Endothelial defect mediates attenuated vasorelaxant response to isoproterenol after lung transplantation

We have previously demonstrated that pulmonary vasodilation in response to isoproterenol is attenuated in conscious dogs after left lung autotransplan tation (LLA). Our present goal was to identify the cellular mechanism respo nsible for this dysfunction. Size- and position-matched pulmonary arterial rings were isolated from the right (control) and left (LLA) lungs of 23 dog s 1-14 mo post-LLA. The rings were suspended for isometric tension recordin g and precontracted, and the vasorelaxant responses to activators of the be ta-adrenoreceptor signaling pathway were examined. With the endothelium int act the maximal pulmonary vasorelaxant response to isoproterenol was reduce d (P < 0.02) to 57 +/- 9% in LLA rings, compared with 87 +/- 3% in control rings. Responses to the G(s) protein activator cholera toxin were also atte nuated post-LLA, with the concentration-effect curve shifted to the right ( P < 0.01) and no change in the maximal response. In contrast, the vasorelax ant responses to forskolin (adenylyl cyclase activator) or dibutyryl cAMP w ere similar in endothelium-intact control and LLA rings. In endothelium-den uded rings the maximal vasorelaxant responses to isoproterenol were reduced (P < 0.01) to similar to 25% in both control and LLA rings. In denuded rin gs cholera toxin, forskolin, and dibutyryl cAMP caused 100% vasorelaxation, and the IC50 values for these agonists were similar in control and LLA rin gs. Isoproterenol increased (P < 0.05) tissue cAMP to the same extent in co ntrol and LLA rings with or without endothelium. In contrast, isoproterenol increased (P < 0.05) tissue cGMP only in endothelium-intact rings, and thi s effect was reduced (P < 0.05) similar to 50% in LLA rings compared with c ontrol. Oxypurinol (endothelial xanthine oxidase inhibitor) restored the pu lmonary vasorelaxant response to isoproterenol in endothelium-intact LLA ri ngs. Our results provide the first evidence that activation of the beta-adr enoreceptor signaling pathway in endothelium-intact pulmonary arterial ring s results in an increase in cGMP. Moreover, the attenuation in beta-adrenor eceptor-mediated pulmonary vasorelaxation post-LLA is due to inactivation o f nitric oxide by endothelium-derived superoxide anion.