The Uptake And Membrane Transport Of Rare Alkali Metal Ions (Rb～+/Cs～+) In Human Erythrocyte

Rare alkali metal rubidium and cesium occurs in human body at trace level. It is shown that the abnormal behaviour of Rb⁺/Cs⁺ concentration is associated with many diseases, such as cardiovascular disease, senile dementia, and tumours. Rubdium and cesium play the same functional role of potassium in many aspects, there even exist competition between each other. Further study on the Rb⁺/Cs⁺ transmembrane mechanisms are urgently needed which can help understanding molecular interactions and of great significance for designing relevant medicines.It is observed under the microscope (x320) that the shape of human erythrocytes almost unchanged after incubation. The erythrocyte shrinkage appeared in a little extent when the extracellular concentration of Rb⁺/Cs⁺ at 90 mmol·L^-1.As a determination of the toxicity of Rb⁺/Cs⁺ for erythrocyte, hemolysis is examined by the release of hemoglobin (Hb) form erythrocyte at its characteristic absorption of 540 nm spectrophotometrically. The results show that rubidium and cesium hardly induce hemolysis, therefore they have a very weak toxicity for human erythrocyte.With the help of atomic absorption spectrometry, the uptake of Rb⁺/Cs⁺ by human erythrocyte and their transmembrane mechanisms were systematically investigated in vitro. The effects of extracellular cation concentration (3x10^-4molL^-1-9x10^-2 molL^-1), incubation temperature (33 ℃-43 ℃), incubation time (1.5h-18h), and medium pH (6.2-9.2) on the cellular uptake were studied. Inhibition experiments were carried out as follows: (1) 0.1 mmol·L^-1 ouabain was used for Na⁺/K⁺-ATPase, (2) 0.1 mmol·L^-1 ouabain with choline chloride were used for countertransport and "leak" mechanism, (3) 50 μmol·L^-1 DIDS and 10 mmol·L^-1 NaHCO₃ for anion exchanger, (4) 0.1 mmol·L^-1 Nifedipine for Ca²⁺ channel. Similar procedures were operated for Rb⁺.The results show that every mentioned factor has a positive effect on the uptake of Rb⁺/Cs⁺ by human erythrocyte. The Rb⁺ uptake followed a first-order kinetic process with the transmembrane rate constant k=5.025×10^-2 h^-1 Muring the beginning 2 hours. With the increase of incubation hours saturated kinetics characteristic appeared from both element incubation systems.Most of extracellular Rb⁺ and Cs⁺ were transported into erythrocytes byNa+/K+-ATPase, however, the transport of Rb+/Cs+ through Ca2+ channel is unavailable. Other pathways for Cs+ were found by further studies: (1) Na+/Cs+-countertransport systems;(2) ClVCsCCV exchanger pathway stimulated by bicarbonate through anion channel (band 3 protein);(3) "leak" process through pores in the membrane.The amount of Rb+/Cs+ which adhere to biomembrane was much less than transmembrane Rb+/Cs+. The membrane structure of human erythrocyte which analysed by laser raman spectrometry proved that both 6 mmol-L"1 and 90 mmol-L"1 Rb+/Cs+ can not induce damage of erythrocyte membrane.