Question

What effects do epinephrine, pilocarpine, atropine, digitalis, calcium ions, sodium ions, and potassium ions have on heart rate?

Answer 1

Epinephrine or adrenaline hormone is secreted by the adrenal medulla. This hormone is released into the bloodstream during strong emotional situations, such as fear or anger. Epinephrine affects cardiac muscle fibers and increase both heart rate and contractility. This condition is also known as the “Flight or Fight Response”, which prepares the body for strenuous activity. Because of these properties, epinephrine is used as a stimulant in cardiac arrest.

Pilocarpine has a negative chronotropic (decrease in heart rate) effect on heart. Pilocarpine has agonistic effects on muscarinic receptors that enhance the activity of muscarinic acetylcholine receptor or in other words pilocarpine enhances the effects of acetylcholine. As acetylcholine is a neurotransmitter in the parasympathetic nervous system, it slows down the heart rate.

Atropine is a anticholinergic drug, which blocks inhibitory effects of acetylcholine (parasympathetic neurotransmitter) on heart rate, and results in tachycardia (increase in heart rate). Atropine is used in cardiac disorders for the treatment of patients with bradycardia (slow heart rate). Atropine also improves the atrioventricular conduction by blocking the parasympathetic effects on the heart.

Digitalis increases the cardiac output as it has a vagal effect on the parasympathetic nervous system. Digitalis inhibits the sodium-potassium ATPase enzyme leading to an increased intracellular concentration of sodium ions. This increase in intracellular sodium ions leads to an increase in cytoplasmic calcium concentration, which improves cardiac contractility. Digitalis is used to improve cardiac contractility (as a positive inotrope) and also used as an antiarrhythmic drug to control the irregular heart rate.

Calcium ions enter the cardiac muscle cells during each heartbeat and causes contraction of muscle fibers. The levels of calcium ion have great influence upon both heart rate and its contractility and increased levels of calcium ions increase both of these. A moderate increase in interstitial calcium level increases the heart rate and strengthens the heartbeat. Extremely high levels of calcium ions may induce cardiac arrest.

Sodium ions are crucial for the production of action potential in cardiac muscle fibers. Increased blood levels of sodium ions decrease heart rate and contractility. Excess sodium ions block the inward movement of calcium ions during cardiac action potentials, which results in decreasing the force of contraction and slow heart rate.

Potassium ions in excessive amount blocks generation of action potentials and slow the heart rate. The differences between intracellular and extracellular concentrations of potassium ions are crucial for the production of action potentials in cardiac muscles. Large quantities of potassium ions can block conduction of the heart impulse from the atria to the ventricles through the A-V bundle.