Membrane capacitance changes induced by thrombin and calcium in single endothelial cells cultured from human...

1. Vesicular secretion from single human umbilical vein endothelial cells (HUVECs) was monitored by changes in membrane capacitance (Cm). Secretion was evoked by dialysis with strongly buffered intracellular free Ca2+ concentrations ([Ca2+]i), flash photolysis of Ca2+-loaded DM-nitrophen or caged InsP3, or by thrombin. [Ca2+]i was monitored spectrofluorimetrically with furaptra. The results show that a large, slowly rising component of vesicular secretion requires prolonged exposure to high [Ca2+]i. 2. Cm increased during intracellular perfusion with [Ca2+] buffered in the range 1.0-20 microM. Changes in Cm comprised an initial slowly rising small component of 0.1-0.5 pF followed by a faster rising larger component of up to approximately 7 pF, seen when [Ca2+]i > 2 microM and which was maximal at 10-20 microM Ca2+. 3. Thrombin evoked rapid initial elevations of [Ca2+]i to a peak of 7.1 +/- 1.5 microM (mean +/- s.e. m., n = 5) that declined within approximately 20-30 s with thrombin present either to resting levels or to a maintained elevated level of 2.0 +/- 0.7 microM (mean +/- s.e.m., range 1.0-3.6 microM, n = 3). Transient [Ca2+]i rises were associated with small, slowly rising increases in Cm of 0.1-0.2 pF, that recovered to pre-application levels over 2-3 min. Maintained elevations of [Ca2+]i caused larger, faster-rising sustained increases in Cm to 1.14 +/- 0.12 pF (mean +/- s.e.m., n = 3). Separate specific enzyme-linked immunosorbent assay (ELISA) showed that 1.0 U ml-1 thrombin produced secretion of von Willebrand factor in HUVEC cultures. 4. Short-lived [Ca2+]i elevations with a peak of 3-25 microM and a duration of approximately 20 s generated by flash photolysis of caged InsP3 or DM-nitrophen produced either no net change in Cm, or small slow increases of approximately 0.1-0.6 pF at up to 5 fF s-1 that recovered to pre-flash levels over 2-3 min. 5. Maintained elevations of [Ca2+]i in the range 1-28 microM produced by flash photolysis of DM-nitrophen caused large increases in Cm, up to approximately 4 pF, corresponding to approximately 25-30 % of the initial cell Cm. The maximum rate of change of Cm was up to 50 fF s-1 at steady [Ca2+] up to 20 microM; Cm recovered towards pre-flash levels only when [Ca2+] had declined.