Signalling pathways involved in the contractile response to 5-HT in the human pulmonary artery

Serotonin (5-hydroxytryptamine; 5-HT) is a potent pulmonary vasoconstrictor and mitogenic agent whose plasma level is increased in pulmonary hypertensive patients. Thus, we explored the signalling pathways involved in the contractile response to 5-HT in human pulmonary arteries (HPAs). Intact and beta-escin permeabilised rings from HPAs mounted in an organ bath system were used to assess both tension and myofilament Ca2+-sensitisation. Microspectrofluorimetry was used for intracellular Ca2+ recordings in cultured HPA smooth muscle cells. Voltage-operated Ca2+ channel blockers (nitrendipine and nifedipine) partially reduced the contraction to 5-HT. Thapsigargin or cyclopiazonic acid (CPA), known to deplete sarcoplasmic reticulum Ca2+ stores, also partially inhibited the contraction, whereas removal of extracellular Ca2+ under these conditions further inhibited the contraction. Changing from Ca2+-free to Ca2+ containing solution, in the presence of nitrendipine and CPA, a protocol known to stimulate store-operated Ca2+ channels, induced HPA contractions that were blocked by nickel. Nickel or gadolinium also reduced the contraction to 5-HT. Finally, 5-HT increased intracellular Ca2+ responses in cultured HPA smooth muscle cells and myofilament Ca2+-sensitisation in HPA rings. Collectively, these results indicate that voltage-operated and voltage-independent Ca2+ channels, as well as Ca2+ release and myofilament Ca2+-sensitisation, participate in 5-HT-induced contraction in HPAs.