The stimulatory effect of hypoxia upon ventilatory CO₂ chemosensitivity can be blocked by ATP receptor antagonists (Al-Hashem & Kumar, 2001). To ascertain whether this effect was mediated through an action at peripheral chemoreceptors, we examined the effect of ATP upon carotid body CO₂ chemosensitivity in vitro.

Carotid bifurcations and their associated tissues were removed from adult male Wistar rats under halothane anaesthesia (2.5 % in O₂), superfused with bicarbonate-buffered saline and prepared for single fibre chemoreceptor afferent nerve recording (Pepper et al. 1995). All animals were killed humanely whilst under anaesthesia. Data are expressed as means ± S.E.M. and significance was taken as P < 0.05 using ANOVA with a post-hoc Sheffe F-test.

ATP (disodium salt) significantly increased chemo-receptor discharge in hyperoxic normocapnia in a dose-dependent manner at concentrations up to 300 µM (P < 0.001, n = 5). At a concentration of 100 µM, ATP increased chemodischarge from a control value of 0.29 ± 0.08 to 0.73 ± 0.04 Hz. This stimulatory effect was abolished by prior application of the P2 purinoceptor antagonist, pyridoxalphosphate-6-azophenyl-2Ì,4Ì-disulphonic acid (PPADS) at a dose of 30 µM. ATP significantly increased steady-state CO₂ sensitivity (derived from normoxic discharge at 40 and 80 mmHg P_CO2) from 0.017 ± 0.007 to 0.030 ± 0.009 Hz mmHg^-1 (P < 0.03, n = 5). Thus ATP sensitivity was more than doubled from 0.017 ± 0.003 Hz µM^-1 at 40 mmHg P_CO2 to 0.036 ± 0.006 Hz µM^-1 at 80 mmHg P_CO2 (P < 0.003). Discharge during ramp decreases in P_O2 (between ca 300-100 mmHg), at two steady-state levels of P_CO2 (40 and 80 mmHg), revealed that control CO₂ sensitivity increased non-linearly and significantly, as expected, with decreasing P_O2 from a minimum of 0.032 ± 0.005 Hz mmHg^-1 at 300 mmHg P_O2 to a maximum of 0.142 ± 0.050 Hz mmHg^-1 at 100 mmHg P_O2 (P < 0.001, n = 8). In contrast, in the presence of 30 µM PPADS, CO₂ sensitivity was abolished at all levels of P_O2 (P > 0.60, n = 8).

Our results confirm the chemoexcitatory effect of ATP upon carotid body chemodischarge (Jarish et al. 1952) and demonstrate mediation via P2 purinoceptors. We have shown that ATP and CO₂ exhibit a greater than additive interaction at the carotid body and suggest that this may occur in a manner similar to that proposed for central respiratory neurones (Thomas et al. 1999) whereby ATP receptor sensitivity is increased by reductions in extracellular pH (King et al. 1996). This peripheral action of ATP most likely accounts for our previous findings in vivo (Al-Hashem & Kumar, 2001).We acknowledge the support of The Wellcome Trust and King Khalid University, Saudia Arabia.

Al-Hashem, F. & Kumar, P. (2001). J. Physiol. 531.P, 96P.

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Pepper, D.R., Landauer, R.C. & Kumar, P. (1995). J. Physiol. 485, 531-541. abstract

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