Cyclic GMP in Vascular Relaxation: Export Is of Similar Importance as Degradation

In the vascular system, cyclic GMP (cGMP) in smooth muscle cells plays an important role for blood vessel relaxation. Intracellular concentrations of cGMP are thought to be determined by the action of cGMP-generating guanylyl cyclases (sensitive to nitric oxide or natriuretic peptides) and cGMP-degrading phosphodiesterases (PDE1, PDE3, and PDE5). Because functionally relevant cGMP elevations are not accessible to conventional methods, we applied real-time imaging with a fluorescence resonance energy transfer (FRET)-based cGMP indicator to follow nitric oxide- and natriuretic peptide-induced cGMP signals in living smooth muscle cells and analyzed the contribution of the miscellaneous cGMP-generating and cGMP-degrading enzymes.

By comparison of cGMP signals in living smooth muscle cells and vascular relaxation of aortic strips in organ bath experiments, we show for the first time that FRET-based cGMP indicators permit the measurement of functionally relevant cGMP signals. PDE5 was the major cGMP phosphodiesterase responsible for reducing nitric oxide- and natriuretic peptide-induced cGMP signals. In contrast, PDE3-involved in the degradation of lower cGMP concentrations-displayed a preference for natriuretic peptide-stimulated cGMP. Unexpectedly, we found that cGMP is transported out of the cells by the ABC transporter multidrug resistance-associated protein 4 and this export turned out to be of similar importance for intracellular cGMP signals as degradation by PDE5. Functionally, inhibition of cGMP export enhanced vascular relaxation as much as inhibition of PDE5.

The findings indicate that cGMP export out of smooth muscle cells is a key player in the regulation of smooth muscle cGMP signals and blood vessel relaxation.