Phospholipid metabolism in the cochlea: differences between base and apex.

Phospholipid-derived second messenger systems are one of the primary means for the transduction of extracellular signals to intracellular effector sites. We have investigated the longitudinal distribution of phospholipid metabolism in the guinea pig cochlea because of increasing evidence that the apex and base process auditory signals differently. Phospholipid metabolism was assayed by measuring the incorporation of radioactive phosphate (32Pi) into lipids of the organ of Corti and the lateral wall tissues (stria vascularis and spiral ligament). 32P-labeling of total phospholipids was higher in the apex than the base, and individual phospholipids exhibited a tissue-specific base/apex distribution. Phosphatidylinositol was the most abundant of the labeled lipids in all tissues except the basal lateral wall, where phosphatidylinositol and phosphatidylcholine were labeled to a similar extent. Experiments on the availability of [32P]-ATP and other non-lipid substrates (inositol, choline, and cytidine) suggested that the base/apex distribution of phospholipid metabolism is based on differences in enzymatic activities. Additional evidence for this is an increased hydrolysis of phosphoinositides in the apex. The base/apex distribution of lipid metabolism suggests that physiological and pathological mechanisms involving phospholipids differ between the turns of the cochlea.