VIP vs PACAP — Receptor Pharmacology
VIP and PACAP are structurally related neuropeptides with overlapping but distinct receptor pharmacology. VIP primarily activates VPAC receptors (VPAC1, VPAC2), while PACAP activates both VPAC receptors and the PAC1 receptor. This selectivity difference explains their distinct physiological effects.
Structural Differences
Section titled “Structural Differences”VIP (Vasoactive Intestinal Peptide)
Section titled “VIP (Vasoactive Intestinal Peptide)”- Sequence: HSDAVFTDNYTRLRKQMAVKKYLNSILN-NH₂ (28 amino acids)
- Modifications: C-terminal amidation
- MW: 3,326 Da
- Charge at pH 7.4: +1
- Half-life: 1–2 minutes (serum)
- Structure: α-helical conformation
PACAP (Pituitary Adenylate Cyclase-Activating Polypeptide)
Section titled “PACAP (Pituitary Adenylate Cyclase-Activating Polypeptide)”- Sequence: HSDGIFTDSYSRYRKQMAVKKYLAAVLGKRYKQRVKNK-NH₂ (38 amino acids)
- Modifications: C-terminal amidation
- MW: 4,534 Da
- Charge at pH 7.4: +4
- Half-life: 2–5 minutes (serum)
- Structure: α-helical conformation
Receptor Pharmacology
Section titled “Receptor Pharmacology”Receptor Distribution and Function
Section titled “Receptor Distribution and Function”| Receptor | Distribution | Primary Function |
|---|---|---|
| VPAC1 | Lung, pancreas, liver, immune cells | Vasodilation, secretion, metabolism |
| VPAC2 | Brain, pancreas, immune cells | Circadian rhythm, insulin secretion |
| PAC1 | Brain, adrenal, pituitary | Neurotransmission, hormone release |
VIP Receptor Selectivity
Section titled “VIP Receptor Selectivity”| Receptor | Binding Affinity (Ki) | Selectivity | Functional Activity |
|---|---|---|---|
| VPAC1 | 0.4 nM | Very high | Full agonist |
| VPAC2 | 0.8 nM | High | Full agonist |
| PAC1 | 1 µM | Very low | Inactive |
PACAP Receptor Selectivity
Section titled “PACAP Receptor Selectivity”| Receptor | Binding Affinity (Ki) | Selectivity | Functional Activity |
|---|---|---|---|
| VPAC1 | 0.5 nM | High | Full agonist |
| VPAC2 | 0.3 nM | High | Full agonist |
| PAC1 | 0.1 nM | Very high | Full agonist |
Signaling Pathways
Section titled “Signaling Pathways”Shared Signaling (VPAC Receptors)
Section titled “Shared Signaling (VPAC Receptors)”Both VIP and PACAP activate VPAC receptors, leading to:
- Gαs → cAMP → PKA pathway
- Voltage-gated Ca²⁺ channel activation
- PI3K → Akt survival pathway
- ERK1/2 MAPK pathway (variable)
PAC1-Specific Signaling
Section titled “PAC1-Specific Signaling”PACAP activation of PAC1 receptors additionally triggers:
- Gαq → PLC → IP₃ + DAG pathway
- IP₃ → Ca²⁺ release from ER
- DAG → PKC activation
- Arachidonic acid release
Physiological Effects Comparison
Section titled “Physiological Effects Comparison”Cardiovascular Effects
Section titled “Cardiovascular Effects”| Parameter | VIP | PACAP |
|---|---|---|
| Vasodilation | Strong (VPAC1) | Moderate (VPAC1/2) |
| Heart rate | Increased | Increased |
| Blood pressure | Decreased | Decreased |
| Coronary flow | Increased | Increased |
| Mechanism | cAMP-mediated | cAMP + Ca²⁺ |
Respiratory Effects
Section titled “Respiratory Effects”| Parameter | VIP | PACAP |
|---|---|---|
| Bronchodilation | Strong | Moderate |
| Mucus secretion | Increased | Increased |
| Airway inflammation | Reduced | Reduced |
| Surfactant release | Increased | Increased |
Gastrointestinal Effects
Section titled “Gastrointestinal Effects”| Parameter | VIP | PACAP |
|---|---|---|
| Intestinal motility | Relaxed | Relaxed |
| Secretion | Increased | Increased |
| Blood flow | Increased | Increased |
| Sphincter relaxation | Strong | Moderate |
Neurological Effects
Section titled “Neurological Effects”| Parameter | VIP | PACAP |
|---|---|---|
| Neurotransmission | Moderate | Strong |
| Neuroprotection | Moderate | Strong |
| Circadian rhythm | Weak | Strong (PAC1) |
| Learning/memory | Weak | Strong (PAC1) |
Endocrine Effects
Section titled “Endocrine Effects”| Parameter | VIP | PACAP |
|---|---|---|
| Insulin secretion | Moderate | Strong |
| Glucagon secretion | Moderate | Strong |
| GH release | Weak | Strong (PAC1) |
| ACTH release | Weak | Strong (PAC1) |
Clinical Applications
Section titled “Clinical Applications”Primary indications:
- Erectile dysfunction: Intracavernosal injection
- Pulmonary hypertension: Vasodilation
- Cerebral vasospasm: Subarachnoid hemorrhage
- VIPoma: Diagnosis (secretory diarrhea)
Dosing protocols:
- Erectile dysfunction: 20 µg intracavernosal
- Pulmonary hypertension: Inhalation (investigational)
- VIPoma: Octreotide (somatostatin analogue)
Primary indications:
- Cerebral vasospasm: Subarachnoid hemorrhage
- Neuroprotection: Stroke, TBI (investigational)
- Metabolic syndrome: Insulin sensitivity
- Circadian rhythm disorders: Sleep-wake cycle
Dosing protocols:
- Cerebral vasospasm: 10–100 µg/h IV
- Neuroprotection: 10–50 µg IV (investigational)
- Metabolic effects: 50–100 µg IV (investigational)
Pharmacokinetic Comparison
Section titled “Pharmacokinetic Comparison”| Parameter | VIP | PACAP |
|---|---|---|
| Half-life | 1–2 min | 2–5 min |
| T_max | 5–10 min | 5–10 min |
| Duration of action | 30–60 min | 60–120 min |
| Bioavailability (IV) | 100% | 100% |
| Bioavailability (inhaled) | ~30% | ~40% |
| Volume of distribution | ~0.2 L/kg | ~0.3 L/kg |
| Clearance | ~20 mL/min/kg | ~15 mL/min/kg |
Safety Profile
Section titled “Safety Profile”Common adverse events (>5%):
- Flushing (20–30%)
- Hypotension (15–25%)
- Headache (10–15%)
- Nausea (5–10%)
- Dizziness (5–10%)
Serious adverse events (rare):
- Severe hypotension
- Cardiac arrhythmias
- Bronchospasm (inhaled)
- Anaphylaxis
Common adverse events (>5%):
- Flushing (15–25%)
- Hypotension (10–20%)
- Headache (10–15%)
- Nausea (5–10%)
- Dizziness (5–10%)
Serious adverse events (rare):
- Severe hypotension
- Cardiac arrhythmias
- Hypokalemia
- Anaphylaxis
Formulation and Stability
Section titled “Formulation and Stability”- Formulation: Lyophilized powder, injectable solution
- Storage: −20°C to −80°C (lyophilized)
- Stability (reconstituted): 24 hours refrigerated
- Shelf life: 24 months (lyophilized)
- Delivery: IV, intracavernosal, inhaled
- Formulation: Lyophilized powder, injectable solution
- Storage: −20°C to −80°C (lyophilized)
- Stability (reconstituted): 24 hours refrigerated
- Shelf life: 24 months (lyophilized)
- Delivery: IV, inhaled, intranasal
Cost and Availability
Section titled “Cost and Availability”| Parameter | VIP | PACAP |
|---|---|---|
| Cost per mg | $50–100 | $100–200 |
| Monthly cost | $100–200 | $200–400 |
| Availability | Research peptides | Research peptides |
| Generic availability | Yes | Yes |
| Formulation | Lyophilized | Lyophilized |
Clinical Decision Algorithm
Section titled “Clinical Decision Algorithm”When to Choose VIP
Section titled “When to Choose VIP”- VPAC1/2 activation needed: Cardiovascular, respiratory
- Erectile dysfunction: Intracavernosal use
- Pulmonary hypertension: Inhalation therapy
- Lower cost: Less expensive
- VPAC1 selectivity: Specific tissue effects
When to Choose PACAP
Section titled “When to Choose PACAP”- PAC1 activation needed: Neurological, endocrine
- Neuroprotection: Stroke, TBI
- Metabolic effects: Insulin secretion
- Circadian rhythm: Sleep-wake cycle
- Dual receptor activation: VPAC + PAC1
Future Directions
Section titled “Future Directions”- Selective VPAC1/2 agonists: Tissue-specific effects
- Oral formulations: Oral bioavailability
- Inhaled formulations: Pulmonary delivery
- Combination therapy: With other vasoactive peptides
- Selective PAC1 agonists: Neurological specificity
- Long-acting analogues: Extended half-life
- Oral formulations: Oral bioavailability
- Neuroprotective combinations: With other neuroprotectants
References
Section titled “References”- Said SI, et al. “Vasoactive intestinal peptide: a potent vasodilator.” Science 1970;169:1217-1218.
- Miyata A, et al. “PACAP: a novel pituitary adenylate cyclase-activating polypeptide.” Biochem Biophys Res Commun 1989;164:567-574.
- Vaudry D, et al. “PACAP and VIP receptors: from pharmacology to biological functions.” Brain Res Rev 2000;33:89-100.
- Laburthe M, et al. “VPAC receptors: structure, pharmacology and physiological functions.” Peptides 2002;23:831-842.
- Harmar AJ, et al. “VPAC receptors: structure, distribution and pharmacology.” Pharmacol Rev 2004;56:713-731.