Anoxia differentially modulates multiple K + currents and depolarizes neonatal rat adrenal chromaffin cells

1 Using perforated‐patch, whole cell recording, we investigated the membrane mechanisms underlying O 2 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. 2 Under voltage clamp, low P O2 (0–15 mmHg) caused a graded and reversible suppression in macroscopic outward K + current. The suppression during anoxia ( P O2 = 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, Ca 2+ ‐dependent K + current, I K(CaO2) ; (ii) suppression of a Ca 2+ ‐insensitive, delayed rectifier type K + current, I K(VO2) ; (iii) activation of a glibenclamide‐ (and Ca 2+ )‐sensitive current, I K(ATP) . 3 During normoxia ( P O2 = 150 mmHg), application of pinacidil (100 μ m ), an ATP‐sensitive potassium channel (K ATP ) activator, increased outward current density by 45.0 ± 7.0 pA pF −1 (step from −60 to + 30 mV), whereas the K ATP 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 I K(ATP) by anoxia appears to reduce the overall K + current suppression attributable to the combined effects of I K(CaO2) and I K(VO2) . 4 Pharmacological tests revealed that I K(CaO2) was carried predominantly by maxi‐K + or BK potassium channels, sensitive to 50–100 n m iberiotoxin; 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. I K(CaO2) , I K(VO2) and I K(ATP) . 5 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, Cd 2+ , 4‐aminopyridine or charybdotoxin, suggesting the presence of anoxia‐sensitive mechanisms additionalto those revealed under voltage clamp. In Na + /Ca 2+ ‐free solutions, the membrane hyperpolarized, though at least a portion of the anoxia‐induced depolarization persisted. 6 In the presence of glibenclamide, the anoxia‐induced depolarization increased significantly to ∼25 mV, suggesting that activation of K ATP channels may function to attenuate the anoxia‐induced depolarization or receptor potential.