Pinacidil-induced Relaxation in Pulmonary Arteries Isolated From Pulmonary Hypertensive and Normotensive Rats and Pre-contracted With Different Spasmogens

Summary: Vasorelaxant responses to the potassium channel opening drug, pinacidil, were obtained on preparations of pulmonary artery and aorta taken from rats with pulmonary hypertension (induced by chronic hypoxia or monocrotaline) and pre-contracted either submaximally with endothelin-1 (ET-1), PGF_2α, U46619 (thromboxane-mimetic) or noradrenaline (NA), or with 80 mM K⁺. In pulmonary artery, but not aorta, from pulmonary hypertensive rats the maximum relaxant response to pinacidil was increased, when compared with data in control rats, irrespective of the spasmogen used to precontract the tissues. The increase in maximum was associated with relaxation to below the tissue resting baseline and probably reflected the presence of inherent contractile tone in arteries from pulmonary hypertensive rats. In addition the potency (-log EC₅₀) of pinacidil was increased in pulmonary arteries from pulmonary hypertensive rats but this was seen only in preparations contracted with ET-1 (30-fold increase) or NA (seven-fold increase) and not in those contracted with PGF_2α, U46619 or K⁺. As a result, in ET-1-contracted preparations from pulmonary hypertensive rats pinacidil was 29-fold more potent on pulmonary artery than on aorta. To explain the increase in potency it is speculated that during the development of pulmonary hypertension the mechanism whereby ET-1 and NA contract pulmonary arteries may change from one in which Ca²⁺ influx plays only a minor role to one in which Ca²⁺ influx predominates, although no direct evidence to support this speculation has yet been obtained.