Physiological and morphological effects of perfusing isolated rat kidneys with hyperosmolal mannitol solutions

Area of Science:

• Nephrology
• Renal Physiology
• Membrane Transport

Background:

• Modifying glomerular charge barrier with varying ionic strengths previously showed reversible reduction in fixed charges without affecting size selectivity.
• Potential confounding effects of mannitol on glomerular permeability, when used to maintain osmolality, required further investigation.

Purpose of the Study:

• To investigate the effects of hyperosmolar mannitol solutions on isolated kidney perfusion and glomerular permeability.
• To compare the impact of hyperosmolar mannitol with standard albumin solutions on kidney function and histology.

Main Methods:

• Isolated kidneys were perfused at 8°C with either hyperosmolar mannitol (560 mOsm) or standard albumin (295 mOsm) solutions.
• Vascular resistance, blood flow, glomerular filtration rate (GFR), and fractional albumin clearance were measured.
• Kidney histology and glomeruli perfusion (using FITC dextran) were assessed.

Main Results:

• Hyperosmolar mannitol perfusion decreased vascular resistance and increased blood flow and GFR by 50%.
• Fractional albumin clearance remained unchanged, indicating no significant alteration to the glomerular barrier.
• Mannitol-perfused kidneys showed preserved histology and even glomeruli perfusion, contrasting with collapsed cortical tissue and tubular swelling in control kidneys.

Conclusions:

• Perfusion with hyperosmolar mannitol solution enhances GFR by increasing the number of active nephrons.
• This effect is distinct from alterations in ionic strength and does not compromise the glomerular barrier or albumin handling.
• Mannitol perfusion preserves kidney histology and improves glomeruli perfusion compared to standard perfusates.