The cellular mechanisms that determine differences in reactivity of arteries of varying size and origin are unknown. We evaluated the hypothesis that there is diversity in the distribution of K+ channels between vascular smooth muscle (VSM) cells within a single segment of the pulmonary arteries (PAs) and that there are differences in the prevalence of these cell types between conduit and resistance arteries, which contribute to segmental differences in the vascular response to NO and hypoxia. Three types of VSM cells can be identified in rat PAs on the basis of their whole-cell electrophysiological properties- current density and the pharmacological dissection of whole-cell K+ current(I(K))-and morphology. Cells are referred to as "K(Ca), K(Dr), or mixed," acknowledging the type of K+ channel that dominates the IK: the Ca2+-sensitive (K(Ca)) channel, delayed rectifier (K(Dr)) channel, or a mixture of both. The three cell types were identified by light and electron microscopy. K(Ca) cells are large and elongated, and they have low current density and currents that are inhibited by tetraethylammonium (5 mmol/L) or charybdotoxin (100 nmol/L). K(Dr) cells are smaller, with a perinuclear bulge, but have high current density and currents that are inhibited by 4-aminopyridine (5 mmol/L). Conduit arteries contain significant numbers of K(Ca) cells, whereas resistance arteries have a majority of K(Dr) cells and few K(Ca) cells. NO rapidly and reversibly increases I(K) and hyperpolarizes K(Ca) cells because of an increase in open probability of a 170-pS K(Ca) channel. Hypoxia depolarizes K(Dr) cells by rapidly and reversibly inhibiting one or more of the tonically active K(Dr) channels (including a 37-pS channel) that control resting membrane potential. The effects of both hypoxia and NO on K+ channels are evident at negative membrane potentials, supporting their physiological relevance. The functional correlate of this electrophysiological diversity is that K(Dr)-enriched resistance vessels constrict to hypoxia, whereas conduit arteries have a biphasic response predominated by relaxation. Although effective in both segments, NO relaxes conduit more than resistance rings, in both cases by a cGMP-dependent mechanism. We conclude that regional electrophysiological diversity among smooth muscle cells is a major determinant of segmental differences in vascular reactivity.

Differential Distribution of Electrophysiologically Distinct Myocytes in Conduit and Resistance Arteries Determines Their Response to Nitric Oxide and Hypoxia

Chronic pulmonary hypertension is a serious complication of a number of chronic lung and heart diseases. In addition to vasoconstriction, its pathogenesis includes injury to the peripheral pulmonary arteries leading to their structural remodeling. Increased pulmonary vascular synthesis of an endogenous vasodilator, nitric oxide (NO), opposes excessive increases of intravascular pressure during acute pulmonary vasoconstriction and chronic pulmonary hypertension, although evidence for reduced NO activity in pulmonary hypertension has also been presented. NO can modulate the degree of vascular injury and subsequent fibroproduction, which both underlie the development of chronic pulmonary hypertension. On one hand, NO can interrupt vascular wall injury by oxygen radicals produced in increased amounts in pulmonary hypertension. NO can also inhibit pulmonary vascular smooth muscle and fibroblast proliferative response to the injury. On the other hand, NO may combine with oxygen radicals to yield peroxynitrite and other related, highly reactive compounds. The oxidants formed in this manner may exert cytotoxic and collagenolytic effects and, therefore, promote the process of reparative vascular remodeling. The balance between the protective and adverse effects of NO is determined by the relative amounts of NO and reactive oxygen species. We speculate that this balance may be shifted toward more severe injury especially during exacerbations of chronic diseases associated with pulmonary hypertension. Targeting these adverse effects of NO-derived radicals on vascular structure represents a potential novel therapeutic approach to pulmonary hypertension in chronic lung diseases.

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.4.1337

Role of Nitric Oxide in the Pathogenesis of Chronic Pulmonary Hypertension

Oxidation and nitrosation of thiols at low micromolar exposure to nitric oxide. Evidence for a free radical mechanism.

Mechanisms of Inhibition of Aldehyde Dehydrogenase by Nitroxyl, the Active Metabolite of the Alcohol Deterrent Agent Cyanamide

Nitric oxide (NO) generated by diethylamine nonoate (DEA/NO), an NO donor, readily oxidized the free sulfhydryl group of human serum albumin (HSA) as well as the sulfhydryl groups of reduced glutathione (GSH) and dithiothreitol (DTT) at pH 7.4 and 37 degrees C. Under anaerobic conditions, the major products of the oxidation of HSA thiol by NO were the sulfenic acid (RSOH) of HSA and nitrous oxide (N2O). The stoichiometry for this reaction, viz., 1 mol of HSA sulfhydryl oxidized to 1 mol of N2O produced, is consistent with a net two-electron oxidation of the protein thiol to a sulfenic acid. The sulfenic acid product of HSA was shown to react with dimedone and GSH, two known reactions of sulfenic acids. In contrast, anaerobic oxidation of GSH and DTT by NO gave a stoichiometry close to the expected ratio of 2:1 (sulfhydryl oxidized to N2O produced) for the oxidation of these thiols to their disulfides and N2O. Under aerobic conditions, significant fractions of the sulfhydryl groups of HSA, GSH, and DTT were oxidized to their respective thionitrites, presumably by N2O3. Thionitrite formation was not observed in the absence of oxygen. The production of HSA-sulfenic acid by NO, as well as by other oxidizing agents such as H2O2 and peroxynitrite, followed by its reaction with circulating GSH or L-Cys may account for the mixed disulfides of HSA observed in plasma.

John B. Warren

Papers

Preparation of Standards and Measurement of Nitric Oxide, Nitroxyl, and Related Oxidation Products