Thermal dependence of chemosensory activity in the carotid body superfused in vitro

We studied the relationship between chemosensory activity and temperature in carotid bodies excised from pentobarbitone-anesthetized cats, and superfused in vitro at flows between 0.4 and 2.0 ml/min with modified Tyrode's solution buffered with HEPES at pH 7.43. The basal frequencies of chemosensory discharges were recorded from the entire carotid nerve at different steady thermal conditions. For preparations superfused with saline equilibrated with 100% O₂, thermally dependent increases in frequency were observed, with significant differences between all nearby thermal stages separated by 0.5°C steps between 36.0 and 38.5°C. The larger gains were recorded between higher temperatures at high flows, between mid temperatures at intermediate flows, and between lower temperatures at low flows. The critical temperature for the calculated maximal gain was directly correlated to superfusion flow. The basal frequencies were consistently elevated when switching to saline equilibrated with 20% O₂ and no significant differences in mean ranks were recorded between 36 and 37°C, as between 38 and 39°C, but frequencies at 36–37°C were significantly higher than those at 38–39°C. Brief rises in chemosensory discharges were evoked by injections of NaCN applied to carotid bodies superfused with saline equilibrated with 100% O₂. The least effective dose was lower at 40°C than at 37.5° or 35.0°C, but the reactivity and slope were not significantly different. It is concluded that the carotid body chemoreceptors fulfill the criteria for being considered as thermosensors, and that their frequency of discharges is thermally modulated within a range close to physiological body temperature.