Peptides Autacoids Dr Mahmoud Khattab

Angiotensins, Angiotensin II, and Blockers The renin-angiotensin (Ang)-aldosterone system (RAAS) has vital pathophysiological & therapeutic importance Angiotensins peptides derived from a high-molecular weight angiotensinogen, a plasma 2-globulin Renin, released from the kidneys & others, converts angiotensinogen into a the decapeptide, angiotensin I (AngI, inactive) Angiotensin-converting enzyme (ACE, peptidyly dipeptidase, kiniase), found in plasma & tissues including glands, kidneys, endothelium & heart

Renin Release Renin release is stimulated by: Β1-adrenergic stimulation Decreased BP/Blood Volume, Renal Ischemia, Sodium Depletion Drugs: ACEIs, ARBs Diuretics, Vasodilators Renin release is inhibited by: NSAIDs, Β-adrenergic blockers

Angiotensins, Angiotensin II, and Blockers ACE cleaves two carboxy terminal amino acids (9 and 10) of AngI to form the octapeptide AngII, the most active form AngII is converted to a less active heptapeptide AngIII & other inactive fragments by a carboxy-peptidase A new homolog of the enzyme, "ACE2" was identified AngI/AngII are substrates for the enzyme, the products are the nonapeptide Ang-(1-9), or a heptapeptide Ang(1-7)

Actions of ACE

Metabolism of Angiotensin Peptides Ang II can be converted to the heptapeptide Ang III by glutamylaminopeptidase or to ang (1-7) by prolylendopeptidase Ang III generally is less potent than ang II but is equally potent as a stimulator of aldosterone secretion Ang III can be metabolized to a hexapeptide (ang 3−8, ang IV) fragment by an arginylaminopeptidase Ang IV is currently being studied as a biologically active peptide with potential physiologic significance

Angiotensin II Receptors Two main receptors are identified: Type-1 & Type-2 angiotensin II receptors Both are G-protein coupled receptors Type-1 Type-2 Agonist AngII>AngIII AngII=AngIII Antag. Losartan __________ Trans-duction - Gq-PLC-IP3 - Gi-AC-c.AMP Phosphotyrosine phosphatase

Angiotensin II Type-1 Receptor Distribution & Function Physiologic functions of AT1 receptors according to their location LOCATION FUNCTION Kidney Glomerulus Mesangial cell contraction Proximal tubule Increased re-absorption of sodium Juxtaglomerular apparatus Decreased renin secretion Heart Inotropic effect and release of growth factors with ensuing stimulation of cardiac myocyte hypertrophy and increased extracellular matrix production Blood vessels Vasconstriction with an increase in afterload as well as local release of growth factors Adrenal gland Aldosterone and catecholamine release Brain Vasopressin release, stimulation of thirst; autonomic activity and cardiovascular reflexes Sympathetic nervous system Increased sympathetic outflow

AT-1 Receptor-Mediated Actions of Angiotensin II Vasculature: Vascular hypertrophy Potent vasoconstriction of pre- & post-capillary blood vessels in skin, kidney & splanchnic areas (minor in brain & skeletal muscle) Cardiac hypertrophy & fibrosis, positive inotropy Increased NE release from sympathetic nerve terminals, adrenal medulla & brain (indirect vasoconstriction) Aldosterone release from adrenal cortex Antdiuretic hormone (ADH, vasopressin) release from pituitary gland

Paracrine Renal Effects of Angiotensin AngI (AI) is generated in the afferent arteriole, converted to ang II (AII) by ACE AII can cause mesangial cell contraction & efferent arteriolar constriction AII can also be filtered at the glomerulus & subsequently act at the proximal tubular cells to increase sodium re-absorption In the renal interstitium, renin produces angiotensin peptides that act at vascular and tubular structures Angiotensin peptides may also be synthesized in and released from renal juxtaglomerular cells

AT-2 Receptor-Mediated Actions of Angiotensin II Release of bradykinin Production of nitric oxide, and increased cGMP Vasodilation Antiproliferative effect on vascular smooth and cardiac muscles Stimulation of Apoptosis Cell differentiation

The Natriuretic Peptide Family Include atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP) ANP derived from a 126 amino acids prohormone, secreted primarily from cardiac atria BNP, identified initially in brain, is secreted from both atria & ventricles CNP identified in brain & in vascular endothelial cells Stretch receptors in the atria and ventricles detect changes in cardiac chamber volume related to increased cardiac filling pressures, resulting in release of both ANP and BNP but not CNP

The Natriuretic Peptide Family The actions of the natriuretic peptides are mediated by natriuretic peptide receptors (NPRs), NPR-A/B/C NPR-A & NPR-B are coupled to membrane-bound GC, increases levels of cGMP NPRs are localized in vascular SM, endothelium, platelets, the adrenal glomerulosa, & the kidney ANP & BNP increase urine volume & sodium excretion, decrease vascular resistance, and inhibit release of renin and secretion of aldosterone & vasopressin Neutral endopeptidases (NEPs) inactivate NPs

Renin-Angiotensin-Aldosterone System (RAAS) Antagonists Angiotensin Converting Enzyme Inhibitors (ACEIs) SH- compounds: captopril, zofenopril, pivalapril Carboxyl compounds: benazepril, cilazapril, enalapril, lisinopril, perindopril, quinapril, ramipril Phosphoryl compounds: fosinpril, ceronapril Mechanism & Site of Action: Competitive inhibition of ACE, being structural analogs to the di-peptides cleaved by the enzyme In exp animals & humans, they inhibit the pressor effect of IV angI denoting inhibition of conversion to angII Inhibition of bradykinin degradation by ACE leading to increased kinins levels (both therapeutic & side effect)

RAAS Antagonists Angiotensin Converting Enzyme Inhibitors (ACEIs) Main Effects Reduced formation of ang II leading to inhibition of most AT-1 receptor-mediated actions Reduction of direct angII-induced vasoconstriction Inhibition of angII-mediated sympathetic NS stimulation that might contribute to vasodilation Significant reduction of plasma aldosterone levels leading to hyperkalemia & sodium depletion Reduced vasopressin release→ increased water depletion (enhance diuretic effect)

Therapeutic Uses & Adverse Effects RAAS Antagonists Angiotensin Converting Enzyme Inhibitors (ACEIs) Main Effects Increased levels of bradykinin → increased vasodilatation & induction of cough & itching Increased plasma renin activity as a result of impaired angII-mediated feedback inhibition Therapeutic Uses & Adverse Effects Main uses are treatment of hypertension (HTN) & CHF, discussed later in respective lectures Detailed actions in HTN & CHF, and adverse effects, are to be discussed later with Prof Matrafi under respective topics

RAAS Antagonists Angiotensin II Type-1 (AT-1) receptor Antagonists They are competitive antagonists at AT-1 receptors with only low affinity to AT-2 R They include drugs like losartan & valsartan They block AT-1-mediated actions, similar to ACEIs but: No effect on bradykinin, substance P, ACE Feedback inhibition of angII is blocked→ ↑ renin release → increased angII Detailed actions in HTN & CHF, and adverse effects, are to be discussed later with Prof Matrafi under respective topics

RAAS Antagonists Vasopeptidase Inhibitors Dual inhibitors of ACE & neutral endopeptidase Examples: omapatrilat & ilepatril (Sanofi-Aventis Pharma) They decrease circulating angIi and increase levels of ANP and bradykinin They under clinical trials for treatment of hypertension, CHF, and diabetic nephropathy Renin Inhibitors Enalkiren & ditekiren are inhibitors of renin They lower BP hypertensive subjects They are still investigated

Kinins Bradykinin is an endogenous vasodilator nonapeptide, released from plasma α2-globulins kininogens High molecular weight (HMW) form present in plasma & a low molecular weight (LMW) form present in tissues Hydrolysis of plasma HMW kininogen, by plasma kallikrein, a protease, gives bradykinin

Kinins Hydrolysis of LMW kininogen by tissue kallikrein →kallidin Kallidin a vasodilator decapeptide with similar properties to those of bradykinin Bradykinin is cataboloised by kiniase II, identical to peptidyl dipeptidase (ACE)

Kinins Receptors, Actions & Therapy The activate B1, B2, B3 receptors linked to PLC/A2 Powerful Vasodilation→ decreased blood pressure via B2 receptor stimulation (NO-dependent) Increase in capillary permeability inducing edema. It produces inflammation & algesia (B2) Cardiac stimulation: Compensatory indirect & direct tachycardia & increase in cardiac output It produces coronary vasodilation Bradykinin has a cardiac anti-ischemic effect, inhibited by B2 antagonists (NO & PI2 dependent)

Kinins Actions & Therapy Kinins produce broncho-constriction & itching in respiratory system (antagonized by ASA) Therapeutic Use: No current use of kinin analogues Increased bradykinin is possibly involved in the therapeutic efficiency & cough produced by ACEIs Aprotinin (Trasylolol), a kallekrein inhibitor, used in treatment of acute pancreatitis, carcinoid syndrome & hyperfibrinolysis

Endothelium-Derived Contracting Factor (EDCF) Endothelins Vascular Endothelium ↓ Endothelium-Derived Contracting Factor (EDCF) (De Mey & Vanhoutte, 1982) Peptidergic EDCF (Hickey Et al., 1985) A 21-Amino Acid Peptide, Endothelin-1 (Yanagisawa et al., 1988

Endothelin Receptors ETA Receptors Agonist: ET-1=ET-2>ET-3 Smooth muscles Actions: Contractile activity Vasopressor response Aldosterone release ETB Receptors Agonist: ET-1=ET-3 Smooth & endothelial cells Actions: Initial depressor effect Initial relaxant response Ex vivo platelet inhibition ETB receptors located on endothelium mediate NO- & PGI2-dependent vasodilatation

Endothelin Therapeutic Potential Endothelin-based therapy is Investigated for HTN, CHF Bosentan is a dual ETA/ET B receptor antagonist Receptor affinity ratio (ET A /ET B ) of 20:1, enhancing vasodilatory role in PAH by reducing ET A receptor activation Bosentan reduces vascular remodeling Adverse Effects: Elevated liver enzymes headache (22%), nasopharyngitis (11%), flushing (9%), hepatic function abnormality (8%) and lower extremity oedema (8%)

Substance P Substance P is an undecapeptide present in brain, afferent neurons, dorsal horn of spinal cord It plays a role in pain transmission It is present in intestine It activates substance P GPCRs linked to EDHF It produces vasodilation & contracts SM of GIT MK-869 is a substance P antagonist tested for treatment of chemotherapy-induced emesis