ABSTRACT-A newly discovered antifungal agent, pramanicin, within the therapeutically effective concentration range (4 -100 mM), inhibits the tone of phenylephrine (PE)-precontracted dog carotid arterial rings in a concentration-dependent manner and leads to gradual development of relaxation. However, pramanicin had no effect on rings precontracted with 100 mM KCl or on endothelium-denuded rings. Thus, inhibition by pramanicin of PE-induced contraction was endothelium-dependent. Preincubation of 100 mM pramanicin with carotid arterial rings for 30 min did not significantly affect the concentration-contraction response to PE, but almost completely inhibited the endothelium-dependent relaxation response to subsequent addition of 3 m M carbachol or 100 m M pramanicin. This irreversible inhibition of endotheliumdependent relaxation, which is independent of extracellular Ca 2+, suggests possible endothelial cell damage by pramanicin. Pretreatment of the endothelium-intact vascular rings with L-N G -nitro-arginine (100 mM) inhibited the relaxation of PE-precontracted rings induced by 3 mM carbachol or 100 mM pramanicin, suggesting that the generation of nitric oxide (NO) in endothelial cells mediates the slow vascular relaxation induced by pramanicin. We conclude that pramanicin has little direct effect on the contractility of smooth muscle cells, but causes an initial slow endothelium-dependent, NO-mediated vascular relaxation. This is followed by a cytotoxic effect on vascular endothelial cells, eventually resulting in the loss of vasorelaxant function.Keywords: Antifungal agent, Pramanicin, Endothelial cell, Nitric oxide, Vascular smooth muscle Antifungal agents are widely used not only in human and veterinary medicine, but are also applied as disinfectants or preservatives in industrial environments where control of fungal growth is desirable. One class of antifungal compounds, having the general structure of a polar, highly functionalized head group and a simple fatty acid side chain (1 -5), act through a variety of mechanisms including the inhibition of sterol biosynthesis, sphingofungin formation and cell wall generation. However, the actions and toxicities of such agents in mammals have not been systematically studied. Indeed, some potent antimicrobial agents exhibit high levels of toxicity and have limited therapeutic value when used systemically, such as cyclopiazonic acid and thapsigargin. Since they have specific and potent inhibitory actions on sarcoplasmic recticulum Ca 2+ -ATPase, they are now widely used as pharmacological tools for studying the regulation of cell Ca 2+ (6 -9) Recently, a potent antifungal agent, pramanicin (see Fig. 1 for chemical structure), with a highly functionalized polar head group and a simple, long aliphatic side chain was isolated from a fungal fermentation (10) and biosynthesized (11). Pramanicin exerts a potent anti-growth effect on a wide range of fungal organisms with minimal inhibitory concentrations (MIC) of 20 -100 mM (10). Despite the suggestion of various clinical app...