Solubility of lead as lead (II) chloride in HEPES-Ringer and artificial seawater (Ca-ASW) solutions

Total dissolved Pb was measured in a number of commonly used physiological solutions by electrothermal atomization atomic absorption spectrometry (ETA-AAS). In HEPES-buffered solutions (pH 7.30) the concentration of total Pb in solution ("measured" Pb) was only 77% of nominal Pb up to 20 microM added Pb, where experiments were undertaken at room temperature (22 +/- 1 degree C). However, test solutions equilibrated at 37 +/- 1 degree C contained 99% of added Pb up to 2 microM. Above this nominal concentration, percentage recoveries dropped to approximately 72% at a nominal concentration of 20 microM. Tris-buffered artificial seawater (CaASW) solutions (pH 7.60) contained more dissolved Pb compared to HEPES-buffered solutions at 22 degrees C. However, increasing the calcium concentration in ASW appeared to increase precipitation of PbCl2. Concentration-corrected dose-response relationships were plotted from previously published data on the effects of Pb on voltage-activated calcium channels of rat dorsal root ganglion (DRG) cells and Aplysia neurons. The plots suggest that the inhibitory effects of Pb on rat DRG cells may prevail at concentrations of Pb even lower than reported previously when measurements were made at 22 degrees C. However, increasing the temperature to 37 degrees C resulted in closer agreement between measured and nominal dose-response curves. The measured dose-response curves for the Tris-buffered ASW solutions closely followed those of the nominal up to 200 microM Pb. In ASW solutions containing 40 mM calcium, PbCl2 was precipitated at Pb concentrations greater than 200 microM.