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Beta adrenergic receptor agonists



β1- agonists

Dobutamine is a β1 adrenergic receptor agonists that specifically acts on cardiac muscle increasing both rate and force of contraction. It increases cAMP levels within the cardiac muscle which stimulates protein kinases increasing intracellular calcium levels. This results increased force of contraction.

action of beta1 agonists

Two advantages of dobutamine are

  • It is selective for β1 adrenergic receptors
  • It doesn’t increase cardiac oxygen consumption
Note: Epinephrine also increases force of contraction but it is not preferred in heart failure for two reasons. First it is not specific for β1 receptors, second it increases cardiac work and oxygen consumption which are deleterious effects in heart failure.

Side effects

Many of the side effects are due to its cardiac stimulatory action.

  • Palpitations
  • Headache
  • Nervousness
  • Rapid heartbeat
  • Increased blood pressure
  • Nausea

Indications

Dobutamine can be used in two conditions

  • Congestive heart failure
  • Cardiogenic shock

β2- agonists

Again these drugs are specific to the β2 adrenergic receptors particularly acting on bronchioles producing relaxation. Four drugs are available in this category including

  • Terbutaline
  • Salbutamol
  • Salmeterol
  • Formoterol

Already we have discussed the two structural modifications responsible for β2 selectivity that can be observed in these drugs. Terbutaline has resorcinol hydroxyl groups and whereas salbutamol and salmeterol have have hydoxymethyl group at 3rd position. Formoterol has entirely different formyl group at 3rd position.

Terbutaline and salbutamol are short acting bronchodilators with duration of action is around 3-5 hrs. On the other hand, salmeterol and formoterol are long acting with duration of action about 12 hrs.

Note: Salbutamol is now called as albuterol.

Why salmeterol is long acting?

One more structural modification within this class is alkyl substitution on aliphatic amine.

As the chain length of alkyl group in amine increases, the duration of action of β2 agonists increases.

duration of beta2 agonists

Terbutaline and salbutamol have tertiary butyl groups on nitrogen whereas salmeterol and formoterol have more lipophilic carbon chain hence long acting.


How they act?

These drugs selectively bind to the β2 receptors thereby increase cAMP within the cell. This increase cAMP causes phosphorylation of myosin-light chain kinases (MLCK) which are actually required for contraction.

beta2 agonists producing relaxation

As MLCK is phosphorylated, it becomes inactive preventing contraction. This results in relaxation of bronchial smooth muscle.

Remember! These drugs act as bronchodilators but don’t have any significant anti-inflammatory actions.


Side effects

Side effects are less when they given by inhalation route due to less systemic absorption.

Eventhough some amount of the drug reaches into systemic circulation producing following effects.

  • Tachycardia
  • Tremors
side effects of beta2 agonists

Tachycardia is due to action of these drugs on β1 receptors. Tremor is a repeated contraction skeletal muscle which is increased by β2 agonists as hey increase blood flow thereby enhance twitch response of skeletal muscle.


Indications

These drugs are mainly used as bronchodilators in

  • Asthma
  • Chronic obstructive pulmonary disorder (COPD)
  • Exercise induced bronchospasm

Salbutamol also relaxes uterine in pregnant woman hence used to delay the labor in premature pregnancy.






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