Pathogenesis of cardiovascular alterations in dogs treated with minoxidil

Human Vascular Wall Microfluidic Model for Preclinical Evaluation of Drug-Induced Vascular Injury

Drug-induced vascular injury (DIVI) in preclinical animal models often leads to candidate compound termination during drug development. DIVI has not been documented in human clinical trials with drugs that cause DIVI in preclinical animals. A robust human preclinical assay for DIVI is needed as an early vascular injury screen. A human vascular wall microfluidic tissue chip was developed with a human umbilical vein endothelial cell (HUVEC)-umbilical artery smooth muscle cell (vascular smooth muscle cell, VSMC) bilayer matured under physiological shear stress. Optimized temporal flow profiles produced HUVEC-VSMC bilayers with quiescent endothelial cell (EC) monolayers, EC tight junctions, and contractile VSMC morphology. Dose-response testing (3-30 μM concentration) was conducted with minoxidil and tadalafil vasodilators. Both drugs have demonstrated preclinical DIVI but lack clinical evidence. The permeability of severely damaged engineered bilayers (30 μM tadalafil) was 4.1 times that of the untreated controls. Immunohistochemical protein assays revealed contrasting perspectives on tadalafil and minoxidil-induced damage. Tadalafil impacted the endothelial monolayer with minor injury to the contractile VSMCs, whereas minoxidil demonstrated minor EC barrier injury but damaged VSMCs and activated ECs in a dose-response manner. This proof-of-concept human vascular wall bilayer model of DIVI is a critical step toward developing a preclinical human screening assay for drug development. Impact statement More than 90% of drug candidates fail during clinical trials due to human efficacy and toxicity concerns. Preclinical studies rely heavily on animal models, although animal toxicity and drug metabolism responses often differ from humans. During the drug development process, perfused in vitro human tissue chips could model the clinical drug response and potential toxicity of candidate compounds. Our long-term objective is to develop a human vascular wall tissue chip to screen for drug-induced vascular injury. Its application could ultimately reduce drug development delays and costs, and improve patient safety.

Tissue-engineered blood vessels as promising tools for testing drug toxicity

Drug-induced vascular injury (DIVI) is a serious problem in preclinical studies of vasoactive molecules and for survivors of pediatric cancers. DIVI is often observed in rodents and some larger animals, primarily with drugs affecting vascular tone, but not in humans; however, DIVI observed in animal studies often precludes a drug candidate from continuing along the development pipeline. Thus, there is great interest by the pharmaceutical industry to identify quantifiable human biomarkers of DIVI. Small-scale endothelialized tissue-engineered blood vessels using human cells represent a promising approach to screen drug candidates and develop alternatives to cancer therapeutics in vitro. We identify several technical challenges that remain to be addressed, including high-throughput systems to screen large numbers of candidates, identification of suitable cell sources and establishing and maintaining a differentiated state of the vessel wall cells. Adequately addressing these challenges should yield novel platforms to screen drugs and develop new therapeutics to treat cardiovascular disease.

Scientific and Regulatory Policy Committee Points-to-consider Paper*

Drug-induced vascular injury (DIVI) is a recurrent challenge in the development of novel pharmaceutical agents. Although DIVI in laboratory animal species has been well characterized for vasoactive small molecules, there is little available information regarding DIVI associated with biotherapeutics such as peptides/proteins or antibodies. Because of the uncertainty about whether DIVI in preclinical studies is predictive of effects in humans and the lack of robust biomarkers of DIVI, preclinical DIVI findings can cause considerable delays in or even halt development of promising new drugs. This review discusses standard terminology, characteristics, and mechanisms of DIVI associated with biotherapeutics. Guidance and points to consider for the toxicologist and pathologist facing preclinical cases of biotherapeutic-related DIVI are outlined, and examples of regulatory feedback for each of the mechanistic types of DIVI are included to provide insight into risk assessment.

Roflumilast-induced Local Vascular Injury Is Associated with a Coordinated Proteome and Microparticle Change in the Systemic Circulation in Pigs

Drug-induced vascular injury (DIVI) is commonly associated with phosphodiesterase (PDE) inhibitors. Despite histological characterization, qualified biomarkers for DIVI detection are lacking. We investigated whether a single administration of roflumilast (PDE-IV inhibitor) induces vascular damage and identified novel surrogate biomarkers of acute vascular injury. Pigs received postoperative 250, 375, or 500 μg of roflumilast or placebo/control. After 1.5 hr, coronary reactivity was determined by catheter-based administration of acetylcholine and sodium nitroprusside (SNP) in the coronary sinus. Immunohistochemical analysis of vessel integrity (von Willebrand factor [vWF]) and fibrin(ogen) deposition was performed in the coronary artery and aorta. Peripheral blood was collected for differential proteomics and microparticles analysis. Circulating interleukin (IL)-6 was analyzed. Roflumilast-treated animals displayed higher vasodilation to acetylcholine and SNP versus controls (p < .05). Roflumilast-treated animals showed a dose-dependent (p < .05) decrease in vessel integrity and dose-dependent increase in fibrin deposition forming a continuous layer at roflumilast-500 μg. Peripheral blood of roflumilast-500-μg-treated animals showed increased levels of total and endothelial-derived microparticles and exhibited a coordinated change in proteins kininogen-1, endothelin-1, gelsolin, apolipoprotein A-I, and apolipoprotein-J associated with vascular injury (p < .05 vs. controls). IL-6 remained unaltered. Roflumilast-induced vascular injury can be detected by novel markers in peripheral blood. Validation of these surrogate markers in human samples seems required.

Nonclinical Safety Biomarkers of Drug-induced Vascular Injury

Better biomarkers are needed to identify, characterize, and/or monitor drug-induced vascular injury (DIVI) in nonclinical species and patients. The Predictive Safety Testing Consortium (PSTC), a precompetitive collaboration of pharmaceutical companies and the U.S. Food and Drug Administration (FDA), formed the Vascular Injury Working Group (VIWG) to develop and qualify translatable biomarkers of DIVI. The VIWG focused its research on acute DIVI because early detection for clinical and nonclinical safety monitoring is desirable. The VIWG developed a strategy based on the premise that biomarkers of DIVI in rat would be translatable to humans due to the morphologic similarity of vascular injury between species regardless of mechanism. The histomorphologic lexicon for DIVI in rat defines degenerative and adaptive findings of the vascular endothelium and smooth muscles, and characterizes inflammatory components. We describe the mechanisms of these changes and their associations with candidate biomarkers for which advanced analytical method validation was completed. Further development is recommended for circulating microRNAs, endothelial microparticles, and imaging techniques. Recommendations for sample collection and processing, analytical methods, and confirmation of target localization using immunohistochemistry and in situ hybridization are described. The methods described are anticipated to aid in the identification and qualification of translational biomarkers for DIVI.

Translation Strategy for the Qualification of Drug-induced Vascular Injury Biomarkers

Drug-induced vascular injury (DIVI) is a common preclinical toxicity usually characterized by hemorrhage, vascular endothelial and smooth muscle damage, and inflammation. DIVI findings can cause delays or termination of drug candidates due to low safety margins. The situation is complicated by the absence of sensitive, noninvasive biomarkers for monitoring vascular injury and the uncertain relevance to humans. The Safer And Faster Evidence-based Translation (SAFE-T) consortium is a public–private partnership funded within the European Commission’s Innovative Medicines Initiative (IMI) aiming to accelerate drug development by qualifying biomarkers for drug-induced organ injuries, including DIVI. The group is using patients with vascular diseases that have key histomorphologic features (endothelial damage, smooth muscle damage, and inflammation) in common with those observed in DIVI, and has selected candidate biomarkers associated with these features. Studied populations include healthy volunteers, patients with spontaneous vasculitides and other vascular disorders. Initial results from studies with healthy volunteers and patients with vasculitides show that a panel of biomarkers can successfully discriminate the population groups. The SAFE-T group plans to seek endorsement from health authorities (European Medicines Agency and Food and Drug Administration) to qualify the biomarkers for use in regulatory decision-making processes.

Hemodynamic Correlates of Drug-induced Vascular Injury in the Rat Using High-frequency Ultrasound Imaging

Several classes of drugs have been shown to cause drug-induced vascular injury (DIVI) in preclinical toxicity studies. Measurement of blood flow and vessel diameter in numerous vessels and across various tissues by ultrasound imaging has the potential to be a noninvasive translatable biomarker of DIVI. Our objective was to demonstrate the utility of high-frequency ultrasound imaging for measuring changes in vascular function by evaluating blood flow and vessel diameter in the superior mesenteric arteries (SMA) of rats treated with compounds that are known to cause DIVI and are known vasodilators in rat: fenoldopam, CI-1044, and SK&F 95654. Blood flow, vessel diameter, and other parameters were measured in the SMA at 4, 8, and 24 hr after dosing. Mild to moderate perivascular accumulations of mononuclear cells, neutrophils in tunica adventitia, and superficial tunica media as well as multifocal hemorrhage and necrosis in the tunica media were found in animals 24 hr after treatment with fenoldopam and SK&F 95654. Each compound caused marked increases in blood flow and shear stress as early as 4 hr after dosing. These results suggest that ultrasound imaging may constitute a functional correlate for the microscopic finding of DIVI in the rat.

Evaluation of von Willebrand factor and von Willebrand factor propeptide in models of vascular endothelial cell activation, perturbation, and/or injury

Pharmacologically, vasoactive agents targeting endothelial and/or smooth muscle cells (SMC) are known to cause acute drug-induced vascular injury (DIVI) and the resulting pathology is due to endothelial cell (EC) perturbation, activation, and/or injury. Alteration in EC structure and/or function may be a critical event in vascular injury and, therefore, evaluation of the circulatory kinetic profile and secretory pattern of EC-specific proteins such as VWF and VWFpp could serve as acute vascular injury biomarkers. In rat and dog models of DIVI, this profile was determined using pharmacologically diverse agents associated with functional stimulation/perturbation (DDAVP), pathological activation (lipopolysaccharide [LPS]/endotoxin), and structural damage (fenoldopam [FD], dopamine [DA], and potassium channel opener (PCO) ZD6169). In rats, FD caused moderate DIVI and time-related increase in plasma VWF levels ∼33% while in control rats VWF increased ∼5%. In dogs, VWF levels transiently increased ∼30% when there was morphologic evidence of DIVI by DA or ZD6169. However, in dogs, VWFpp increased >60-fold (LPS) and >6-fold (DDAVP), respectively. This was in comparison to smaller dynamic 1.38-fold (LPS) and 0.54-fold (DDAVP) increases seen in plasma VWF. Furthermore, DA was associated with a dose-dependent increase in plasma VWFpp. In summary, VWF and VWFpp can discriminate between physiological and pathological perturbation, activation, and injury to ECs.

Incidental Histopathological Findings in Hearts of Control Beagle Dogs in Toxicity Studies

In preclinical studies of pharmaceutical agents, the beagle dog is a commonly used model for the detection of cardiotoxicity. Incidental findings, postmortem changes, and artifacts must be distinguished histopathologically from test item–related findings in the heart. In this retrospective analysis, cardiac sections from 88 control beagles (41 male, 47 female; ages 5–18 months) in preclinical studies were examined histopathologically. The most common finding was thickening of the tunica media of intramural coronary arteries, most likely a postmortem change. The second most common finding was the presence of vacuoles within Purkinje fibers. Dilated lymphatic and blood vessels at the insertion of chordae tendineae were noted more commonly in males than in females and were considered a normal anatomic feature. Mesothelial-lined papillary fronds along the epicardial surface of the atria were present in several dogs, as were small infiltrates of inflammatory cells usually within the myocardium. In summary, control beagles’ hearts frequently have incidental findings that must be differentiated from test item–related pathologic changes. Historical control data can be useful for the interpretation of incidental and test item–related findings in the beagle heart.

Coronary and systemic arterial physiology and immunohistochemical markers related to early coronary arterial lesions in beagle dogs given the potassium channel opener, ZD6169, or the endothelin receptor antagonist, ZD1611

We evaluated immunohistochemistry (von Willebrand Factor [vWF] or fibrinogen) and systemic and coronary arterial physiological parameters in beagle dogs to investigate early arterial lesions induced by the potassium channel opener, ZD6169, or the endothelin receptor antagonist, ZD1611. Dogs given an oral dose of ZD6169 (experiment 1) were terminated 1 day later and showed arterial and myocardial lesions. Minimal arterial lesions exhibited few condensed medial smooth muscle cells only, with others showing segmental medial necrosis occasionally with medial/adventitial acute inflammation. Intercellular immunostaining was seen in ostensibly normal tissue, where no pathology was present in conventionally stained sections. vWF and fibrinogen are valuable tools for detecting disruption of arterial integrity. In experiment 2, 2 dogs were given a single high dose of ZD6169 or ZD1611 and BP/HR monitored by conventional measures or telemetry. Substantially reduced systolic/diastolic BP and increased HR occurred within 10 min of ZD6169 infusion: ZD1611 caused minor BP decrease and HR increase. In experiment 3, both drugs given to anaesthetized dogs induced markedly exaggerated systolic phasic forward and reverse flow in left descending and right coronary arteries. Diastolic coronary artery flows were unaffected with ZD1611 and increased slightly with ZD6169. In both coronary arteries, the ZD1611-induced increase in flows paralleled decreased resistance.

Evidence for the nitric oxide pathway as a potential mode of action in fenoldopam-induced vascular injury

Fenoldopam, a dopaminergic DA1 agonist, induces vasodilatation via nitric oxide (NO), and this may be associated with mesenteric arterial injury. NO is produced from the enzymatic action of nitric oxide synthase (NOS), which is regulated by the shear-stress mediating protein caveolin-1. Profound vasodilatation and accompanied decreased shear are early events that could initiate vascular injury. Therefore, it is of interest to determine the role of caveolin-1 and the NO pathway in fenoldopam-induced vascular injury. At sites of fenoldopam-induced mesenteric arterial injury, decreased caveolin-1 expression and apoptosis were prominent immunohistochemical findings. An additional finding at these sites of injury were loss and/or reduced expression of caveolin-1 regulated structural proteins, connexin-43, (gap junction) ZO-1, and claudin (tight junctions). Because functional loss of caveolin-1 is associated with increased NOS activity and vasodilatation via NO, studies were conducted to show a NO donor produced vascular lesions in the mesenteric arteries morphologically similar to those induced by fenoldopam. Moreover, the incidence and severity of fenoldopam-induced vascular injury were reduced when an NOS inhibitor or a scavenger of NO-generated free radicals were coadministered with fenoldopam. Collectively, these data suggest that caveolin-1 and its regulated NO pathway may play an important role in vasodilatory drug-induced vascular injury.

Successful drug development despite adverse preclinical findings part 2: examples

To illustrate the process of addressing adverse preclinical findings (APFs) as outlined in the first part of this review, a number of cases with unexpected APF in toxicity studies with drug candidates is discussed in this second part. The emphasis is on risk characterization, especially regarding the mode of action (MoA), and risk evaluation regarding relevance for man. While severe APFs such as retinal toxicity may turn out to be of little human relevance, minor findings particularly in early toxicity studies, such as vasculitis, may later pose a real problem. Rodents are imperfect models for endocrine APFs, non-rodents for human cardiac effects. Liver and kidney toxicities are frequent, but they can often be monitored in man and do not necessarily result in early termination of drug candidates. Novel findings such as the unusual lesions in the gastrointestinal tract and the bones presented in this review can be difficult to explain. It will be shown that well known issues such as phospholipidosis and carcinogenicity by agonists of peroxisome proliferator-activated receptors (PPAR) need to be evaluated on a case-by-case basis. The latter is of particular interest because the new PPAR α and dual α/γ agonists resulted in a change of the safety paradigm established with the older PPAR α agonists. General toxicologists and pathologists need some understanding of the principles of genotoxicity and reproductive toxicity testing. Both types of preclinical toxicities are major APF and clinical monitoring is difficult, generally leading to permanent use restrictions.

Myocardial and Reproductive System Toxicity of SCH 351591, a Selective Phosphodiesterase-4 Inhibitor, in CD-1 Mice

This report describes the findings of preclinical testing of SCH 351591, a selective phosphodiesterase 4 inhibitor, in CD-1 mice over a wide range of doses, in which the heart and reproductive organs of both sexes demonstrated toxic effects. Repeat-dose toxicity studies assessed 5, 15, 50, 100, 200, 400, and 800 mg/kg/day, orally by gavage, for one or three months. Findings included higher testes and ovary weights and lower uterus weights (≥200 mg/kg), small ovaries/uterus (≥400 mg/kg), and histopathologic changes of large corpora lutea and ovarian atrophy at 200 and 800 mg/kg, respectively. In addition, chronic myocardial inflammation of the heart base occurred at 100 mg/kg. Vaginal staging of the estrous cycle revealed persistent diestrus. There was no histopathologic correlate or morphometric change to explain higher testes weights. A pilot fertility and early embryonic developmental toxicity study assessing doses of 100, 200, 400, and 800 mg/kg/day produced complementary results. Females had prolonged or abnormal estrous cycles, fewer successful pregnancies, increased ovarian corpora lutea, and decreased size of live litters owing to fetal resorptions. Male fertility was not affected. However, males had a 25% increase in testes weights at all doses. The pharmacology of specific PDE4 isoenzymes may explain both the reproductive and cardiac findings.

Cardiovascular toxicity of minoxidil in the marmoset

The aim of the experiments was to assess the toxicity of minoxidil, a potent vasodilator, in marmosets. The animals were treated either at escalating doses from 2 to 40 mg/kg, escalating doses from 40 to 200 mg/kg or single doses of 150 mg/kg or 200 mg/kg. ECG recording and echocardiographic examination were conducted before and 1h after treatment. Necropsy and histopathology were performed 24h after the last dose. The treatment with minoxidil induced myocardial necrosis, coronary arteriopathy and degeneration of renal tubules in animals treated with 150 mg/kg or 200 mg/kg. Myocardial necrosis associated with fibrosis in some animals was located mainly in the left and right ventricles (including papillary muscles), but also in the right atrium, left atrium and/or interventricular septum. Arteriopathy was observed in small coronary arteries of the right or left atrium. ECG and echocardiographic examinations showed that in animals treated with 150 mg/kg or 200 mg/kg, there were positive chronotropic and inotropic effects that compensated for the hypotensive effect of the drug and were considered to have played a key role in the pathogenesis of the cardiovascular lesions. The cardiotoxicity of minoxidil in marmosets was similar to that described in dogs, but occurred at much higher doses. In conclusion minoxidil produced cardiovascular toxicity in the marmoset, which was probably due to the marked changes in the cardiac function associated with exaggerated pharmacological effects of the compound. The marmosets were found to be less sensitive than dogs to the cardiotoxicity of minoxidil.

The application of discovery toxicology and pathology towards the design of safer pharmaceutical lead candidates

Toxicity is a leading cause of attrition at all stages of the drug development process. The majority of safety-related attrition occurs preclinically, suggesting that approaches to identify 'predictable' preclinical safety liabilities earlier in the drug development process could lead to the design and/or selection of better drug candidates that have increased probabilities of becoming marketed drugs. In this Review, we discuss how the early application of preclinical safety assessment--both new molecular technologies as well as more established approaches such as standard repeat-dose rodent toxicology studies--can identify predictable safety issues earlier in the testing paradigm. The earlier identification of dose-limiting toxicities will provide chemists and toxicologists the opportunity to characterize the dose-limiting toxicities, determine structure-toxicity relationships and minimize or circumvent adverse safety liabilities.

Biomarkers and mechanisms of drug-induced vascular injury in non-rodents

In preclinical safety studies, drug-induced vascular injury can negatively impact candidate-drug selection because there are no obvious diagnostic markers for monitoring this pathology preclinically or clinically. Furthermore, our current understanding of the pathogenesis of this lesion is limited. While vasodilatation and increased shear stress appear to play a role, the exact mechanism(s) of injury to the primary target cells, smooth muscle (SMC) and endothelial cell (EC), are unknown. Evaluation of potential novel markers for clinical monitoring with a mechanistic underpinning would add value in risk assessment and risk management. This mini review focuses on the efforts and progress to identify diagnostic markers as well as understanding the mechanism of action in nonrodent drug-induced vascular injury.

Effect of milrinone on echocardiographic parameters after single dose in Beagle dogs and relationship with drug-induced cardiotoxicity

The aim of this study was to further investigate the mechanism of development of cardiac lesions occurring under treatment with milrinone in dogs, by using echocardiography for assessing the effects of this drug on cardiac function. Milrinone is a cAMP phosphodiesterase 3 inhibitor having positive inotropic and vasodilatory effects. We treated groups of three dogs with milrinone at a single dose of 0.5 or 1 mg/kg and recorded M-mode and Doppler parameters at different time points before and after treatment. The hearts of the high-dose animals were histopathologically examined. The treatment with milrinone at 1 mg/kg produced mild cardiac lesions at two different locations. In the left ventricle, haemorrhages in the subendocardium and myocardium occurred in all three dogs. In the right atrium, subepicardial haemorrhages occurred in one dog and inflammation of the epicardium was observed in two dogs. These lesions were considered to be related to changes in the cardiac function, which were investigated by echocardiography. Milrinone treatment produced a moderate tachycardia and changes in M-mode parameters indicating an increase in contractility, in particular, a decrease in end-systolic volume, an increase in ejection fraction and an increase in the rate of circumferential fiber shortening. In addition, there was an increase in the maximal aortic flow velocity evaluated by Doppler measurements, which is thought to represent a haemodynamic correlate of an increase in left ventricular contractility. This increase in myocardial work is considered to play a key role in the development of the lesions observed in the left ventricle. Doppler measurements also revealed changes in the right atrioventricular flow, probably resulting from cardiac stimulation produced by milrinone. In particular, there was an increase in the Vmax of the A-wave of the tricuspid flow, suggesting an increase in contractility of the right atrium. This change, by increasing blood flow in atrial wall, may be involved in the induction of the lesions observed in the right atrium. In conclusion, Doppler and M-mode echocardiography are useful tools to assess haemodynamic changes occurring upon treatment with vasodilators or cardiac stimulants in order to further understand the mechanism of development of cardiac lesions produced by such compounds.

The toxicity of SCH 351591, a novel phosphodiesterase-4 inhibitor, in Cynomolgus monkeys

SCH351591, a novel phosphodiesterase-4 inhibitor under investigation as a potential therapeutic for asthma and chronic obstructive pulmonary disease (COPD), was evaluated in a 3-month rising-dose study in Cynomolgus monkeys. Four groups, containing four monkeys/sex, received vehicle control or rising doses up to 12, 24, or 48 mg/kg of SCH351591 daily. Although initial exposure produced clinical signs of emesis, reduced food intake, and reduced body weight, tachyphylaxis to the emesis allowed dose escalation up to 48 mg/kg/day. Two monkeys died and 3 were sacrificed in moribund condition over the course of the study. Early mortality, involving monkeys dosed with 12 or 24 mg/kg, was attributed to sepsis (2 monkeys) or colon inflammation (3 monkeys). Leukocyte function assays on low- and mid-dose group survivors revealed an inhibition of T lymphocyte proliferation for 12 mg/kg group males and 24 mg/kg group monkeys of both sexes. Necropsy findings, unassociated with early mortality, included reduced size and weight of the thymus, depletion of body fat, red discoloration of the gastric mucosa, and perivascular hemorrhage of the stomach and heart. Stomach and heart gross findings were present in the high-dose group only. Histopathologic lesions, in addition to those attributed to concurrent bacterial infection, included thymic atrophy, serous atrophy of fat, myocardial degeneration and acute to chronic inflammation of small to medium-sized arteries in various organs and tissues including the heart, kidneys, stomach, salivary glands, pancreas, esophagus, gallbladder, and mesentery. The findings of this study demonstrate the potential of a PDE4 inhibitor to alter immunologic response as well as to produce arteriopathy in nonhuman primates.

Chronic Effects of Flutamide in Male Beagle Dogs

Flutamide, a potent nonsteroidal antiandrogen, was administered orally to male beagle dogs for 2,3, or 4 years at doses of 10,20, or 40 mg/kg/day. At each study interval, the results of clinical pathology examinations, organ weight determinations, necropsy, and histopathologic examinations generally were similar and included atrophy of the prostate gland, testicular interstitial cell hyperplasia, and seminiferous tubular atrophy and degeneration. After 3 years of drug exposure, there were 3 dogs with testicular interstitial cell adenomas and a few dogs with 1 or more enlarged mammary gland nipples. Based upon the pharmacologic activity of flutamide, these findings were expected and considered the consequence of long-term blocking of testosterone receptors and an exaggerated compensatory response to increased secretion of luteinizing hormone. The findings of this study were consistent with other examples of dysregulated hormone stimulation of target tissues noted during the nonclinical safety assessment of flutamide. In consideration of the clinical indication of flutamide for advanced prostatic carcinoma and based upon reports of minimal flutamide-related adverse clinical responses, the findings of this study pose no concern for human risk assessment.

Effects of minoxidil on ischemia-induced mechanical and metabolic dysfunction in dog myocardium

Effects of minoxidil on ischemia-induced myocardial mechanical and metabolic dysfunction were examined in anesthetized open-chest dogs. A regional portion of the left ventricle was made ischemic for 20 min by ligating the left anterior descending coronary artery, and then reperfused for 120 min. Dimethylsulfoxide or minoxidil (0.3, or 1.0 mg/kg) was injected intravenously 10 min before ligation. Ischemia decreased regional myocardial contraction, and reperfusion recovered it but incompletely. Myocardial metabolic derangement was observed during ischemia, such as decreases in the myocardial levels of ATP and creatine phosphate. These metabolic changes caused by ischemia were restored by reperfusion. Minoxidil injection at 0.3 and 1.0 mg/kg significantly decreased blood pressures but increased coronary flow. Pretreatment with minoxidil significantly enhanced the recovery of myocardial contraction during reperfusion after ischemia. The levels of ATP and creatine phosphate in the ischemic myocardium were significantly preserved by minoxidil at 0.3 mg/kg. No significant effect of minoxidil on the metabolism was observed in the 120 min reperfused myocardium. In conclusion, minoxidil improved the mechanical dysfunction in the reperfused heart and the drug at low dose preserved high-energy phosphates during ischemia.

Studies on coronary arteriopathy in dogs following administration of CI-1020, an endothelin A receptor antagonist

A selective nonpeptide endothelin A (ETA) receptor antagonist, CI-1020, was administered to beagle dogs intravenously (i.v.) for 4 hours to 4 weeks. One animal/sex received CI-1020 at 1 mg/kg/hr intravenously for 4, 8, or 24 hours to investigate onset of arteriopathy. Control animals (1/sex) received the vehicle only. To determine reversibility of arteriopathy, 8 dogs/sex were given CI-1020 at 1 mg/kg/hr for 4 days. Two dogs/sex were sacrificed 1, 3, 8, and 29 days following cessation of infusion. Lesion development with prolonged exposure was investigated in 1 male dog. It was given CI-1020 by i.v. bolus at 120 mg/kg/day for 4 weeks and Monastral blue dye was administered i.v. to facilitate localization of vascular lesions. Coronary blood flow was determined in 4 dogs infused with CI-1020 at 0.3, 3, and 30 mg/kg for one hour at each dose. Macroscopically, hemorrhage or blue discoloration of Monastral blue was noted in the extramural coronary arteries along the coronary groove and atrium. Histologically, the earliest coronary changes were noted in animals sacrificed after 24 hours of treatment and characterized by medial hemorrhage and necrosis with a few infiltrating neutrophils. In the reversibility study, incidence and severity of arteriopathy was dependent on time of sacrifice following cessation of infusion. Acute necrotizing inflammation of arteries was present in all animals (n = 4) on day 1 postinfusion, whereas on day 8 postinfusion, lesions characterized by medial small pockets of trapped red cells, cell debris, and adventitial thickening were seen in 1 dog/sex. By day 29 postinfusion, coronary arteries were similar to controls. In the dog given daily i.v. bolus injections of CI-1020 for 4 weeks, arterial inflammatory lesions varied from acute to chronic, although most lesions were considered chronic active. Monastral blue pigments were noted in the wall of most arteries with chronic or chronic active lesions. Acute lesions were similar to those noted in day 1 postinfusion of the reversibility study. Medial smooth muscle necrosis and/or fibrosis with mixed inflammatory cell infiltrates characterized chronic or chronic active lesions. Smooth muscle proliferation and migration into the intima were also noted. There were no significant changes in coronary blood flow, coronary vascular resistance, or mean arterial blood pressure following CI-1020 infusion for 3 hours. In the 24-hour infusion study, plasma endothelin 1 (ET-1) levels were mildly elevated (1.5-4 fold) during CI-1020 infusion when compared to either pretest or control values. These results indicate that administration of endothelin antagonist (CI-1020) to dogs was associated with development of coronary arteriopathy, which was completely resolved within 29 days following cessation of treatment. With prolonged (4-week) CI-1020 treatment, arterial lesions at varying stages of development (acute, chronic active, chronic) were seen, suggesting that tolerance to treatment (up to 4 weeks) does not occur.

Histopathological and immunohistochemical studies on arteritis induced by fenoldopam, a vasodilator, in rats

Fenoldopam, a dopaminergic (DA1) agonist, has been reported to induce medial necrosis and adventitial inflammatory response in the splanchnic arteries in rats. This study was carried out to clarify the detailed time course of the inflammatory responses, using antibodies for the inflammatory cell markers, CD3 (T cell), CD20 (B cell) and ED-1 (macrophage), and inflammatory serum factors, IgG, IgM and C3. Rats were administered fenoldopam for 24 hours by intravenous infusion. Histopathologically, medial necrosis with hemorrhage was observed at the end of infusion, but it almost disappeared on day 7 post-infusion. Adventitial inflammatory responses with ED-1-, CD3- and CD20-positive cells were very slight at the end of infusion, became prominent with marked fibrosis on days 3 and 5, decreased on day 7, and subsided on day 14. The serum factors were first present in the area of medial necrosis, then shifted to the subendothelial space or cytoplasm of smooth muscle cells, and disappeared on day 14 post-infusion. Gaps in the external elastic lamina were observed on days 3 and 5 post-infusion, and IgG and IgM were present outside the gaps in the adventitia. These results provided us with more detailed information on the inflammatory responses following medial damage induced by vasodilators.

Endothelin receptor subtype distribution predisposes coronary arteries to damage

Several vasoactive drugs that lower blood pressure and increase heart rate induce regional cardiotoxicity in the dog, most frequently of right coronary arteries and right atrium. The basis for this selective damage is thought to result from local changes in vascular tone and blood flow. Administration of an endothelin receptor antagonist (ETRA, SB 209670) to dogs induced damage most frequent and severe in the right coronary artery and right atrium. Because site predisposition may correlate with distribution of vasoactive receptors, the objectives of this study were to map endothelin (ET) receptor distribution and density within regions of dog heart using both gene (mRNA) and protein expression endpoints for dog ET(A) and ET(B) receptors, and, additionally, correlate ET receptor subtype density with regional cardiac blood flow. A 10- to 15-mmHg reduction in mean arterial pressure with a concomitant increase in heart rate (10-20%), a six- and twofold increase in regional blood flow to the right and left atrium, respectively, and acute hemorrhage, medial necrosis, and inflammation were observed in the right coronary arteries and arteries of the right atrium after ETRA infusion for 5 days. Radioligand protein binding to quantify both ET receptors in normal dog heart indicated a twofold greater density of ET receptors in atrial regions versus ventricular regions. Importantly, ET receptor density in coronary arteries was markedly (about five- to sixfold) increased above that in atrial or ventricular tissues. ET receptor subtype characterization indicated ET(B) receptors were three times more prevalent in right coronary arteries compared to left coronary arteries and in situ hybridization confirmed localization of ET(B) in vascular smooth muscle. ET(A) receptor density was comparable in right and left coronary arteries. Quantitative real-time polymerase chain reaction for ET(A) and ET(B) receptor mRNA transcripts supported the site prevalence for message distribution. Consequently, the composite of protein and message expression profiles for ET(A) and ET(B) receptors indicated a disproportionate distribution of ET(B) receptors within right coronary artery of dog and this, along with functional measures of blood flow after ETRA infusion indicated a predisposition for exaggerated pharmacological responses and subsequent damage to right coronary arteries by ET and/or ETRAs.

Arterial lesions induced by phosphodiesterase III (PDE III) inhibitors and DA(1) agonists

Compounds that inhibit the low Km, cGMP-inhibitable form of phosphodiesterase (type III) and the DA(1) agonist, fenoldopam, are potent vasodilators that have been associated with segmental medial haemorrhagic necrosis in susceptible arterial beds following administration of suprapharmacological doses to dogs and/or rats. Morphological and haemodynamic investigative studies with PDE III inhibitors support the hypothesis that the arterial toxicity is the consequence of the vasodilator pharmacology of these compounds. Investigative data also suggest that similar mechanisms are involved in the pathogenesis of arterial lesions induced by fenoldopam and the K(+) channel opener, minoxidil.

Coronary arteriopathy in monkeys following administration of CI-1020, an endothelin A receptor antagonist

A selective non-peptide endothelin A (ETA) receptor antagonist, CI-1020, was administered to cynomolgus monkeys intravenously (i.v.) for 2 or 4 wk and orally for 4 wk. Groups consisting of 3 animals of each sex received CI-1020 at 1, 5, and 10 mg/kg/hr (i.v.) or orally at 250, 500, and 750 mg/kg body weight for 4 wk. Control animals received the vehicle only. In a separate experiment, 1 male was infused with 10 mg/kg/hr for 2 wk, and Monastral blue dye was administered i.v. to facilitate localization of lesions to the vascular walls. One female was administered saline and the dye and served as a control. One female at 1 mg/kg/hr was found dead at week 2, and 1 female at 5 mg/kg/hr was euthanatized during week 4 as a result of severe thigh swelling at the catheter site. Macroscopically, extramural coronary arteries appeared thickened and nodular in the 4-wk i.v. study in the female found dead at 1 mg/kg/hr, in 1 male and 1 female at 5 mg/kg/hr, and in 2 females at 10 mg/kg/hr. Histologically, Monastral blue pigment trapped in the walls of coronary arteries with arteriopathy was observed in the male treated with CI-1020 at 10 mg/kg/hr for 2 wk. Extramural coronary arteriopathy occurred at all doses in the 4-wk i.v. study, with higher incidence occurring in females than in males (7 of 9 treated females compared with 3 of 9 treated males). In the oral study, 1 female at 500 mg/kg/day and 1 male and 2 females at 750 mg/kg/day had coronary arteriopathy. Histological changes after 2 wk of treatment were characterized by intimal thickening, fragmentation of the internal elastic lamina, necrosis and edema of the media, and mixed inflammatory-cell infiltrates in the intima, media, and adventitia. After 4 wk of i.v. administration, arteriopathy was characterized by segmental disruption of the elastic lamina and intimal and medial fibrosis with complete replacement of smooth muscle with fibrous tissue. The adventitia was thickened as a result of fibrosis and mixed or mononuclear inflammatory-cell infiltrates. CI-1020 concentrations were higher in males (1.57 to 29 micrograms/ml) than in females (0.974 to 24.4 micrograms/ml) in the i.v. study. Transient systemic exposure with high maximum plasma concentration (Cmax) (120-352 micrograms/ml) in the oral study was insufficient to provoke arterial changes of the same magnitude as those noted with continuous i.v. administration. The regeneration of the media by fibrous tissue and the disruption of the elastic lamina may weaken the arterial wall and increase the susceptibility of the artery to the development of aneurysm.

Coronary arterial lesions in dogs treated with an endothelin receptor antagonist

Structurally and pharmacologically diverse vasodilators are known to lower blood pressure, increase heart rate, and produce acute injury to right coronary arteries in the dog. Administration of low concentrations of endothelin-1 (ET-1) to anesthetized dogs causes coronary vasoconstriction and reductions in coronary blood flow. Therefore, pharmacologic blockade of endothelin receptors (ETA and ETB) with the mixed ET receptor antagonist SB 209670 could lead to coronary vasodilatation. In toxicology studies, continuous administration of SB 209670 to dogs for 5 days at 50 micrograms/kg/min was associated with minor but sustained increases in heart rate (10-30 beats/min), slight decreases in mean arterial pressure (10-15 mm Hg), and medial hemorrhage and necrosis of extramural coronary arteries in the right atria. Doses of 10 micrograms/kg/min had no effect. The lesions in the right atrium were associated with the highest density of ET receptors, approximately 470 fmol/mg compared to 170-200 fmol/mg in the ventricles and septum. Because changes in systemic cardiovascular parameters are minimal, the coronary arterial lesion is most likely due to a local vasodilatory effect in the coronary bed.

Arteriopathy induced by an adenosine agonist-antihypertensive in monkeys

An adenosine agonist, designated chemically as (R)-N-(2,3-dihydro-1H-inden-1-yl) adenosine or CI-947, was administered orally to 2 males and 2 female cynomolgus monkeys each at 5, 10, 20, and 50 mg/kg of body weight for 2 wk. One male and 1 female given 50 mg/kg were euthanatized on days 10 and 8, respectively, because of poor clinical condition. Emesis was present at 10, 20, and 50 mg/kg. Decreased heart rate and QT prolongation were present at 50 mg/kg. Extramural coronary arterial lesions consisting of medial necrosis with cellular debris and mixed inflammatory cell response in the intima, media, and adventitia were present in 1 male at 20 mg/kg and 1 male at 50 mg/kg at study termination. Similar arterial lesions were present in the small and large intestines and testis of the male at 50 mg/kg. Colonic mucosal erosions with mixed inflammatory cell infiltrates in the lamina propria were seen in this male and in all CI-947 treated females at 10, 20, and 50 mg/kg. Myocardial degeneration and necrosis of myocardial fibers with mononuclear cell infiltrates in the interstititum were noted in the left ventricle of 1 female at 20 mg/kg and in all animals at 50 mg/kg. Renal cortical tubular dilatation with increases in serum creatinine and/or blood urea nitrogen were noted in a control female and animals at 10 and 50 mg/kg. Plasma CI-947 concentration increased with increasing dose. Coronary vascular injury in the monkey was similar to the arterial lesion in CI-947-treated dogs and may relate to the pharmacologic/hemodynamic effects induced by CI-947. When compared with the dog, the monkey appears to be less sensitive to development of arteriopathy, as indicated by lower incidence, at similar systemic exposure levels.

Characterization of coronary arterial lesions in the dog following administration of SK&F 95654, a phosphodiesterase III inhibitor

Drugs that inhibit the low-Km, cGMP-inhibitable form of phosphodiesterase III (PDE III) are associated with arterial lesions in the extramural coronary arteries of dogs following oral and intravenous administration at high doses. Acute coronary arterial lesions have been investigated following administration to the dog of SK&F 95654, a potent PDE III inhibitor, and the progression of the lesion defined. Groups of 3 male beagle dogs received a single 2-hr infusion of SK&F 95654 at 8 mg/kg/hr and the characteristics of the coronary arterial lesions were evaluated at 1, 3, 10, and 34 days postdosing by light, scanning, and transmission electron microscopy. At 24 hr postdosing, the arterial lesion was characterized by segmental or circumferential necrosis of medial smooth muscle cells and hemorrhage; adventitial hemorrhage was also noted, particularly in the right atrial artery. Ultrastructural evaluation showed extensive medial necrosis, characterized by loss of smooth muscle cells and their replacement by cellular debris with ingress of erythrocytes, platelets, and inflammatory cells into the media. Associated with medial changes, significant endothelial effects were observed consisting of widening of intercellular boundaries, loss of normal elongated cellular appearance, and the attachment of numerous leukocytes and platelets. During the 10-34-day postdosing period, substantial repair of the arterial lesions occurred such that by day 34 all sections of extramural coronary artery were normal. The lesions induced in the dog are consistent with a hemodynamic effect induced by the pharmacological action of SK&F 95654.

Mesenteric arteriopathy in the rat induced by phosphodiesterase III inhibitors: an investigation of morphological, ultrastructural, and hemodynamic changes

A reproducible model of a phosphodiesterase III (PDE III) inhibitor-induced arteriopathy has been developed in the rat after subcutaneous administration of SK&F 95654. Administration of this potent PDE III inhibitor induced an arteriopathy of mesenteric arteries within 24 hr that was dose-related in intensity and incidence over the range 0.174, 0.348, 0.523, and 0.697 mmol/kg. The arteriopathy was restricted to muscular arteries of external diameter of 100-800 microns and was shown microscopically to be focal or segmental medial necrosis and hemorrhage. A time-course experiment, conducted from 3 to 24 hr postdosing, showed that the first changes observed 6 hr postdosing were on the endothelium followed by focal hemorrhages into the media at 12 hr postdosing, causing compression, degeneration, and necrosis of myocytes. From 16 hr postdosing, there was focal endothelial cell necrosis and loss of confluence. Leukocytes and activated platelets were found adhering to exposed basement lamina and seen to pass through endothelial gaps into the subintima. By 24 hr postdosing, medial necrosis was extensive with large areas of media replaced by erythrocytes, cell debris, and a few leukocytes and platelets. The effect of 3 structurally dissimilar PDE III inhibitors administered subcutaneously at a dose of 0.697 mmol/kg was compared with that of SK&F 95654. The arteriopathy induced by these compounds were identical to that produced by SK&F 95654 with the incidence and severity of lesions ranked in the following order: SK&F 95654 > WIN 62582 > SK&F 94836, with no macroscopic lesions observed for SK&F 94120. Systolic blood pressure was measured for these 4 PDE III inhibitors at regular intervals over the 24-hr period postadministration by a plesthymographic method. The severity of the arterial lesions correlated with the magnitude of hypotension induced by these agents. It is postulated that the arterial damage is a consequence of profound vasodilation resulting in abnormal endothelial permeability and increased wall tension, resulting in progressive medial necrosis and hemorrhage.

Age dependence of the cardiac lesions induced by minoxidil in the rat

To evaluate the age- and dose-dependence of the cardiotoxicity induced by minoxidil, histologic studies were made of the hearts of 3-, 6-, 15- and 24-month-old Sprague Dawley rats treated with either 10, 50 or 250 mg/kg of the drug p.o. daily for two consecutive days. The 10 mg/kg dose of minoxidil induced myocyte necrosis in each of the 24-month-old rats but only in one other animal (6-month-old). The 50 mg/kg dose produced necrosis in all the 15- and 24-month-old rats, but in only one of the other animals (6-month-old), while the 250 mg/kg dose induced necrosis in animals of all ages. Inflammation was present in all minoxidil-treated animals, but at each dose level it was most severe in the oldest rats. Interstitial hemorrhages were observed at all dose levels, but increased in frequency and severity with the dose of minoxidil, and at each dose level they were more severe in the oldest animals. Vascular lesions consisting of arteriolar damage and calcification were observed only in the 24-month-old animals. Thus, these data demonstrate that the cardiac lesions induced by minoxidil are more frequent and severe in older than in younger rats.

Pathologic features of naturally occurring juvenile polyarteritis in beagle dogs

Eighteen young Beagle dogs (eight males and 10 females), ages 6-40 months, with canine juvenile polyarteritis syndrome (CJPS), a naturally occurring vasculitis and perivasculitis of unknown etiology, were necropsied, and their tissues were examined by histopathologic and histochemical methods. The condition is characterized by recurring episodes of an acute onset of fever (> 40 C) and neck pain that persist for 3-7 days. The major histopathologic alterations were a systemic vasculitis and perivasculitis. During the febrile, painful period of CJPS, the vascular lesions ranged from a histiocytic-lymphocytic periarterial infiltration to transmural arterial inflammation with concomitant fibrinoid necrosis and vascular thrombosis. Massive periarterial accumulations of inflammatory cells were common and often extended into adjacent tissues. The small- to medium-sized muscular arteries of the heart, cranial mediastinum, and cervical spinal meninges were consistently involved. Vasculitis occasionally occurred in other organ systems. The vascular lesions in dogs examined during clinically normal periods consisted of intimal and medial fibrosis, ruptured elastic laminae, and mild perivasculitis; these lesions were probably related to previous episodes of vasculitis. Eight dogs that had experienced repeated acute episodes also developed splenic, hepatic, and renal amyloidosis. The clinical signs, laboratory abnormalities, and the vascular lesions suggest that the condition may be immune-system mediated. CJPS may serve as a naturally occurring animal model of human immune-system-mediated vasculitides such as polyarteritis nodosa, infantile polyarteritis, and Kawasaki disease.

The pharmacologic basis of the cardiovascular toxicity of minoxidil in the dog

Minoxidil (MNX), like several other vasoactive drugs, causes cardiovascular toxicity in dogs by undetermined mechanisms. We studied the mechanism of cardiovascular toxicity of MNX [an adenosine triphosphate (ATP)-sensitive potassium channel opener] by blocking its pharmacologic effects with glyburide (an ATP-sensitive potassium channel blocker) in groups of 5 female beagle dogs treated orally for 2 days with 1.0 mg/kg/day of MNX alone or with glyburide given in 5 or 6 divided doses of 300 mg/kg at 2 hr before and after each dose of MNX and at 3-6-hr intervals thereafter. A third group of 5 dogs received glyburide alone in the same dosing regimen as in the combination group. Mean arterial pressure (MAP), heart rate (HR), the pharmacokinetics of MNX, and gross and microscopic changes in the heart were evaluated. Glyburide did not influence the pharmacokinetics of MNX but prevented or markedly attenuated the MNX-induced cardiovascular lesions (right atrial hemorrhagic lesions, subendocardial necrosis, or coronary arteritis) occurred in dogs whose MNX-induced hemodynamic effects were effectively blocked by glyburide. In conclusion, the cardiovascular toxicity of MNX in dogs is not caused by a direct toxic effect of MNX on the heart but apparently is related to the exaggerated pharmacologic/profound hemodynamic effects it elicits in the dog.

Coronary vascular and myocardial lesions due to experimental constriction of the abdominal aorta

The development and evolution of arterial and myocardial lesions were morphologically evaluated in Wistar rats submitted to constriction of the abdominal aorta. The control (sham-operated) and operated groups were evaluated 1, 2, 4 and 6 weeks after surgery. The aorta-constricted groups developed arterial hypertension followed by myocardial hypertrophy evidenced from the first week onwards by the increase in ventricular weight and in the diameters of left and right ventricular myofibers. The histopathologic study of the myocardium revealed in aorta-constricted groups, since the first week, widespread necrotizing changes of the intramural coronary branches surrounded by multifocal areas of myofiber degeneration and necrosis. The lesions were more extensive in the wall of the right ventricle and were gradually replaced by scar tissue. At the 6th week patchy focal fibrotic scars were found scattered in the myocardium of both ventricles. There were no systemic lesions in aorta-constricted or sham-operated groups. The close association between the arterial and myocardial lesions shows that muscle necrosis and scars are due to ischemia. They may influence the contractile performance of the myocardium in this model of pressure-induced hypertrophy of the heart.

Cardiotoxicity of vasodilators and positive inotropic/vasodilating drugs in dogs: an overview

Standard toxicological studies in dogs using high doses of vasodilators and positive inotropic/vasodilating agents give rise to a species-specific cardiotoxicity. The reason may be the extreme sensitivity of the dog to the pharmacological effects of these drugs; exaggerated pharmacodynamic effects and prolonged disturbance of homeostasis mechanisms often are responsible for the observed organ lesions. An assessment of the toxicological relevance and the risk for patients taking the drugs at therapeutic doses cannot be made without taking into account their pathomechanisms and the pathophysiological basis of the exceptional reaction patterns occurring in dogs. A large series of vasodilating and positive inotropic agents are presented, their pharmacological properties are described, and toxicological effects in dogs are compared. In view of the poor correlation between the distinct cardiac lesions induced in dogs and a lack of comparable toxicity in humans, it appears desirable to reassess the adequacy of the standard toxicological approaches for these substances.

Acute cardiovascular toxicity induced by an adenosine agonist-antihypertensive in beagles

An adenosine agonist, designated chemically as (R)-N-(2,3-dihydro-1H-inden- 1-yl) adenosine, or CI-947, was administered to 3 male and 3 female beagles in oral doses of 5 mg/kg body weight. Multiple episodes of arrhythmia were recorded electrocardiographically with Holter monitors in 2 males and 2 females monitored up to 48 hr. One male and 1 female were necropsied at 24 hr and the remaining dogs were necropsied at 48 hr post-dosing. At 48 hr, multifocal perivascular epicardial and myocardial hemorrhage was noted grossly in 1 female. Microscopic coronary arterial alterations were present in all treated dogs irrespective of the occurrence of arrhythmias. At 24 hr, proteinic material and red cells were present in the media accompanied by minimal adventitial accumulation of neutrophils. At 48 hr, coronary arterial lesions progressed to media vacuolation, transmural necrosis, and perivascular accumulation of neutrophils. Ultrastructural alterations included: endothelial retraction, subendothelial accumulation of fibrin and platelets, necrosis of smooth muscle cells, and mural infiltration of granulocytes and monocytes. Coronary vascular injury may be due to altered hemodynamics associated with the coronary vasodilator properties of adenosine agonist compounds.

Potassium channel openers and vascular smooth muscle relaxation

Potassium channel openers comprise a diverse group of chemical agents which open plasma-lemmal K-channels. They show selectivity for smooth muscle, although K-channels in cardiac and skeletal muscle, neurones and the pancreatic beta-cell are also affected at relatively high concentrations. In addition, at least one endogenous K-channel opener of vascular origin--endothelium-derived hyperpolarizing factor--exists and in man plays a role in modulating blood vessel tone. The type of K-channel involved in the actions of both exogenous and endogenous K-channel openers is still uncertain, although a prime candidate in smooth muscle seems similar to the [ATPi]-modulated K-channel in the pancreatic beta-cell. This review focuses attention on the action of these agents in vascular smooth muscle and on the possible clinical exploitation of their powerful vasorelaxant properties.