Imaging of Ca²+ Transients in Endothelial Cells of Single Perfused Capillaries: Correlation of Peak [Ca²⁺]_i with Sites of Macromolecule Leakage

Abstract

Objective: To investigate the mechanisms responsible for variation in the macromolecular leakage (formation of localized leaky sites) in venular microvessels with increased permeability, we examined the hypothesis that cytoplasmic calcium concentration [Ca²⁺]_i, does not increase uniformly within microvessel endothelial cells.

Methods: We loaded the endothelial cells forming the walls of venular microvessels in frog mesentery with fura-2, and imaged [Ca²⁺]_i using a cooled CCD camera.

Results: Control [Ca²⁺]_i was close to 60 nM in all regions. Control permeability was uniformly low in all microvessels. Exposure to ionomycin (5 mM) increased [Ca²⁺]_i in a biphasic manner, but not uniformly. There was variation in both time to peak (bimodal distribution) and peak [Ca²⁺]_i (274 ± 13 nM; mean variation above or below the peak value was 110 nM). Raising extracellular calcium from 1.1 to 5 mM increased the mean variation of [Ca²⁺]_i about peak values. Extravascular leakage of fluorescently labeled albumin or low-density lipoproteins was most prominent at sites where increases in [Ca²⁺]_i were largest.

Conclusions: These data indicate that variation in [Ca²⁺]_i within individual endothelial cells or groups of cells could account, at least in part, for the distribution of localized leakage sites for macromolecules in venular microvessels in the high-permeability state.

Key Words:

• Capillary permeability
• fura-2
• macromolecular leaking sites
• endothelial cell calcium
• cell-cell communication