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Chapter 6: Introduction to Autonomic Pharmacology

A 3-year-old child has been admitted to the emergency department having swallowed the contents of 2 bottles of a nasal decongestant. The active ingredient of the medication is phenylephrine, a potent, selective α-adrenoceptor agonist drug. Which of the following is a sign of α-receptor activation that may occur in this child?

(A) Bronchodilation

(B) Cardiac acceleration (tachycardia)

(C) Pupillary dilation (mydriasis)

(D) Renin release from the kidneys

(E) Vasodilation of blood vessels

Mydriasis can be caused by contraction of the radial fibers of the iris; these smooth muscle cells have α receptors. All the other listed responses are mediated by β adrenoceptors (Table 6–3). The answer is C.

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Table 6–3. Direct effects of autonomic nerve activity on some organ systems.
  Effect of
  Sympathetic Parasympathetic
Organ Actiona Receptorb Actiona Receptorb
Eye        
 Iris        
  Radial muscle Contracts α1
  Circular muscle Contracts M3
 Ciliary muscle [Relaxes] β Contracts M3
Heart        
 Sinoatrial node Accelerates β1, β2 Decelerates M2
 Ectopic pacemakers Accelerates β1, β2
 Contractility Increases β1, β2 Decreases (atria) [M2]
Blood vessels        
 Skin, splanchnic vessels Contracts α
 Skeletal muscle vessels Relaxes β2
  Contracts α
  [Relaxes] [Mc]
Bronchiolar smooth muscle Relaxes β2 Contracts M3
Gastrointestinal tract        
 Smooth muscle        
  Walls Relaxes α2,d β2 Contracts M3
  Sphincters Contracts α1 Relaxes M3
 Secretion Inhibits α2 Increases M3
 Myenteric plexus Activates M1
Genitourinary smooth muscle        
 Bladder wall Relaxes β2 Contracts M3
 Sphincter Contracts α1 Relaxes M3
 Uterus, pregnant Relaxes β2
  Contracts α Contracts M3
 Penis, seminal vesicles Ejaculation α Erection M
Skin        
 Pilomotor smooth muscle Contracts α
 Sweat glands    
  Thermoregulatory Increases M
  Apocrine (stress) Increases α
Metabolic functions        
 Liver Gluconeogenesis β2, α
 Liver Glycogenolysis β2, α
 Fat cells Lipolysis β3
 Kidney Renin release β1
Autonomic nerve endings        
 Sympathetic Decreases NE release Me
 Parasympathetic Decreases ACh release α

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