5.2 Salivary Secretion, Flow Rates and Autonomic Control

2. Salivary Secretion: Flow Rates, Two-Stage Formation, and Autonomic Control

Saliva is an ultrafiltrate of blood that is subsequently modified during ductal transit. Healthy adults produce between 0.5 and 1.5 litres of saliva daily.

Resting vs Stimulated Flow Rates and Clinical Diagnostic Thresholds

• Unstimulated Whole Saliva (UWS / Resting):
  • Normal physiological baseline: 0.3 to 0.4 mL/min.
  • Objective diagnostic threshold for hyposalivation: < 0.1 mL/min.
  • Governed primarily by baseline spontaneous firing of parasympathetic tone directed to the submandibular and sublingual glands.
• Stimulated Whole Saliva (SWS):
  • Normal physiological flow rate: 1.5 to 2.0 mL/min.
  • Objective diagnostic threshold for stimulated hyposalivation: < 0.5 to 0.7 mL/min.
  • Elicited by masticatory mechanical stimulation (periodontal mechanoreceptors) and gustatory chemical stimulation (especially sour citric acid taste pathways), shifting the parotid gland to the primary contributor.

The Two-Stage Secretion Hypothesis

Salivary fluid generation operates according to the classic model established by Thaysen:

Stage 1 (Acinus): Isotonic Primary Secretion
Plasma Ultrafiltration + Active Cl- Transport (NKCC1, TMEM16A) + Paracellular Na+ + Transcellular H2O (Aquaporin-5)
                                     │
                                     ▼
Stage 2 (Striated & Excretory Ducts): Duct Modification
Active Reabsorption of Na+ (ENaC) & Cl- (CFTR) │ Active Secretion of K+ & HCO3-
                      [Water-Impermeable Epithelium]
                                     │
                                     ▼
Final Secretion: Hypotonic Oral Fluid (Low Na+, Low Cl-, Variable K+ and HCO3-)

1. Stage 1 — Primary Acinar Secretion (Isotonic):

   • Acinar basolateral membranes express the Na⁺/K⁺-ATPase pump, which establishes an electrochemical gradient by pumping 3 Na⁺ out and 2 K⁺ in.
   • This gradient powers the Na⁺/K⁺/2Cl⁻ cotransporter (NKCC1), accumulating intracellular chloride (Cl⁻) against its chemical gradient.
   • Parasympathetic activation elevates intracellular free calcium ([Ca²⁺]ᵢ), opening apical calcium-activated chloride channels (TMEM16A / Anoctamin-1). Chloride rushes into the acinar lumen.
   • The negative transepithelial luminal potential draws Na⁺ passively across the paracellular tight junctions.
   • The accumulated luminal NaCl creates a localized osmotic gradient, driving water transcellularly into the lumen through apical Aquaporin-5 (AQP5) water channels. The primary fluid in the acinar lumen is isotonic with blood plasma.

2. Stage 2 — Ductal Modification (Hypotonic Conversion):

   • As primary saliva traverses the striated and interlobular excretory ducts, luminal Na⁺ is actively reabsorbed via the epithelial sodium channel (ENaC), and Cl⁻ is reabsorbed via the CFTR channel and Cl⁻/HCO₃⁻ anion exchangers.
   • Simultaneously, ductal cells actively secrete K⁺ and bicarbonate (HCO₃⁻) into the lumen.
   • Because the ductal epithelium is impermeable to water, solutes are reabsorbed in far greater quantities than water. Consequently, the final saliva discharged into the oral cavity is decidedly hypotonic.

The Flow Rate Dependency of Salivary Electrolytes

The ionic composition of saliva changes drastically as a function of flow rate:

• At low (resting) flow rates, saliva moves slowly through the striated ducts, providing extended contact time for maximal Na⁺ and Cl⁻ reabsorption. Resting saliva has very low Na⁺ (~5–10 mmol/L) and Cl⁻ (~15 mmol/L), while K⁺ remains elevated (~20 mmol/L).
• At high (stimulated) flow rates, saliva rushes through the ductal system with minimal contact time, outstripping the reabsorptive capacity of ductal ENaC channels. Luminal Na⁺ and Cl⁻ concentrations rise substantially (approaching 40–60 mmol/L). Concurrently, parasympathetic stimulation actively enhances ductal carbonic anhydrase-mediated secretion of bicarbonate (HCO₃⁻), driving salivary HCO₃⁻ concentration from ~5 mmol/L up to 30–60 mmol/L, elevating salivary pH from resting ~6.7 to stimulated 7.8–8.0.

Autonomic Nervous Regulation: Parasympathetic vs Sympathetic

Salivary secretion is entirely under neural reflex control; there is no direct hormonal secretomotor control.

• Parasympathetic Pathway (Fluid Volume Driver):
  • Primary neurotransmitter: Acetylcholine (ACh); co-transmitter: Vasoactive Intestinal Peptide (VIP).
  • Receptors: Muscarinic M3 receptors on acinar and ductal cells.
  • Signal Transduction: ACh binding couples to the Gq protein, activating phospholipase C (PLC). PLC hydrolyses phosphatidylinositol 4,5-bisphosphate (PIP₂) into inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ binds receptors on the endoplasmic reticulum, releasing a massive surge of intracellular Ca²⁺. Elevated [Ca²⁺]ᵢ opens apical Cl⁻ channels (TMEM16A) and basolateral K⁺ channels, initiating the transcellular osmotic water cascade.
  • Result: Copious, watery saliva with low protein content. VIP concurrently stimulates profound local arteriolar vasodilation, increasing capillary blood flow to sustain prolonged secretion.
• Sympathetic Pathway (Macromolecular Protein Driver):
  • Primary neurotransmitter: Noradrenaline (Norepinephrine); acting on β1 and β2-adrenergic receptors.
  • Signal Transduction: β-receptors couple to the Gs protein, activating adenylyl cyclase, which catalyses the synthesis of cyclic adenosine monophosphate (cAMP) from ATP. cAMP activates Protein Kinase A (PKA), phosphorylating cytoskeletal proteins that trigger exocytosis of preformed zymogen storage granules.
  • Result: Low volume, viscous, highly concentrated saliva rich in amylase, mucins, and organic defensive proteins. Intense sympathetic stimulation (e.g., acute stress response) causes vasoconstriction via α1-adrenergic receptors, resulting in transient sensations of oral dryness.