Part 8 · Chapter 8.3

RVD & RVI

After swelling or shrinkage, cells restore volume over minutes by regulatory volume decrease (RVD) or regulatory volume increase (RVI). These responses pump osmotic load out of or into the cell; water follows. This chapter maps the ion- transport machinery activated by volume sensors (M8.2).

1. Regulatory Volume Decrease (RVD)

Hypotonic swelling activates: (a) VRAC/LRRC8 Cl^- efflux; (b) K-Cl cotransporter (KCC) effluxing K⁺Cl^-; (c) volume-activated K⁺ channels (IK_vol). Net: KCl + water exit. Completes over 3–10 min, returning volume to baseline even with sustained hypotonic challenge. WNK kinases (low Cl^- activation) phosphorylate KCC to activate it.

2. Regulatory Volume Increase (RVI)

Hypertonic shrinkage activates: (a) NKCC1 (2Cl + Na + K influx), (b) NHE (Na⁺/H⁺ exchange), (c) anion exchanger (Cl^-/ HCO₃^-). Net: NaCl + H₂O uptake. WNK activation by high Cl^- phosphorylates SPAK/OSR1 which activate NKCC1. Completes over similar minutes timescale.

\[ \text{RVD: KCC + VRAC}\quad\quad \text{RVI: NKCC1 + NHE + AE} \]

Simulation: RVD and RVI Time Courses

Python

script.py37 lines

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

# RVD after hypotonic swelling
t = np.linspace(-1, 20, 400)
V = np.ones_like(t)
hypotonic = t > 0
# Initial swelling to 1.3x
V[hypotonic] = 1 + 0.3 * (1 - np.exp(-(t[hypotonic])/0.3))
# RVD phase: K+Cl efflux via KCC + VRAC -> back to baseline over ~5 min
rvd_start = t > 1
V[rvd_start] = 1.3 * np.exp(-(t[rvd_start]-1)/3) + 1*(1 - np.exp(-(t[rvd_start]-1)/3))
V = np.clip(V, 0.95, 1.3)

# RVI after hypertonic shrinkage
V_rvi = np.ones_like(t)
V_rvi[hypotonic] = 1 - 0.25 * (1 - np.exp(-(t[hypotonic])/0.3))
V_rvi[rvd_start] = 0.75 + 0.25 * (1 - np.exp(-(t[rvd_start]-1)/3))

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, V, color='#60a5fa', lw=2.5, label='Hypotonic challenge -> RVD')
ax.plot(t, V_rvi, color='#fbbf24', lw=2.5, label='Hypertonic challenge -> RVI')
ax.axhline(1, color='#94a3b8', ls='--', label='Baseline volume')
ax.axvline(0, color='#f87171', ls=':', label='Challenge onset')
ax.set_xlabel('Time (min)', color='#bae6fd')
ax.set_ylabel('V / V0', color='#bae6fd')
ax.set_title('Regulatory volume decrease (RVD) and increase (RVI)',
             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('RVD: K+Cl efflux via KCC + VRAC -> water follows')
print('RVI: NKCC1 + Na/H and Cl/HCO3 exchangers bring osmotic load in')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. Tissue Specificity

All cells have some RVD/RVI capacity, but the dominant machinery varies. Red blood cells emphasise K-Cl cotransport. Kidney cells (renal medulla) face extreme osmolarity swings and use organic osmolytes (M8.4) for long-term adaptation. Hepatocytes couple volume regulation to metabolic flux. Reduced RVD/RVI capacity contributes to ischaemic swelling-induced injury.

Key References

• Hoffmann, E. K., Lambert, I. H. & Pedersen, S. F. (2009). “Physiology of cell volume regulation in vertebrates.” Physiol. Rev., 89, 193–277.

• Pedersen, S. F. et al. (2016). “Biophysics and physiology of the volume-regulated anion channel (VRAC/VSOR).” Pflügers Arch., 468, 371–383.

• Hoffmann, E. K. & Pedersen, S. F. (2011). “Cell volume homeostatic mechanisms: effectors and signalling pathways.” Acta Physiol., 202, 465–485.

← 8.2 Sensors 8.4 Organic Osmolytes →