In all eukaryotic cells there are two general mechanisms to generate a Ca2+ signal, that is either activation of Ca2+ entry through the cellular plasma membrane and/or the release of Ca2+ from specialised intracellular organelles (endoplasmic reticulum, in particular), generally known as intracellular calcium stores. Intracellular sources deliver most of the Ca2+ ions for the Ca2+ signal in non-excitable cells, as well as in several types of excitable cells (eg, muscle and secretory cells). Neurones are the notable exception, in which the plasmalemmal Ca2+ entry apparently dominates, with the intracellular calcium stores playing mostly a modulatory role.
So far, the endoplasmic reticulum (ER) Ca2+ stores are the best characterised Ca2+ homeostatic organelles in nerve cells. In general terms, ER calcium stores may modulate the plasma membrane-generated Ca2+ signal either acting as a Ca2+ buffering system (`Ca2+ sink') removing cytoplasmic Ca2+ loads by SERCA (SarcoEndoplasmic Calcium ATPase) pumping, or by amplifying this signal via Ca2+-induced Ca2+ release (CICR). These processes - Ca2+ buffering and Ca2+ release - are functionally tightly coupled: Ca2+ buffering increases the releasable Ca2+ content of the ER, which may significantly modulate the availability and/or magnitude of stimulation-induced Ca2+ release from these stores.
The relevance of neuronal ER Ca2+ stores appears to differ depending on the neuronal model used. In peripheral neurones caffeine, an established agent to trigger CICR, induce reliably an intracellular Ca2+ signal. In contrast, in central neurones the prerequisite for a caffeine-induced CICR is a conditioning depolarisation pulse, which loaded the neurones, and the ER stores with Ca2+ ions. This led to the proposal that in central neurones the ER stores are devoid of releasable Ca2+ under resting conditions. None the less they are ready to accumulate Ca2+ entering the cell during neuronal activity; after being `charged' they became able to fire a full-blown Ca2+ release.
We are going to increase our knowledge of the regulatory function of intraluminal Ca2+ concentration on the availability of Ca2+ release machinery. The planned experiments will combine intraneuronal Ca2+ recordings (in cytosol ([Ca2+]i, and in stores, [Ca2+]L) with electrophysiological measurements of transmembrane currents, which will require controlled access to both cell cytoplasm (intracellular dialysis) and cellular membrane (adequate voltage clamp in patch-clamp experiments). We are going to measure net calcium fluxes through both plasma membrane and membrane of the ER. We will also establish how the intra-ER Ca2+ content may influence the properties of Ca2+ release from intracellular calcium stores.