Answer
**Chronotropic agents** influence the heart rate (chronos = time) by either increasing (positive chronotropic) or decreasing (negative chronotropic) its rate of contraction, thereby affecting the heart's pacing.
1. **Positive Chronotropic Agents**: These substances or factors increase the heart rate.
Examples:
- **Epinephrine**: This hormone, released by the adrenal glands in response to stress or excitement, binds to beta-adrenergic receptors on the heart, leading to an increase in heart rate and contractility.
- **Thyroid Hormones**: Thyroid hormones like thyroxine (T4) and triiodothyronine (T3) can increase heart rate by increasing the sensitivity of the heart to sympathetic stimulation.
2. **Negative Chronotropic Agents**: These substances or factors decrease the heart rate.
Examples:
- **Acetylcholine**: Released by the parasympathetic nervous system, acetylcholine binds to muscarinic receptors on the heart, slowing down the heart rate and reducing its contractility.
- **Beta-Blockers**: Medications like propranolol block beta-adrenergic receptors, reducing the effects of sympathetic stimulation and resulting in a decreased heart rate.
**Inotropic agents** influence the force of contraction of the heart muscles (inotropy = force), affecting the heart's ability to pump blood effectively.
1. **Positive Inotropic Agents**: These substances or factors increase the force of cardiac contraction.
Examples:
- **Digoxin**: A medication derived from the foxglove plant, digoxin increases intracellular calcium levels in heart muscle cells, leading to stronger contractions.
- **Sympathetic Stimulation**: Sympathetic nerves release norepinephrine, which increases the calcium influx into cardiac cells and enhances the force of contraction.
2. **Negative Inotropic Agents**: These substances or factors decrease the force of cardiac contraction.
Examples:
- **Calcium Channel Blockers**: Medications like verapamil and diltiazem block calcium channels in cardiac cells, reducing the influx of calcium ions and weakening the force of contraction.
- **Acidosis**: When the blood becomes acidic (low pH), it can interfere with calcium handling in the heart cells, leading to decreased contractility.
In summary, chronotropic agents affect heart rate, with positive chronotropic agents increasing it and negative chronotropic agents decreasing it. Inotropic agents influence the force of cardiac contractions, with positive inotropic agents increasing contraction strength and negative inotropic agents decreasing it. These agents play a crucial role in regulating cardiac function and overall cardiovascular health.
Work Step by Step
**Chronotropic agents** influence the heart rate (chronos = time) by either increasing (positive chronotropic) or decreasing (negative chronotropic) its rate of contraction, thereby affecting the heart's pacing.
1. **Positive Chronotropic Agents**: These substances or factors increase the heart rate.
Examples:
- **Epinephrine**: This hormone, released by the adrenal glands in response to stress or excitement, binds to beta-adrenergic receptors on the heart, leading to an increase in heart rate and contractility.
- **Thyroid Hormones**: Thyroid hormones like thyroxine (T4) and triiodothyronine (T3) can increase heart rate by increasing the sensitivity of the heart to sympathetic stimulation.
2. **Negative Chronotropic Agents**: These substances or factors decrease the heart rate.
Examples:
- **Acetylcholine**: Released by the parasympathetic nervous system, acetylcholine binds to muscarinic receptors on the heart, slowing down the heart rate and reducing its contractility.
- **Beta-Blockers**: Medications like propranolol block beta-adrenergic receptors, reducing the effects of sympathetic stimulation and resulting in a decreased heart rate.
**Inotropic agents** influence the force of contraction of the heart muscles (inotropy = force), affecting the heart's ability to pump blood effectively.
1. **Positive Inotropic Agents**: These substances or factors increase the force of cardiac contraction.
Examples:
- **Digoxin**: A medication derived from the foxglove plant, digoxin increases intracellular calcium levels in heart muscle cells, leading to stronger contractions.
- **Sympathetic Stimulation**: Sympathetic nerves release norepinephrine, which increases the calcium influx into cardiac cells and enhances the force of contraction.
2. **Negative Inotropic Agents**: These substances or factors decrease the force of cardiac contraction.
Examples:
- **Calcium Channel Blockers**: Medications like verapamil and diltiazem block calcium channels in cardiac cells, reducing the influx of calcium ions and weakening the force of contraction.
- **Acidosis**: When the blood becomes acidic (low pH), it can interfere with calcium handling in the heart cells, leading to decreased contractility.
In summary, chronotropic agents affect heart rate, with positive chronotropic agents increasing it and negative chronotropic agents decreasing it. Inotropic agents influence the force of cardiac contractions, with positive inotropic agents increasing contraction strength and negative inotropic agents decreasing it. These agents play a crucial role in regulating cardiac function and overall cardiovascular health.
