Pulmonary and systemic haemodynamic effects of cromakalim in conditions of normoxia and hypoxia in dogs

Hypoxic pulmonary vasoconstriction

Hypoxic vasoconstriction is unique to pulmonary circulation. The pulmonary response is part of a self-regulatory mechanism by which pulmonary capillary blood flow is automatically adjusted to alveolar ventilation for maintaining the optimal balance of ventilation and perfusion. In pathological conditions, hypoxic pulmonary vasoconstriction may occur as an acute episode or as a sustained response with pulmonary hypertension and vascular remodeling. Vasoactive substances produced from the endothelial cells (prostanoids, nitric oxide, or endothelin) or other mediators such as 5 hydroxytryptamine have been examined as possible mediators of hypoxic vasoconstriction. These appear more likely to be modulators than mediators of the vasoconstrictor response to hypoxia. Recent hypotheses have emerged indicating that O2 levels per se can regulate ion channel activity. The modulation of both K+ and Ca2+ channels differs according to the conduit or resistance pulmonary vessel type, tending to extend the former and contract the latter, thereby opposing the ventilation to perfusion mismatching. In the absence of drugs that act selectively on pulmonary circulation, inhaled therapy is an alternative in the treatment of pulmonary hypertension. According to its short half-life and to its potential cytotoxicity, nitric oxide is only of value in the management of patients with acute respiratory disease. Aerosolized prostacyclin and iloprost result in a sustained efficacy of the inhaled vasodilator regimen in patients with severe pulmonary hypertension and offer a new strategy for treatment of this disease. At the moment, therapy aimed at reversing the structural remodeling and matrix deposition in pulmonary arteries remains experimental. New drugs such as potassium channel openers or endothelin receptor antagonists warrant further investigations as possible therapeutic candidates in the treatment of pulmonary hypertension.

The pulmonary vasodilator properties of potassium channel opening drugs

1. This article reviews the effects of potassium channel opening drugs (KCOs) on blood vessels of the pulmonary circulation. KCOs are effective pulmonary vasodilators in vitro (isolated arteries and perfused lungs) and in vivo in a variety of animal species. They prevent or reverse pulmonary vasoconstriction/contraction induced by a range of vasoconstrictor spasmogens or by alveolar hypoxia. 2. The pulmonary vasorelaxant effects of the KCO drugs are blocked by glibenclamide, do not depend on the endothelium, are dependent on the vasoconstrictor spasmogen used to contract the preparations and are enhanced in preparations taken from pulmonary hypertensive rats. 3. Selectivity for pulmonary compared with systemic vessels is seen in vessels from pulmonary hypertensive rats but not in the absence of pulmonary hypertension. 4. The pulmonary vasodilatation that is induced by (a) endothelium derived hyperpolarising factor, (b) endothelin, (c) increased pulmonary blood flow or (d) prolonged, severe hypoxia is probably due to potassium efflux through the same population of potassium channels as those on which the KCOs act. 5. Acute hypoxic pulmonary vasoconstriction, and also the depolarisation seen in arteries from chronically hypoxic rats, each involve inhibition of potassium efflux through glibenclamide-insensitive potassium channels. 6. It is suggested that the KCOs warrant investigation as possible therapeutic agents in the treatment of pulmonary hypertension.

Effects of two K+ channel openers, aprikalim and pinacidil, on hypoxic pulmonary vasoconstriction

This study investigated the effects of two K+ channel openers, aprikalim and pinacidil, on hypoxic pulmonary vasoconstriction induced in isolated rat lung perfused at constant flow. In order to evaluate the mechanism of the hypoxic vasoconstriction we also studied the effects of an inhibitor of the endothelium-derived relaxing factor (EDRF), NG-nitro-L-arginine methyl ester (100 microM), an inhibitor of the guanylate cyclase, methylene blue (30 microM), two K+ channel blockers, glibenclamide (1 microM) and tetraethylammonium (20 mM). In normoxia, NG-nitro-L-arginine methyl ester, methylene blue, glibenclamide or tetraethylammonium did not enhance significantly the baseline perfusion pressure, suggesting that neither EDRF nor K+ channels are involved in the modulation of the low basal pulmonary vascular tone. In hypoxia, aprikalim and pinacidil (0.03-3 microM) induced a concentration-dependent decrease of pulmonary pressure, exhibiting their spasmolytic effects in acute hypoxia. The hypoxic pressure response was significantly increased by NG-nitro-L-arginine methyl ester, methylene blue and tetraethylammonium, but not by glibenclamide suggesting that EDRF and K+ channels other than ATP-sensitive K+ channels are involved in the modulation of the hypoxic pressure response. The spasmolytic effects of aprikalim and pinacidil (1 microM) were not modified by NG-nitro-L-arginine methyl ester, but were partially reduced by tetraethylammonium and completely abolished by glibenclamide, suggesting that these effects are mainly but not exclusively mediated through ATP-sensitive K+ channel opening.

Potassium channel openers and other regulators of KATP channels

1. Interest in ATP-sensitive K (KATP) channels first arose when it was shown that hypoglycaemic sulphonylureas, such as glibenclamide, closed these channels in pancreatic beta-cells to cause insulin release. The demonstration that certain smooth muscle relaxants (K channel openers) may exert their actions through opening a similar channel in vascular smooth muscle fueled further investigation of these channels and their physiological role in a variety of tissue types, including various types of smooth muscle, cardiac and skeletal muscle and neural and endocrine organ function. 2. The K channel openers have a variety of potential therapeutic applications, including disorders of smooth muscle hyperreactivity, such as hypertension, and a great deal of research has focused on this field. More recently, attention has turned to the cardiac actions of these compounds and this area is discussed in detail. One of the current problems is the lack of selectivity of KATP channel regulators. However, there have been a number of recent encouraging reports suggesting that, under certain pathophysiological conditions, the action of the K channel openers may be enhanced, conferring upon them some degree of selectivity. 3. A number of endogenous regulators of these channels have been identified, particularly in the category of endogenous openers of these channels. At present though, the physiological role of these channels and the endogenous regulators identified, is unclear. 4. It is evident that, although advances have been made, much work is still required to increase our understanding and ultimately to allow selective pharmacological manipulation of these channels to become a therapeutic reality.

Effects of three K+ channel openers on airways and pulmonary circulation in the isolated guinea-pig lung

The antispasmodic and spasmolytic effects of levcromakalim (BRL 38227), aprikalim (RP 52891) and pinacidil were investigated in airways and pulmonary vessels of the isolated guinea-pig perfused lung. In airways, the three drugs exhibited modest antispasmodic properties, and pinacidil was more potent than levcromakalim and aprikalim against the contractions induced by carbachol (0.001-10 microM) or K+ (5-50 mM). Whereas levcromakalim and aprikalim acted only at low concentrations of K+, the rightward shift of the K+ concentration-effect curve produced by pinacidil was observed at all K+ concentrations (5-50 mM), suggesting that a mechanism of action other than K+ channel opening is involved in the effects of pinacidil. Pinacidil (0.3-100 microM) had the greatest spasmolytic effect in airways precontracted by carbachol (0.3 microM). The three K+ channel openers were equipotent against the sustained contractions of airways induced by 25 or 30 mM K+ and their spasmolytic activity was more marked against contractions induced by low rather low than high K+ concentrations. Levcromakalim and aprikalim were more effective as relaxant than as antispasmodic drugs. In K(+)-precontracted pulmonary vessels, the relaxant activity of pinacidil and levcromakalim was more pronounced than that observed in airways, suggesting a vascular selectivity of these drugs.