Ca2+ ionophore

Br > Cl > I. This conductance was not time-dependent. On the other hand, Cl conductance stimulated by the Ca2+ ionophore A23187 (2.5 pM) or elevation in the free Ca2+ levels in the pipet from 100 to 500 nM was time-dependent and exhibited a nonlinear current-voltage relationship that was outwardly rectifying. The permselectivity of this Ca2+-stimulated conductance was 1 > Br > cr, distinguishing it from the cAMP-stimulated Cfr conductance. As both calcium ionophore A23187 and cAMP were shown to elevate the Cl conductance of the cornea, it is quite likely that these two types of Cr channels are present in the corneal epithelium [96,106,114],... 

The Ca2+ taken up in the TG-insensitive compartment could be released by the Ca2+ ionophore A23187. Only a partial release was observed with L1SP3. The concentration of InsP3 required for half-maximal release was about two times higher than that required for half-maximal activation of Ca2+ release from the compartment filled by the SERCA pump. A very similar pattern was seen for the InsP3 effect on the additional Ca2+ compartment created in COS-1 cells by overexpression of the worm Pmrl. Although still indirect, these data strongly... 

Constitutive activity of PDE was found in cultured carrot cells this activity did not depend on either Ca2+ or CAM. By contrast, a CAM-dependent isoform of PDE (CAM-PDE) was induced in the cells by adding forskolin or Bt2cAMP to the culture [30]. Induction of CAM-PDE activity in Bt2cAMP-treated carrot cells was markedly inhibited in the presence of verapamil, and addition of Ca2+-ionophore A23187 induced CAM-PDE [34]. These results suggest that increased Ca2+, but not cAMP, in the stimulated carrot cells triggers induction of the PDE isoenzyme. Affinity of CAM-PDE to the substrate was low compared to constitutive PDE Km values, 0.14 and 0.07 pM, respectively) however, V for the induced PDE was approximately 2.7 times higher than for the constitutive isoenzyme. 

Our understanding of the details of these processes will probably increase in the near future, due to the development of better techniques for the determination of intracellular [Ca2+], One example involves the use of the fluorescent indicator quin 2. This allows quantitative determination of [Ca2+] rather than the mere confirmation of increase or decrease in [Ca2+].412,413 Quin 2 binds Ca2+ with high selectivity, and its fluorescence increases more than four-fold upon saturation with Ca2". The indicator is added in a lipophilic form, the tetraacetoxymethyl ester, which is taken up by cells. Hydrolysis within the cells gives quin 2, which will then remain in the cell cytosol without binding to cytoplasmic proteins or uptake into organelles. An example of its use lies in the demonstration that release of insulin was mediated by increase in cytosolic Ca2+ produced by use of the Ca2+-ionophore ionomycin.414... 

Samples were made either by adding lipid to known amounts of salt solution or by adding lipid to dextran-plus-salt solutions. Mixtures were allowed to equilibrate for 48 hr and then were mounted between mica windows. Mixing and waiting at 50°C for up to 48 hr provided no advantage in reacning equilibria nor did the addition of a Ca2+ ionophore, A23187. All measurements were at room temperature (25°C). 

In the foregoing discussion (Sections 2.1.1 and 2.1.2), we have considered the effects of PKC activation in the regulation of neurotransmitter release by modulation of nerve terminal excitability. There are numerous instances, however, of regulation of neurotransmitter by PKC downstream of Ca2+ entry, evinced by studies using Ca2+ ionophores to bypass ion-channel involvement (see Majewski and Iannazzo... 

On stimulation of quiescent cells with growth factors or serum there is a rapid increase in the transmembrane flux of Na+, K+ and H+ and a mobilisation of Ca2+ from intracellular stores. The increase in Ca2+ concentration can be mimicked by treating cells with the Ca2+ ionophore, A23187. There quickly follows a series of events leading to changes in gene expression and cell structure and eventually to DNA replication and cell division. 

The second criterion to accept Ca2+ as a second messenger is its ability to stimulate LH release when intracellular concentrations are elevated by any means, even in the absence of GnRH. Incubation of gonadotropes with Ca2+ ionophores (A23187 or ionomycin) stimulates LH secretion in a dose- and time-dependent manner... 

In CHO cells, D4 receptors were shown to potentiate the increase in arachidonic acid release induced by ATP or Ca2+ ionophore. This effect appeared to require PTX-sensitive G proteins and to depend on PKC (Chio et al., 1994b Huff 1996). In the same cells, D4 receptor stimulation produced H+ excretion resulting from the activation of an amiloride-sensitive Na+/H+ exchanger through a PTX-sensitive mechanism (Chio et al., 1994b Coldwell et al., 1999). D4 receptor stimulation also increased the proliferation of CHO cells (Huff, 1996), showing that the signal transduction pathways stimulated by D4 receptors are identical to those stimulated by D2 receptors in this cell type. 

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