pharmacology Beta receptor blockers Beta 1 receptors distribution 

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					Beta receptor blockers

Beta 1 receptors distribution

       Heart – increased HR, force of contraction, AV nodal conductivity
       Juxtaglomerular cells of kidney – increased renin secretion

Beta 2 receptors distribution

       Smooth muscle(vascular, bronchi, GI, genitourinary) – relaxation
       Skeletal muscle – tremors, glycogenolysis, uptake of K+
       Liver – glycogenolysis, gluconeogenesis

Actions on CVS – cardiac depressants

       Slows heart rate, decreases myocardial contractility and cardiac output
       Slows conduction in atria, AV node and increases ERP (antiarrhythmic effect)
       Drugs with ‘partial agonist’ activity increase heart rate at rest, but decrease it in exercise
       Coronary flow is reduced, but less than the oxygen consumption

Anti-hypertensive effects

       Block beta 1 receptors in myocardium  decreased HR, FC, CO  decreased BP
       Central action
       Reduced renin synthesis

Action on respiratory system

       In asthmatics, non-selective beta blockers(propanolol) can cause severe bronchoconstriction
       Local anesthetic action aka ‘membrane stabilizing action’

Classification

       Non –selective – beta 1 and beta 2
            o Propanolol, sotalol, timolol – without intrinsic sympathomimetic activity
                (partial agonist)
            o Acebutalol, pindolol – with intrinsic sympathomimetic activity
       Beta 1 selective
            o Metaprolol, Atenolol, Acebutalol, Esmolol, Betaxolol, Nebivolol
            o Oxprendolol, Pindolol, Propanolol – membrane stabilizing activity – local anesthetic
                action
       Beta blockers with additional alpha blocking activity
            o Carvedilol, Labetalol, Medroxolol, Bucindolol

Propanolol – prototype

       Extensive 1st pass metabolism
       Highly lipophilic

Atenolol – hydrophilic, doesn’t cross BBB
Selective beta 1 blockers – safe in asthma

         Esmolol – ultra-short acting ; half life- 8mins
              o Given IV
              o Hydrolyzed rapidly by esterases in erythrocytes
         Nebivolol – also causes vasodilation through endothelium-mediated mechanism
         Celiprolol – beta 1 selective with beta 2 agonism

Sotalol

         Non-selective with anti-arrhythmic actions independent of its ability to block beta receptors
          (due to K+ channel blockade)

Carvedilol

         Attenuate oxygen- free radical –initiated lipid peroxidation
         Inhibits vascular smooth muscle mitogenesis

Therapeutic uses

         Cardiovascular diseases
              o Hypertension – 1st line drugs for mild hypertension
              o Angina – classical angina
              o Supraventricular and ventricular arrhythmias
              o Acute MI – decreases myocardial oxygen demand, causes redistribution of myocardial
                  blood flow, antiarrhythmic action
              o Hypertrophic obstructive cardiomyopathy – increases stroke volume but slowing
                  ventricular ejection and decreased outflow resistance
              o Acute dissecting aortic aneurysm
              o To combat arrhythmias in pheochromocytoma
              o Congestive cardiac failure
                       Metaprolol, carvedilol, bisoprolol
                       Blocks harmful effects of catecholamines
                       Inhibits central sympathetic flow, direct vasodilator effects, prevents cardiac
                          hypertrophy
                       Reduces risk of sudden death
                       Reduces myocardial remodelling
         Hyperthyroidism – adjunct to definitive treatment (preoperatively) and in thyroid storm
         Prophylaxis of migraine – propanolol, timolol, metoprolol
         Anxiety – to control somatic symptoms
         Glaucoma – drugs of choice
         Variceal bleeding in patients with portal hypertension – propanolol, nodolol
Esmolol – beta 1 selective blocker

Uses

      Supraventricular arrhythmias
      Arrhythmia associated with thyrotoxicosis
      Perioperative hypertension
      MI in acute ill patients

Precautions

      May induce congestive heart failure in susceptible patients – with compensated heart failure,
       acute MI, cardiomegaly
      May precipitate attack in asthmatics

Adverse effects

      Bradycardia
      Cold extremities
      Risk of sudden death on discontinuation – in IHD, or renovascular hypertension
      CNS effects
           o Fatigue
           o Sleep disturbances – insomnia, nightmares
           o depression
      effects in DM
           o masks symptoms of hypoglycaemia
           o delays recovery from insulin-induced hypoglycaemia
           o increases likelihood of exercise induced hypoglycaemia
      in asthmatics
           o life-threatening increase in airway resistance
           o worsening of claudication

Drug interactions

      antihypertensive effects of b-blockers can be opposed by indomethacin and other NSAIDs
      avoid with cardiac depressants like verapamil
      reduces metabolism of lignocaine

Contraindications

      asthma
      poorly controlled diabetes mellitus
      bradycardia
      heart block
      Prinzmetals angina
      Severe unstable left heart failure

				
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posted:6/5/2012
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