Beta-1 adrenergic agonists
According to their specificity of action on adrenergic receptors, beta-1 (cardiac effects primarily) or beta-2 (vasodilation, bronchodilatation and uterine relaxation), beta agonists are classified into beta-1-mimetic, beta-2-mimetic and mixed betamimetic, i.e. at the same time beta-1 and beta-2-mimetic. The majority of their effects result from activation of adenylcyclase and increase in the intracellular concentration of cAMP.
The drug of reference of this group is dobutamine which causes cardiac stimulation with, in particular, positive inotropic and chronotropic effects with increase in cardiac output. It increases also cardiac excitability and can cause rhythm disorders. The complex pharmacological properties of dobutamine are explained partly by the existence of two enantiomers which do not have exactly the same effects.
Dobutamine is administered by intravenous infusion for management of congestive heart failure, shock with low cardiac output.
Contraindications for its employment are mechanical obstacles to blood circulation.
Increase in the intracellular concentration of cAMP by beta stimulation results from an increase in its synthesis by adenylcyclase activation.
But such an increase can also be obtained by inhibition of phosphodiesterases which normally inactivate cAMP by transformation into inactive 5 ' AMP. There are nonspecific inhibitors of phosphodiesterases like caffeine and theophylline which have moreover other mechanisms of action ( See “Adenosine, antagonists, ATP”.) and specific inhibitors of type III phosphodiesterases present especially in the myocardium such as amrinone ,milrinone and enoximone and inhibitors of type V phosphodiesterases such as sildenafil.
Because of their positive inotropic effect and their vasodilator effect, milrinone and enoximone are used to treat acute myocardial failures. They did not give adequate results in chronic administration. A more recent derivative, vesnarinone, which seemed promising during initial studies, in fact increased mortality.