Diet-induced changes in rabbit serum potassium do not alter size or number of brain capillary endothelial cell...

Previous studies have shown brain extracellular (EC) [K+] to remain constant during wide variations of the plasma-brain [K+] gradient. High serum [K+] is hypothesized to increase K+ pumping by the blood-brain barrier (BBB) to maintain normal EC levels. This changing metabolic work could be reflected in endothelial cell mitochondria which have been shown to be three to four times more numerous in the BBB than in other capillary endothelial cells. We measured several morphologic parameters of brain capillary endothelial cells in white male New Zealand rabbits following dietary manipulation of their serum [K+] levels. Four groups, based on serum [K+] levels, were formed: Deficient, Low, Normal, and High. Following the final day's weight and blood samples, rabbits were anesthetized and perfused for transmission electron microscopic planimetry. Variables included number and area of mitochondria, areas of capillary lumen and capillary diameter, area of endothelial cell, and proportion of endothelial cell taken up by mitochondria. With the exception of the area of the endothelial cell (P less than 0.01), which decreased with increasing serum [K+] levels, no differences appeared among the variables in any of the four groups. Lack of anticipated differences in mitochondrial number or proportional area indicates either that pumping K+ occupies a minor fraction of cell energy, or changes in mitochondrial size or number are not sufficiently sensitive indicators of metabolic workload in this model.