ELSEVIER Hearing Research 91 (1995) 79-86 I.IMIII IIl aKl.! Combined effects of adrenalectomy and noise exposure on compound action potentials, endocochlear potentials and endolymphatic potassium concentrations Yi Long Ma a,1 Kenneth J. Gerhardt b Lisa M. Curtis a Leonard P. Rybak d Craig Whitworth d, Kyle E. Rarey a,c,* a Department of Anatomy and Cell Biology, University of Florida, Gainesville, FL 32610, USA b Department of Communication Processes and Disorders, University of Florida, Gainesville, FL 32610, USA c Department ofOtolaryngology, Uniuersity of Florida, Gainesville, FL 32610, USA d Department of Surgery, School of Medicine, Southern Illinois UniL,ersity, Springfield, IL 62794, USA Received 27 December 1994; revised 12 May 1995; accepted 12 August 1995 Abstract The effects of removal of endogenous corticosteroids via bilateral adrenalectomy in combination with noise exposure (30 min at 100 dB) were determined by recording compound action potentials (CAP) and endocochlear potentials (EP), and by measuring potassium concentrations (K +) within the endolymph. Thirty-eight Long-Evans rats were divided into groups according to experimental treatments: adrenalectomy (ADX) or non-ADX and noise exposure or non-noise exposure. CAP thresholds, EP and K + values were subjected to repeated-measures analysis of variance with group and time as factors classifying the measurements. Noise exposure resulted in significant elevations of CAP thresholds in both the ADX and non-ADX animals, but had no effect on either EP or endolymphatic K + . Recovery was noted during all post-exposure measurement periods and was significantly faster for ADX animals. EP and K + did not change during or after noise exposure. ADX animals showed a non-significant reduction of EP and a statistically significant increase of K + during all measurement periods as compared to non-ADX animals. Keywords: Endocochlear potential; Potassium; Compound action potential; Noise exposure; Adrenalectomy I. Introduction The presence of Na,K-ATPase within the cochlea (Kuijpers and Bonting, 1969; Kuijpers, 1974; Kerr et al., 1982, 1986; Burnham and Stirling, 1984a, b; ten Cate et al., 1994), primarily in the stria vascularis, has been con- sidered to be an important factor in maintaining high K + concentration (K~ +) in the endolymph and a positive endo- cochlear potential (EP). Regulation of Na,K-ATPase in the inner ear, as well as in other systems, has been shown to be influenced by adrenal steroids and intracellular Na ÷ (Garg et al., 1981, 1985; Mujais et al., 1985; Rarey et al., * Corresponding author: Department of Anatomy and Cell Biology and Otolaryngology, University of Florida, Gainesville, FL 32610-0235, USA. Tel.: (904) 392-3420; Fax: (904) 392-3305. 1 On leave from Institute of Environmental Medicine, Tongji Medical University, Wuhan, Hubei, P.R. China. 0378-5955/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0378-5955(95)00172-7 1988). Removal of endogenous corticosteroids via bilateral adrenalectomy (ADX) leads to a significant reduction in Na,K-ATPase activity (Rarey et al., 1988, 1989). For example, Na,K-ATPase activity in stria vascularis tissues of adrenalectomized rats was reduced by 60% as compared to that of control rats (Rarey et al., 1988, 1989). Presum- ably, bilateral ADX should also produce decreased Ke + accompanied by decreased EP. However, Bismuth et al. reported an opposite result in 1993. In that study, EP and endocochlear Ke + were unchanged in the adrenalectomized rat. Intense noise exposure reportedly decreases Na,K- ATPase activity in the stria vascularis and reduces EP (Chen, 1991), although the effects of moderate and low- level noise exposure on Na,K-ATPase have not been re- ported. Theoretically, noise exposure stresses the structures within the cochlea. Thus, the combined effect of noise and ADX may result in greater changes in EP and K~ + . The present study was designed to determine if the effects of