DIVALENT ION BLOCK OF INWARD RECTIFIER CURRENT IN HUMAN CAPILLARY ENDOTHELIAL-CELLS AND EFFECTS ON RESTING MEMBRANE-POTENTIAL

1. Cultured human capillary endothelial cells (HCEC) contain a large i nward rectifier current, I-K(IR), that can be abolished by removing ex ternal K+ or by adding 50 mu M Ba2+. 2. We show that I-K(IR) is respon sible for maintaining the hyperpolarized potential (-60.6+/-0.5 mV, n = 83) of HCEC. Blocking I-K(IR) with 50 mu M Ba2+ shifts the zero curr ent 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 rel ationship 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 b lock of peak current at -120mV being 85.3+/-3.2% (n = 5) for 50 mu M B a2+, 62.9+/-2.2% (n = 5) for 5 mM Sr2+, 40.7+/-2.5% (n = 9) for 5 mM M g2+, 33.4+/-2.1% (n = 5) for 5 mM Mn2+ and 32.9+/-2.1% (n = 5) for 5 m M Ca2+. 5. The voltage dependence of Sr(2+)block of peak I-K(IR) occur red 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 (de lta) of 0.44 from the outside.