Using perforated‐patch, whole cell recording, we investigated the membrane mechanisms underlying O2 chemosensitivity in neonatal rat adrenomedullary chromaffin cells (AMC) bathed in extracellular solution containing tetrodotoxin (TTX; 0.5–1 μ
m), with or without blockers of calcium entry.
Under voltage clamp, low
PO2 (0–15 mmHg) caused a graded and reversible suppression in macroscopic outward K+ current. The suppression during anoxia ( PO2= 0 mmHg) was ∼35 % (voltage step from −60 to +30 mV) and was due to a combination of several factors: (i) suppression of a cadmium‐sensitive, Ca2+‐dependent K+ current, IK(CaO2); (ii) suppression of a Ca2+‐insensitive, delayed rectifier type K+ current, IK(VO2); (iii) activationof a glibenclamide‐ (and Ca2+)‐sensitive current, IK(ATP).
During normoxia (
PO2= 150 mmHg), application of pinacidil (100 μ m), an ATP‐sensitive potassium channel (KATP) activator, increased outward current density by 45.0 ± 7.0 pA pF−1 (step from −60 to + 30 mV), whereas the KATP blocker glibenclamide (50 μ m) caused only a small suppression by 6.3 ± 4.0 pA pF−1. In contrast, during anoxia the presence of glibenclamide resulted in a substantial reduction in outward current density by 24.9 ± 7.9 pA pF−1, which far exceeded that seen in its absence. Thus, activation of IK(ATP) by anoxia appears to reduce the overall K+ current suppression attributable to the combined effects of IK(CaO2) and IK(VO2).
Pharmacological tests revealed that
IK(CaO2) was carried predominantly by maxi‐K+ or BK potassium channels, sensitive to 50–100 n miberiotoxin; this current also accounted for the major portion (∼60 %) of the anoxic suppression of outward current. Tetraethylammonium (TEA; 10–20 m m) blocked all of the anoxia‐sensitive K+ currents recorded under voltage clamp, i.e. IK(CaO2), IK(VO2) and IK(ATP).
Under current clamp, anoxia depolarized neonatal AMC by 10–15 mV from a resting potential of ∼‐55 mV. At least part of this depolarization persisted in the presence of either TEA, Cd2+, 4‐aminopyridine or charybdotoxin, suggesting the presence of anoxia‐sensitive mechanisms additionalto those revealed under voltage clamp. In Na+/Ca2+‐free solutions, the membrane hyperpolarized, though at least a portion of the anoxia‐induced depolarization persisted.
In the presence of glibenclamide, the anoxia‐induced depolarization increased significantly to ∼25 mV, suggesting that activation of KATP channels may function to attenuate the anoxia‐induced depolarization or receptor potential.