Activation of Ca2+ entry into acinar cells by a non-phosphorylatable inositol trisphosphate

Depletion of intracellular calcium stores activates a calcium current in mast cells

In many cell types, receptor-mediated Ca2+ release from internal stores is followed by Ca2+ influx across the plasma membrane. The sustained entry of Ca2+ is thought to result partly from the depletion of intracellular Ca2+ pools. Most investigations have characterized Ca2+ influx indirectly by measuring Ca(2+)-activated currents or using Fura-2 quenching by Mn2+, which in some cells enters the cells by the same influx pathway. But only a few studies have investigated this Ca2+ entry pathway more directly. We have combined patch-clamp and Fura-2 measurements to monitor membrane currents in mast cells under conditions where intracellular Ca2+ stores were emptied by either inositol 1,4,5-trisphosphate, ionomycin, or excess of the Ca2+ chelator EGTA. The depletion of Ca2+ pools by these independent mechanisms commonly induced activation of a sustained calcium inward current that was highly selective for Ca2+ ions over Ba2+, Sr2+ and Mn2+. This Ca2+ current, which we term ICRAC (calcium release-activated calcium), is not voltage-activated and shows a characteristic inward rectification. It may be the mechanism by which electrically nonexcitable cells maintain raised intracellular Ca2+ concentrations and replenish their empty Ca2+ stores after receptor stimulation.

Inositol 1,3,4,5-tetrakisphosphate activates an endothelial Ca(2+)-permeable channel

Receptor-mediated increases in the cytosolic free calcium ion concentration in most mammalian cells result from mobilization of Ca2+ from intracellular stores as well as transmembrane Ca2+ influx. Inositol 1,4,5-trisphosphate (InsP3) releases calcium from intracellular stores by opening a Ca(2+)-permeable channel in the endoplasmic reticulum. But the mechanism and regulation of Ca2+ entry into nonexcitable cells has remained elusive because the entry pathway has not been defined. Here we characterize a novel inositol 1,3,4,5-tetrakisphosphate (InsP4) and Ca(2+)-sensitive Ca(2+)-permeable channel in endothelial cells. We find that InsP4, which induces Ca2+ influx into acinar cells, enhances the activity of the Ca(2+)-permeable channel when exposed to the intracellular surface of endothelial cell inside-out patches. Our results suggest a molecular mechanism which is likely to be important for receptor-mediated Ca2+ entry.

Muscarinic receptor-operated Ca2+ influx in transfected fibroblast cells is independent of inositol phosphates and release of intracellular Ca2+

Receptor-mediated changes in cytoplasmic calcium concentrations occur either through release from intracellular calcium stores or by the opening of channels in the plasma membrane, allowing influx of calcium from the extracellular fluid. Carbachol, a muscarinic receptor agonist, stimulated both calcium influx and inositol 1,4,5-trisphosphate (InsP3)-mediated intracellular calcium release in A9 fibroblast cells expressing a m3 muscarinic receptor clone. The calcium influx persisted even after pretreatment of cells with phorbol 12-myristate 13-acetate, which completely prevented the rise in inositol phosphates and intracellular calcium levels. The calcium influx was blocked by divalent cations but was not affected by inhibitors of voltage-dependent calcium channels or high potassium depolarization, indicating the presence of a receptor-operated and voltage-insensitive calcium channel in these cells. Calcium influx was not stimulated by the addition of cAMP analogs or arachidonic acid. To examine the possible involvement of G proteins in m3 receptor-activated calcium influx, two chimeric m2 and m3 muscarinic receptors were expressed in A9 cells in which the third cytoplasmic loop (the primary structural determinant in G protein coupling selectivity of muscarinic receptors) had been exchanged between the m2 receptor, which has no effect on calcium influx, and the m3 receptor. Calcium influx was found to be associated with a structural component of the m3 muscarinic receptor other than the third cytoplasmic loop.

The path of calcium in cytosolic calcium oscillations: a unifying hypothesis

Data from 42 systems have been assembled in which the overall spatial course of relatively natural, intracellular calcium pulses has been or can be determined. These include 21 cases of solitary pulses in activating eggs and 21 cases of periodic (as well as solitary) pulses in various fully active cells. In all cases, these pulses prove to be waves of elevated calcium that travel from one pole of a cell to the other or from the periphery inward. The velocities of these waves are remarkably conserved--at approximately 10 microns/sec in activating eggs and approximately 25 microns/sec in other cells at room temperature. Moreover, in three cases, the data suffice to show that these velocities fit the Luther equation for a reaction/diffusion wave of calcium through the cytosol. It is proposed that (i) natural intracellular calcium pulses quite generally take the form of cytosolic calcium waves and (ii) cytoplasmically controlled calcium waves are triggered and then propagated by the successive action of two distinct modes of calcium-induced calcium release. First, in the lumenal mode, a slow increase of calcium within the lumen of the endoplasmic reticulum reaches a level that triggers fast lumenal release as well as fast localized release into the cytosol. Then, the well-known cytosolic mode drives a reaction/diffusion wave across or into the cell.