Failure of forskolin to elevate the endocochlear potential in kanamycin-poisoned animals

cAMP-stimulated Cl- secretion is increased by glucocorticoids and inhibited by bumetanide in semicircular canal duct epithelium

Background

The vestibular system controls the ion composition of its luminal fluid through several epithelial cell transport mechanisms under hormonal regulation. The semicircular canal duct (SCCD) epithelium has been shown to secrete Cl^- under β₂-adrenergic stimulation. In the current study, we sought to determine the ion transporters involved in Cl^- secretion and whether secretion is regulated by PKA and glucocorticoids.

Results

Short circuit current (I_sc) from rat SCCD epithelia demonstrated stimulation by forskolin (EC₅₀: 0.8 μM), 8-Br-cAMP (EC₅₀: 180 μM), 8-pCPT-cAMP (100 μM), IBMX (250 μM), and RO-20-1724 (100 μM). The PKA activator N6-BNZ-cAMP (0.1, 0.3 & 1 mM) also stimulated I_sc. Partial inhibition of stimulated I_sc individually by bumetanide (10 & 50 μM), and [(dihydroindenyl)oxy]alkanoic acid (DIOA, 100 μM) were additive and complete. Stimulated I_sc was also partially inhibited by CFTR_inh-172 (5 & 30 μM), flufenamic acid (5 μM) and diphenylamine-2,2^′-dicarboxylic acid (DPC; 1 mM). Native canals of CFTR^+/− mice showed a stimulation of I_sc from isoproterenol and forskolin+IBMX but not in the presence of both bumetanide and DIOA, while canals from CFTR^−/− mice had no responses. Nonetheless, CFTR^−/− mice showed no difference from CFTR^+/− mice in their ability to balance (rota-rod). Stimulated I_sc was greater after chronic incubation (24 hr) with the glucocorticoids dexamethasone (0.1 & 0.3 μM), prednisolone (0.3, 1 & 3 μM), hydrocortisone (0.01, 0.1 & 1 μM), and corticosterone (0.1 & 1 μM) and mineralocorticoid aldosterone (1 μM). Steroid action was blocked by mifepristone but not by spironolactone, indicating all the steroids activated the glucocorticoid, but not mineralocorticoid, receptor. Expression of transcripts for CFTR; for KCC1, KCC3a, KCC3b and KCC4, but not KCC2; for NKCC1 but not NKCC2 and for WNK1 but only very low WNK4 was determined.

Conclusions

These results are consistent with a model of Cl^- secretion whereby Cl^- is taken up across the basolateral membrane by a Na⁺-K⁺-2Cl^- cotransporter (NKCC) and potentially another transporter, is secreted across the apical membrane via a Cl^- channel, likely CFTR, and demonstrate the regulation of Cl^- secretion by protein kinase A and glucocorticoids.

The effect of forskolin on the sound-evoked potentials in the guinea pig cochlea

The effect of forskolin (FSK) on cochlear sound-evoked potentials was examined in the guinea pig. The perfusion of the scala vestibuli (SV) with FSK (2 x 10(-4) M) produced a significant increase in the amplitude of negative summating potential (- SP) with no change in cochlear microphonics (CM) amplitude, and a significant decrease in the amplitude of compound action potential (CAP) with a significant prolongation of N1 latency and a 20 dB CAP threshold elevation. The results lead us to speculate that FSK-induced changes may be involved in the transient formation of endolymphatic hydrops.

Comparison of endolymph cross-sectional area measured histologically with that measured in vivo with an ionic volume marker

In order to establish how endolymph volume is regulated, it is essential to be able to measure endolymph volume or cross-sectional area in vivo. We have developed methods to accomplish this by injecting the volume marker ion hexafluoroarsenate (AsF6) into endolymph by iontophoresis. For an injection at a constant rate, the endolymph concentration is inversely dependent on the cross-sectional area of the scala into which injection occurred. Marker concentrations were monitored by inserting ion-selective microelectrodes into endolymph near the injection site. In a previous study we quantified the degree of hydrops in animals following ablation of the endolymphatic sac. In the present study we validated the technique by comparing the endolymphatic cross-sectional area measured in vivo with AsF6 with that measured by established histologic procedures. The correlation between the two measures was good, with a coefficient of .903, although the area measured histologically was a little lower than that measured in vivo.

Effects of kanamycin ototoxicity and hair cell regeneration on the DC endocochlear potential in adult chickens

High doses of aminoglycoside antibiotics cause massive damage to the avian basilar papilla. The resulting functional loss could conceivably arise from the reduction in the DC endocochlear potential (EP) due to impairment of the tegmentum vasculosum (TV) or to shunting of current through the damaged sensory epithelium. To test this hypothesis, the EP was measured in adult chickens after destroying hair cells in the basal half of the cochlea with a high dose (400 mg/kg per day for 10 days) of kanamycin (KM). KM treatment caused an increase in the steady-state EP from +18.1 to +23.3 mV and a decrease in the magnitude of the negative EP from -42.0 to -19.2 mV. The EP showed almost no change between 1 and 2 days and 1 week post-KM treatment. After 4 weeks of recovery, most hair cells had regenerated; however, the steady-state EP was still elevated by 13% and the negative EP was depressed by 37%. These results suggest that functional loss as shown by the large reduction in cochlear microphonic (CM) and the elevated thresholds of compound action potential (CAP) following KM treatment is not due to a reduction in the EP but may arise from functional deficits in the hair cells and/or the auditory nerve.

Role of endolymphatic anion transport in forskolin-induced Cl- activity increase of scala media

To determine the role of anion transport in the forskolin-induced Cl- increase of scala media (SM), effects of forskolin on the EP (endocochlear potential) and Cl- activity (ACl) in SM were examined with double-barrelled Cl(-)-selective microelectrodes. The experiments were carried out on guinea pig cochleae, using a few anion transport inhibitors: IAA-94 for a Cl- channel blocker, bumetanide (BU) for an Na+/K+/2Cl- cotransport blocker, and SITS and DIDS for Cl-/HCO3- exchange blockers. The application of forskolin (200 microM) into scala vestibuli (SV) caused a 20 mEq increase of endolymphatic ACl and a 15 mV elevation of EP, and IAA-94 with forskolin completely abolished these responses. Although each application of BU, SITS or DIDS did not completely suppress EP elevation, the concurrent application of these inhibitors completely suppressed EP with endolymphatic ACl increase. The results indicate the involvement of Cl- channels, Na+/K+/2Cl- cotransport and Cl-/HCO3- exchange in forskolin-induced increase of ACl and EP. The role of adenylate cyclase activation and Cl- transport in endolymph homeostasis was discussed.