Measurement of cytoplasmic calcium in single microvessels with increased permeability

We investigated the hypothesis that an increase in cytoplasmic calcium ion concentration, [Ca²⁺](i), is one of the mechanisms responsible for increased microvessel permeability. We loaded the cells forming the walls of individually perfused microvessels in frog mesentery with fura-2 and measured [Ca²⁺](i) in the control state and after adding the Ca²⁺ ionophore ionomycin to the perfusate. [Ca²⁺](i) in the control state was 65±6 nM and increased to an initial peak of 285±29 nM after 1-3 min. After 4, 6, and 10 min, [Ca²⁺](i) was 199±18, 163±16, and 129±9 nM, respectively. [Ca²⁺](i) fell back to 77±7 nM after ionophore was removed. In similar experiments, hydraulic conductivity (L(p)) increased to a peak of 9.5 times control after 1-3 min, then fell to 2.0 times control after 6 min. L(p) remained elevated at this level for as long as ionophore was present in the perfusate. [Ca²⁺](i) modulates the initial and sustained phases of the permeability increase. Both processes depend on external Ca²⁺ influx. Our experiments provide the first direct measurement of [Ca²⁺](i) during a change in the permeability of an intact microvessel.