The role of nitric oxide and its redox variants in the regulation of vascular smooth muscle tone

The role of nitric oxide (NO) and its redox variant nitroxyl (HNO) in the regulation of vascular smooth muscle tone was investigated in isolated rat mesenteric arteries, using a combination of wire myography, electrophysiology and immunohistochemistry. The suitability of the HNO donor isopropylamine NONOate (IPA/NO) as an alternative to Angeli's (AS) salt in organ bath studies was examined. Previous studies of IPA/NO have been primarily in vivo in nature. IPA/NO had a similar profile to AS for both vasorelaxation and hyperpolarization of rat mesenteric arteries and was therefore a suitable alternative to AS. In addition, the presence of a sGC/cGMP-independent pathway, through which HNO exerted a significant vasorelaxatory action, was confirmed together with the importance of K+ channels as the main component of this secondary pathway. Repeated applications of either AS or IPA/NO did not effect relaxation produced by these donors, but the hyperpolarization was significantly reduced. Following the characterization of IPA/NO, the potential role of the HNO-TRPA1- CGRP pathway was investigated in isolated rat mesenteric arteries. The presence of TRPA1 was confirmed in perivascular nerves' using immunohistochemistry and a role in hyperpolarization of vascular smooth muscle cells was demonstrated. However, neither the TRPA1 antagonist HC030031 (50 μM), nor depletion of CGRP affected vasorelaxation, suggesting the HNO-TRPA1-CGRP pathway may not be physiologically important. Finally, the role of HNO and NO in the regulation of vasomotion was examined. Addition of the NO synthase inhibitor L-NAME (100 μM) unveiled rapid, depolarizing spikes in the membrane potential of vascular smooth muscle cells. However, the complexity of membrane potential recordings in constricted arteries prevented a conclusive determination of which redox variant of NO is responsible for supressing these spikes. Tension studies suggest that the dominant redox variant involved in regulation of vasomotion is NO, but this remains to be confirmed. Together these data confirm a role for NO and HNO as important regulators of vascular tone and as such they both provide potential as therapeutic agents in the treatment of cardiovascular disorders such as hypertension.