Part 6 · Chapter 6.2

Transcellular Transport

Transcellular transport moves solutes from one side of an epithelium to the other by sequential apical and basolateral transporters. Epithelial polarity — different proteins on the two membrane faces — is the architecture that makes directional transport possible. Primary active Na-K ATPase sets up ion gradients that drive secondary active cotransport via SGLT, NHE, and related carriers.

1. Epithelial Polarity

Epithelial cells display distinct apical(luminal, ~10% of membrane) and basolateraldomains separated by the tight junction (M6.1). Vesicular trafficking of membrane proteins is sorted at the trans-Golgi network by apical vs. basolateral sorting signals. This asymmetry is what distinguishes epithelial transport from simple diffusion.

2. The Crane Gradient Hypothesis

Crane 1960 proposed that Na⁺ cotransport drives uphill solute uptake. In enterocyte sugar absorption: Na-K ATPase on basolateral membrane pumps 3 Na⁺ out / 2 K⁺ in (ATP hydrolysis, primary active); this creates [Na⁺]_cell ~10 mM vs [Na⁺]_lumen~140 mM. Apical SGLT1 cotransports Na⁺ + glucose down the Na⁺gradient (secondary active), concentrating glucose inside the cell. Basolateral GLUT2 facilitates glucose exit down its gradient. The full circuit absorbs glucose against its blood-side gradient.

Simulation: SGLT1 & Oral Rehydration

Python

script.py45 lines

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

# Glucose-Na cotransport in small-intestine enterocyte
# SGLT1 apical: Na + glucose in
# GLUT2 basolateral: glucose out
# Na-K ATPase basolateral: 3 Na out, 2 K in
# This is the Crane 1960 gradient hypothesis

Na_lumen = 140
Na_cell = np.linspace(5, 140, 200)
# SGLT1 flux scales with (Na_l - Na_c) * [Glc]
V_SGLT = np.maximum(0, (Na_lumen - Na_cell)) * 0.3

# Glucose transport (crude)
Glc_cell = V_SGLT / 0.2   # steady state vs GLUT2 clearance

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(14, 5), facecolor='#0a0f1e')
for ax in (ax1, ax2):
    ax.set_facecolor('#111e2f'); ax.tick_params(colors='#bae6fd')
    for s in ax.spines.values(): s.set_color('#334155')
    ax.grid(True, color='#334155', alpha=0.3)

ax1.plot(Na_cell, V_SGLT, color='#34d399', lw=2.5)
ax1.axvline(10, color='#fbbf24', ls='--', label='Physiological [Na]_cell ~10 mM')
ax1.set_xlabel('[Na]_cell (mM)', color='#bae6fd')
ax1.set_ylabel('SGLT1 glucose flux (rel)', color='#bae6fd')
ax1.set_title('SGLT1: Na gradient drives uphill glucose uptake',
              color='#67e8f9', fontweight='bold')
ax1.legend(facecolor='#1e293b', edgecolor='#334155', labelcolor='#cbd5e1')

# ORS: WHO glucose-salt oral rehydration
rate = [0.3, 0.8, 1.0, 0.7, 0.4]
labels = ['0% glc', '1% glc', '2% glc (WHO)', '5% glc', '10% glc']
ax2.bar(labels, rate, color='#60a5fa', edgecolor='#bae6fd')
ax2.set_ylabel('H2O absorption (L/h)', color='#bae6fd')
ax2.set_title('WHO ORS: 2% glucose optimal for Na/H2O uptake',
              color='#67e8f9', fontweight='bold')
plt.setp(ax2.get_xticklabels(), rotation=15)

plt.tight_layout()
plt.savefig('output.png', dpi=120, bbox_inches='tight', facecolor='#0a0f1e')
print('SGLT1 is the basis for WHO oral rehydration solution; saved ~50M lives/yr historically')
print('Na-K ATPase maintains low [Na]_cell; gradient drives secondary active glucose uptake')

Click Run to execute the Python code

Code will be executed with Python 3 on the server

3. Cotransporter Families

SGLT1/2: Na/glucose. SGLT2 inhibitors (empagliflozin, dapagliflozin) are major type-2 diabetes drugs.
NHE3: Na/H exchanger in proximal tubule and jejunum.
NKCC2: Na/K/2Cl in thick ascending limb of Henle. Loop diuretics (furosemide) target this.
NCC: Na/Cl in distal tubule. Thiazide diuretic target.
PepT1: H/peptide cotransport in enterocyte.

4. Oral Rehydration Therapy

Cholera produces massive intestinal fluid loss by CFTR hyperactivation (M6.5) but leaves SGLT1 intact. Hirschhorn & Pierce 1968 showed that glucose + NaCl solution drives Na⁺/H₂O absorption via SGLT1, recovering fluid. WHO oral rehydration solution (2% glucose + salts) saves an estimated 50 million lives per year and is perhaps the single most important medical therapy ever developed.

Key References

• Crane, R. K. (1960). “Intestinal absorption of sugars.” Physiol. Rev., 40, 789–825.

• Wright, E. M. & Loo, D. D. F. (2000). “Coupling between Na⁺, sugar, and water transport across the intestine.” Ann. NY Acad. Sci., 915, 54–66.

• Wright, E. M. et al. (2017). “Novel and unexpected functions of SGLTs.” Physiology, 32, 435–443.

← 6.1 Tight Junctions 6.3 Paracellular →