Vascular biology implies understanding mechanisms involved in vascular tone, hemostasis, thrombosis and pathobiology of ischemic syndrome, remodeling, and plaque formation. It involves functional assessment of endothelial and smooth muscle cells in the vascular system.
The major regulator of vascular homeostasis is the endothelium, which is a thin layer of cells that line the entire circulatory system. The endothelium was originally thought to act as a simple passive barrier for water and electrolytes. Enormous advances since the 80s have led to understanding the complex functions of this large organ that exerts vasoprotective effects such as vasodilation, prevention of thrombosis and inhibition of inflammatory response.
Vascular function assessment and Atherosclerosis characterization
The impact of different drugs on relaxation or contraction of the endothelium or the vascular smooth muscle can be investigated using isolated tissue bath protocols. Discoveries related to the implication of NO (Nitric Oxide) in vasodilation have been largely done using ex vivo experiments in organ/tissue baths.
The more understanding we gain about vascular tone regulation, hemostasis, and thrombosis, the more promising the therapeutic development. With the development of novel therapies based on small molecules, gene and cell therapy or tissue engineering, vascular biology studies are more than fundamental.
Atherosclerosis is a chronic progressive inflammatory disease characterized by accumulation of lipids and inflammatory cells in medium and large size arteries such as aorta, carotid and coronary arteries. This results in thickening and hardening of the arterial wall, ultimately leading to blood flow restriction. The buildup of the atherosclerotic plaque can rapidly become unstable and rupture, triggering thrombus formation. Atherosclerosis is the underlying pathology of numerous cardiovascular diseases such as myocardial infarction and stroke. Genetic manipulation on mice (knockouts of LDL receptor, Apolipoprotein E or PCSK9) or high fat diet in mice and rabbits are the main used models to mimic pathophysiology of atherosclerosis.
One of the earliest markers of the pathology is the endothelial dysfunction where endothelial cells become activated and display a pro-inflammatory and pro-thrombotic phenotype. Identification of the pathways underlying the interaction of endothelial cells and vascular smooth muscle cells in the vascular homeostasis can offer strategic insights to prevent and treat atherosclerotic disease. Extensive studies have been performed using organ/tissue baths to characterize this dysfunction and the impact of different drugs on arteries derived from animal models of atherosclerosis.
Vascular reactivity studies and Ischemia-reperfusion (I/R) injury
The isolated heart is used to study vascular reactivity, endothelial and smooth muscle function and the effect of a variety of interventions on coronary flow and its distribution. Researchers investigate the response of the heart to agonists or antagonists added to the perfusate, and study the inotropic, lusitropic or chronotropic responses, generating a Concentration Response Curve (CRC).
Two techniques are used to study the isolated heart, Langendorff heart (LH) and working heart (WH), both using the left side of the heart.
Vascular reactivity studies are performed preferentially using the Langendorff perfused heart mode, measuring coronary flow variations in response to vasoactive substances.
Ischemia-reperfusion (I/R) injury can be experimented using the isolated heart retrieved from healthy or diseased animals in order to study the impact of I/R on the myocardial tissue and its function. A period of global (blocked flow from aorta) or regional (occluded or ligated coronary artery) ischemia can be induced for some period followed by reperfusion. Studies on vascular biology are based on variations in the coronary flow that will be related to vasodilation or vasoconstriction in response to different stimuli. I/R studies allow researchers to document the susceptibility of the endothelium to myocardial infarction.
The ex vivo perfusion of isolated kidney has been used for decades to study renal physiology and evaluate different pathophysiologic conditions. This system offers a heated oxygenated environment and ensures highly reproducible conditions without confounding effects of endogenous neuro-hormonal substances. It can be used under constant flow or constant pressure with a pressure flow regulator. The investigation of vasoregulatory properties of receptors in response to agonists or antagonists may offer insights into renal vascular function. Quantification of different proteins and metabolites in the perfusate enables further characterization of the involved mechanisms and specific signaling pathways. Hence, this system is useful for metabolic studies and Pharmacokinetics assessment. It is mainly useful for toxicological studies, knowing the ureter can be cannulated to collect urine. The isolated perfusion system can also be applied to liver or mesenteric bed for vascular and toxicological studies.
The liver is perfused through the portal vein while collecting in the vena cava. Here again the biliary secretory process can be investigated through cannulation of the biliary canal. Liver injury can be well characterized using this system.
Blood flow and thrombosis monitoring
Real-time monitoring and even direct visualization of thrombus formation in mice makes it possible to evaluate genetic and pharmacological interventions. Genetic studies, for which the mouse is the favored species, have yielded valuable insights into the mechanisms underlying thrombus formation. Pharmacological interventions allow the assessment of fibrinolytic and thrombolytic properties of drugs.
Methods for inducing arterial thrombosis include both mechanical and chemically-induced injury, among others. In the latter category, two of the most widely used techniques are:
– Topical application of ferric chloride (FeCl3) directly on the artery
– intravenous injection of a substance called Rose Bengal, which subsequently accumulates in membranes of endothelial cells. Segments of arteries exposed to green light trigger a reaction that leads to the formation of substances harmful to the epithelium.
In anesthetized subject applications, thrombus formation is monitored by measuring flow. An extensive range of flowmeters from Transonic is available to measure volume flow through arteries, veins and ducts of different species and of diameter 0.25 mm and upwards.
Hypertension is characterized by changes in the vasculature including remodelling and endothelial dysfunction, all these features defining a vascular phenotype for hypertension The variations in blood pressure and heart rate are signals that can be monitored in vivo using implanted telemetry. Standard parameters such as systolic, diastolic and mean blood pressure may be retrieved through an intraarterial catheter. Telemetry methods allow for continuous and stress-free measurement of variations in these parameters
Monitoring blood pressure in rodents has helped researchers unravel various mechanisms involved different pathological conditions and is therefore very useful for basic comprehension of disease. Further analysis can be performed using ecgAUTO such as the baroreflex sensitivity index, which is known for its implication in various cardiovascular diseases including hypertension where the baroreflex control on heart rate is reduced.
High precision blood pressure measurement
Vascular remodelling is the dominant component of cardiovascular disease such as hypertension. Investigating variations in blood pressure in vivo using pressure catheters equipped with solid state sensors on the tip provides high degree precision measurements. Monitoring blood pressure variations is key to discovering new therapeutic interventions.
Intracardiac pressure is also of great interest for hemodynamic evaluation of animal models.
Non-invasive blood pressure from rodents
Vascular function is important in maintaining blood pressure homeostasis. It is therefore of primary interest to monitor variations in blood pressure. This can be performed non-invasively on rodents, using CODA system. The CODA system performs blood pressure measurements from the tail cuff of the animal and provides six parameters including systolic, diastolic and mean blood pressure, heart rate, blood flow and blood volume. The VPR technology uses a pressure transducer to non-invasively measure blood volume in the tail. The main advantages of this system are it requires no surgery and provides accurate and consistent blood pressure measurements over time in long term studies on large numbers of animals.
The volume-pressure recording technology used in the sensors is clinically validated and provides virtually 100% correlation with direct blood pressure measurements.
Tissue baths systems
emkaBATH2, ideal for education
emkaBATH4, fully automated & modular