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 mu 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 similar to 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, I-K(CaO2); (ii) suppression of a Ca2+-insensitive, delayed rectifier type K+ current, I-K(VO2); (iii) activation of a glibenclamide- (and Ca2+)-sensitive current, I-K(ATP). 3. During normoxia (P-O2 = 150 mmHg), application of pinacidil (100 mu M), an ATP-sensitive potassium channel 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 mu 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 nM iberiotoxin; this current also accounted for the major portion (similar to 60%) of the anoxic suppression of outward current. Tetraethylammonium (TEA; 10-20 mM) blocked all of the anoxia-sensitive KC 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 similar to-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 additional to those revealed under voltage clamp. In Na+/Ca2+-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 similar to-25 mV, suggesting that activation of K-ATP channels may function to attenuate the anoxia-induced depolarization or receptor potential.