Pharmacology of the Pituitary and Endocrine Regulation

Fundamental Concepts of the Endocrine System and Negative Feedback

• Negative Feedback Mechanism:
      * Thermostat Metaphor: Consider a house on a nice day. When you enter and the temperature is recorded at $58$, you set the thermostat to a desired temperature, such as $70$. The heater activates to reach the goal. Once the house reaches $70$, the heater turns off. This cessation of activity once the target is reached is the essence of negative feedback.
      * Physiological Application: Hormones in the body operate similarly; the production of a hormone is turned off once there is a sufficient amount already present in the body.
      * Exercise Metaphor: When running, the body begins to sweat. Sweating ceases when the individual stops running and cools off. The body recognizes it no longer needs the output (sweat) because the initial condition (overheating) has been resolved.
      * The Goal of Homeostasis: Negative feedback is the primary method used to maintain a stable internal environment where "everything's right."

• Communication Systems in the Human Body:
      * The Nervous System: Communicates through neurons and "action potentials."
      * The Endocrine System: Communicates through hormones that circulate through the bloodstream.
      * Target Specificity: Hormones act like a "lock and key" or a receptor system. They only interact with specific target organs or cells. There are approximately $50,000,000$ target cells in the human body.
      * Example: Follicle Stimulating Hormone ($FSH$) acts specifically on the ovaries and does not affect other unrelated tissues.

Anatomy and the Neuroendocrine Bridge

• The Hypothalamus:
      * Location: Located in the brain.
      * Function: Acts as the bridge between the nervous system and the endocrine system. It creates hormones that are sent to the pituitary gland to stimulate further hormone release. It is effectively "in charge" of the entire hormonal process.

• The Pituitary Gland:
      * Location: Located in the brain.
      * Structure: Consists of two distinct lobes: the Anterior Pituitary and the Posterior Pituitary.

• Other Endocrine Glands:
      * Adrenal Glands: Located above the kidneys; these react to hormones to secrete corticosteroids.
      * Thyroid Gland: Located in the middle of the neck, near the larynx. It is responsible for metabolism via hormones $T3$ and $T4$.
      * Parathyroid Glands: Exist as two pairs of two (four total glands).

Anterior Pituitary Hormones

• Overview: The anterior pituitary secretes six primary hormones.

• Adrenocorticotropic Hormone ($ACTH$):
      * Path: Sent from the anterior pituitary to the adrenal glands (above the kidneys).
      * Result: Stimulates the secretion of glucocorticoids and mineralocorticoids.
      * Mineralocorticoids (e.g., Aldosterone): Retains sodium ($Na^+$) in the body, which pulls in water ($H_2O$). This increases blood pressure and fluid volume, especially during stress.
      * Glucocorticoids (e.g., Cortisol):
          * Promotes sodium retention leading to hypertension and edema.
          * Provides anti-inflammatory effects (similar to how hydrocortisone is used to prevent scarring).
          * Releases glucose to provide energy during stressful situations.

• Gonadotropins ($FSH$ and $LH$):
      * Definition: These are "sex hormones."
      * Follicle Stimulating Hormone ($FSH$) and Luteinizing Hormone ($LH$) act on the gonads (testes in men, ovaries in women).
      * In Men: Stimulates testosterone production and sperm cell development.
      * In Women: Impacts the menstrual cycle and the production of progesterone and estrogen.

• Prolactin:
      * Function: Responsible for milk production.
      * Clinical Note: Medications can be used to suppress prolactin if there is abnormal or excessive milk production (leaking).

• Growth Hormone ($GH$):
      * Function: Facilitates skeletal and muscle growth.

• Thyroid Stimulating Hormone ($TSH$):
      * Function: Travels to the thyroid gland to stimulate the release of thyroid hormones ($T3$ and $T4$), which regulate metabolism.

Posterior Pituitary Hormones

• Overview: The posterior pituitary is governed by nervous system communication (neuroendocrine) rather than the complex "telephone game" of releasing hormones seen in the anterior lobe.

• Oxytocin:
      * Function: Causes uterine contractions during labor.
      * Milk Letdown: Responsible for the milk letdown reflex (ejection of milk) when a baby begins breastfeeding.
      * Psychological Effect: Often referred to as a "happy drug."

• Antidiuretic Hormone ($ADH$ / Vasopressin):
      * Function: Stimulates the kidneys to retain fluid by increasing water absorption.
      * Mechanism: Reduces water excretion by up to $90\%$, resulting in less, more concentrated urine.
      * Triggers for Release: Low blood pressure, low fluid volume, and dehydration.
      * Secondary Effect: Causes vasoconstriction.

Pharmacological Concepts: Terminology and Diagnostic Use

• Suffix "-tropin": Means to "act on" or "stimulate."
      * Example: Gonadotropin-releasing hormone ($GnRH$) is released by the hypothalamus to act on the pituitary.

• Inhibitory Factors: Some hypothalamic hormones suppress release, such as Somatostatin, which inhibits growth hormone.

• General Purpose of Pituitary Drugs:
      * Replacement Therapy: For patients with hormone deficiencies.
      * Antagonizing: To suppress overactive hormone production (hypersecretion).
      * Diagnostic Aid: Used to determine if a deficiency exists within the hormonal feedback loop.

Specific Medications: Anterior Pituitary Agents

• Cosyntropin:
      * Classification: Synthetic $ACTH$.
      * Use: Diagnostic test for adrenocortical insufficiency.
      * Diagnostic Procedure:
          1. Draw baseline labs for cortisol and $ACTH$.
          2. Administer Cosyntropin.
          3. Wait exactly $60$ minutes.
          4. Re-draw labs to check for a rise in cortisol and aldosterone.
      * Interpretation: If there is no rise in cortisol after administration, the patient has an adrenal deficiency and requires hormone replacement.

• Octreotide:
      * Classification: Synthetic form of somatostatin (inhibitor of Growth Hormone).
      * Uses:
          1. Acromegaly: To suppress excess GH in "giant" patients.
          2. Bleeding Esophageal Varices.
          3. Vipomas: Specified pancreatic neuroendocrine tumors that secrete Vasoactive Intestinal Peptide ($VIP$). Excess $VIP$ causes diarrhea and flushing; Octreotide inhibits this hormone to control symptoms.
      * Nursing Considerations:
          * Monitor blood sugars in all patients (diabetic and non-diabetic).
          * Monitor for cardiac abnormalities; an EKG may be necessary.

• Somatropin:
      * Classification: Synthetic Growth Hormone ($GH$).
      * Indications:
          1. Growth Hormone Deficiency: May be idiopathic or caused by a pituitary adenoma (a non-cancerous tumor that disrupts homeostasis).
          2. HIV Infection: Used to increase body mass, weight, and physical endurance.
      * Condition for Linear Growth: Must be administered before the growth plates (epiphyses) have closed. If growth plates are closed, it will not increase height but may still be used for body structure or glucose regulation.
      * Administration/Education: Given via SubQ or IM injection. Parents and patients must be taught injection techniques and should maintain a journal of height and weight.

Specific Medications: Posterior Pituitary Agents

• Vasopressin:
      * Function: Mimics naturally occurring Antidiuretic Hormone ($ADH$).
      * Primary Use: Diabetes Insipidus ($DI$), a condition characterized by excess diuresis (excessive peeing and thirst).
      * Emergency Uses: Used in septic shock or hypotensive crisis as a potent vasoconstrictor. Can also be used to stop certain types of bleeding.
      * Nursing Warning (IV Administration): Because it is a potent vasoconstrictor, extravasation (infiltration) can cause localized necrosis due to lack of blood flow. A central line or large peripheral IV is preferred.

• Desmopressin:
      * Classification: Synthetic form of Vasopressin, but with less aggressive effects on blood pressure.
      * Uses:
          1. Diabetes Insipidus.
          2. Bleeding Disorders: Treats Hemophilia A and von Willebrand’s disease by increasing Factor VIII.
          3. Nocturnal Enuresis: Used for bed-wetting in children.
      * Administration Considerations: Available as oral, IV, SubQ, or nasal. When using the nasal route, ensure passages are clear and not blocked by mucus/snot to ensure proper absorption.

Questions & Discussion

• Q: Where exactly are the adrenals?
      * A: Directly above your kidneys.

• Q: What is a Vipoma?
      * A: It is a type of neuroendocrine tumor, specifically in the pancreas, that secretes vasoactive intestinal peptide ($VIP$), leading to motility issues and diarrhea.

• Q: Can you use Somatropin forever?
      * A: Not for height. Once growth plates close, it is no longer effective for increasing height.

• Q: How do we check if Octreotide is working for a Vipoma?
      * A: You would watch for a decrease in symptoms like flushing and diarrhea.

• Q: Is Vasopressin more aggressive than a standard pressor?
      * A: Yes, it is very potent and often seen in ICU settings for hypotensive crises.

• Q: Why do we check nasal passages for Desmopressin?
      * A: If the patient is stuffy, the medication will stick to the mucus and not the tissue, preventing absorption (similar to a suppository placed in stool).

• Q: What was the drug from last test I shouldn't put in a Styrofoam cup?
      * A: Cyclosporine (used for organ transplants).