The mechanism of relaxation induced by atrial natriuretic peptide in the porcine renal artery

1 The mechanisms underlying the relaxation of the porcine renal artery induced by atrial natriuretic peptide (ANP) were investigated, using front‐surface fluorimetry with fura‐2 and receptor‐coupled permeabilization by α‐toxin. 2 ANP decreased the cytosolic Ca2+ concentration ([Ca2+]i) and tension during the contraction induced by a high external K+ solution, in a concentration‐dependent manner. This ANP‐induced decrease in [Ca2+]i during the contraction induced by high K+ solution was composed of two phases, an initial rapid phase, followed by a maintenance phase. The initial rapid decrease in [Ca2+]i, but not the maintained decrease in [Ca2+]i, was inhibited when the tissue was treated with thapsigargin, a selective Ca2+ pump inhibitor of the sarcoplasmic reticulum. When the tissues were treated with thapsigargin and external Ca2+ was replaced by Ba2+, which cannot be transported by the Ca2+ pump, ANP did not induce a decrease in [Ba2+]i, even though the elevation of tension induced by Ba2+ was strongly inhibited. 3 In the absence of extracellular Ca2+, ANP inhibited the release of Ca2+ from the intracellular store induced by noradrenaline (NA). 4 The [Ca2+]i (abscissa scale)‐tension (ordinate scale) relationship observed during the contraction induced by various concentrations of high external K+ solution was shifted downwards by the addition of 10−8 m ANP, indicating that, at any given [Ca2+]i, the tension generated by high K+ solution was considerably inhibited by the additon of 10‐8 m ANP. The [Ca2+]i‐tension curve of the contraction obtained by the cumulative application of external Ca2+ (0–3.75 mM) during depolarization with 118 mM K+ solution was shifted to the left by 3 × 10−7 m NA. This NA‐induced [Ca2+]i‐tension relationship was shifted to the right by 10−8 m ANP, indicating that the ANP‐induced reduction of Ca2+‐sensitivity operates during the contraction induced by NA. 5 In α‐toxin‐permeabilized preparations, ANP induced relaxation of tissues precontracted with a mixture of 3 × 10−7 m Ca2+, 10−5 m guanosine 5′‐triphosphate (GTP) and 10−6 m NA. Thus a component of ANP‐induced relaxation took place by way of a reduction in the Ca2+ sensitivity of the myofilaments, independent of changes in [Ca2+]i. 6 These results indicate that ANP induces relaxation of the porcine renal artery by: (1) reducing [Ca2+]i mainly via the activation of the Ca2+ pumps located on the sarcoplasmic reticulum and sarcolemma, as well as via inhibition of agoinist‐induced