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

Abstract

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⁺. 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). 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. 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⁺. 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 (δ) of 0.44 from the outside.