What is the effect of norepinephrine on the heart?

Norepinephrine has a significant effect on the heart. When it is released into the bloodstream, it acts on alpha and beta-adrenergic receptors, but its effect on the heart is primarily due to its action on beta-1 adrenergic receptors. The activation of these receptors leads to several effects:

1. Increased Heart Rate (Chronotropy): Norepinephrine stimulates the sinoatrial (SA) node, which increases the rate at which the pacemaker cells depolarize. This results in an increase in heart rate.

2. Increased Contractility (Inotropy): Norepinephrine enhances the force of the heart's contractions by causing more calcium to enter the cardiomyocytes (heart muscle cells) during each heartbeat. This results in stronger contractions.

3. Increased Conductivity (Dromotropy): Norepinephrine can also enhance the speed of electrical conduction through the heart, which can precipitate the coordination of contractions.

4. Increased Relaxation Rate (Lusitropy): It improves the heart's ability to relax during diastole, which is important for the heart chambers to fill properly with blood for the next cycle.