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

Abstract

1. 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 μM glibenclamide the resting potential (-50 ± 4 mV, n = 18) was unaffected by 10 μM tetraethylammonium ions, 200 nM charybdotoxin, 200 nM iberiotoxin, 100 μM ouabain or 100 μM digitoxin. The negative potential was therefore maintained without ATP-sensitive (K(ATP)) or large conductance Ca2+-sensitive (BK(Ca)) K channels, and without the Na+-K+ATPase. 3. The resting potential, the delayed rectifier current (I(K(V))) and the A-like K+ current (I(K(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 I(K(V)), 10 mM causing depolarization from -44 mV to -22 mV with accompanying inhibition of I(K(V)) by 56% and I(K(A)) by 79%. In marked contrast, the effects of quinine on resting potential were poorly correlated with its effects on both I(K(A)) and I(K(V)). At 10 mM, quinine reduced I(K(V)) and I(K(A)) by 47% and 38%, respectively, with no change in the resting potential. At 100 μM, both currents were almost abolished while the resting potential was reduced < 50%. Raising the concentration to 1 mM had little further effect on I(K(A)) or I(K(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, I(K(N)). Thus, 100 μM quinine reduced both I(K(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 I(K(A)) and I(K(V)), but not I(K(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 I(K(V)) or I(K(A)), but it was accompanied by 60% inhibition of I(K(N)) at 0 mV. 8. Our findings suggest that the resting potential of rabbit pulmonary artery myocytes depends on I(K(N)), and that inhibition of I(K(N)) may mediate the depolarization induced by hypoxia.