Uptake of cicletanine by vascular smooth muscle

Abstract

Cicletanine is an antihypertensive agent with vasorelaxant/natriuretic properties. The mechanism of vasorelaxation by cicletanine is unknown and may be related to effects on prostacyclin synthesis or cGMP phosphodiesterase inhibition. Direct links of vasorelaxation to biochemical mechanisms have been hampered by an approximate tenfold difference in potency between intact tissue and isolated enzyme systems. Therefore, the purpose of this study was to determine the relationship between cicletanine concentrations in vascular smooth muscle and in tissue bath superfusates. A high-performance liquid chromatography (HPLC) method was developed that allowed for detection of 1–2 ng of cicletanine in vascular tissue. Incubation of denuded aortic smooth muscle rings from spontaneously hypertensive rats with vasorelaxant concentrations (10–100 μM) of cicletanine resulted in a seven- to 12-fold accumulation of cicletanine in the tissue relative to the tissue bath. Uptake was maximal after 15 min of incubation and was maintained for at least 4 hr in the presence of continuous exposure to cicletanine. Uptake was not stereoselective, since both enantiomers of cicletanine were concentrated to the same extent and at a similar rate. In further studies, the rate of efflux of cicletanine from aortic rings was determined to be roughly the same as the rate of uptake, with a T1/2 of 3 min. These studies demonstrate that cicletanine can be concentrated within vascular smooth muscle to concentrations that are seven- to 12-fold higher than those in a tissue bath. The mechanism responsible for this effect is not known but apparently is not related to differences in the rate of uptake/rate of efflux. These data further suggest that there may be a link between clinically attainable plasma concentrations of cicletanine (roughly 12–80 μM) and vasorelaxation.