ECTO-NUCLEOTIDASES TERMINATE PURINERGIC SIGNALING IN THE COCHLEAR ENDOLYMPHATIC COMPARTMENT

THERE is strong evidence for a purinergic signalling system in the inn er ear which regulates auditory sensitivity. This study describes the terminating mechanism for purinergic signalling in the cochlear endoly mphatic compartment via ecto-nucleotidases. Exogenous ATP was introduc ed into the scala media (SM) of the isolated, perfused guinea-pig coch lea, and the effluent was assayed for the adenine nucleotide metabolit es by reverse-phase HPLC. Tissue viability was confirmed by fluorescen ce imaging of cochlear tissues. Extracellular ATP degradation to adeno sine was Ca2+/Mg(2+)dependent, and was not affected by inhibitors of i ntracellular ATPases and nonspecific alkaline phosphatase. High azide concentration (5 mM) and suramin produced an inhibitory effect on ATP hydrolysis, consistent with inhibition of E-type ATPase activity. The V-max of ATP hydrolysis (2564 mu mol min(-1) SM-1) was indicative of h igh ecto-ATPase activity. Our results support the role of ecto-nucleot idases as a principal mechanism for termination of purinergic signalli ng within SM, a compartment of the cochlea showing considerable P2X re ceptor expression. (C) 1998 Rapid Science Ltd.