Free cytosolic Ca2+ measured with Ca(2+)-selective electrodes and fura 2 in rat mesenteric resistance arteries.

Free cytosolic Ca2+ was measured with sub-micrometer-tip, double-barrelled, Ca(2+)-selective electrodes and fura 2 in rat mesenteric resistance arteries. The purpose was to establish intracellular free Ca2+ concentration ([Ca2+]i) values in resting and stimulated vessels. Isolated vessels were mounted for isometric force measurements. Measured with electrodes, mean [Ca2+]i was 115 and 708 nM under resting and norepinephrine-activated conditions, respectively. Fura 2 was calibrated intracellularly including determination of the intracellular dissociation constant (Kd) of the fura 2:Ca2+ complex. The intracellular Kd was 342 nM. With this value of Kd, fura 2 measurements of mean [Ca2+]i were 129 and 537 nM under resting and norepinephrine-activated conditions, respectively. The values measured with the two techniques were thus in good accordance.