Now we will discuss the widely prescribed agents in the adrenergic system i.e. beta blockers.
Even though beta blockers are classified as non-selective and selective, all beta blockers have a common structural feature aryloxypropanol amine.
Non-selective vs selective
Non-selective beta blockers act on all the beta receptors and prevent the action of catechol amines. They block β1 receptors thereby inhibit activation of adenylyl cyclase and subsequent rise in cAMP. As the cAMP levels fall, cAMP mediated protein kinases are not activated resulting in inhibition of contraction. This results in decrease of both rate and force of contraction.
On the other hand, these drugs also block β2 receptors on smooth muscle and again decrease cAMP levels. Here the role of cAMP in smooth muscle is quite different inactivating MLCK thereby producing relaxation. As cAMP levels are decreased by beta blockers, MLCK is not inactivated instead active and produces contraction.
So, non-selective beta blockers produce cardiac inhibition and smooth muscle stimulation. They produce bronchoconstriction which is more important in patients with asthma and chronic obstructive pulmonary disorder (COPD).
Selective beta blockers
Selective beta1 blockers are cardioselective specifically acting on heart thereby inhibit cardiac stimulation by catechol amines. Therefore they decrease both rate and force of contraction.
As they are selective for β1 receptors they show negligible action on and produce less bronchospasm.
So which is better?
From the above discussion, we may expect that selective beta blockers are advantageous, but this is not always true and it depends on the context of their use.
For example, for treating hypertension, selective beta blockers are preferred as they mainly decrease blood pressure. Similarly in asthmatic patients, selective beta blockers are preferred.
On the other hand, in few of the situations, the purpose of beta blocker is to prevent sympathetic stimulation where non-selective beta blockers are preferred. For example, in conditions like anxiety, tremor non-selective beta blocker like propranolol is used.