Identification and quantitative analysis of the cell membrane MgィイD1[2+]ィエD1 transporter in cardiac and smooth muscles.

A fluorescent Mg-D12--D1 indicator，furaptra(mag-fura-2)，was loaded into smooth muscle cells of guinea-pig tenia cecum，and cytoplasmic Mg-D12-D1concentration([Mg-D12-D1]-D2i-D2)was measured。Lowering extracellular Na D 1 I D 1 concentration([Na I D 1 I D 1]I D 2 o I D 2)in the absence of Ca I D 12 I D 1 caused gradual elevation of[Mg I D 12 I D 1]I D 2 i I D 2，the rate was strongly dependent on[Na I D 1 I D 1]I D 2 o I D 2 with the maximal rate of rise of 0.16µM./s.We also measured[Mg D 12 E D 1]I D 2 i i d d./s.We also measured[Mg D 12 I D 1]I D 2 I D 2。[Mg I D 12 I D 1]I D 2 i I D 2 was elevated when cells were treated with ionomycin 10 mM extracellular Mg I D 12 I D 1([Mg I D 12 I D 1]I D 2 o I D 2)。The rat of rise of[Mg I D 12 I D 1]I D 2 i i D 2 was reduced in the higher[Na I D 1 I D 1]I D 2 o I D 2.The net efflux of Mg-D12--D1from the loaded cells was also markedly inhibited by the presence of 140 mM Na-D1-D1.These results suggest the exixtence of the Na-D1-dependent Mg D12-D1 transport(Na D1-Mg D1-Mg D12 D1exchange)in smooth muscle and cardiac muscle cells.The rate of Mg I D 12 I D 1 transport was estimated to be 0.026 pmol/cm I D 12 I D 1·s in tenia and 0.085 pmol/cm I D 12 I D 1·s in cardiac myocytes at 25℃.It is unlikely that the Na I D1 I D 1-Ca I D 12 I D 1 exchanger is mainly responsible for this Mg I D 12 I D 1 transport，because only the slight Na I D 1 I D 1-Mg I D 12 I D 1 exchange activity was observed in the CCL 39 cells overexpressing the Na I D 1 I D 1-Ca D 12 I D 1 exchanger.

A fluorescent MgィイD12+ィエD1 indicator, furaptra (mag-fura-2), was loaded into smooth muscle cells of guinea-pig tenia cecum, and cytoplasmic MgィイD12+ィエD1 concentration ([MgィイD12+ィエD1]ィイD2iィエD2) was measured. Lowering extracellular NaィイD1+ィエD1 concentration ([NaィイD1+ィエD1]ィイD2oィエD2)in the absence of CaィイD12+ィエD1 caused gradual elevation of [MgィイD12+ィエD1]ィイD2iィエD2,the rate was strongly dependent on [NaィイD1+ィエD1]ィイD2oィエD2 with the maximal rate of rise of 0.16μM./s.We also measured [MgィイD12+ィエD1]ィイD2iィエD2 in rat ventricular myocytes. [MgィイD12+ィエD1]ィイD2iィエD2 was elevated when cells were treated with ionomycin in 10 mM extracellular MgィイD12+ィエD1([MgィイD12+ィエD1]ィイD2oィエD2). The rat of rise of [MgィイD12+ィエD1]ィイD2iィエD2 was reduced in the higher [NaィイD1+ィエD1]ィイD2oィエD2. The net efflux of MgィイD12+ィエD1 from the loaded cells was also markedly inhibited by the presence of 140 mM NaィイD1+ィエD1.These results suggest the exixtence of the NaィイD1+ィエD1-dependent MgィイD12+ィエD1 transport (NaィイD1+ィエD1-MgィイD12+ィエD1 exchange) in smooth muscle and cardiac muscle cells. The rate of MgィイD12+ィエD1 transport was estimated to be 0.026 pmol/cmィイD12ィエD1・s in tenia and 0.085 pmol/cmィイD12ィエD1・ s in cardiac myocytes at 25℃. It is unlikely that the NaィイD1+ィエD1-CaィイD12+ィエD1 exchanger is mainly responsible for this MgィイD12+ィエD1 transport, because only the slight NaィイD1+ィエD1-MgィイD12+ィエD1 exchange activity was observed in the CCL39 cells overexpressing the NaィイD1+ィエD1-CaィイD12+ィエD1 exchanger.