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.

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