Prolonged NO Treatment Does Not Result in cGMP‐dependent Protein Kinase Isoform Switching

Prolonged nitric oxide (NO) treatment in pulmonary artery (PA) results in NO hyporesponsiveness due to both reduced cGMP production after acute treatment with NO and decreased cGMP responsiveness. The reduction in cGMP responsiveness is associated with a significant reduction in cGMP‐dependent protein kinase (cGK) total and specific activity. The effect of prolonged NO treatment on cGKI isoform mRNA and protein expression was determined in this study with the hypothesis that reduced cGK activity was related in part to isoform switching from a more cGMP responsive cGKI isoform (alpha) to one that is less cGMP responsive (beta). Both cGKI isoforms are found in PA, with the beta isoform accounting for 70% of the cGKI mRNA and protein expressed. A similar predominance of cGKI beta was also observed in aortic and tracheal smooth muscle. The finding of different levels of mRNA and protein expression for the two cGKI isoforms in these tissues suggests that their expression is, in part, independently regulated. Total cGKI mRNA and protein expression are both reduced by prolonged NO treatment without, however, any change in the relative amounts of the two cGKI isoforms. The reduction in cGKI activity associated with prolonged NO treatment is therefore not explained by cGKI isoform switching. Since NO‐mediated changes in cGKI expression are indistinguishable, other factors must account for the differential expression of cGKI isoforms in smooth muscle. This work was supported by NIH R01 69968.