Local and circulating concentrations of serotonin (5-HT) increase in response to pulmonary hypoxia and may contribute to pulmonary hypertension (MacLean et al. 2000). Under normoxic conditions, 5-HT induces a pulmonary vasoconstriction but there is little information on the 5-HT-induced calcium signal in small intrapulmonary arteries (IPA), which play a central role in blood flow regulation in the lung. We, therefore, investigated the effects of 5-HT on the calcium concentration ([Ca²⁺]_i) in isolated small IPA from rat. Rats were humanely killed according to the national guidelines. IPA with an external diameter of 228.6 ± 8.5 mm (n = 22) were isolated by dissection from the left lung. The vessels were loaded with the calcium-sensitive fluorescent dye fura-PE3 (2 mM) for 1.5 h at 37 °C. Digital imaging was used to record [Ca²⁺]_i. Vessels were pressurized at 10 mmHg and the experiments were performed at 37 °C. Significance was tested with Student’s paired t test (P < 0.05). Data are expressed as the means ± S.E.M.

Bath application of 5-HT induced a concentration-dependent intracellular calcium increase with a maximum and a half-maximum effect occurring at 10 and 2.1 mM, respectively (n = 6). This 5-HT-induced sustained calcium signal was not significantly modified by L-type calcium channel antagonists nitrendipine (1 mM, P = 0.13, n = 4) or nicardipine (1 mM, P = 0.45, n = 5). Depletion of the intracellular calcium store by pretreatment with a sarcoplasmic reticulum Ca²⁺-ATPase inhibitor (cyclopiazonic acid, CPA, 10 mM) did not affect the calcium response to 5-HT (n = 6). Store-operated calcium channel antagonists (LOE 908, SKF 96365 and gadolinium 10 mM, n = 6, 7 and 5, respectively) did not modify the 5-HT-induced calcium signal. CPA alone activated a capacitative-like channel sensitive to LOE 908 (inhibition of 71 %, n = 4) and insensitive to SKF 96365 (n = 4) and gadolinium (n = 5), suggesting that 5-HT activates a non-capacitative calcium channel. On the other hand, inhibitors of arachidonic acid production such as a DAG lipase inhibitor, RHC 80267, 50 mM or a G-protein-coupled phospholipase A2 antagonist, isotetrandrine, 10 mM, partially blocked the 5-HT-induced calcium increase by 58 % (n = 6) and 51 % (n = 5), respectively. Arachidonic acid 100 mM induced a small calcium increase in two out of five vessels, suggesting a synergic mechanism is involved in the calcium response to 5-HT.

Although a capacitative-like calcium influx appears to be present in IPA, the calcium response to 5-HT depends on the activation of a non-capacitative calcium channel. The activation of this channel may partly involve the production of arachidonic acid.

This work was funded by INSERM and Conseil Regional d’Aquitaine.

All procedures accord with current National guidelines.