Characteristics of the Hypoxic Response in Isolated Pulmonary Arteries

In isolated pulmonary arteries, HPV is biphasic when induced by switching from a normoxic to a hypoxic gas mixture (Fig. 1A). In agreement with Ward and co-workers (35, 51), we find that hypoxia induces an initial transient constriction (phase 1), followed by a slow tonic constriction (phase 2) (12, 13, 66). The initial transient constriction peaks within 5-10 min of the hypoxic challenge, whilst the secondary tonic constriction peaks after 30-40 min (12, 13, 35, 51, 66). Others describe a transient, monophasic constriction of isolated arteries by hypoxia, which has been referred to by some as biphasic (11, 23, 29, 69). Importantly, the time course of the records of HPV presented in the latter investigations, and/or the duration of exposure to hypoxia, indicate that they describe the initial phase 1 constriction only.

Figure 1. The effect on HPV of removing the endothelium and extracellular Ca2+. A: A representative record indicating phase 1 and phase 2 of the response of an intact pulmonary artery ring to hypoxia. B: Constriction by hypoxia of a pulmonary artery ring without endothelium. C: Constriction by hypoxia of an intact pulmonary artery ring in the absence of extracellular Ca2+. D: Constriction by hypoxia of a pulmonary artery ring without endothelium and in the absence

Figure 1. The effect on HPV of removing the endothelium and extracellular Ca2+. A: A representative record indicating phase 1 and phase 2 of the response of an intact pulmonary artery ring to hypoxia. B: Constriction by hypoxia of a pulmonary artery ring without endothelium. C: Constriction by hypoxia of an intact pulmonary artery ring in the absence of extracellular Ca2+. D: Constriction by hypoxia of a pulmonary artery ring without endothelium and in the absence

These variations in qualitative description of HPV demonstrate the significance ofthe basic methodology to our interpretation of events. We should be acutely aware, therefore, of the exact parameters within which each experiment was carried out before arriving at any definitive conclusion as to the mechanisms and/or mediators involved.

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