Hyperforin Stimulates Intracellular Calcium Mobilisation and Enhances Extracellular Acidification in DDT₁-MF2 Smooth Muscle Cells

Hyperforin, an acylphloroglucinol derivative, is a major constituent of St. John’s wort extract (Hypericum perforatum L.), which is used in treating depressive disorders. Hyperforin has been demonstrated as a modulator of several neuronal ion channels, and inhibits smooth-muscle contraction induced by various neurotransmitters. To evaluate the spasmolytic properties of hyperforin in more detail, we performed studies on the hamster vas deferens smooth muscle cell line DDT₁-MF2. In a first series of experiments, we determined the effect of hyperforin on intracellular Ca²⁺ concentration ([Ca²⁺]_i) using the fluorochrome fura-2. These investigations were supplemented in a second series of assays, where the effects on cellular metabolism were analysed by measuring the rate of extracellular release of acidic metabolites with the help of a microphysiometer. Hyperforin (0.3 - 10 µg/ml) caused a concentration-dependent elevation of [Ca²⁺]_i and extracellular acidification rate (ECAR). Both of these effects were independent of extracellular Ca²⁺. To elucidate whether the increase of [Ca²⁺]_i by hyperforin causes or results from its ECAR-stimulating properties, we used various pharmacological tools to reveal the sequence of events and the molecular mechanisms involved. Our results suggest that hyperforin induces release of Ca²⁺ from as yet unidentified sources. Since the ECAR stimulation was inhibited to a different extent by the intracellular Ca²⁺ chelator BAPTA as well as by inhibitors of plasmalemmal and mitochondrial Na⁺/Ca⁺ exchange, but not by inhibitors of Na⁺/H⁺ antiport, the intracellular Ca²⁺ increase seems to be essential for this hyperforin effect. However, further studies are needed to establish the exact mode of action, and to deduce whether this aspect of hyperforin activity contributes to its antidepressant and neuroprotective effects.

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