The Na+/Ca2+ exchanger is active and working in the reverse mode in human umbilical artery smooth muscle cells

The data presented in this work suggest that in human umbilical artery (HUA) smooth muscle cells, the Na+/Ca2+ exchanger (NCX) is active and working in the reverse mode. This supposition is based on the following results: (i) microfluorimetry in HUA smooth muscle cells in situ showed that a Ca2+-free extracellular solution diminished intracellular Ca2+ ([Ca2+]i), and KB-R7943 (5 μM), a specific inhibitor of the Ca2+ entry mode of the exchanger, also decreased [Ca2+]i (40.6 ± 4.5% of Ca2+-free effect); (ii) KB-R7943 produced the relaxation of HUA rings (−24.7 ± 7.3 gF/gW, n = 8, p < 0.05); (iii) stimulation of the NCX by lowering extracellular Na+ increases basal [Ca2+]i proportionally to Na+ reduction (Δ fluorescence ratio = 0.593 ± 0.141 for Na+-free solution, n = 8) and HUA rings’ contraction (peak force = 181.5 ± 39.7 for 130 mM reduction, n = 8), both inhibited by KB-R7943 and a Ca2+-free extracellular solution. In conclusion, the NCX represents an important Ca2+ entry route in HUA smooth muscle cells.