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

Flora Jow and Randy Numann

1. Cultured human capillary endothelial cells (HCEC) contain a large inward rectifier current, I_K(IR), that can be abolished by removing external K⁺ or by adding 50 µM Ba²⁺.

2. We show that I_K(IR) is responsible for maintaining the hyperpolarized potential (-60·6 ± 0·5 mV, n = 83) of HCEC. Blocking I_K(IR) with 50 µM 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 I_K(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 I_K(IR) by 6·9 ± 1·9 % (n = 4). The relationship between [Ca²⁺]_o and the percentage block of I_K(IR) gives a K_d value of 5·4 ± 0·6 mM at -120 mV.

4. I_K(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 µM 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 I_K(IR) occurred with a K_d 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 () of 0·44 from the outside.

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