Pressor response and respiratory depression evoked by intravertebral arterial injections of hypoxic saline.

Cardio-respiratory effects of intravertebral arterial injections of hypoxic and/or hypercapnic solutions have been studied in anesthetized (dial-urethane), sino-aortic denervated, vagotomized, artificially ventilated cats. Injection of a small amount (0.8-1.5 ml) of CO2-saturated saline (hypercapnic and hypoxic loading) caused increases in arterial pressure (AP) and heart rate (HR) accompanied by an excitation of phrenic nerve activity (PNA). By contrast, injection of the same amount of N2-saturated saline (hypoxic loading) evoked dissociated cardio-respiratory effects, which were characterized by a transient depression of PNA accompanied by a small but clear increase in AP: the AP rise (18.5 +/- 6.7 mm Hg) was smaller than that (31.3 +/- 7.7 mm Hg) during the aforementioned hypercapnic and hypoxic loading with the CO2-saturated saline. The control injections (normocapnic loading) evoked negligible effects. These differences in cardio-respiratory effects suggest that there may exist hypoxia-sensitive neural substrates, independent of the central chemoreceptors, which would, directly or indirectly, influence the vasomotor center and/or the respiratory center.