Developmental expression of monocarboxylate transporter in the gerbil inner ear

The expression of H+-monocarboxylate cotransporters (MCTs) that facilitate cell uptake of lactate, pyruvate and other monocarboxylates was investigated in the adult and postnatally developing gerbil inner ear. In the mature cochlea, immunoreactive MCT1 was present in marginal cells of the stria vascularis and in type II, suprastrial and limbal fibrocytes. In the adult vestibular system, dark cells and a subpopulation of fibrocytes immediately underlying maculae and cristae stained strongly for MCT1. Satellite cells surrounding mature spiral and vestibular ganglia neurons also expressed MCT1. MCT1 immunoreactivity was present at birth in marginal and dark cells, at 8 days after birth in fibrocytes and at 12 days after birth in satellite cells, and coincided precisely with the developmental expression of Na,K-ATPase in these sites. The coexpression of MCT1 and Na,K-ATPase in these cell types points to MCT1 as an important source of energy to drive inner ear Na,K-ATPase activity. In the adult inner ear, MCT2 was detectable only in tectal cells of the cochlea and supporting cells of the crista ampullaris. Immunostaining was first observed at 16 days after birth in tectal and at 20 days after birth in supporting cells, and at the same time immunoreactive aquaporin 4 appeared in these cells. The coexpression of MCT2 and aquaporin 4 suggests a possible role for MCT2 in regulating transcellular water movement. Because MCT2 facilitates the transport of acidic intermediates, its biological significance also could relate to modulation of cell pH and volume. Maintenance of the inner ear's unique ion and fluid gradients is essential to normal hearing and balance and requires the expenditure of large amounts of energy. The cellular distribution of MCT1 and MCT2 points to their participation in generating these electrochemical gradients and their potential involvement in sensory deficits associated with various inner ear disorders.