Endothelin in the central control of cardiovascular and respiratory functions

1. Exogenously administered endothelin (ET) modulates the activity of cardiovascular and respiratory neurons in the central nervous system (CNS) and, thus, affects arterial blood pressure (ABP) and ventilation. However, a physiological role(s) for endogenous ET in the CNS has not been elucidated. To address this question, we examined ABP and ventilation in mutant mice deficient in ET-1, ET(A) and ET(B) receptors and endothelin-converting enzyme-1, which were made by gene targeting. 2. Respiratory frequency and volume was measured in mice by whole body plethysmography when animals breathed normal room air and hypoxic and hypercapnic gas mixtures. A few days after respiratory measurements, a catheter was implanted into the femoral artery under halothane anaesthesia. On the following day, the ABP of awake mice was measured through the indwelling catheter and heart rate was calculated from the ABP signal. After 2 h ABP measurement, arterial blood was collected through the catheter and pH and the partial pressures of O₂ and CO₂ were measured by a blood gas analyser. 3. Compared with corresponding controls, the mean (±SEM) ABP in ET-1(+/-) and ET(B)-deficient mice was significantly higher (118±2 vs 106±3 mmHg for ET-1(+/-) (n = 22) and ET- 1(+/+) (n = 17) mice, respectively; 127±3 vs 109±4 mmHg for ET(B)(-/s) (n = 9) and ET(B)(+/s) (n = 9) mice, respectively; P < 0.05 for both). In ET-1(+/- ) mice, PCO₂ tended to be higher and PO₂ was significantly lower than corresponding values in ET-1(+/+) mice. Under resting conditions, there was no significant difference in respiratory parameters between mutants and their corresponding controls. However, reflex increases of ventilation to hypoxia and hypercapnia were significantly attenuated in ET-1(+/-), ET-1(-/-) and ETA(-/-) mice. 4. In another series of experiments in ET-1(+/-) mice, we found that sympathetic nerve activity (SNA) was augmented and reflex excitation of phrenic nerve activity (PNA) in response to hypoxia and hypercapnia was blunted. Attenuation of the reflex PNA response to hypercapnia was also observed in the medulla-spinal cord preparation from ET- 1(-/-) mice. 5. Elevation of ABP in ET(B)-deficient mice was most likely due to a peripheral mechanism, because SNA and respiratory reflexes were not different from those in control animals. 6. We conclude that endogenous ET-1 plays an important role in the central neural control of circulation and respiration and that ETA receptors mediate this mechanism.