CELLULAR BIOLOGY OF BLOOD VESSELS

•The biology of the vascular wall is essential to understanding the pathophysiology of atherosclerosis, vasospasm, and hypertension, as well as the rationale behind the development and application of new therapeutic strategies for vascular disease.

CELLULAR BIOLOGY OF BLOOD VESSELS

• Physiologically, the two most important cell types in the vascular system are the endothelial cell and the vascular smooth muscle cell.

CELLULAR BIOLOGY OF BLOOD VESSELS

•The vascular endothelial cell (VEC) is generally oriented with the direction of blood flow parallel to the main axis of the vessel.

•Endothelial cells are held together by junctional complexes that regulate permeability and control cell-to-cell communication.

VEC

VEC

Vascular Smooth Muscle (VSM)

•The smooth muscle cell is a spindle-shaped cell the orientation of which varies with the type of artery.

•It is generally helical in large, elastic arteries and concentric in muscular arteries.

SMOOTH MUSCLE

Vascular Smooth Muscle (VSM)

• In normal arteries, the smooth muscle cells are primarily in the ''contractile'' phenotype.

•Under conditions where smooth muscle cells are proliferating, such as atherosclerotic plaques, intimal hyperplasia as a result of angioplasty, or placement in culture—these cells morphologically and biochemically convert into a growth mode and lose their differential contractile features.

Vascular Smooth Muscle (VSM)

• In normal arteries, the smooth muscle cells are primarily in the ''contractile'' phenotype.

VSM responds to systemically secreted substances, ie epinephrine

VSM associated with arteriolar vessels responds to nerve input

VSM responds to VEC derived substances

How do circulating mediators access the VSM?

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Sympathetic input into VSM

Sympathetic input into VSM

Capillaries – no VSM

Pericytesmay serve therole of VSMin capillaries-however ithas never beenproven.

Vascular Pericytes

Vascular Pericytes• Pericytes, also known as Rouget cells or

mural cells, are associated abluminally with all vascular capillaries and post-capillary venules.

• Based on their location and their complement of muscle cytoskeletal proteins, pericytes have been proposed to play a role in the regulation of blood flow.

• Physical contact mediated by cell adhesion molecules, integrins and gap junctions appear to contribute to the control of vascular growth and function.

ENDOTHELIAL CELL–VASCULAR

SMOOTH MUSCLE INTERACTIONS

•The endothelium serves a dual function in the control of vascular tone.

•It secretes relaxing factors such as nitric oxide and adenosine and constricting factors such as the endothelins.

•Vessel tone is thus dependent on the balance between these factors as well as upon the ability of the smooth muscle cell to respond to them.

ENDOTHELIUM-DERIVED RELAXING FACTOR/NITRIC OXIDE

• An endothelium-derived relaxing factor

(EDRF) was first described by Furchgott and Zawadzki, who observed that aortic rings dilated in response to acetylcholine only when the rings maintained an intact endothelium.

•The predominant form of EDRF, derived from L-arginine by the action of the enzyme nitric oxide synthase, is nitric oxide (NO), or a closely related nitroso compound

ENDOTHELIUM-DERIVED RELAXING FACTOR/NITRIC OXIDE

• NO easily crosses the smooth muscle

cell membrane and binds to the heme moiety of the soluble guanylate cyclase (sGC), thereby enhancing the formation of the cyclic guanosine monophosphate (cGMP).

•Cyclic GMP, in turn reduces intracellular Ca2+ concentrations, leading to dephosphorylation of the myosin light chain and relaxation.

ENDOTHELIUM-DERIVED RELAXING FACTOR/NITRIC OXIDE

•It should be noted that the drug

nitroglycerin and nitroprusside exert vasodilator effects by being converted to NO, thus substituting for a natural product.

•Deficiency in release of active NO is an important contributing factor leading to vasospasm.

ENDOTHELIUM-DERIVED RELAXING FACTOR/NITRIC OXIDE

•NO is produced by the action of the

enzyme NO synthase, which oxidizes the guanidino nitrogens of L-arginine to form citrulline and NO.

•There are 3 isotypes of this enzyme

–(1) brain (bNOS, type I),

–(2) macrophages (iNOS, for inducible NOS,type II), and

–(3)endothelial cell (eNOS, type III).

ENDOTHELIUM-DERIVED RELAXING FACTOR/NITRIC OXIDE

•NOS can be induced by

inflammatory mediatorsIL-1, TNF-, and LPS.

•NO inhibits VSM proliferation and platelet aggregation.

PROSTACYCLIN • Prostacyclin, or PGI2, is a prostanoid derived from

the action of cyclooxygenase on arachidonic acid.

•It is released by the endothelium and relaxes vascular smooth muscle by increasing its intracellular content of cyclic adenosine monophosphate (AMP).

•Prostacyclin is also platelet-suppressant and antithrombotic, and it reduces the release of growth factors from endothelial cells and macrophages.

ADENOSINE

•Both adenine nucleosides (adenosine) and nucleotides (adenosine diphosphate, or ADP, and ATP) are released by the endothelium in response to such stimuli as thrombin and flow.

•Adenine nucleosides bind to P1 purinergic receptors that activate cyclic AMP, leading to relaxation,

ENDOTHELIN

•The endothelins are a family of closely related peptides made and secreted by endothelial cells in some but not all vascular beds.

•There are three endothelins (endothelin-1, -2, and -3), all of which are 18 amino acid peptides.

•Endothelins are initially synthesized as preproendothelin, released in precursor form, and activated by endothelin-converting enzyme.

ENDOTHELIN

• Endothelin-1 is the most potent endogenous vasoconstrictor ever identified.

ANGIOTENSIN-CONVERTING ENZYME

•Endothelial cells synthesize and express on their surface angiotensin-converting enzyme (ACE), the protein that converts angiotensin I to the potent vasoconstrictor angiotensin II and that degrades and inactivates bradykinin.

PHYSIOLOGY OF THE VASCULAR SMOOTH MUSCLE

CELL •The smooth muscle cell

normally responds to hormonal stimulation with contraction or relaxation.

Comparisonof vsmand cardiacmuscle

Transduction of signals from VEC to VSM

Know

PHYSIOLOGY OF THE VASCULAR SMOOTH MUSCLE

CELL •The earliest signals generated within

the cell following stimulation with calcium-mobilizing vasoactive agonists involve hydrolysis of a specific class of membrane lipids, the phosphoinositides.

•There are three major inositol phospholipids in the plasma membrane that serve as substrates for the enzyme phospholipase C.

PHYSIOLOGY OF THE VASCULAR SMOOTH MUSCLE

CELL • Phospholipase C cleaves phospholipids to

liberate the water-soluble head group inositol trisphosphate (IP3), and the lipophilic molecule diacylglycerol.

•The most important for signal generation is inositol trisphosphate (IP3), which has been shown to release Ca2+ from intracellular stores.

•Ca2+, in turn, activates a cascade of enzymes leading to contraction or growth.

PHYSIOLOGY OF THE VASCULAR SMOOTH MUSCLE

CELL • Diacylglycerol is a potent activator

of protein kinase C, a Ca2+- and phospholipid-dependent enzyme that phosphorylates numerous cellular proteins.

MYOSIN LIGHT CHAINS

•Phasic contraction of smooth muscle is proposed to be regulated by a sliding-filament mechanism similar to that seen in skeletal muscle.

•Force generation is accomplished by attachment of the myosin heads (or cross bridges) to actin filaments. .

MYOSIN LIGHT CHAINS • Myosin light chain phosphorylation is mediated

by an enzyme known as myosin light chain kinase (MLCK).

•When Ca2+ increases with in the cell in response to hormonal stimulation, it binds to calmodulin, which, in turn, associates with MLCK, converting it from an inactive to an active form.

•MLCK then phosphorylates the myosin light chain, permitting actin activation of the Mg2+-ATPase and resulting in cross-bridge formation.

2ATP

cAMP

Know

Unifying model of VSM tonal control

VSM: Hypertrophy versus

Hyperplasia

• Vascular smooth muscle cell growth takes two forms: hypertrophy and hyperplasia.

•In general, hypertrophy appears to occur in response to long-term stimulation with vasoconstrictor-type agents, while hyperplasia occurs in response to the classical growth factors

Vascular Disease

Vascular DiseaseAtherosclerosis

VEC Control of VSM Growth

ANGIOGENESIS

• Angiogenesis = formation of new blood vessels.

• in vivo occurs during normal wound healing and during the vascularization of solid tumors.

•It is a complex process involving degradation of the basement membrane, the migration and proliferation of endothelial cells, and tube formation.

ANGIOGENESIS

•Factors have been shown to stimulate angiogensis ,including:–Vascular endothelial growth factor

(VEGF)

–fibroblast growth factor (FGF),

–vascular permeability factor (VPF),

–transforming growth factor- (TGF- ),

–angiogenin,

–tumor necrosis factor-(TNF-),

–insulin-like growthfactor I (IGF-I).

Role of VEC and Clotting

Thrombosis formationVersus

Thrombosis inhibition

THE ENDOTHELIAL CELL AND

THROMBOSIS FORMATION

• Quiescent endothelial cells normally present an anti-thrombotic surface that resists platelet adhesion and does not activate coagulation.

•The continuity of the endothelium is essential to this function, and non-thrombogenicity has been attributed in part to the negative charge on the surface of these cells.

THE ENDOTHELIAL CELL AND

THROMBOSIS FORMATION

•Endothelial cells are, however, capable of synthesizing and secreting pro-thrombotic factors, especially when stimulated with cytokines or other inflammatory agents.

•The endothelium thus represents a functional antithrombotic-thrombolytic/thrombotic balance

Thrombosis

Thrombosis

Thrombosis

There is arecent thrombosisin this narrowedcoronaryartery

THE ENDOTHELIAL CELL AND

THROMBOSIS FORMATION • Potent anticoagulants elaborated by the

endothelium include NO and prostacyclin, which inhibits platelet aggregation, heparin-like molecules, and thrombomodulin, which activates protein C.

•In addition, antithrombin III binds to the surface-bound heparin-like molecules and serves as a clearance molecule for thrombin, as well as a thrombin inhibitor.

THE ENDOTHELIAL CELL AND

THROMBOSIS FORMATION

•Endothelial cells are, however, capable of synthesizing and secreting pro-thrombotic factors, especially when stimulated with cytokines or other inflammatory agents.

•The endothelium thus represents a functional antithrombotic-thrombolytic/thrombotic balance

Control of Thrombosisby VEC