PROTEIN-KINASE-G IS NOT ESSENTIAL TO NO-CGMP MODULATION OF BASAL TONEIN RAT PULMONARY CIRCULATION

Nitric oxide (NO) is important in modulating increased pulmonary vascu lar tone. Whereas in other systems it is believed that the action of N O is mediated through guanosine 3',5'-cyclic monophosphate (cGMP) and protein kinase G (PKG), the validity of this pathway in the pulmonary circulation has not been established. Using isolated salt-perfused nor motensive and hypertensive rat lungs, we studied the effects of the so luble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxali n-1-one (ODQ), and the PKG inhibitors, KT5823, Rp-8-pCPT-cGMPS, and et hylamino)ethyl]-5-isoquinolinesulfonamide](H-8), on pulmonary vascular resistance. In isolated normotensive lungs, ODQ-mediated inhibition o f soluble guanylyl cyclase augmented hypoxic pulmonary vasoconstructio n, whereas the PKG inhibitors had no effect. Despite the marked differ ences in the physiological effect, ODQ and Rp-8-pCPT-cGMPS inhibited P KG activity to a similar degree as determined by a back-phosphorylatio n assay showing decreased PKG-mediated phosphorylation of serine 1755 on the D-myo-inositol 1,4,5-trisphosphate receptor. In hypertensive lu ngs, inhibition of soluble guanylyl cyclase by ODQ increased perfusion pressure by 101 '' 20% (P < 0.05), an increase similar to that seen w ith inhibition of NO synthase (NOS), confirming an essential role for cGMP. In contrast, KT5823, Rp-8-pCPT-cGMPS, and H-8 (used in doses 5- to 100-fold in excess of their reported inhibitory concentrations for PKG) caused only a small increase in baseline perfusion pressure (14 /- 2%, P = not significant from vehicle control), Effectiveness of PKG inhibition in the hypertensive lungs was also confirmed with the back -phosphorylation assay. These studies suggest that whereas NO-mediated modulation of vascular tone in the normotensive and hypertensive pulm onary circulation is dependent on cGMP formation, activation of PKG ma y not be essential.