Background and Objectives Endothelial cells are active participants in inflammatory processes. They are involved in diverse activities including the regulation of leucocyte extravasation, angiogenesis, cytokine production, protease and extracellular matrix synthesis, vasodilation, etc. The small gaseous molecule hydrogen sulphide (H₂S) is involved in a variety of physiological processes like vascular relaxation, angiogenesis, neurotransmission and inflammation. In the vascular system, ATP-sensitive K⁺-channels are a major target for H₂S but over the last few years evidence has accumulated that several Na⁺- and Ca²⁺-permeable channels are also sensitive to H₂S. In the present study we investigated the effect of H₂S on Ca²⁺ signalling in cultured endothelial and smooth muscle cells with special emphasis given to the role of H₂S in modulating store-operated Ca²⁺ channels.

Materials and Methods Experiments were performed with human microvascular endothelial cells (HMEC-1), endothelial cells isolated from porcine aorta, and smooth muscle cells isolated from rat aorta and rat trachea. Mobilisation of intracellular Ca²⁺ and Ca²⁺ entry was monitored by measuring the intracellular free Ca²⁺ concentration with FURA-2 in the absence and presence extracellular Ca²⁺, respectively. Activity of endothelial nitric oxide synthase (eNOS) in intact cells was determined as conversion of incorporated L-[³H]-arginine into L-[³H]-citrulline.

Results Incubation of human and porcine endothelial cells with the H₂S-donor NaHS (100 µM, 10–45 min) evoked a release of Ca²⁺ from intracellular stores that was not accompanied by Ca²⁺ influx from the extracellular space. In accordance with these data suggesting that H₂S may inhibit store-operated Ca²⁺ entry, incubation of cells with NaHS attenuated Ca²⁺ influx induced by depletion of Ca²⁺ stores with receptor agonists (ATP, histamine) or the endoplasmatic reticulase ATPase inhibitor, thapsigargin. As a consequence, the stimulatory effect of these agonists on endothelial NO formation was strongly diminished, whereas the response to the Ca²⁺ ionophore A23187 was barely affected. Similar to the results obtained with endothelial cells, depletion of intracellular Ca²⁺ stores in smooth cells isolated from rat aorta or rat trachea also resulted in a pronounced Ca²⁺ entry that was completely blocked upon pre-treatment of cells with NaHS.

Conclusions H₂S inhibits the stimulatory effect of Ca²⁺ of mobilising agonists on endothelial NO formation by attenuating store-operated Ca²⁺ entry. Inhibition of store-operated Ca²⁺ channels by H₂S is not peculiarity of endothelial cells but also occurs in vascular and tracheal smooth muscle cells. These hitherto undescribed effects may be in part possible for the beneficial effects of H₂S in sulphur bath therapy.