This increases the blood pressure and the blood flow returning to the heart. The blood vessels in skeletal muscles lack alpha-receptors because they need to stay open to utilize the increased blood pumped by the heart.
Remember the fight or flight response? It would not make sense to take blood from other parts of the body and pump it to the muscles so we can run away or defend ourselves if the blood vessels in the skeletal muscles are also constricted and cannot benefit from the increased blood circulation providing extra oxygen and nutrients. So what do you think happens if we block these alpha-receptors? Right, the arteries dilate. Thus an alpha-blocker medication causes vasodilation and can be used to treat hypertension.
Next are the beta receptors. Beta-1 receptors are located in the heart. When beta-1 receptors are stimulated they increase the heart rate and increase the heart's strength of contraction or contractility. The beta-2 receptors are located in the bronchioles of the lungs and the arteries of the skeletal muscles. When these receptors are stimulated, they increase the diameter of the bronchioles to let more air in and out during breathing and they dilate the vessels of the skeletal muscles so they can receive the increased blood flow produced by stimulating the alpha and beta 1 receptors.
So reflect for a moment: If norepinephrine or epinephrine is the neurotransmitter of the sympathetic nervous system and it interacts with all the receptors we just described, then we know that norepinephrine or epinephrine stimulates the alpha, beta-1 and beta-2 receptors and thus it is an alpha agonist, a beta-1 agonist and a beta-2 agonist.
When we administer epinephrine or adrenaline to a patient, we expect alpha, beta-1 and beta-2 agonist effects; we expect an:. We can also stimulate a single receptor site such as a beta-2 agonist medication like an albuterol inhaler that stimulates beta-2 receptors in the lungs then we can dilate the bronchioles in the patient with bronchospasm without causing excessive stimulation of the heart.
The sympathetic receptors can be over-stimulated by the non-therapeutic use of substances like cocaine and methamphetamines. Or the excessive use or overdose of sympathomimetic medication like pseudoephedrine or those used to treat attention deficit disorders. Severe alcohol withdrawal may also induce sympathetic overdrive. Excessive stimulation of the sympathetic receptors can result in dangerously high blood pressure, tachycardia, dysrhythmias and hyperthermia, any one of which may cause organ damage with the real potential for organism death.
Now let's switch to the parasympathetic or cholinergic receptors. These are easier since there are only two types, muscarinic receptors and nicotinic receptors. And I will make it even easier by getting rid of the nicotinic receptors after I tell you they are involved in muscle contraction and are affected by substances such as curare used on those poison-tipped arrows that cause muscle paralysis by blocking these nicotinic receptors.
Medications such as succinlycholine are available to block the nicotinic receptors and induce paralysis necessary for certain medical procedures. Cells in the adrenal medulla synthesize and secrete epinephrine and norepinephrine. Following release into blood, these hormones bind adrenergic receptors on target cells, where they induce essentially the same effects as direct sympathetic nervous stimulation.
Synthesis of catecholamines begins with the amino acid tyrosine, which is taken up by chromaffin cells in the medulla and converted to norepinephrine and epinephrine through the following steps:. Norepinephine and epinephrine are stored in electron-dense granules which also contain ATP and several neuropeptides. Secretion of these hormones is stimulated by acetylcholine release from preganglionic sympathetic fibers innervating the medulla.
Many types of "stresses" stimulate such secretion, including exercise, hypoglycemia and trauma. When this happens, epinephrine can be injected directly into your heart to help make it start beating again. Not all people will experience all possible side effects of each adrenergic drug. Likewise, not every adrenergic drug is right for every person.
Health conditions other than what you need to treat with an adrenergic drug can play a role in deciding which drug is right for you. You can discuss all of these factors with your doctor to find a good choice.
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