Carotid chemoreceptor discharge responses to hypoxia and hypercapnia in normotensive and spontaneously hypertensive rats.

The carotid chemoreceptor discharge responses to hypoxia and hypercapnia were quantitatively compared between normotensive (NTR) and spontaneously hypertensive rats (SHR). For this purpose we recorded afferent mass discharges from the carotid sinus nerve (CSN) at various levels of end-tidal O2 and CO2 concentrations (FetO2, FetCO2 (%)) in the urethane-anesthetized, vagotomized and artificially ventilated rats. The CSN chemoreceptor discharge was evaluated by subtracting the small activity remaining in acute hyperoxia (chemoreceptor inactivation), which was estimated as baroreceptor in origin, from the large total CSN activity. The CSN chemoreceptor discharges at various levels of FetO2 or FetCO2 were expressed as the percent of control activity measured in normoxic and normocapnic conditions (FetO2, 15-16%; FetCO2, 4.5-5.1%). There was an exponential increase in the CSN chemoreceptor discharge as FetO2 was decreased from hyperoxic to various hypoxic levels (maximally 6%) at a maintained FetCO2 (normocapnia). The relationship between the CSN chemoreceptor discharge and the hypoxic stimulus was quantitatively assessed by the regression analysis using an exponential function. Exponential increases in the CSN chemoreceptor discharge by hypoxia and the parameters in the exponential function reflecting the sensitivity to hypoxia were significantly higher in the SHR than in the NTR, which indicated a high carotid chemoreceptor discharge response to hypoxia in the SHR. The CSN chemoreceptor discharge was increased linearly by increasing the FetCO2 from the normocapnic level up to about 10% at a maintained FetO2 (normoxia). Increases in discharge produced by severe hypercapnia were, however, much smaller than that caused by hypoxia. The slope of the CO2 stimulus-CSN chemoreceptor discharge response line was almost the same in NTR and SHR. The results demonstrated that the responsiveness of rat carotid chemoreceptor to hypoxia is augmented in the SHR. The role of carotid chemoreceptor afferents in ventilatory reflex responses to hypoxia and their alterations in the SHR are discussed.