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 permeabiliz ation by alpha-toxin. 2 ANP decreased the cytosolic Ca2+ concentration ([Ca2+](i)) and tension during the contraction induced by a high exte rnal K+ solution, in a concentration-dependent manner. This ANP-induce d decrease in [Ca-2i](i) during the contraction induced by high K+ sol ution 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 wa s treated with thapsigargin, a selective Ca2+ pump inhibitor of the sa rcoplasmic 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 thoug h the elevation of tension induced by Ba2+ was strongly inhibited. 3 I n the absence of extracellular Ca2+, ANP inhibited the release of Ca2 from the intracellular store induced by noradrenaline (NA). 4 The [Ca 2+](i) (abscissa scale)-tension (ordinate scale) relationship observed during the contraction induced by various concentrations of high exte rnal K+ solution was shifted downwards by the addition of 10(-8) M ANP , indicating that, at any given [Ca2+](i), the tension generated by hi gh K+ solution was considerably inhibited by the additon of 10(-8) M A NP. The [Ca2+](i)-tension curve of the contraction obtained by the cum ulative application of external Ca2+ (0-3.75 mM) during depolarization with 118 mM K+ solution was shifted to the left by 3 x 10(-7) M NA. T his NA-induced [Ca2+](i)-tension relationship was shifted to the right by 10(-8) M ANP, indicating that the ANP-induced reduction of Ca2+-se nsitivity operates during the contraction induced by NA. 5 In alpha-to xin-permeabilized preparations, ANP induced relaxation of tissues prec ontracted with a mixture of 3x10(-7) M Ca2+, 10(-5) M guanosine 5'-tri phosphate (GTP) and 10(-6) M NA. Thus a component of ANP-induced relax ation took place by way of a reduction in the Ca2+ sensitivity of the myofilaments, independent of changes in [Ca2+](i). 6 These results ind icate 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 inhibiti on of agoinist-induced release of Ca2+ from the intracellular store; a nd (2) decreasing the Ca2+-sensitivity of the contractile elements.