Therapeutic effect of mesenchymal human stem cells transplantation (MSCT) has been evaluated in a whole-body irradiated (6 Gy) rats. Experimental design of the study comprised large conductance Ca²⁺-dependent K⁺ channels (BKCa) activity measurements in aortic smooth muscle cells using patch clamp technique in whole-cell modification, non-invasive systolic arterial blood pressure measurement and simultaneous measurement of contractile force and [Ca²⁺]i. Bone marrow was aspirated in heparin from the sternum of healthy volunteers after informed consent (with 1%lidocaine as a local anaesthetic). Mesenchymal stem cells (MSC) were separated using negative selection procedure with monoclonal antibodies (Human RosetteSep Mesenchymal Stem Cell Enrichment Cocktail, StemCell Inc.). The isolated MSC after Ficoll-Hypaque centrifugation were resuspended in MesenCult medium (StemCell Inc.) supplemented with appropriate Mesenchymal Stem Cell Stimulatory Supplements and cultivated 20 – 32 days in the same medium with additional recombinant human growth factors: SDF-1a, EGF, and PDGF-AA (CHO-grade). After two passages MSC were transplanted intravenously to irradiated rats on the 7th day of post-irradiation in a single dose of 16-20×106 cells per rat. Whole-body irradiation produced a decrease of BKCa activity in aortic myocytes. This was paralleled by a reduction of the NO-dependent ACh-induced vascular relaxation and arterial hypertension development. Thus, the vasorelaxing force of BKCa was diminished in irradiated myocytes. Simultaneous measurements of contractile force and [Ca²⁺]i showed that myofilament Ca²⁺ sensitivity defined as the ratio of force change to [Ca²⁺]i significantly increased following irradiation. MSCT effectively restored outward currents (from 13±1 to 24±1 pA/pF, P<0.05, n=12) mainly due to paxilline-sensitive BKCa component, and led to an increase in amplitude of maximal ACh-induced endothelium-dependent relaxation in irradiated vascular tissues from 43±3% to 87±6% (P<0.05, n=12). MSCT normalized myofilament Ca²⁺ sensitivity (from 0.068±0.007 to 0.030±0.004 mN/nM, P<0.05, n=12) and arterial blood pressure from 152±4 to 121±2 mm Hg (P< 0.05, n=12). The data obtained suggest that MSC demonstrate a clearly expressed therapeutic potential to normalize vascular abnormalities induced by ionized irradiation and appear to be worthwhile therapeutic approach in case of vascular malfunction induced by ionizing irradiation.