Activity of aortic chemoreceptors in the anaesthetized rat

Abstract

1. It has been widely accepted that the rat aortic depressor nerve contains only baroreceptors. However, the experiments which have provided these negative data have employed whole aortic nerve recording. In the present study, the technical difficulties associated with recording single fibres in vivo, from the rat aortic nerve (diameter 25-50 microm), have been surmounted using a small tip, glass suction electrode technique. 2. Upon switching from normocapnic hyperoxia to hypercapnic hypoxia, irregularly firing units (n = 13) appeared and these were significantly excited by intravenous injections of sodium cyanide (20 microg) but not by rises in arterial blood pressure induced by methoxamine (an alpha1-adrenoreceptor agonist; 10 microg). Inhalation of 100 % oxygen rapidly and reversibly silenced, or profoundly reduced, ongoing activity. 3. Intravenous injection of phenylbiguanide (PBG; a 5-HT3 receptor agonist; 8 microg) strongly stimulated the chemoreceptors and was followed by a period of chemodepression (3-21 s). In contrast none of the single fibre baroreceptors recorded (n = 15) were excited by PBG but all significantly increased their discharge in response to the increases in arterial blood pressure associated with methoxamine and cyanide. Both the excitatory and inhibitory effects of PBG on the chemoreceptor fibres were abolished by ondansetron (a 5-HT3 receptor antagonist: 1 mg kg-1 i.v.; n = 5 animals) whilst the chemoexcitatory action of cyanide was preserved. 4. It is concluded that there are chemoreceptor afferents contained in the aortic nerve of the Sprague-Dawley rat. The 5-HT3 receptor appears not to be a pre-requisite for aortic body chemoexcitation.

Figure 1. Schematic drawing of the experimental preparation

At the arrow negative pressure is applied to obtain a high resistance seal between the suction electrode and the nerve fascicle of the right aortic nerve. SLN, superior laryngeal nerve.

Figure 2. Formaldehyde-induced fluorescence of the right aortic nerve

The arrow is pointing to one small intensely fluorescent (SIF) cell. Note the blood vessel seen entering the caudal pole of the glomus. Scale bar: 50 μm.

Figure 3. Single chemoreceptor fibre of the aortic nerve

Integrated activity of a single chemoreceptor afferent fibre following injection (10-20 μl) of phenylbiguanide (PBG, a 5-HT₃ receptor agonist), cyanide and methoxamine into the superior vena cava. (Blood gas variables: pH 7.43; P_a,O2 64 mmHg; P_a,CO2 51 mmHg; haematocrit 40 %.)

Figure 5. Latency to excitation of a chemoreceptor fibre

Raw and integrated activity of a single chemoreceptor afferent fibre following injection (20 μl) of phenylbiguanide (PBG, a 5-HT₃ receptor agonist) and cyanide into the superior vena cava. Note the difference in latency to the excitation of the fibre following injection of the two different chemicals. (Blood gas variables: pH 7.46; P_a,O2 94 mmHg; P_a,CO2 42 mmHg; haematocrit 46.5 %.)

Figure 6. P_O2-response curves of single (_•) and few fibre (○) aortic chemoreceptors

The F_I,O2 was set to 0.2, 0.15 and 0.1 and the P_a,CO2 was 41 ± 2.3 mmHg (mean ±s.d.) for all tests; n = 3 animals.

Figure 7. Cyanide excitation of aortic chemoreceptor is P_O2 sensitive

Left panel shows four superimposed sweeps of a single aortic chemoreceptor afferent recorded with a suction electrode. Right panel shows integrated activity of a single chemoreceptor (illustrated in the left panel) during hypercapnic hypoxia (pH 7.37, P_a,O2 55 mmHg; P_a,CO2 56 mmHg; haematocrit 34 %). The figure is a complete data file with two episodes where the data was paused. The spaces between the panels are temporally accurate with regard to the time bar. At the arrows CN indicates the injection times of 20 μg cyanide into the superior vena cava. The horizontal bar indicates the time of application of 100 % oxygen to the ventilator inlet. Note the subsequent silencing of the chemoreceptor and rise in blood pressure. During the pause in the recording the discharge was tracked on the oscilloscope and audio-amplifier, the fibre was silenced for almost two minutes before the second injection of CN. The response to cyanide is almost abolished but recovers after the animal is returned to original hypoxic conditions (third panel).

Figure 4. Single baroreceptor fibre of the aortic nerve

Integrated activity of a single baroreceptor afferent fibre following injection (10-20 μl) of phenylbiguanide (PBG, a 5-HT₃ receptor agonist), cyanide and methoxamine into the superior vena cava.