Antihypertensives

Antihypertensives or How not to blow your cork

Background • Cardiovascular pharmacology must always deal with two problems 1. Treating the disease state (e.g. reducing elevated blood pressure) 2. Accounting for the body’s homeostatic response to the treatment • Individual variation in response, and probable drug interactions, will dictate the correct regimen of drugs to be administered • Goal is to develop regimen using fewest drugs at lowest effective doses • Reduces number and severity of side effects • Increases patient compliance

Hypertension • Defined as elevation of arterial blood pressure above a normal value (120/80 mmHg). • Highest risk factor associated with cardiovascular disease  risk doubles with each 20 mm Hg increase in systolic bp over 140 mm Hg • Most cases of hypertension (90%) are due to unknown etiology  called essential hypertension • Normal increase in bp with age (most cases diagnosed in middle age) Note: for this class BP = AP

Hypertension is asymptomatic but may increase risk of other pathologies: • Atherosclerosis • Coronary artery disease • Congestive heart failure • Diabetes • Insulin resistance • Stroke • Renal disease • Retinal disease (easiest condition to diagnose)

Recall: Regulation of blood pressure due to combination of • Renin-angiotensin-aldosterone system • Sympathetic nervous system • Vasopressin (ADH) system • Fluid retention/excretion by the kidney Note: the most effective antihypertensive drug regimens will impair the function of one or more of the above systems

Compensatory mechanisms counteracting decreased blood pressure

Classes of Antihypertensive Drugs • βblockers • Peripherally acting sympatholytics • Centrally acting sympatholytics • (Diuretics) • Angiotensin inhibitors • Calcium channel blockers • Direct vasodilators

Figure 12-2 Summary of sites and mechanisms of action antihypertensives

Stages of Hypertension Heart failure Angina Post-myocardial infarction Extensive coronary artery disease Diabetes Chronic kidney failure Recurrent stroke prevention

Mechanisms of Action: Diuretics • Will talk about specifics later but generally reduce blood volume by decreasing electrolyte, and thus water, reabsorption in the kidney (increase urine excretion) • Causes reduced plasma volume which decreases CO, which lowers BP

Diuretics • amiloride • Thiazides* • burnetanide • chlorthalidone • eplerenone • furosemide (also used in race horses, altitude sickness) • indapamide • metolazone • spironolactone • triamterene

Mechanisms of Action: Angiotensin Inhibitors • Angiotensin converting enzyme (ACE) inhibitor • blocks conversion of angiotensin I to angiotensin II • Angiotensin receptor blockers • Reversibly bind to the Ang. I subtype of Ang. II receptors in blood vessels  reduce physiological effect of Ang. II Note: both above have similar antihypertensive effect

Angiotensin Inhibitors • ACEs • captopril • enalapril • lisinopril • benazepril • ramipril • Angiotensin receptor blockers • losartan • valsartan • candesartan • telmisartan

Mechanisms of Action: Drugs affecting the SNS – Adrenergic β, αreceptor antagonists • Many types of β blockers • All competitively antagonize the effects of epinephrine and norepinephrine on β1 –adrenergic receptors in the heart, and renin-secreting cells of the kidney • αreceptor antagonists work only by blocking α1receptors on vascular smooth muscle

Mechanisms of Action: Drugs affecting the SNS – Sympatholytics CNS active • Work by reducing the firing rate of sympathetic nerves • Mediated by activation of α2-adrenergic receptors in the CNS but exact site is unclear • Enter brain after absorption into bloodstream Peripherally acting • Interfere with norepinephrine release from sympathetic nerve terminals • May inhibit formation of catecholamines

Adrenergic receptor antagonists • β-blockers • propanolol • atenolol • sotalol • pindolol • labetalol • Carvedilol • α1 receptor antagonists • clonidine • α-methyldopa • guanfacine • guanabenz • Reserpine – 1st widely used antihypertensive

Mechanisms of Action: Ca2+ Channel Blockers • All excitable tissue contains voltage-dependent Ca2+ channels • Inhibit inward movement of Ca2+ through specific (L-type) voltage-dependent Ca2+ channels • This type of channel prevalent in cardiac and vascular smooth muscle • When Ca2+ channels are inactivated, Ca2+ is pumped out of cell, actin dissociates from myosin and muscle relaxes, opening vascular lumen and decreasing resistance, which decreases BP • Major effect is on coronary and peripheral arterioles

Ca2+ channel blockers • Verapamil (1st one used to treat hypertension) • nifedipine • diltiazem

Mechanisms of Action: Direct vasodilators • Most powerful antihypertensive drugs • May cause strong compensatory reactions to bring BP back up • Fluid retention • Increase in • renin-release • heart rate • contractility • Usually used only in severe hypertension or for patients not responding to other antihypertensives

Direct vasodilators • Hydralazine • Minoxidil • Pinacidil • Diazoxide

Clinical considerations: diuretics • Usually well tolerated, relatively cheap, and work as well as other methods • They are especially effective in African-Americans • At initial treatment urinary excretion increase significantly but after several days returns close to normal, and BP remains depressed

Clinical considerations: angiotensin inhibitors • Most effective in patients with elevated plasma renin levels (but this condition is rare) • Still effective in hypertensive patients with normal or even low levels of renin • Useful for treating hypertension associated with other cardiovascular risk factors, like heart failure, stroke, myocardial infarctions, diabetes, and kidney disease

Clinical considerations: SNS drugs • The long-term decrease in CO is usually most responsible for lowering BP • For some patients CO returns to normal as TPR decreases  decreased BP continues • β–blockers also inhibit renin release which contributes significantly to decreased BP, especially if renin levels are elevated • Effect on two different systems causes β–blockers to often be used in combination with other antihypertensives (direct vasodilators, α1 adrenergic receptor blockers) because get three types of effects with only two drugs • β–blockers may also counteract reflex compensatory responses (that increase CO) caused by these other drugs

Clinical considerations: SNS drugs (con’t) • Peripheral α1 adrenergic receptor blockers (prazosin, doxazosin) reduce TPR  may cause fluid retention  may then need to give diuretics to counteract

Clinical considerations: Ca2+ channel blockers • All excitable tissue contains receptors for Ca2+ channel blockers but not all tissue affected equally • Dependence of tissue on exogenous Ca2+ dictates sensitivity to blockers • High in cardiac tissue (especially AV node), lower in skeletal muscle • Some may be contraindicated due to other disease states or if using specific drugs • Example - do not use verapamil in cases of heart failure associated with increased TPR  will slow down an already poorly pumping heart • Example - do not use certain β-blockers in combination with Ca2+ channel blockers in heart failure

Drugs for hypertensive emergencies • May have to reduce BP quickly but temporarily • Unexpected side effects of other drugs • Side effects of illegal drugs • Accidental poisoning • Above may cause severe tachycardia  can reduce BP (and HR) by i.v. infusion of nitroprusside • Full effect in seconds • Recovery from effect within a few minutes • Or repeated low-dose i.v. injections of diazoxide • Full effect in 1 to 5 minutes • Recovery within a day

Treatment of Hypertension and see Table 12-1

Antihypertensives