Intracellular Zinc Elevation Measured with a “Calcium-Specific” Indicator during Ischemia and Reperfusion in Rat Hippocampus: A Question on Calcium Overload

In this paper, Stork and Li describe intraneuronal zinc changes following oxygen glucose deprivation (OGD) in hippocampal slices. They make several claims, which include the suggestion that the detection of zinc by calcium sensitive dyes has been ignored, and that the role of calcium may be over-stated. These claims are exaggerated, and ignore a substantial body of literature that has both recognized and utilized the zinc sensitivity of dyes designed to detect calcium. A recent clear example is the paper of Devinney and colleagues (2005), who clearly delineated the zinc sensitivity of calcium dyes in the context of excitoxicity. Indeed, this paper has a section in the discussion entitled "How are calcium and zinc distinguished?". This and a number of papers cited therein belie the claim that the topic has largely been ignored by others. A second concern is that the key experiment, where TPEN decreases the fluorescence signal of the calcium sensitive dye, may be conceptually flawed. As noted by the authors, TPEN does bind calcium but with a much lower affinity than zinc. However, as discussed by Dineley and colleagues (2002), when chelators are present in excess over the free ion concentration, binding is driven by the chelator concentration rather than the affinity. At the high concentrations used here, TPEN could be having an important impact on the calcium concentration, and may not be specifically chelating zinc at all. The simple experiment of using a much lower concentration of TPEN was not reported. Although we certainly agree that zinc may make an under- appreciated contribution to excitotoxic neuronal injury, this paper does a disservice to the concept by failing to recognize the substantial body of work that has gone before, and by basing key conclusions on flawed experiments.

Dineley KE, Malayandi LM and Reynolds, IJ. (2002) A reevaluation of intraneuronal zinc measurements: artifacts associated with high intracellular dye concentrations. Mol. Pharmacol. 62:618-627. Devinney, MJ, Reynolds IJ and Dineley, KE (2005) Simultaneous detection of intracellular free calcium and zinc using fura2FF and fluozin3. Cell Calcium 37:225-232.