ECTONUCLEOTIDASE ACTIVITY IN THE PERILYMPHATIC COMPARTMENT OF THE GUINEA-PIG COCHLEA

It has been clearly demonstrated that extracellular adenosine 5'-triph osphate (ATP) exerts a potent modulatory activity in the cochlea throu gh its interaction with P-2 purinoceptors. However, little is known re garding the metabolism of extracellular ATP in cochlear tissues via ec tonucleotidases. This study provides evidence for the presence of ecto nucleotidases in the perilymphatic compartment of the guinea pig cochl ea. Using microperfusion, ATP (500 mu M) was introduced into the cochl ear perilymph through the basal turn scala tympani, and effluent was c ollected from the basal turn scala vestibuli. Samples were subsequentl y analysed for the presence of adenine metabolites using high performa nce liquid chromatography (HPLC). Cell viability was evaluated by the activity of the intracellular enzyme lactate dehydrogenase (LDH) in th e perfusate. ATP was degraded to 122.8 +/- 9.9 mu M (25.0 +/- 5.8%) du ring the passage through the cochlear perilymphatic compartment. Break down of ATP resulted in the formation of adenosine 5'-diphosphate (41. 5 +/- 9.0 mu M), adenosine 5'-monophosphate (201.3 +/- 15.5 mu M), ade nosine (108.6 +/- 8.3) and inosine (15.0 +/- 1.5 mu M). The degradatio n of ATP was significantly (P < 0.001, Student's t-test) inhibited in the absence of divalent cations, Ca2+ and Mg2+ in the perfusate. In co ntrol experiments, no spontaneous degradation of ATP was observed in v itro. LDH activity was similar during ATP perfusions (2.9 +/- 0.9%) to control perfusions with artificial perilymph (4.2 +/- 1.0%) indicatin g well preserved cell integrity in the cochlear perilymphatic compartm ent. The degradation of extracellular ATP in the presence of intact ti ssues and its inhibition in the absence of divalent cations, a cofacto r for ectonucleotidases, provides evidence for ectonucleotidase activi ty in the perilymphatic fluid space of the cochlea.