Optical detection of calcium dependent ATP release from stimulated medullary chromaffin cells

Evanescent-wave microscopy: a new tool to gain insight into the control of transmitter release

Evanescent-wave excitation was used to visualize individual fluorescently labelled vesicles in an optical slice near the plasma membrane of bovine adrenal chromaffin cells. A standard upright microscope was modified to accommodate the optics used for directing a laser beam under a supracritical angle on to the glass-water interface on top of which the cells are grown. Whereas epi-illumination images appeared blurred and structureless, evanescent-wave excitation highlighted acridine orange-labelled vesicles as individual pinpoints. Three-dimensional (3D) trajectories of individual vesicles were obtained from time-resolved image stacks and used to characterize vesicles in terms of their average fluorescence F and mobility, expressed here as the 3D diffusion coefficient D(3). Based on the single-vesicle analysis, two groups of vesicles were identified. Transitions between these states were studied before and after stimulation of exocytosis by repetitive or maintained membrane depolarizations by elevated extracellular [K+]. Findings were interpreted as sequential transitions between the previously characterized pools of vesicles preceding the fusion step. The observed approach of vesicles to their docking sites was not explained in terms of free diffusion: most vesicles moved unidirectionally as if directed to their binding sites at the plasma membrane. Vesicle mobility at the membrane was low, such that the sites of docking and fusion were in close vicinity. Both the rim region and confined areas in the centre of the footprint region were the site of intense vesicle trafficking.

Elemental composition of secretory granules in pancreatic islets of Langerhans

We have characterized, by electron probe microanalysis, rapidly frozen cultured rat islets at the level of individual secretory granules. Elemental analysis of thin, dried cryosections showed that beta granules could be distinguished by high Zn, Ca, and S, whereas non-beta (mainly alpha) granules contained elevated P and Mg. Although a single granule type predominated in a particular cell, some rebel granules were found in A cells that had the compositional fingerprint of B cell granules. Zn, which was found in millimolar concentrations in B cell granules, was considered a marker for the insulin storage complex. The data indicate that non-B islet cells in the adult pancreas may produce insulin-containing organelles and that, when glucagon and insulin are coexpressed, these hormones are packaged in separate granules.

Endoplasmic reticulum as a source of Ca2+ in neurotransmitter secretion

Depolarization of the synaptosomal membrane by a rapid elevation of [K+]0 induces secretion of adenosine-5'-triphosphate (ATP) as well as the specific neurotransmitters. In addition to the classical [Ca2+]0-dependent mode, we have found that ATP secretion also occurred in the absence of extracellular calcium [( Ca2+]0 less than 1 microM). The extent of both modalities of secretion depended on membrane potential, and the [Ca2+]0-independent secretion proceeded at a rate that was substantially smaller than that of the [Ca2+]0-dependent mode at all membrane potentials examined. We propose that intracellular stores may provide the Ca2+ required for exocytosis in the [Ca2+]0-independent mode of ATP secretion. To test this hypothesis, we searched for the presence of Ca(2+)-release channels gated by intracellular messengers in our synaptosomal preparation. We fused membrane vesicles from lysed synaptosomes with acidic phospholipid bilayers formed at the tip of a patch pipette and found that these membranes contained a Ca(2+)-selective channel. The properties of this channel resemble those of the Ca(2+)-release channel reconstituted from sarcoplasmic reticulum membrane vesicles. These include size of the single open-channel conductance (75 pS Cs+ as the main current carrier), activation by adenine nucleotides (ATP), ryanodine and caffeine, and inhibition by ruthenium red.

Calcium dependency of muscarinic and nicotinic agonist-induced ATP and catecholamine secretion from porcine adrenal chromaffin cells

The secretion of catecholamines and ATP induced by cholinergic agonists and its dependence on extracellular Ca2+ were studied in cultured porcine adrenal chromaffin cells. Both nicotine and methacholine (a selective muscarinic agonist) induced secretion and increases in cytosolic free Ca2+ concentration ([Ca2+]in), although the activation of nicotinic receptors produced responses that were larger than those produced by activation of muscarinic receptors. The secretion and the increase in [Ca2+]in evoked by nicotine were completely dependent on extracellular Ca2+ and were blocked by prior depolarization of the cells with high extracellular K+ levels. In addition, nicotine induced significant 45Ca2+ influx. In contrast, the secretion and the increase in [Ca2+]in evoked by methacholine were partially dependent on extracellular Ca2+; methacholine also induced 45Ca2+ influx. Prior depolarization of the cells with high extracellular K+ levels did not block methacholine-induced secretion. In general, nicotinic responses were mediated by Ca2+ influx through voltage-dependent pathways. In contrast, muscarinic responses were dependent on both Ca2+ influx through an unknown mechanism that could not be inactivated by high K+ concentration-induced depolarization and presumably also intracellular Ca2+ mobilization.

Kinetic characteristics of calcium-dependent, cholinergic receptor controlled ATP secretion from adrenal medullary chromaffin cells

Adrenal chromaffin cells secrete catecholamines (CA) and ATP in response to acetylcholine (ACh) and high [K+]o. The release process is relatively fast making it difficult to measure the early phase of the secretory response. Recently we were able to resolve the time course of the secretory response by measuring the release of ATP using luciferin-luciferase included in the extracellular medium. For the three secretagogues studied, ACh, nicotine and high [K+]o, the early phase of release followed a complex kinetics. Allowing for an initial delay of the secretory response, the kinetics could be described as the sum of two power exponential processes. Increasing the temperature from 23 to 37 degrees C induced a marked decrease in the two time constants needed to fit the early time course of the ATP secretion. The activation energies, estimated from Arrhenius plots, were approx. 20 and 16 kcal/mol for both phases of ATP release induced by either cholinergic agonists or high [K+]o. These results suggest that cholinergic receptor activation and membrane depolarization induce ATP (and CA) secretion through a common pathway. The initial delay in the onset of the secretory response decreased with increasing doses of secretagogue and with temperature. We propose that the delay preceding the actual onset of ATP release represents the time required for generation of intracellular second messengers. The effective concentration attained by these messengers depend apparently on both receptor occupancy by the agonist and the ensuing Ca2+ channel activation.