Inhibitory effect of phloretin on histamine release from isolated rat mast cells

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Abstract
The ability of the flavonoid phloretin to inhibit histamine release from rat mast cells varied considerably with the releasing agent investigated. The response to the combination of the ionophore A23187 and the phorbol ester TPA and to suboptimal concentrations of the ionophore (0.5 μ M) was potently inhibited (IC50 about 5 μM), whereas phloretin was less potent against responses to the ionophore (1 μM) IC50 of 17 μM), to antigen alone and in combination with TPA (IC50 of 30–50 μM), to TPA in the absence of calcium (IC50 of 50 μM) and to compound 48/80 in the absence and presence of calcium (IC50 of 60–90 μM). The inhibition by phloretin at concentrations above 10μM was partly counteracted by glucose (5 mM) indicating effects on oxidative metabolism. The flavonoid quercetin was equally potent in inhibiting histamine release induced by antigen, the ionophore at different concentrations and in combination with TPA (IC50 of 20μM). Although not conclusive, the results are consistent with an inhibition of protein kinase C by phloretin at concentrations below 10 μM. At higher concentrations unspecific actions become apparent and phloretin therefore seems to be of limited utility as a probe for signal-pathways in cell responses.