Lysosomal Calcium: Impact on Serotonin‐Induced Contractions of Ovine Pulmonary Arteries

Abstract

Mobilisation of intracellular Ca2+ from lysosomes is involved in smooth muscle contraction in response to several agonists but serotonin (5-HT) has not been examined. Given the proposed role of 5-HT in pulmonary arterial hypertension, this study aimed to investigate the role of lysosomal Ca2+ stores in pulmonary artery (PA) smooth muscle 5-HT responses. Responses of endothelium-denuded ovine PAs were measured using myography. 5-HT cumulative concentration response curves (CCRCs) were investigated in Ca2+-free medium and with nimodipine (a lysosomal Ca2+ channel blocker). Contractions are normalised to 80mM KCl responses and expressed as mean ± S.E.M. PAs showed dose-dependent contractions to 5-HT. Fourth order PAs were more responsive than third order; maximal contractions were significantly higher (P<0.05). Responses to 5-HT were reduced in the absence of extracellular Ca2+ (statistically significant at [5-HT] of 30μM and 100μM (P<0.05)). Maximal contraction: Ca2+-free medium 174±20%KCl (n=5); Ca2+-containing medium 282±17%KCl (n=5). PAs in Ca2+-free medium containing nimodipine displayed reduced contractions to 5-HT compared to control (statistically significant (P<0.05) at [5-HT] of 10μM, 30μM and 100μM). Nimodipine reduced maximal contraction at 30μM 5-HT by 56±4%: nimodipine-exposed PAs 73±7%KCl (n=5); control 174±20%KCl (n=5). Anatomical order in ovine PAs is an important factor in 5-HT responsiveness. Results also suggest both extracellular Ca2+ and mobilisation of Ca2+ from intracellular lysosomal stores play a significant role in 5-HT-mediated contraction of ovine PAs. Research supported by Institute for Health & Wellbeing Research, Robert Gordon University.