Effects of β-adrenoceptor activation on haemodynamics during hypoxic stress in rats

Effects of different nitric oxide synthases on pulmonary and systemic hemodynamics in hypoxic stress rat model

BACKGROUND

Under hypoxia, exaggerated compensatory responses may lead to acute mountain sickness. The excessive vasodilatory effect of nitric oxide (NO) can lower the hypoxic pulmonary vasoconstriction (HPV) and peripheral blood pressure. While NO is catalyzed by various nitric oxide synthase (NOS) isoforms, the regulatory roles of these types in the hemodynamics of pulmonary and systemic circulation in living hypoxic animals remain unclear. Therefore, this study aims to investigate the regulatory effects of different NOS isoforms on pulmonary and systemic circulation in hypoxic rats by employing selective NOS inhibitors and continuously monitoring hemodynamic parameters of both pulmonary and systemic circulation.

METHODS

Forty healthy male Sprague-Dawley (SD) rats were randomly divided into four groups: Control group (NG-nitro-D-arginine methyl ester, D-NAME), L-NAME group (non-selective NOS inhibitor, NG-nitro-L-arginine methyl ester), AG group (inducible NOS inhibitor group, aminoguanidine), and 7-NI group (neurological NOS inhibitor, 7-nitroindazole). Hemodynamic parameters of rats were monitored for 10 min after inhibitor administration and 5 min after induction of hypoxia [15% O2, 2200 m a. sl., 582 mmHg (76.5 kPa), Xining, China] using the real-time dynamic monitoring model for pulmonary and systemic circulation hemodynamics in vivo. Serum NO concentrations and blood gas analysis were measured.

RESULTS

Under normoxia, mean arterial pressure and total peripheral vascular resistance were increased, and ascending aortic blood flow and serum NO concentration were decreased in the L-NAME and AG groups. During hypoxia, pulmonary arterial pressure and pulmonary vascular resistance were significantly increased in the L-NAME and AG groups.

CONCLUSIONS

This compensatory mechanism activated by inducible NOS and endothelial NOS effectively counteracts the pulmonary hemodynamic changes induced by hypoxic stress. It plays a crucial role in alleviating hypoxia-induced pulmonary arterial hypertension.

Effects of CO2 on time-dependent changes in cardiorespiratory functions under sustained hypoxia

Hypercapnia in addition to hypoxia affects the mammalian cardiorespiratory system and has been suggested to exert its effects on cardiorespiratory function by slightly different mechanisms to hypoxia. In the present study, we examined cardiorespiratory changes in urethane-anesthetized rats under hypocapnic (Hypo, 10% O₂), isocapnic (Iso, 10% O₂ and 4% CO₂), and hypercapnic (Hyper, 10% O₂ and 8% CO₂) hypoxia for 2 h to clarify the effects of CO₂ on sustained hypoxia-induced cardiorespiratory responses. Respiratory frequency increased the most in Hypo and tidal volume in Hyper. Minute ventilation, a product of respiratory frequency and tidal volume, increased the most in the latter group. Regarding cardiovascular variables during the hypoxic exposure period, heart rate and mean blood pressure both markedly decreased in Hypo. However, decreases in these parameters were small in Iso, and both increased over the pre-exposure level in Hyper. The present results suggest that CO₂ interferes with the hypoxia-activated neural pathway via another pathway under sustained exposure to hypoxia.