Vascular actions of 'caged' phenylephrine analogs depend on the structure and site of attachment of the 2-nitrobenzyl group.

In the experiments presented in this article, the effects of four caged analogs of the alpha 1-adrenergic agonist phenylephrine (PE) on the properties of small (100-200 microns outer diameter), isolated rat mesenteric arteries were compared. The four caged PE analogs contained either an unsubstituted (analogs I and II) or an alpha-carboxy substituted (analogs III and IV) 2-nitrobenzyl group attached to the phenolic oxygen atom (O-linked; analogs II and IV) or to the amino group (N-linked; analogs I and III) of PE. The structure of each caged PE analog was confirmed by UV, IR and 1H NMR spectral analysis. For physiological experiments, photolysis of the caged PE analogs was accomplished with a Hi-Tech Scientific flashlamp, and vascular smooth muscle contraction was measured with a computer-based image analysis system. In some experiments, the fura-2 ratiometric technique was used to examine the effects of the caged PE analogs on intracellular Ca2+ levels. At concentration < or = 10(-6) M, none of the four analogs displayed measurable intrinsic vasoconstricting activity, that is, vasoconstrictions were only observed following light flashes, consistent with the release of free PE. At concentrations > or = 10(-5) M, however, both O-linked compounds (analogs II and IV) and the alpha-carboxy substituted N-linked caged PE (analog III) produced vasoconstriction prior to photolysis. In contrast, no intrinsic vasoconstricting activity was evident with the unsubstituted N-linked caged PE (analog I) at concentrations up to 300 microM (the highest concentration tested). At concentrations > or = 10 microM, the O-linked unsubstituted caged PE (analog II) also had intrinsic vasodilating activity and markedly attenuated vasoconstrictions and increases in intracellular Ca2+ produced by high KCl. Similar effects were observed with the N-linked caged PE analogs (I and III) at > or = 100 microM, whereas no measurable relaxations were seen with the alpha-carboxy O-linked caged PE analog (i.v.) at concentrations up to 300 microM (the highest concentration tested). Taken together, the results presented here demonstrate that the N-linked unsubstituted caged PE analog (I) can be used reliably at concentrations up to 100 microM and is, therefore, the analog of choice for physiological studies of alpha 1-receptor-mediated events.