BACKGROUND AND PURPOSE L-tryptophan (L-W) is a precursor of the vasoconstrictor, 5-HT. However, acute administration of L-W ethyl ester (L-Wee) lowered blood pressure. The mechanism of action is unknown. This study compares the vascular effects of L-W and L-Wee in intact animals, isolated aortic rings, small mesenteric arteries (MA) and explores possible mechanisms by studies in vascular smooth muscle cells (VSMC) of MA.
EXPERIMENTAL APPROACH Effects of L-W or L-Wee (5–50 mg kg−1, i.v.) on mean arterial pressure (MAP) and heart rate (HR) were determined in male Sprague-Dawley rats. The effects of L-W and L-Wee on basal tone and of phenylephrine- or KCl-induced contractions of aortic and MA rings were assessed. Effects of L-Wee and L-W on voltage-operated calcium channels (VOCC) of VSMC of MA were also examined in patch-clamp studies.
KEY RESULTS Administration of L-Wee, but not L-W, evoked a rapid and transient dose-dependent decrease in MAP and HR. While both agents failed to affect basal tone, L-Wee decreased, concentration-dependently, (Imax > 98%) tension responses to phenylephrine and KCl in an endothelium-independent manner in aorta (IC50 2 mM) and MA (IC50 17 µM). L-Wee evoked concentration-dependent inhibition of VOCC currents (IC50 12 µM; Imax 90%) in VSMC of MA.
CONCLUSIONS AND IMPLICATIONS Esterified L-W (L-Wee), but not L-W, preferentially relaxed resistance vessels rather than conduit vessels. These effects were associated with blockade of VOCC by L-Wee. Our findings suggest that the falls in MAP and HR induced by L-Wee were due to blockade of VOCC by L-Wee.