Choroid plexus structure and function in young rats on a high-potassium diet.

Rats, 20 days old, were fed a diet containing 20% KCl for 10 days. At 30 days these rats had lower body, brain and choroid plexus weights than matched controls and plasma [K+] was increased to 7.5 mM compared to 4.1 mM for controls. Cerebrospinal fluid (CSF) [K+] and bulk fluid secretion were not changed by the high K+ diet but there was a mild metabolic acidosis. The fourth ventricle choroid plexus was prepared for transmission electron microscopy and the structure of the epithelium analysed by quantitative stereology. The high K+ rats had a large increase in the mitochondrial volume fraction of the epithelium due to a 36% increase in individual mitochondrial volume. The high-K+ rats also had a decrease in the apical surface density of the epithelium due to a 41% decrease in the height of the microvilli. It is concluded that the increase in mitochondrial volume may provide the additional energy required for the increased transport of K+ out of the CSF in hyperkalaemia. The reduction in height of the microvilli could be a means for maintaining a normal CSF bulk secretion rate despite the increase in K+ transport required to maintain normal CSF [K+] in hyperkalaemia.