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 mu M), a specific inhibitor of the Ca2+ entry mode of the exchanger, also decreased [Ca2+], (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 (Delta 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. (c) 2005 Elsevier Inc. All rights reserved.