Endothelial dysfunction in a murine model of mild hyperhomocyst(e)inemia.

Homocysteine is a risk factor for the development of atherosclerosis and its thrombotic complications. We have employed an animal model to explore the hypothesis that an increase in reactive oxygen species and a subsequent loss of nitric oxide bioactivity contribute to endothelial dysfunction in mild hyperhomocysteinemia. We examined endothelial function and in vivo oxidant burden in mice heterozygous for a deletion in the cystathionine beta-synthase (CBS) gene, by studying isolated, precontracted aortic rings and mesenteric arterioles in situ. CBS(-/+) mice demonstrated impaired acetylcholine-induced aortic relaxation and a paradoxical vasoconstriction of mesenteric microvessels in response to superfusion of methacholine and bradykinin. Cyclic GMP accumulation following acetylcholine treatment was also impaired in isolated aortic segments from CBS(-/+) mice, but aortic relaxation and mesenteric arteriolar dilation in response to sodium nitroprusside were similar to wild-type. Plasma levels of 8-epi-PGF(2alpha) (8-IP) were somewhat increased in CBS(-/+) mice, but liver levels of 8-IP and phospholipid hydroperoxides, another marker of oxidative stress, were normal. Aortic tissue from CBS(-/+) mice also demonstrated greater superoxide production and greater immunostaining for 3-nitrotyrosine, particularly on the endothelial surface. Importantly, endothelial dysfunction appears early in CBS(-/+) mice in the absence of structural arterial abnormalities. Hence, mild hyperhomocysteinemia due to reduced CBS expression impairs endothelium-dependent vasodilation, likely due to impaired nitric oxide bioactivity, and increased oxidative stress apparently contributes to inactivating nitric oxide in chronic, mild hyperhomocysteinemia.

Drug treatment of peripheral vascular disease.

There are a limited number of clinically effective pharmacotherapeutic agents for treatment of peripheral vascular disorders. Pentoxifylline and cilostazol are available for the symptomatic treatment of intermittent claudication. Analogs of carnitine and L-arginine are being evaluated for treatment of the symptoms of intermittent claudication, and prostaglandins and growth factors are being evaluated for critical limb ischemia. Calcium channel blockers remain the treatment of choice for Raynaud's phenomenon. Alternative vasodilators may be used selectively to treat individuals with Raynaud's phenomenon who are intolerant of calcium channel blockers or in whom such therapy has been unsuccessful. Prostaglandins have been evaluated in patients with refractory Raynaud's phenomenon who also have digital ulceration. The mainstay of short-term treatment (and prophylaxis) of deep vein thrombosis (DVT) has been unfractionated heparin, but now the use of low molecular weight heparin (LMWH) has emerged. Newer agents, such as heparinoids and direct thrombin inhibitors, hold promise for the prevention and treatment of DVT. Prolonged treatment with warfarin is still required to prevent recurrent thrombosis, although the duration of treatment has come under debate.

Differential desensitization of thromboxane A2 receptor subtypes.

Two subtypes of the thromboxane A2 (TxA2) receptor (TxA2R-E and TxA2R-P), which differ in their alternatively spliced cytoplasmic tails, have been identified. The initial concentration of the TxA2 mimetic IBOP required to reduce peak intracellular Ca2+ concentration ([Ca2+]i) induced by a second addition of IBOP (100 nmol/L) was similar (IC50 for TxA2R-E and TxA2R-P, 0.46 +/- 0.16 and 0.40 +/- 0.07 nmol/L) in fibroblasts overexpressing either the TxA2R-E or -P subtype. Although the number of TxA2 binding sites decreased in TxA2R-P cells after prolonged stimulation with a TxA2 mimetic, those in the TxA2R-E cells increased markedly. To determine whether the mechanism for desensitization differs between subtypes, the effect of activation of protein kinase C (PKC) or cAMP-dependent kinase on TxA2-induced [Ca2+]i mobilization was measured. Forskolin reduced the IBOP-induced peak [Ca2+]i in neither TxA2R-E nor TxA2R-P cells; however, treatment with phorbol esters (IC50, 0.57 +/- 0.70 nmol/L) strongly prevented IBOP-mediated [Ca2+]i rise in TxA2R-E but not in TxA2R-P cells. Desensitization of TxA2R-E by phorbol esters was prevented by the PKC inhibitor calphostin C or by downregulation of PKC-alpha. Thus, the response of TxA2R-E to prolonged stimulation differs from that of TxA2R-P in both the regulation of the number of binding sites and the mechanism for desensitization; agonists that activate PKC-alpha might interfere with TxA2R-E-mediated signaling.

Intranasal delivery of cardiovascular agents: an innovative approach to cardiovascular pharmacotherapy.

The intranasal administration of drugs has long been used for the topical treatment of various nasal disorders. Many features of the intranasal mucosa also make it useful for delivery of systemically active agents. It has been shown that intranasal drug administration can provide plasma drug levels similar to those observed with comparable doses of parenteral drugs. The feasibility of intranasal administration of propranolol, nifedipine, and nitroglycerin has been investigated in several small clinical studies. Intranasal propranolol has been shown to improve exercise tolerance in patients with angina pectoris. Intranasal nifedipine has been used to treat patients with perioperative hypertension and hypertensive crisis. Intranasal administration of nitroglycerin was shown to blunt the hypertensive response to endotracheal intubation. These studies and others suggest that intranasal delivery of cardiovascular drug treatment could be used in those clinical situations where a rapid or intermittent drug effect is desired and can potentially serve as an alternative to parenteral drug administration.

Angiotensin II receptor antagonists: a new approach to blockade of the renin-angiotensin system.

A-II exerts its activity on various target tissues by binding to its receptors. The discovery of local RASs and A-II receptors within various tissues has generated interest in the clinical usefulness of RAS inhibition by directly blocking the action of A-II at the receptor level. Different A-II receptor subtypes have been identified and subsequently termed AT1 and AT2. AT1-receptor subtypes are the predominant receptor subtypes existing in most organs and, by coupling to a transmembrane G protein, seem to be the main subtypes participating in the vasoactive responses of A-II. Saralasin, a peptide with specific A-II receptor-antagonistic activity, had limited practical long-term usefulness as a result of its short half-life, significant agonistic properties, and lack of oral bioavailability. The discovery of simple benzyl-substituted imidazoles, which possess weak but highly selective A-II receptor antagonistic properties, led to the development of losartan (DuP 753). Losartan is a potent, orally active, specific, competitive nonpeptide A-II receptor antagonist that appears to be an effective antihypertensive agent both in animal studies and in preliminary clinical trials. The therapeutic usefulness of losartan, however, is not limited to its antihypertensive effects. The potential benefits of A-II receptor antagonists include roles in postmyocardial infarction therapy, slowing A-II-induced cardiac hypertrophy, 154, 155 slowing the progression of heart failure, preventing postangioplasty restenosis, and in slowing the progression of renal disease. Furthermore, losartan, a selective A-II type 1 (AT1) receptor antagonist, has also been a valuable pharmacologic probe for studying the mechanism of A-II stimulation of its receptors. A-II receptor antagonism appears to be as effective as ACE inhibition in the treatment of hypertension and other pathologic processes that involve the RAS and may offer an alternative to those patients who cannot tolerate ACE inhibitors because of their side effects.

Angiotensin II receptor blockade: an innovative approach to cardiovascular pharmacotherapy.

Through the multiple actions of angiotensin II (AII), the renin-angiotensin system (RAS) participates in cardiovascular homeostasis. Angiotensin II acts by binding to specific membrane-bound receptors, which are coupled to one of several signal transduction pathways. These AII receptors exhibit heterogeneity, represented by AT1 and AT2 receptor subtypes. The AT1 receptor mediates the major cardiovascular action of the RAS. This receptor has been cloned from multiple species, disclosing features consistent with a transmembrane, G-protein-linked receptor. Further AII receptor heterogeneity is evident by the cloning of isotypes of the AT1 receptor. Blocking the interaction of AII with its receptor is the most direct site to inhibit the actions of the RAS. Many AII receptor antagonists, including peptide analogs of AII and antibodies directed against AII, possess unfavorable properties that have limited their clinical utility. The discovery and further development of imidazole compounds with AII antagonist properties and favorable characteristics, however, has promise for clinical utility. The leader in this field is a selective AT1 receptor antagonist losartan (previously known as DuP 753 or MK-954). Losartan was demonstrated to be an effective antagonist of many AII-induced actions and an effective antihypertensive agent in many animal models of hypertension (HTN). Losartan also demonstrated secondary benefits in preventing stroke, treating congestive heart failure (CHF), and delaying the progression of renal disease in animal models. Clinical studies confirm the AII antagonist action of losartan and suggest that losartan will be effective in the treatment of essential HTN. AII antagonism is likely to provide useful treatment in essential HTN and CHF, conditions in which the RAS is known to play a major role. The utility of AII antagonism may extend beyond that of HTN and CHF, as suggested by the potential usefulness of angiotensin-converting enzyme (ACE) inhibition in the treatment or prevention of many other diseases. The key advantage AII antagonists provide over ACE inhibitors is that they may avoid unwanted side effects, related to bradykinin potentiation with the latter drugs. The AII antagonists will help determine the role of the RAS in physiologic regulation and in the pathophysiology of various disease states.