Medical Pharmacology: Autonomic nervous System (Adrenergic)

β₂-Adrenergic Receptors

Adrenergic Receptor Subtypes continued

β₂-adrenergic receptors have been identified in liver, skeletal muscle, and in smooth muscle.⁹

In smooth muscle, activation of this β-adrenergic receptor subtype results in stimulation (or activation) of G_s, increased adenylyl cyclase activity, increased [cAMP], and increased protein kinase A activity.
- Recall that kinases phosphorylate target sites and in this case, increasing protein kinase A catalytic activity promotes phosphorylation of a protein important in smooth muscle contraction and relaxation, myosin light chain kinase.
- Increased phosphorylation of myosin light chain kinase reduces its tendency to bind calcium-calmodulin and this favors smooth muscle relaxation.
  - Dephosphorylation reactions are catalyzed by phosphatases and balance the actions of kinases in regulation of target site phosphorylation state.
- Sometimes one associates β₂-adrenergic agonists with treatment for asthma.
  - Relaxation of bronchial smooth muscle, mediated by β₂-adrenergic receptors, may proceed by a G_s-independent increase in K⁺ channel conductance.
  - In this model, increasing K⁺ conductance enhances movement of potassium ions out of the cell.
    - Having fewer positive charged ions inside the cell results in membrane hyperpolarization, i.e. movement of the membrane potential to more negative potentials, thus opposing membrane depolarization.
    - Depolarization itself is associated with contraction; therefore, membrane hyperpolarization tends to reduce bronchiolar smooth muscle tone.
Activation of β₂-adrenergic receptors in the liver utilizes the G_s signaling system which, through subsequent phosphorylation steps, increases catalytic activity of glycogen phosphorylase and glycogen catabolism. One result of this activity is increased plasma glucose.
- For skeletal muscle, the same process stimulates both glycogenolysis and K⁺ uptake.⁹
β₂-adrenergic receptor activation sometimes is associated with the G_q G protein subunit.
- This association results multiple kinase activations which influence many intracellular signaling pathways.³⁵
Subdivision of β-adrenergic receptors reflects potency differences of various adrenergic agonists.⁴⁶
- For example, the subdivision of β receptors into β₁ and β₂-adrenergic receptor subtypes with β₁ (myocardial) and β₂ (smooth muscle and most other locations) differing with respect to epinephrine and norepinephrine potencies.⁴⁶
  - That is, epinephrine and norepinephrine exhibit similar potencies at β₁; however, epinephrine is about 10-50 times more potent at β₂ receptors.⁴⁶
  - Differences between adrenergic receptors are also reflected in antagonist affinities. ⁴⁶

**Crystallographic structure of the β₂-adrenergic receptor⁴⁵**

"Crystallographic structure of the β₂-adrenergic receptor depicted as a green cartoon and the bound partial inverse agonist carazolol ligand as spheres (carbon atom = grey, oxygen = red, nitrogen = blue). The phospholipid bilayer depicted as blue spheres (phosphate head groups) and yellow lines (lipid sidechains)."^{45,45(a), 45(b)}

β₃-Adrenergic Receptors

The existence of a third β-adrenergic receptor subtype, β₃, was suggested by "atypical" responses to adrenergic receptor antagonists, i.e. the β₃-receptor subtype exhibited low affinity.⁴⁷

Prior to definitive identification of this receptor subtype, the idea of other adrenergic receptor types beyond β₁ and β₂ had been proposed to account for sympathetic regulation of metabolic processes in the digestive tract, adipose tissue and skeletal muscle.⁴⁷
Also, in the 1980s adrenergic receptor agonists were developed that caused metabolic rate stimulation, thermogenesis in adipose tissue, ileus relaxation, and soleus muscle glycogen synthesis.
- These agonists had limited action at β₁ and β₂ adrenergic receptors, however.
  - In 1989, a human gene was isolated that encoded the third β-adrenergic receptor, β₃.⁴⁷
  - Pharmacological properties were investigated in addition to tissue distribution for the β₃ adrenergic receptor. ⁴⁷
  - In cultured cells expressing the β₃ subtype (no expression of β₁ or β₂), most classical β-adrenergic antagonists were unable to inhibit isoproterenol-induced cAMP accumulation.
    - The β₃ adrenergic receptor is about 10 times more sensitive to norepinephrine and epinephrine.

Adipocytes represent a primary location for β₃ receptor localization, although all of the β receptor subtypes are present in white and brown adipose tissue.⁴⁶

Following administration of β₃-receptor agonists, both lipolysis and thermogenic responses are observed in animals.⁴⁶

Small variations in the β₃ receptors gene (genetic polymorphism) might be a factor in susceptibility to type 2 diabetes and obesity risk.⁴⁶

One study considered the time of onset of non-insulin-dependent diabetes mellitus (NIDDM) and genetic variation in the β₃ receptor gene. ⁴⁷

The gene was analyzed in 10 Pima Indians with assessment of single-stranded conformational polymorphism and dideoxy sequence analysis. Also, association studies in 642 Pima individuals (390 with NIDDM and 252 without NIDDM) were undertaken. ⁴⁷

A "missense" mutation in the β₃-adrenergic receptor gene resulted in the substitution of a tryptophan by arginine in the first intracellular loop of the receptor protein. ⁴⁷

**β₃-Adrenergic Receptor Protein Diagram⁴⁷**

Each amino acid is shown as a circle. The substitution of a tryptophan by arginine (due to a missense mutation) is illustrated at the beginning of the first intracellular loop.⁴⁷

In those subjects homogeneous for this mutation the onset of NIDDM was occurred at an earlier age.⁴⁷

Dopamine:
- Dopamine is an important endogenous agent that acts by interacting not only with dopamine receptors (D₁) but also with multiple subtypes of catecholamine receptors in the periphery.⁹ (Other dopamine receptor types (D₂-like) will be discussed later).
  - Dopamine
  - The predominant receptor-mediated effect will depend on the concentration of dopamine infused.
    - This consequence follows in part from differences in affinities between dopamine and various receptors.
  - Accordingly, at low dopamine concentrations the first receptors to be increasingly saturated and activated will be those that exhibit the highest dopamine-receptor affinity.
    - As the dopamine concentration increases other receptor types will be increasingly occupied and effects mediated by activation of those receptors become prominent.
  - For example, at "low dose" dopamine (<2 μg/kg/min infusion rate) the predominant dopamine effect, vasodilitation, is mediated by D₁dopaminergic receptors in the following vascular beds:
    - Mesentery
    - Renal
    - Coronary. ⁹
  - A principal physiological consequence of D₁ receptor activation is vasodilatation.
    - Vasodilatation occurs subsequent to D₁receptor activation which increases adenylyl cyclase activity in vascular smooth muscle that in turn increases [cAMP].⁹
  - However, at higher infusion rates (2-10 μg/kg/min), dopamine exhibits positive inotropism (increased myocardial contractility) by activation of β₁ adrenergic receptors.
  - Even higher dopamine concentrations, seen at increased rates of infusion (>10 μg/kg/min), vasoconstriction is observed due to dopamine-activation of α₁ receptors.⁹
    - The sequence of D₁, β₁, α₁ receptor activation illustrates that one aspect of drug selectivity is concentration dependent.
    - In this example increasing likelihood of first D₁, then β₁and finally α₁ receptor activation is dopamine concentration dependent; however, another type of progression, from desired therapeutic pharmacological actions of a drug to adverse effects of a drug may also progress in accordance with drug concentrations.
    - Firstly, the therapeutically-targeted receptor is activated and then, at higher drug concentrations, the drug activates other sites associated with undesirable effects.

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