Pentoxifylline inhibits agonist-induced vasoconstriction in vascular smooth muscle and spontaneous peristalsis in isolated ileum

Pentoxifylline (PTX) is currently used therapeutically as a tumor oxygenator where it been shown to increase tumor blood flow and potentiate ionizing radiation damage. The clinical benefits of PTX have been primarily attributed to its effect on the rheologic properties of whole blood, although there is speculation that the mechanism for PTX-induced increases in tumor oxygenation may be the direct result of reduced vascular resistance. Therefore, to address the issue of vascular (geometric) resistance directly, we examined the ability of PTX and its hydroxy metabolite, lisofylline (LF), to modulate phenylephrine (PE)-induced constriction in isolated rat tail arteries. PTX or LF significantly attenuated phenylphrine (PE)-induced vasoconstriction in a dose-dependent manner. The EC50 for LF and PTX were 336 and 466 microM, respectively. Gastrointestinal disturbances have been reported following oral ingestion of PTX. To clarify the mechanistic basis for this side effect we examined the potential of PTX to modulate spontaneous peristalsis in isolated rat ileum rings. PTX significantly attenuated the spontaneous contractions (oscillations) in a dose-dependent manner. In comparison to isolated rat arterial vessels, the ileum ring preparations were significantly more sensitive (eightfold) to the relaxing effects of PTX (EC50 58 microM). Our data suggest that PTX- or LF-induced changes in tumor blood flow may be the direct result of vascular smooth muscle relaxation. Furthermore, the gastrointestinal disturbances that have been reported in the literature may be a consequence of PTX-induced inhibition of gut peristalsis.