Noise-induced elevations of plasma endothelin (ET-3)

Protecting the auditory system with glucocorticoids

Glucocorticoids are hormones released following stress-related events and function to maintain homeostasis. Glucocorticoid receptors localize, among others, to hair cells, spiral ligament and spiral ganglion neurons. Glucocorticoid receptor-induced protection against acoustic trauma is found by i) pretreatment with glucocorticoid agonists; ii) acute restraint stress; and iii) sound conditioning. In contrast, glucocorticoid receptor antagonists exacerbate hearing loss. These findings have important clinical significance since synthetic glucocorticoids are commonly used to treat hearing loss. However, this treatment has limited success since hearing improvement is often not maintained once the treatment has ended, a fact that reduces the overall appeal for this treatment. It must be realized that despite the widespread use of glucocorticoids to treat hearing disorders, the molecular mechanisms underlying this treatment are not well characterized. This review will give insight into some physiological and biochemical mechanisms underlying glucocorticoid treatment for preventing hearing loss.

Changes in cochlear blood flow due to intra-arterial infusions of angiotensin II (3-8) (angiotensin IV) in guinea pigs

The effects of a newly discovered form of angiotensin, angiotensin IV (ANGIV), on cochlear blood flow (CBF) have been investigated utilizing the laser Doppler flowmetry (LDF) technique. Two specific questions were addressed: What are the effects of anterior inferior cerebellar artery infusions (AICA) of ANGIV on CBF and do angiotensin fragments other than ANGIV influence CBF in mature male and female guinea pigs. Infusions of ANGIV, and C-terminal shortened fragments were accomplished via micropipette into the AICA and changes in CBF were observed using LDF. The results demonstrated that 10 and 100 pmol/min doses of ANGIV increased CBF 22% and 75% (n = 6; P < 0.01) from baseline, respectively, with little change in mean arterial blood pressure (MAP). Pretreatment with the ANGIV antagonist divalanal-ANGIV (1 nmole/min) blocked increases in CBF due to infusions of 100 pmol/min of ANGIV. The infusion of the C-terminal shortened fragment ANGIV(1-5) and saline had no significant effect on either CBF or MAP. These results provide the evidence for a new subtype of the angiotensin receptor and indicate the likely role of circulating hormones in blood flow regulation in the inner ear.

Role of endogenous endothelin-1 in stress-induced gastric mucosal damage and acid secretion in rats

In rats subjected to 8 h water-immersion stress, gastric and duodenal mucosal hemorrhage and erosions were detected by macroscopic and histopathological examination. Moreover, plasma and gastric mucosal endothelin-1 (ET-1) levels rose appreciably in a time-related manner during water immersion, with a higher concentration detected in gastric mucosa. Thus, the percentage increases in plasma (gastric mucosal) ET-1, relative to basal levels, after 1, 4 and 8 h of water immersion were 86(172), 169(322) and 210(391)%, respectively. Likewise, a marked increase of gastric acid output was demonstrated 30 min after water immersion and lasted for 3 h. Pretreatment with the endothelin ET(A)/ET(B) receptor blocker, bosentan (30 and 100 mg kg(-1)), orally, dose-dependently antagonized gastric and duodenal mucosal damage as indicated by reductions in lesion lengths of 67 and 80%, respectively. Similar protective effects on mucosa were observed when bosentan was given by the intramuscular route. On the other hand, bosentan suppressed the rate of acid output by 30.3+/-2.1% in the stressed rats, but had no such effect in non-stressed animals. Taken together, results from this study implicate the endogenous peptide, ET-1, as a powerful mediator of stress-evoked gastro-duodenal mucosal damage and, moreover, present bosentan as a potential protector against hyperacidity and mucosal erosion that occur as a consequence of stress.

Endothelin-1 causes luminal constrictions in rat cochlear veins

Serum levels of the vasoconstrictor endothelin-1 (ET-1) increase in ischemia and systemic hypertension. We examined the effects of ET-1 on the cochlear microvasculature. Blood vessels were cast with methacrylate in adult male Wistar Kyoto rats, 10 min after intravenous injection of ET-1 (1.0 microg/kg); control animals received saline. Systemic blood pressure was recorded continuously. ET-1 increased the average systolic pressure by 18% and average diastolic pressure by 22% (P < 0.01). Scanning electron microscopy of cast vessels showed multiple circumscribed luminal constrictions on: (1) postcapillary venules; (2) collecting veins; (3) where collecting veins merged with the spiral modiolar vein; (4) on the spiral modiolar vein itself. Circumscribed constrictions in arteries were not observed. In ET-1 injected animals focal contractions of collecting veins reduced luminal width by 13.4% +/- 2.9 (P < 0.01). In control rats, constrictions on venous casts were minimal and constrictions on arteries were not observed. The present study shows that ET-1 is involved in local control of cochlear blood flow in that it focally contracts cochlear veins. It is suggested that this might be due to the high affinity of ET-1 receptors and/or the large number of ET-1 receptors on contractile cells in venous walls.