Does slow-reacting substance of anaphylaxis mediate hypoxic pulmonary vasoconstriction?

We have previously established a relationship between mast cell degranulation and hypoxic pulmonary vasoconstriction (HPV). In the present study, we investigated the possible role of slow-reacting substance of anaphylaxis (SRS-A) in the mediation of HPV. In 18 conscious sheep, pulmonary artery pressure, pulmonary arterial wedge pressure, and cardiac output were measured for the calculation of pulmonary vascular resistance (PVR) along with arterial oxygen tension (PaO₂ while breathing room air and while breathing 13% O₂ (balance, N₂). Before and during 13% O₂ breathing (pretreatment), Group 1 received an intravenous infusion of cromolyn sodium (3 mg/kg^-1/min^-1) and Group 2 was infused with FPL-57231, a SRS-A antagonist (2 mg/kg^-1/min^-1); and Group 3 and 4 received infusions of cromolyn sodium or FPL-57231 after induction of HPV. During 13% O₂ breathing (mean PaO₂, 47 mmHg), mean PVR increased to 190% of baseline. Pretreatment with cromolyn sodium prevented HPV, whereas infusion of cromolyn sodium after induction of HPV failed to reverse it; FPL-57231 both prevented HPV (pretreatment) and reversed it when infused after induction of HPV. Pretreatment with the prostaglandin synthetase inhibitor indomethacn (2 mg/kg) 1 h before the experiment failed to modify the FPL-57231-induced reversal of hypoxic vasoconstriction, thus excluding the release of inhibitory prostaglandins by this compound. We conclude that cromolyn sodium prevented HPV presumably by inhibiting the release of SRS-A, which mediates pulmonary vasoconstriction directly or indirectly through other mechanisms.