ANS Critical Thinking Question

Question

Propranolol is a drug known as an Adrenergic Antagonist. Specifically, when it binds to β receptors (a subcategory of adrenergic receptors), it antagonizes any effects that result when norepinephrine or epinephrine bind. Because of this, propranolol is referred to as a β blocker. Based on your knowledge of the autonomic nervous system, select all possible effects of propranolol binding to β receptors on the specified target effectors/organs/variables listed

1) Heart:

A. Increased HR, Increased contractility, Faster Conduction

B. Decreased HR, Decreased contractility, Slower Conduction

C. Decreased HR only

2) Blood Pressure:

A. Decrease

B. Increase

C. No effect

3) Bronchioles

A. Bronchodilation

B. Bronchoconstriction

C. No effect

4) Blood Glucose

A. Increase

B. Decrease

C. No effect

5) Excretion

A. Increase

B. Decrease

C. No effect

Answer 1

1) B

β receptors of the heart normally respond to epinephrine and norepinephrine. β Receptors are found on the SA node, AV node, and ventricular myocardium. SA node controls the pace of heart beat, the AV node controls conduction through the specialized fibers, and the ventricles generate contractile force to pump out blood, creating a stroke volume. Sympathetic Nervous system effects are increased chronotropy (HR), increased inotropy (contractility), and increased dromotropy (conduction speed). By blocking these receptors, the opposite effects result: decreased chronotropy, decreased inotropy, and decreased dromotropy.

2) A

Blood pressure decreases due to the negative inotropy of the heart from the β blocker effect.

CO = HR x SV. MAP = CO x TPR.

CO is cardiac output; HR is heart rate; SV is stroke Volume.

TPR is total peripheral resistance; MAP is mean arterial pressure (a measurement of Blood Pressure)

Because HR + SV decrease, CO decreases. Because CO decreases, so does MAP. Decreased MAP indicated decreased Blood Pressure

In addition, the kidneys have β receptors on granular cells responsible for releasing renin. Activating this pathway, the Renin Angiotensin Aldosterone ADH System (RAAAS), ultimately raises blood pressure. Blocking the kidney receptors results in the opposite effect. This would be NOT raising blood pressure.

3) B

Bronchioles have β receptors on their smooth muscle to control their diameter. When epinephrine binds to the β receptors on the bronchioles, they dilate because their smooth muscle relaxes. Blocking this effect with propranolol causes the opposite reaction. The bronchioles constrict.

4) B

The liver hepatocytes also contain β receptors. Normally, epinephrine binding to their receptors, activates them to perform gluconeogenesis and glycogenolysis. Both of these processes produce glucose. Gluconeogenesis produces glucose from non-glucose sources like amino acids and fats. Glycogenolysis catabolizes glycogen into glucose. Glucose levels in the blood would increase, then. Because propranolol blocks the receptors on the liver, the opposite effect results. This causes decreased blood glucose.

5) A

By blocking the sympathetic nervous system, parasympathetic tone becomes stronger. The core mnemonic for the parasympathetic nervous system is: "Rest, Digest, Read, and Pee." As propranolol decreases sympathetic tone, relatively rising parasympathetic tone, excretion increases. This makes sense with the decrease in blood pressure as well. Blocking granular cells of the kidneys would block the release of renin. Increased excretion from the body means loss of body fluids, which have a diuretic effect, thus lowering blood pressure.