Pulsatile intracellular calcium release does not depend on fluctuations in inositol trisphosphate concentration

Inhibition by Ca2+ of inositol trisphosphate-mediated Ca2+ liberation: a possible mechanism for oscillatory release of Ca2+

Light-flash photolysis of caged inositol 1,4,5-trisphosphate (InsP3) was used to generate reproducible transients of free InsP3 in Xenopus oocytes, and the resulting liberation of Ca2+ from intracellular stores was monitored by recording Ca2+-activated membrane currents and by use of the fluorescent Ca2+ indicator fluo-3. InsP3-mediated Ca2+ release was inhibited by elevating the intracellular free Ca2+ level, either by microinjecting Ca2+ into the cell or by applying conditioning light flashes to liberate Ca2+. This inhibition followed a slow time course, being maximal after about 2 s and subsequently declining over several seconds. Negative feedback of Ca2+ ions on InsP3-mediated Ca2+ liberation may explain the oscillatory release of Ca2+ seen during activation of inositol phospholipid signaling in the oocyte, and the time course of the inhibition is consistent with the period of the oscillations.

A metabolically stable analog of 1,4,5-inositol trisphosphate activates a novel K+ conductance in pyramidal cells of the rat hippocampal slice

IP(s)3, a metabolically stable analog of 1,4,5-inositol trisphosphate (IP3), inhibited action potential firing when injected into hippocampal pyramidal cells. This effect was associated with decreased input resistance, a more negative resting potential, outward rectification at depolarized potentials, and an afterhyperpolarization. The response to IP(s)3 was unaffected by antagonists of Na+, Ca2+, and Cl- conductances, but was sensitive to changes in extracellular K+ concentration. The IP(s)3-induced conductance was voltage-dependent, was activated in 10 ms with depolarization, and was blocked by extracellular Ba2+ or intracellular Ca2+ chelation. It was not suppressed by other K+ conductance antagonists. Thus, IP(s)3 may activate a novel K+ conductance in CA1 pyramidal cells. IP3 itself did not elicit this conductance, suggesting it may be rapidly metabolized in these cells.

Inositol phosphates and cell signalling

Inositol 1,4,5-trisphosphate is a second messenger which regulates intracellular calcium both by mobilizing calcium from internal stores and, perhaps indirectly, by stimulating calcium entry. In these actions it may function with its phosphorylated metabolite, inositol 1,3,4,5-tetrakisphosphate. The subtlety of calcium regulation by inositol phosphates is emphasized by recent studies that have revealed oscillations in calcium concentration which are perhaps part of a frequency-encoded second-messenger system.

Inositol 1,3,4,5-tetrakisphosphate and inositol 1,4,5-trisphosphate act by different mechanisms when controlling Ca2+ in mouse lacrimal acinar cells

In internally perfused single lacrimal acinar cells the competitive inositol 1,4,5-trisphosphate (Ins 1,4,5-P3)-antagonist heparin inhibits the ACh-evoked K+ current response mediated by internal Ca2+ and also blocks both the Ins 1,4,5-P3-evoked transient as well as the sustained K+ current increase evoked by combined stimulation with internal Ins 1,4,5-P3 and inositol 1,3,4,5-tetrakisphosphate (Ins 1,3,4,5-P4). When, during sustained stimulation with both Ins 1,4,5-P3 and Ins 1,3,4,5-P4, one of the inositol polyphosphates is removed, the K+ current declines; whereas removal of Ins 1,4,5-P3 results in an immediate termination of the response, removal of Ins 1,3,4,5-P4 only causes a very gradual and slow reduction in the current. Ins 1,3,4,5-P4 is therefore not an acute controller of Ca2+ release from stores into the cytosol, but modulates the release of Ca2+ induced by Ins 1,4,5,P3 by an unknown mechanism, perhaps by linking Ins 1,4,5 P3-sensitive and insensitive Ca2+ stores.