The role of the Na⁺-Ca²⁺ exchanger (NCX) in the mechanism of the isoprenaline (Iso)-induced vasorelaxation was investigated by simultaneously monitoring the intracellular Ca²⁺ concentration ([Ca²⁺]_i) and tension of fura-2-loaded medial strips of porcine coronary arteries. Normal physiological salt solution (PSS) contained 137.3 mM Na⁺ and 5.9 mM K⁺. During the sustained phase of contraction, Iso induced only a transient decrease in [Ca²⁺]_i when contraction was induced by depolarization with 118 mM K⁺ solution containing 25.2 mM Na⁺. When contraction was induced with 30 mM K⁺ in PSS containing 113.2 mM Na⁺, Iso induced a sustained decrease in [Ca²⁺]_i, whereas in contractions induced by 30 mM K⁺ in a low Na⁺ (25.2 mM Na⁺) PSS, Iso transiently decreased [Ca²⁺]_i. Replacement of Ca²⁺ with Ba²⁺ (which cannot be extruded by the Ca²⁺ pumps but can be extruded through the NCX) resulted in decreased [Ba²⁺]_i induced by Iso in normal but not in low Na⁺ PSS. On the other hand, Iso induced a sustained decrease in [Ca²⁺]_i when strips were pre-contracted by U46619, a thromboxane A₂ analogue, in PSS. Various types of K⁺ channel blockers (iberiotoxin, 4-aminopyridine, apamin or glibenclamide) or combinations of these blockers failed to completely inhibit the Iso-induced decreases in [Ca²⁺]_i and tension. However, Iso-induced sustained decreases in [Ca²⁺]_i during the contraction induced by U46619 were greatly inhibited in a low Na⁺ PSS. The Iso-induced decrease in tension during contraction by U46619 was greatly inhibited by 2â,4â-dichlorobenzamil, a forward- and reverse-mode NCX inhibitor, but not by ouabain, a selective inhibitor of Na⁺,K⁺-ATPase. These results indicate that the NCX is involved in the Iso-induced reduction of [Ca²⁺]_i and tension of the porcine coronary arterial smooth muscle.