Responses of single-unit carotid body chemoreceptors in adult rats

Abstract

1. Our goal was to describe the in situ responses in rats of single-unit carotid body chemoreceptors to changes in arterial PO2 and PCO2. We identified single-unit carotid chemoreceptor activity in male, adult Sprague-Dawley rats by their rapid responses to i.v. NaCN (20 microg) and transient (10 s) asphyxia. 2. Single-unit chemoreceptor responses to isocapnic changes in oxygenation within the arterial oxygen pressure range 34-114 mmHg were described by the power function: f(dis) = 74010(Pa,O2)-2.5; (r2 = 0.6), where f(dis) is the discharge frequency (spikes s-1), P(a,O2) is the arterial oxygen partial pressure (mmHg) and r2 is the correlation coefficient. 3. The responses to iso-oxic changes in CO2, assumed to be linear, had a slope of 0.089 spikes s-1 (mmHg Pa,CO2)-1 (r2 = 0.7). 4. We conclude that carotid body chemoreceptors in adult rats have responses to changes in Pa,O2 and Pa,CO2 similar to those of other species.

Figure 1. Effects of 10 s of asphyxia (A), and an intravenous bolus of NaCN (20 μg, B) upon a chemoreceptor unit

In A, the upper trace is the tracheal tidal pressure with lung inflation indicated by a downward deflection. Asphyxia was produced by turning the ventilator off, allowing the lungs to remain deflated.

Figure 2. The effects of three different levels of oxygenation upon a chemoreceptor unit

The P_a,CO2/pH_a values for A, B and C are 35 mmHg/7.3, 34 mmHg/7.3 and 43 mmHg/7.23, respectively. This unit differs from the one shown in Fig. 1. These records were made after the rat had been at the appropriate inspired oxygen fraction for at least 5 min. Arterial blood was sampled subsequently, during the 5th minute of a steady-state level of discharge.

Figure 3. Effect of different levels of oxygenation upon single-unit chemoreceptor activity

Each of the dashed lines connects the data points for a given unit (n= 20 units). The continuous line is the best fit (r²= 0.6) for all points and represents a function in which f_dis= 74010 (P_a,O2)^−2.5, where f_dis is the discharge frequency (spikes s⁻¹) and P_a,O2 is recorded in mmHg. r² is the correlation coefficient.

Figure 4. Effect of CO₂ upon single-unit chemoreceptor activity

The thick dashed line represents the best linear fit to these data (f_dis= 0.089(P_a,CO2)^-2.3; r²= 0.69, where f_dis is discharge frequency in spikes s⁻¹ and P_a,CO2 is in mmHg; n= 10). The mean P_a,O2 (89 mmHg) during hypercapnia was not significantly different from that (86 mmHg) at the lowest CO₂ level.