed that the initial step of exocytosis was not coupled directly with the microtubule-dependent translocation but with a continuous, slow Brownian fluctuation of granules.

Selective activation of exocytosis by low concentrations of ACh in rat pancreatic acinar cells

1. We have monitored changes in membrane capacitance (delta C) and conductance (delta G) induced by muscarinic acetylcholine stimulation in single rat pancreatic acinar cells. 2. Acetylcholine (ACh, 500 nM) induced simultaneous increases of delta C and delta G. In contrast, a low concentration (50 nM) of ACh exclusively induced delta C increases without delta G. These responses were abolished by the internal perfusion of heparin. This indicates that inositol 1,4,5-trisphosphate-mediated internal Ca2+ mobilization either simultaneously activates exocytosis and ion channels or exclusively initiates exocytosis. In comparison, a low concentration of A23187 selectively activated ion channels but a high concentration activated exocytosis and ion channels simultaneously. 3. These selective response patterns of delta C and delta G depend on the choice of agonist and the internal EGTA concentration. From this, we postulated two explanations for the selective action of muscarinic ACh stimulation on exocytosis. First, an area of high [Ca2+]i, spatially close to secretory granules, activates exocytosis. Second, an as yet unknown signalling factor sensitizes the Ca2+ affinity of the exocytotic apparatus.

A fluorimetric and amperometric study of calcium and secretion in isolated mouse pancreatic beta-cells

We have examined the temporal relationship between intracellular Ca2+ concentration ([Ca2+]i) and secretion in single intact pancreatic beta-cells. Secretion was detected as the release of 5-hydroxytryptamine from pre-loaded beta-cells, using amperometry, and changes in [Ca2+]i were monitored by microfluorimetry. Stimulation of beta-cells by elevation of the extracellular K+ concentration ([K+]o), acetylcholine or glucose increased [Ca2+]i and, after a delay of 2-7 s, evoked amperometric currents. In the presence of glucose, we observed oscillations in [Ca2+]i which were associated with oscillations in the amplitude and frequency of amperometric currents: however, the temporal correlation was not exact, suggesting that there is a significant latency between the increase in average [Ca2+]i and exocytosis. Both the amplitude and frequency of the amperometric currents elicited by 50 mM KCl declined with successive stimulation, but were restored by agents which elevate intracellular adenosine 3':5':cyclic monophosphate (cAMP). This suggests that beta-cells may possess a readily releasable pool of granules which is replenished by cAMP. The variable amplitude of the amperometric currents is discussed in terms of a model in which several secretory granules fuse simultaneously with the plasma membrane.