Part 3 · Chapter 3.4

Second Messengers

Second messengers are small, rapidly-diffusing cytoplasmic molecules that relay and amplify signals from membrane receptors to downstream effectors. cAMP (Sutherland 1958, Nobel 1971), IP₃/DAG, Ca²⁺, cGMP, and NO are the canonical messengers. Each has distinct kinetics, spatial scale, and termination machinery.

1. cAMP

Adenylyl cyclase (AC) is activated by G_s and inhibited by G_i. It converts ATP to cAMP. Phosphodiesterases (PDE, 11 families) hydrolyse cAMP to AMP. cAMP primarily activates protein kinase A (PKA): holoenzyme R₂C₂binds 4 cAMP, dissociates into active C subunits. PKA targets include CREB, glycogen phosphorylase kinase, and ion channels. Epac is a second cAMP effector (Rap1-GEF).

2. IP₃ & DAG

PLCβ (G_q-activated) and PLCγ (RTK-activated) cleave PIP₂into two messengers:

\[ \text{PIP}_2 \xrightarrow{\text{PLC}} \text{IP}_3 + \text{DAG} \]

IP₃ diffuses cytosolically and opens IP₃R Ca²⁺channels on the ER (chapter 7.2). DAG remains membrane-bound and activates protein kinase C (PKC). IP₃ is cleared by 5-phosphatase to IP₂; DAG is cleared by DGK to phosphatidic acid.

Simulation: cAMP, IP₃, Ca²⁺ Transients

Python

script.py43 lines

import numpy as np, matplotlib
matplotlib.use('Agg')
import matplotlib.pyplot as plt

# Second-messenger dynamics after GPCR activation
t = np.linspace(0, 10, 500)
stim = (t > 1) & (t < 6)

# cAMP: rise via Gs/AC, fall via PDE
cAMP = np.zeros_like(t)
for i in range(1, len(t)):
    dt = t[i]-t[i-1]
    cAMP[i] = cAMP[i-1] + dt*(3.0*stim[i] - 0.5*cAMP[i-1])

# IP3: rapid rise and fall
IP3 = np.zeros_like(t)
for i in range(1, len(t)):
    dt = t[i]-t[i-1]
    IP3[i] = IP3[i-1] + dt*(5.0*stim[i] - 1.5*IP3[i-1])

# Cytosolic Ca from IP3R
Ca = np.zeros_like(t)
for i in range(1, len(t)):
    dt = t[i]-t[i-1]
    Ca[i] = Ca[i-1] + dt*(8.0*IP3[i-1] - 2.0*Ca[i-1])

fig, ax = plt.subplots(figsize=(11, 6), facecolor='#0a0f1e')
ax.set_facecolor('#111e2f'); ax.tick_params(colors='#bae6fd')
for s in ax.spines.values(): s.set_color('#334155')
ax.plot(t, cAMP, color='#60a5fa', lw=2.5, label='cAMP (Gs/AC)')
ax.plot(t, IP3, color='#fbbf24', lw=2.5, label='IP3 (Gq/PLC)')
ax.plot(t, Ca, color='#f87171', lw=2.5, label='Cytosolic Ca2+')
ax.axvspan(1, 6, alpha=0.1, color='#60a5fa', label='Agonist')
ax.set_xlabel('Time (s)', color='#bae6fd')
ax.set_ylabel('Relative concentration', color='#bae6fd')
ax.set_title('Second messengers: cAMP slow, IP3 fast, Ca downstream of IP3',
             color='#67e8f9', fontweight='bold')
ax.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')
ax.grid(True, color='#334155', alpha=0.3)
plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0f1e')
print('cAMP rise/fall: ~0.5-2 s time constant; PDE4 clearance')
print('IP3/Ca transient: 10-100 ms onset; shapes spikes and oscillations')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. cGMP & Nitric Oxide

Soluble guanylyl cyclase binds NO, producing cGMP. cGMP activates PKG and cGMP-gated ion channels (rod / cone photoreceptors). NO is the signal transducer in vascular smooth-muscle relaxation (“endothelium-derived relaxing factor”, Furchgott/Murad/Ignarro Nobel 1998). PDE5 degrades cGMP; sildenafil inhibits PDE5.

4. Microdomains & Scaffolds

Second messengers rarely operate freely throughout the cell. AKAPs (A-kinase anchoring proteins) tether PKA to specific subcellular compartments, creating local cAMP microdomains. PDE4D anchors to specific cAMP pools. The result is spatial signalling specificity: a cell can process two independent G_ssignals simultaneously if they generate cAMP in different subcellular locales.

Key References

• Sutherland, E. W. (1972). “Studies on the mechanism of hormone action.” Science, 177, 401–408.

• Berridge, M. J. (2009). “Inositol trisphosphate and calcium signalling mechanisms.” Biochim. Biophys. Acta, 1793, 933–940.

• Zaccolo, M. (2009). “cAMP signal transduction in the heart: understanding spatial control for the development of novel therapeutic strategies.” Br. J. Pharmacol., 158, 50–60.

← 3.3 Tyrosine Kinases 3.5 Signal Integration →