Much attention has been focused on the role of nitric oxide in hypertension and cardiovascular disease. More recently, the role of superoxide anion and its interaction with nitric oxide has been investigated in this context. This review will concentrate on the role of superoxide in human and experimental hypertension, paying particular attention to the potential sources of superoxide within the vasculature and discussing some of the molecular mechanisms surrounding its production and dismutation. We discuss what is known about the human superoxide dismutase enzymes. We conclude that the balance between nitric oxide and superoxide is more important than the absolute levels of either alone.

We undertook these studies to determine whether a deficient nitric oxide production in genetically hypertensive rats could result from its being scavenged by an excess production of superoxide. In one study we used a porphyrinic microsensor to measure nitric oxide concentrations released by cultured endothelial cells from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar-Kyoto rats (WKY). SHRSP cells released only about one third the concentration of nitric oxide as did WKY cells. Treatment of cells with superoxide dismutase increased nitric oxide release, demonstrating that normally nitric oxide is scavenged by endogenous superoxide. The increase in nitric oxide release in response to superoxide dismutase treatment was more than twice as great from SHRSP as from WKY cells, demonstrating the greater amount of superoxide in the hypertensive rats. A direct measure of superoxide with the use of lucigenin demonstrated the presence of 68.1 +/- 7.1 and 27.4 +/- 3.5 nmol/L of this anion in SHRSP and WKY endothelial cells, respectively. The presence of superoxide in the rat aorta was also estimated by quantification of its effect on carbachol relaxation. This relaxation was diminished when endogenous superoxide dismutase was blocked by diethyldithiocarbamic acid. This blockade reduced the relaxation by 51.2 +/- 5.2% in SHRSP aortas and by only 22.0 +/- 8.2% (P = .015) in WKY aortas. Data from these diverse systems are in agreement that superoxide production is excessive in SHRSP tissues. This excess superoxide, by scavenging endothelial nitric oxide, could contribute to the increased vascular smooth muscle contraction and hence to the elevated total peripheral resistance of these rats.

Helical strips of rat tail artery were observed to relax in response to potassium after contraction induced by 10^–7 g/ml norepinephrine in potassium-free solution. After several minutes of relaxation, the strips showed an abrupt redevelopment of tension. The amplitude of the potassium-induced relaxation was employed as an index of the activity of the electrogenic sodium-potassium pump and hence of the Na⁺-K⁺ ATPase. This assumption seemed justified because the observed amplitude of potassium-induced relaxation paralleled known effects of the following variables on Na⁺-K⁺ ATPase: (1) intracellular sodium concentration; (2) ouabain administration; (3) magnesium; (4) temperature, and (5) potassium concentration. The relaxation that occurred in response to potassium is suggested to be due to an enhanced Na⁺-K⁺ ATPase resulting in increased electrogenic transport of sodium and potassium and, consequently, hyperpolarization. We propose that potassium-induced relaxation of rat tail artery may be used as a functional indicator of Na⁺-K⁺ ATPase activity in vascular smooth muscle.

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The first part of the contractile response of rabbit aorta to epinephrine is depressed by elevation of calcium concentration; the second is potentiated. These observations suggest that the rate-limiting factor for the former is membrane excitability (depressed by increased calcium), while that for the latter is the role that calcium plays in coupling membrane excitation with the development of tension by the contractile protein (a function that is augmented by increased calcium).

Responses to catecholamines were studied in isolated helical muscle strips of large and small coronary vessels and in isolated perfused coronary arteries from the dog. Epinephrine and norepinephrine in concentrations well below those normally present in the blood uniformly caused relaxation of small coronary vessels. Norepinephrine was much more potent than epinephrine. The relaxation was reversibly blocked by the beta adrenergic blocker, nethalide. During this blockade, catecholamines either were inactive or produced a slight contraction. Strips contracted by angiotensin, blood or KCl, as well as those completely depolarized by K 2 SO 4 , were relaxed by catecholamines. Strips taken from large coronary vessels, unlike those from small vessels, were, in some cases, contracted by catecholamines; in others, after a transient contraction, they were relaxed. Contraction was blocked by Dibenzyline. This difference in behavior of large and small vessels may account for some of the contradictions found in the literature concerning the response of coronary vessels to catecholamines.

This brief review of the rapidly developing research on vascular smooth muscle presents the state of the art as I see it from within my own frame of reference. For a more objective, detailed insight into the workings of vascular smooth muscle, several substantial reviews and compendiums may be read (1-7). CONTRACTILE PROTEINSThe mechanical events responsible for the contraction of vascular smooth muscle are associated with its contractile proteins. These proteins not only develop the mechanical force responsible for the contraction but also act as the enzyme that catalyzes the release of energy by which this force is developed. They are both the spark plug and the piston of the contractile machine.The contractile proteins of vascular smooth muscle are arranged in well-organized thick and thin filaments (8-10). The thick filaments, presumably bundles of myosin molecules, average 15.5 nm in diameter and have lateral projections suggestive of cross-bridges extending toward adjacent thin filaments. The thin filaments, presumably fibrous actin, average 5-8 nm in diameter and appear to be attached to dense bodies that are usually connected to the cell membrane. Contraction of vascular smooth muscle most probably is effected by some version of the Huxley sliding filament mechanism.The most easily interpretable studies of the functions of the contractile proteins are those performed in isolation with the determinants of the enzymatic and physical responses tightly controlled. There is a qualitative similarity between the actomyosin of vascular smooth muscle and the actomyosin of skeletal muscle (11) evidenced by the observation that a hybrid actomyosin can be prepared by combining myosin from one of these types of muscle with actin from the other; this From the Department of Physiology, University of Michigan, Ann Arbor, Michigan 48104.hybrid provides a functionally active enzyme. Although the adenosinetriphosphatase (ATPase) activity of skeletal muscle actomyosin is many times faster than the activity of vascular smooth muscle actomyosin, the speed of activity of the hybrid preparation is determined by the source of the myosin. This observation correlates with the extensive studies by Barany (12) showing that a direct parallel exists between the maximum velocity of shortening of a muscle and the ATPase activity of its actin-activated myosin. These observations bear the important implication that the shortening velocity of vascular smooth muscle has the actomyosin ATPase activity as its rate-limiting factor. This slow release of chemical energy is reflected in the slow physical changes in the actomyosin molecule observed in superprecipitation studies (13) or in studies of contraction velocity of glycerinated fibers (14).All the indexes of functional activity of native actomyosin from vascular smooth muscle (ATPase activity, superprecipitation, and contraction of glycerinated fibers) have the same low requirement for activator calcium. Half-maximal activity of any of these processes occurs at an ionic calcium concentratio...

The reactivity of vascular smooth muscle in helical strips from femoral arteries of normotensive, spontaneously hypertensive, renal hypertensive, and deoxycorticosterone acetate-(DCA-) hypertensive rats was studied. Spontaneous rhythmic contractions occurred in 25 of the 30 strips from the three groups of hypertensive rats and in only 2 of the 10 strips from normotensive rats. Strips from renal and DCAhypertensive rats had lower thresholds to epinephrine and potassium chloride (KC1) than did strips from spontaneously hypertensive and normotensive rats. Lanthanum (2.5 HIM) caused contraction of all 10 strips from spontaneously hypertensive rats but failed to cause contraction of any strip from the other three groups of rats. Strontium (5 mvc) caused contraction in 8 of 10 strips from spontaneously hypertensive rats but caused contraction in only 7 of the 30 strips from the other three groups. The optimal calcium concentration for tension development in response to a KC1 stimulus was approximately twice as high for strips from hypertensive rats as it was for strips from normotensive rats. Strips from DCA-hypertensive rats showed less tachyphylaxis to angiotensin II than did strips from the other three groups of rats. These results quantify our earlier observation that the reactivity of vascular smooth muscle from hypertensive rats is importantly different from that of normotensive rats. In addition, the study delineates individuality in vascular smooth muscle reactivity in different types of experimental hypertension. The results suggest that the cell membrane of the vascular smooth muscle in the hypertensive rat is more labile than that in the normal rat. KEY WORDSrenal hypertension DCA-hypertension angiotensin spontaneously hypertensive rats calcium artery strip lanthanum strontium From the Department of Physiology, University of Michigan, Ann Arbor, Michigan 48104.