Regulation of the resting potential of rabbit pulmonary artery myocytes by a low threshold, O2‐sensing potassium current*

The contributions of specific K+ currents to the resting membrane potential of rabbit isolated, pulmonary artery myocytes, and their modulation by hypoxia, were investigated by use of the whole-cell, patch-clamp technique. 2. In the presence of 10 microM glibenclamide the resting potential (-50 +/- 4 mV, n = 18) was unaffected by 10 microM tetraethylammonium ions, 200 nM charybdotoxin, 200 nM iberiotoxin, 100 microM ouabain or 100 microM digitoxin. The negative potential was therefore maintained without ATP-sensitive (KATP) or large conductance Ca(2+)-sensitive (BKCa) K channels, and without the Na(+)-K+ ATPase. 3. The resting potential, the delayed rectifier current (IK(V)) and the A-like K+ current (IK(A)) were all reduced in a concentration-dependent manner by 4-aminopyridine (4-AP) and by quinine. 4. 4-AP was equally potent at reducing the resting potential and IK(V), 10 mM causing depolarization from -44 mV to -22 mV with accompanying inhibition of IK(V) by 56% and IK(A) by 79%. In marked contrast, the effects of quinine on resting potential were poorly correlated with its effects on both IK(A) and IK(V). At 10 mM, quinine reduced IK(V) and IK(A) by 47% and 38%, respectively, with no change in the resting potential. At 100 microM, both currents were almost abolished while the resting potential was reduced < 50%. Raising the concentration to 1 mM had little further effect on IK(A) or IK(V), but essentially abolished the resting potential. 5. Reduction of the resting potential by quinine was correlated with inhibition of a voltage-gated, low threshold, non-inactivating K+ current, IK(N). Thus, 100 microM quinine reduced both IK(N) and the resting potential by around 50%. 6. The resting membrane potential was the same whether measured after clamping the cell at -80 mV, or immediately after a prolonged period of depolarization at 0 mV, which inactivated IK(A) and IK(V), but not IK(N). 7. When exposed to a hypoxic solution, the O2 tension near the cell fell from 125 +/- 6 to 14 +/- 2 mmHg (n = 20), resulting in a slow depolarization of the myocyte membrane to -35 +/- 3 mV (n = 16). The depolarization occurred without a change in the amplitude of IK(V) or IK(A), but it was accompanied by 60% inhibition of IK(N) at 0 mV. 8. Our findings suggest that the resting potential of rabbit pulmonary artery myocytes depends on IK(N), and that inhibition of IK(N) may mediate the depolarization induced by hypoxia.