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 cage d analogs of the alpha(1)-adrenergic agonist phenylephrine (PE) on the properties of small (100-200 mu m outer diameter), isolated rat mesen teric arteries were compared. The four caged PE analogs contained eith er an unsubstituted (analogs I and II) or an alpha-carboxy substituted (analogs III and IV) 2-nitrobenzyl group attached to the phenolic oxy gen 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 H-1 NMR spectral analysis. For physiological e xperiments, 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 expe riments, the fura-2 ratiometric technique was used to examine the effe cts of the caged PE analogs on intracellular Ca2+ levels, At concentra tion less than or equal to 10(-6) M, none of the four analogs displaye d measurable intrinsic vasoconstricting activity, that is, vasoconstri ctions were only observed following light flashes, consistent with the release of free PE. At concentrations greater than or equal to 10(-5) M, however, both O-linked compounds (analogs II and IV) and the alpha -carboxy substituted N-linked caged PE (analog III) produced vasoconst riction prior to photolysis. In contrast, no intrinsic vasoconstrictin g activity was evident with the unsubstituted N-linked caged PE (analo g I) at concentrations up to 300 mu M (the highest concentration teste d). At concentrations greater than or equal to 10 mu M, the O-linked u nsubstituted caged PE (analog II) also had intrinsic vasodilating acti vity and markedly attenuated vasoconstrictions and increases in intrac ellular Ca2+ produced by high KCl. Similar effects were observed with the N-linked caged PE analogs (I and III) at greater than or equal to 100 mu M, whereas no measurable relaxations were seen with the alpha-c arboxy O-linked caged PE analog (IV) at concentrations up to 300 mu M (the highest concentration tested). Taken together, the results presen ted here demonstrate that the N-linked unsubstituted caged PE analog ( I) can be used reliably at concentrations up to 100 mu M and is, there fore, the analog of choice for physiological studies of alpha(1)-recep tor-mediated events. (C) 1997 Elsevier Science S.A.