Changes in iare reported for one single cell in oscillation experiments or a group of cells in other protocols.However, in the current study in which wild type HEK cells were used, we failed to consistently produce repetitive transient responses of iwith mM carbachol.However, significantly different results were obtained when cells were stimulated to oscillate with the lower, mM concentration of carbachol.The decay of the iresponse to thapsigargin under these conditions is due <a href="http://inhibit07.online/archives/253"></a>
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entirely to plasma membrane extrusion. Concentrations in the range of to mM did not induce increases in iin all cells.With this protocol, carbachol induced one or two spikes, but sustained oscillations were not observed.Similar findings were obtained in a total of four to six experiments.One piece of evidence for this idea is the sensitivity of capacitative calcium entry to inhibition by APB. As for thapsigargin, mM APB caused a slight reduction of irelease in response to mM arachidonic acid. B and C except that different concentrations of the membranepermeant IP receptor inhibitor, APB, were used.In the top panel, the agonist was mM carbachol, and in the bottom panel the agonist was mM thapsigargin.We found that mM isotetrandrine completely blocked the oscillations in of cells tested, whereas mM blocked completely in of. However, like its close cousin, tetrandrine, isotetrandrine can also function as a calcium channel blocker.The inhibition is apparently not due to action as a nonspecific channel blocker, or to membrane depolarization, because isotetrandrine caused only slight inhibition of entry in response to thapsigargin. These findings call into question the validity of isotetrandrine as a specific tool to demonstrate PLA involvement.Furthermore, the relative insensitivity of the thapsigargininduced entry to isotetrandrine further supports the view that the entry driving the ioscillations is not capacitative.APB has also been shown to modulate IP receptors to conclude that the IP receptor is somehow involved in the activation of capacitative calcium entry.Arachidonic acid, an unsaturated fatty acid produced by the action of PLA or diacylglycerol lipase on membrane lipids, is mobilized in a variety of cell types by the actions of neurotransmitters and hormones.In B, mM isotetrandrine was added during the oscillations, and the oscillations ceased.These results have been considered strong evidence for the conformational coupling model for activation of capacitative calcium entry, because this model invokes an obligatory role for the IP receptor interacting with plasma membrane capacitative calcium entry channels. Clearly, APB is one of the more specific inhibitors of capacitative calcium entry.There is considerable evidence that arachidonic acid can serve as an activator of a noncapacitative pathway in HEK cells. However, the association between arachidonic acidinduced entry and the entry associated with oscillations is lost when the effects of APB are examined.At least some of the previous evidence for PLA involvement. These arguments notwithstanding, it is still possible, given other points of evidence that suggest an involvement of PLA and arachidonic acid, that PLA is involved in the oscillations.In B, mM thapsigargin was used to activate capacitative calcium entry.To our knowledge no other organic antagonist of capacitative calcium entry channels shows this degree of specificity.