Lj. Teppema et al., Low-dose acetazolamide reduces CO2-O-2 stimulus interaction within the peripheral chemoreceptors in the anaesthetised cat, J PHYSL LON, 537(1), 2001, pp. 221-229
1. Using the technique of end-tidal CO2 forcing, we measured the effect of
the carbonic anhydrase inhibitor acetazolamide (4 mg kg(-1), I.V.) on the C
O2, sensitivities of the peripheral and central chemoreflex loops both duri
ng hyperoxia and hypoxia in 10 cats anaesthetised with alpha -chloralose-ur
ethane.
2. In the control situation, going from hyperoxia (arterial P-O2(P-a,P-O2)4
7.40 +/- 3.62 kPa, mean +/- S.D.) into moderate hypoxia (P-a,P-O2 8.02 +/-
0.30 kPa) led to an almost doubling of the peripheral CO2 sensitivity (S-p)
: a rise from 0.09 +/- 0.07 to 0.16 +/- 0.06 1 min(-1) kPa(-1). After aceta
zolamide, however, lowering the P-a,P-O2 from 46.95 +/- 5.19 to 8.02 +/- 0.
66 kPa did not result in a rise in S-p, indicating the absence of a CO2-O-2
stimulus interaction.
3. In hypoxia, acetazolamide reduced S-p from 0.16 +/- 0.06 to 0.07 +/- 0.0
5 1 min(-1) kPa(-1). In hyperoxia, however, the effect on S-p was much smal
ler (an insignificant reduction from 0.09 +/- 0.07 to 0.06 +/- 0.05 1 min(-
1) kPa(-1)).
4. Acetazolamide reduced both the hyperoxic and hypoxic sensitivities (S-p)
of the central chemoreflex loop: from 0.45 +/- 0.16 to 0.27 +/- 0.13 1 min
(-1) kPa(-1) and from 0.40 +/- 0.16 to 0.26 +/- 0.13 1 min(-1) kPa(-1), res
pectively. In hyperoxia, the apnoeic threshold B(X-intercept of the ventila
tory CO2 response curve) decreased from 2.91 +/- 0.57 to 0.78 +/- 1.9 kPa (
P = 0.005). fn hypoxia, B decreased from 1.59 +/- 1.22 to -0.70 +/- 2.99 kP
a (P = 0.03).
5. Because acetazolamide abolished the CO2-O-2 interaction, i.e. the expect
ed increase in S-p when going from hyperoxia into hypoxia, we conclude that
the agent has a direct inhibitory effect Oil the carotid bodies. The exact
mechanism toy which the agent exerts this effect will remain unclear until
more detailed information becomes available on the identity of the carboni
c anhydrase iso-enzymes within the carotid bodies and their precise subcell
ular distribution.