1. We have measured intracellular pH (pHi) in freshly isolated pyramidal neurones from the CAI region of the rat hippocampus using the fluorescent indicator 2',7'-bis(carboxyethyl)-5(and-6)-carboxyfluorescein (BCECF). 2. The neurones selected by our isolation procedure, when studied in the nominal absence of C02-HCO3-, had a mean steady-state pHi of 6-81 + 0-02 (n = 163). The recovery of pH1 from acid loads was very slow. The rate of recovery from acid loads was reduced by Na+ removal, but only very slightly inhibited by 1 mm amiloride.3. The addition of 5 % C02-25mm HC03-caused steady-state pHi to increase from 6-74 + 0 05 to 7 03 + 0 03 (n = 28). In the presence of 5 % C02-25 mm HC03-, the rate of pH1 recovery from acid loads was much faster than in its absence.4. The HC03--induced alkalinization was reversible, and did not occur in the absence of extracellular Na+ or in the presence of DIDS (4,4'-diisothyocyanatostilbene-2,2'-disulphonic acid). 5. In the absence of external Cl-, successive exposures to CO2-HCO3-elicited alkalinizations that were progressively reduced in rate and amplitude. This effect, presumably due to gradual depletion of internal Cl-, was rapidly reversed by returning Cl-to the external medium. 6. We conclude that the major acid-extrusion mechanism in pyramidal CAL neurones is the Na+-dependent Cl--HC03-exchanger. The Na+-dependent mechanism that operates in the nominal absence of HC03-is far less active.

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Our results indicate that a combination of phosphoinositide depletion and acidification of the membrane/boundary layer is sufficient to activate the light-sensitive channels. Together with the demonstration of light-induced, PLC-dependent acidification, this suggests that excitation in Drosophila photoreceptors may be mediated by PLC's dual action of phosphoinositide depletion and proton release.

Aims/hypothesisWe set out to test the hypothesis that insulin secretion from beta cells is targeted towards the vasculature.MethodsThe spatial location of granule fusion was identified by live-cell two-photon imaging of mouse pancreatic beta cells within intact islets, using sulforhodamine B labelling. Three-dimensional (3D) immunofluorescence of pancreatic slices was used to identify the location of proteins associated with neuronal synapses.ResultsWe demonstrated an asymmetric, non-random, distribution of sites of insulin granule fusion in response to glucose and focal targeting of insulin granule secretion to the beta cell membrane facing the vasculature. 3D immunofluorescence of islets showed that structural proteins, such as liprin, piccolo and Rab2-interacting molecule, normally associated with neuronal presynaptic targeting, were present in beta cells and enriched at the vascular face. In contrast, we found that syntaxin 1A and synaptosomal-associated protein 25 kDa (SNAP25) were relatively evenly distributed across the beta cells.Conclusions/interpretationOur results show that beta cells in situ, within intact islets, are polarised and target insulin secretion. This evidence for an ‘endocrine synapse’ has wide implications for our understanding of stimulus–secretion coupling in healthy islets and in disease.Electronic supplementary materialThe online version of this article (doi:10.1007/s00125-014-3252-6) contains peer-reviewed but unedited supplementary material, which is available to authorised users.