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Milrinone attenuates thromboxane receptor‐mediated hyperresponsiveness in hypoxic pulmonary arterial myocytes
Author(s) -
Santhosh KT,
Elkhateeb O,
Nolette N,
Outbih O,
Halayko AJ,
Dakshinamurti S
Publication year - 2011
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.2011.01306.x
Subject(s) - thromboxane receptor , endocrinology , receptor , forskolin , medicine , chemistry , agonist , biology
BACKGROUND AND PURPOSE Neonatal pulmonary hypertension (PPHN) is characterized by pulmonary vasoconstriction, due in part to dysregulation of the thromboxane prostanoid (TP) receptor. Hypoxia induces TP receptor–mediated hyperresponsiveness, whereas serine phosphorylation mediates desensitization of TP receptors. We hypothesized that prostacyclin (IP) receptor activity induces TP receptor phosphorylation and decreases ligand affinity; that TP receptor sensitization in hypoxic myocytes is due to IP receptor inactivation; and that this would be reversible by the cAMP‐specific phosphodiesterase inhibitor milrinone. EXPERIMENTAL APPROACH We examined functional regulation of TP receptors by serine phosphorylation and effects of IP receptor stimulation and protein kinase A (PKA) activity on TP receptor sensitivity in myocytes from neonatal porcine resistance pulmonary arteries after 72 h hypoxia in vitro . Ca 2+ response curves to U46619 (TP receptor agonist) were determined in hypoxic and normoxic myocytes incubated with or without iloprost (IP receptor agonist), forskolin (adenylyl cyclase activator), H8 (PKA inhibitor) or milrinone. TP and IP receptor saturation binding kinetics were measured in presence of iloprost or 8‐bromo‐cAMP. KEY RESULTS Ligand affinity for TP receptors was normalized in vitro by IP receptor signalling intermediates. However, IP receptor affinity was compromised in hypoxic myocytes, decreasing cAMP production. Milrinone normalized TP receptor sensitivity in hypoxic myocytes by restoring PKA‐mediated regulatory TP receptor phosphorylation. CONCLUSIONS AND IMPLICATIONS TP receptor sensitivity and EC 50 for TP receptor agonists was regulated by PKA, as TP receptor serine phosphorylation by PKA down‐regulated Ca 2+ mobilization. Hypoxia decreased IP receptor activity and cAMP generation, inducing TP receptor hyperresponsiveness, which was reversed by milrinone.