Effect of the endothelin receptor antagonist bosentan on chronic hypoxia-induced morphological and physiological changes in rat carotid body.

Previous experiments have repeatedly demonstrated that exposure to chronic hypoxia (CH) elicits remarkable structural changes and chemosensory hypersensitivity in the mammalian carotid body. Moreover, recent studies have shown that CH upregulates the neuroactive peptide, endothelin (ET), in oxygen-sensitive type I cells. The present study examines the possible involvement of ET in adaptation by concurrently exposing rats to hypobaric CH (B(P) = 380 Torr) and bosentan, a potent nonpeptide antagonist that blocks ET(A) and ET(B) receptors. Carotid body weight indicated that 14 days of CH induced organ enlargement, a response that was blunted in bosentan-treated rats (CH: 2.54 +/- 0.19-fold increase; CH plus bosentan: 1.92 +/- 0.14-fold increase; P < 0.05). Morphometric studies revealed that bosentan substantially eliminated CH-induced hyperplasia of chemosensory cell lobules as well as expansion of the connective tissue matrix. Vascular dilation associated with CH was not altered by the drug. In untreated animals exposed to 3 days of CH, expression of proliferating cell nuclear antigen (PCNA), a marker of mitosis, was increased in lobules of oxygen-sensitive type I cells and in extralobular vascular and connective tissue cells. The incidence of PCNA expression was significantly (P < 0.05) reduced in bosentan-treated animals. In vitro assessments of carotid sinus nerve (CSN) activity showed that enhancement of basal and hypoxia-evoked chemosensory activity following 9 days of CH was significantly (P < 0.001) blunted by concurrent treatment with bosentan. Collectively, our data are consistent with the hypothesis that CH-induced adaptation in the carotid body is at least partially mediated by signaling pathways involving ET receptors.