EDITORIAL FOCUS
Focus on "Hypoxic constriction of porcine distal pulmonary arteries: endothelium and endothelin dependence"

Jane A. Madden

Department of Neurology, The Medical College of Wisconsin, Milwaukee 53226; and Research Service, Zablocki Veterans Affairs Medical Center, Milwaukee, Wisconsin 53295


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PROBABLY MANY OF US INVOLVED in pulmonary vascular research have, at some time, begun a grant application or a manuscript with a variation on the phrase "despite considerable research, the mechanism(s) responsible for hypoxic pulmonary vasoconstriction is still unknown." However, even as we lament that the answer to the hypoxic response still eludes us, the quest remains fascinating. With each new work, another clue emerges, and both the questions answered and the questions raised by that work encourage us to continue. This is the case with the current article (14) in this issue of the American Journal of Physiology-Lung Cellular and Molecular Physiology. In this, the most recent of that laboratory's series of studies examining the role of endothelin (ET)-1 in the pulmonary vasculature (14, 20-24), Liu et al. (14) propose that hypoxic pulmonary vasoconstriction depends heavily on the contribution of ET-1, the potent vasoconstrictor released by the vascular endothelium.

Evidence for and against endothelium dependence of the hypoxic response in pulmonary arteries can be found in numerous studies performed in a variety of species. For example, in pigs, the constriction appears to be, at least to some degree, endothelium dependent (10, 14), although smooth muscle cells isolated from distal pulmonary arteries have been shown to constrict slightly to hypoxia (20). In rats, there is evidence both for (13) and against (27) endothelium dependence, and in cats (15, 16), there does not appear to be any dependency at all.

It has been hypothesized that the hypoxic response is due to diminished production of an endothelium-derived dilator substance(s) and/or the elaboration of a constrictor substance(s). Endothelium-derived vasodilators such as prostacyclin, nitric oxide, and possibly an endothelium-derived hyperpolarizing factor have been shown to modulate the hypoxic response, but it now seems likely that hypoxic vasoconstriction is not due to their reduced synthesis. Nevertheless, the question of whether an endothelium-derived constrictor substance such as ET-1 is responsible for hypoxic pulmonary vasoconstriction remains relevant, not only in light of the work by Liu et al. (14) but also by the demonstration of hypoxia-induced ET-1 synthesis by isolated lungs (9) and pulmonary vascular endothelial cells (11) and strong evidence for ET-1 involvement in the pathogenesis of pulmonary hypertension (2).

Liu et al. (14) used isolated, cannulated 100- to 150-µm-diameter pig pulmonary arteries. When these arteries were exposed to hypoxia (PO2 = 30 Torr), their diameter decreased if the endothelium was intact. This decrease was somewhat enhanced by inhibiting vasodilator prostaglandins, and it was greatly enhanced by inhibiting nitric oxide. On the other hand, there was no change in arterial diameter during hypoxia if the vessels were treated with the ETA receptor blocker BQ-123 or if the endothelium was removed. Pretreating the endothelium-denuded vessels with 10-10 M ET-1 restored the hypoxic response close to its original magnitude. This study extended the results of prior work from their group (20), which showed that small contractions to hypoxia by pig pulmonary artery smooth muscle cells were markedly enhanced by pretreatment with 10-10 M ET-1, a dose that under normoxic conditions had no effect on either cell length or intracellular Ca2+ concentration. A similar priming effect by ET-1 on hypoxic pulmonary vasoconstriction has also been seen in rat pulmonary artery myocytes (26). Liu et al. (14) also cited numerous other studies in which ET-1 receptor antagonists blocked hypoxic vasoconstriction and hypoxia resulted in increased ET-1 production in both intact and isolated lungs and pulmonary vascular endothelial cells.

On the basis of the above results, it would seem logical, then, to conclude that ET-1 directly mediates hypoxic pulmonary vasoconstriction. But as ET-1 enters its 13th year in the scientific literature, its actions, like those of many teenagers, are "like totally unpredictable." Thus one can find other studies [many cited by Liu et al. (14)] that just as convincingly argue that ET-1 is not a mediator of the hypoxic response. For example, in contrast to the results of Liu et al., Lazor et al. (12) found that inhibiting the ETA receptor had no effect on the hypoxic response in rat pulmonary arteries and Hasanuma et al. (8) found that ET-1 caused pulmonary vasodilation. It has also been pointed out (2) that the relatively rapid response to hypoxia and the relatively slow synthesis of ET-1 make it difficult to state that ET-1 is responsible for hypoxic pulmonary vasoconstriction. To further complicate the picture, it has also been reported (6) that although bovine pulmonary microvessels produce a contractile factor in vitro, it is not ET-1.

If, however, we acknowledge that ET-1 is probably involved in some capacity in the hypoxic response, then how might it work? (For a thorough treatment of the mechanisms involved in the action of ET-1 in vascular smooth muscle and the pulmonary circulation in particular, see Refs. 3, 5, 7.) Briefly, though, ET-1 directly interacts with ETA and ETB receptor subtypes on vascular smooth muscle and endothelial cells. It is generally accepted that the ETA receptor is involved in the constrictor responses and the ETB receptor in the vasodilatory responses (3, 5, 7). Recent work by Schmeck et al. (19) raises the intriguing possibility that it is not the ETA receptor but rather another form of the ETB receptor that mediates the hypoxic constriction seen during conditions of elevated pulmonary vascular tone. An increase in intracellular Ca2+ is one of the integral components of the ET-1 signaling process (3, 5, 7, 24), but the mechanisms by which intracellular Ca2+ increases or by which ET-1 might increase the sensitivity of the contractile apparatus to Ca2+ are still unclear. In addition to its effects on intracellular Ca2+ release and/or the open probability of Ca2+ channels, ET-1 may alter the activity of one or more types of K+ channels (1, 17, 18, 20, 22, 23) as well as the activity of other ion channels and ion-exchange processes (3).

If ET-1 does not initiate the hypoxic response, then, as Liu et al. (14) state, "the role of ET-1 in hypoxic pulmonary vasoconstriction may be more subtle than direct concentration-dependent activation of smooth muscle contraction." Liu et al. did not assess any potential role for K+ channels in the isolated arteries of this study, nor did Sham et al. (20) measure the effect of ET-1 on voltage-dependent or other K+ channels in pig pulmonary artery myocytes during acute hypoxia. Although Sham et al. suggested that cellular sensitivity to Ca2+ might be enhanced by ET-1, this was also not determined.

Editorial license allows one to propose all kinds of future studies without incurring the obligation of actually having to design and do them. Some of the more intriguing studies would deal with the differences in the hypoxic responses among the various species. We often invoke the different species argument to explain discrepancies between our findings and those of others. But in order for us to truly understand the role of ET-1 in the pulmonary system, similar types of studies should be performed in rats, pigs, cats, dogs, rabbits, and, where possible, humans. Studies should also be done in whole animals as well as in their isolated lungs, arteries, veins, and smooth muscle and endothelial cells.

Perhaps even more perplexing than species differences is the question of the legitimacy and relevance of in vitro responses. When pulmonary vessels are removed from their sheltered existence in the lung parenchyma or smooth muscle and endothelial cells are isolated and grown in culture, are they then predisposed or activated to manifest behaviors not typically seen in vivo or are they merely providing us with a realistic view of possible pathological responses?

While we wrestle with those issues, studies should also be conducted to determine the different types of ET-1 receptor(s) subtypes, their relative populations in different branches of the vascular tree, and the differences in their responsiveness to ET-1. It should also be investigated whether pulmonary vascular smooth muscle and endothelial cells produce the same types and amounts of ETs and whether cells within the same tissue differ in their production and/or responsiveness to them (25). To what extent does the contractile state, intrinsic tone, and intracellular Ca2+ concentration of the pulmonary vasculature determine its response to ET-1? With respect to the mechanisms of action in the pulmonary vasculature, is the effect of ET-1 similar on all types of K+ channels, and how does this effect(s) relate to other membrane mechanisms? What are the effects of acute and prolonged hypoxia on the production of and response to ET-1 (4)? Do the kinetics of conversion from the ET precursor to the active form differ between normoxic and hypoxic animals?

With apologies to the many authors whose excellent work was not cited, this editorial ends with the hope that in the near future, we will all be able to write "due to considerable research, the mechanism(s) responsible for hypoxic pulmonary vasoconstriction is now known."


    FOOTNOTES

Address for reprint requests and other correspondence: J. A. Madden, Neurology Research 151, VAMC, Milwaukee, WI 53295 (E-mail: jmadden{at}mcw.edu).


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Am J Physiol Lung Cell Mol Physiol 280(5):L853-L855
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