U4L4 - Hormonal Regulation of Water Balance

Kidneys: Responsible for water balance through hormonal mechanisms.
- The urinary system is crucial for maintaining ionic and water balance.
- Discussion will focus on mechanisms employed for achieving water balance through hormones.

Key Components of Regulation:
- External Cause of Low Water
  - Stimulus
  - Receptors
  - Regulator
  - Effector
  - Response
  - Effect
- Big Picture: Conceptual framework for understanding hormonal regulation of water balance.

Dehydration Factors:
- Lack of drinking water.
- Exercise leading to low water due to sweating.
- Excessive heat causing increased sweating.
- Diarrhea contributing to loss of water.
- High osmotic pressure in capillaries results from increased solute concentration.

Effects on Blood Composition:
- Dehydration results in lower water concentration and higher solute concentration within blood, leading to a hypertonic solution.
- Water is drawn into blood vessels from surrounding cells through osmosis due to high solute concentration.

Osmoreceptors: Located in the hypothalamus, sensitive to osmotic changes in the blood.
- Surrounded by capillaries that experience high osmotic pressure during dehydration.
- When osmotic pressure is high, water in osmoreceptors crenates (shrinks).
  - Crenation Condition: Plasma osmolarity >280 milli-osmoles/kg.
- Thirst Trigger: Increases to >290 milli-osmoles/kg.
- ADH Secretion Threshold: Approximately a 1% increase in plasma osmolarity leads to increased ADH secretion from the posterior pituitary gland.

Posterior Pituitary Gland: Secretes Antidiuretic Hormone (ADH) via exocytosis into the bloodstream.
- ADH Prevents urine formation, maintaining water balance.

Afferent Nerves: Sensory nerves that carry signals from osmoreceptors to regulators.
Efferent Pathway Mediator: ADH serves as the mediator connecting regulators to effectors.

Distal Convoluted and Collecting Tubule of the Nephron:
- Normally impermeable to water but becomes permeable in the presence of ADH.
- Increased water reabsorption by 15% when ADH is present compared to absence of ADH.

Physiological Changes:
- Increased concentration of water in the bloodstream.
- Blood hypotonicity increases.
- Decreased plasma osmotic pressure reduces water outflow from surrounding cells.
- Results in more concentrated urine, thereby conserving water.

Negative Feedback Mechanism:
- Reduction of stimulus leads to return to homeostasis.

ADH (Vasopressin):
- High concentrations lead to vasocompression (constriction of blood vessels) during low blood volume.
- Produced in the hypothalamus, transported to the posterior pituitary for secretion.
Structure of ADH:
- A nonapeptide consisting of nine amino acids.

Binding to V2 Receptor:
- ADH attaches to the V2 receptor, a membrane-bound protein in the distal convoluted and collecting tubule membranes.
G Protein Activation:
- G protein is activated, converting GDP to GTP, releasing energy for subsequent reactions.
Adenylyl Cyclase Activation:
- G protein activates Adenylyl Cyclase, facilitating conversion of ATP to cyclic AMP (cAMP).
  - cAMP functions as the first messenger entering the nucleus.

Nuclear Action of cAMP:
- cAMP binds to repressor proteins on the gene for protein kinase A (PKA), removing them from DNA, leading to transcription.
Transcription Process:
- The PKA gene is transcribed into mRNA, which is then translated to produce PKA protein.

PKA Function:
- PKA performs complex phosphorylation functions affecting cytoskeleton structure.
- Adjusts to create porous vesicles containing aquaporins.
- Vesicles merge with nephron's apical membrane, forming channels that allow water to enter through the previously water-impermeable membrane.
- Water then diffuses across the membrane, ultimately entering the bloodstream and relieving dehydration effects.