Divalent ion block of inward rectifier current in human capillary endothelial cells and effects on resting membrane potential

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1. Cultured human capillary endothelial cells (HCEC) contain a large inward rectifier current, IK(IR), that can be abolished by removing external K+ or by adding 50 microM Ba2+. 2. We show that IK(IR) is responsible for maintaining the hyperpolarized potential (-60.6 +/- 0.5 mV, n = 83) of HCEC. Blocking IK(IR) with 50 microM Ba2+ shifts the zero current level and depolarizes HCEC by 36.5 +/- 1.3 mV (n = 4). 3. Increasing external Ca2+ concentration ([Ca2+]o) from 0.5 to 7 mM reduces the magnitude of IK(IR) by 36.5 +/- 2.3 % (n = 5) and depolarizes the cells by 10.33 +/- 2.4 mV (n = 3), whereas decreasing [Ca2+]o from 1.8 to 0.5 mM increases the amplitude of IK(IR) by 6.9 +/- 1.9 % (n = 4). The relationship between [Ca2+]o and the percentage block of IK(IR) gives a Kd value of 5.4 +/- 0.6 mM at -120 mV. 4. IK(IR) is also blocked by other divalent ions, with Ba2+ >> Sr2+ > Mg2+ > Mn2+ = Ca2+, and the block of peak current at -120 mV being 85.3 +/- 3.2 % (n = 5) for 50 microM Ba2+, 62.9 +/- 2.2 % (n = 5) for 5 mM Sr2+, 40.7 +/- 2.5 % (n = 9) for 5 mM Mg2+, 33.4 +/- 2.1 % (n = 5) for 5 mM Mn2+ and 32.9 +/- 2.1 % (n = 5) for 5 mM Ca2+. 5. The voltage dependence of Sr2+ block of peak IK(IR) occurred with a Kd value of 1.0 +/- 0.09 mM for -140 mV, 1.9 +/- 0.16 mM for -130 mV, 3.1 +/- 0.28 mM for -120 mV, 4.6 +/- 0.34 mM for -110 mV and 6.4 +/- 0.5 mM for -100 mV (n = 5), with a calculated electrical distance (delta) of 0.44 from the outside.