Recently a receptor for nicotinic acid named GPR A also
Recently, a receptor for nicotinic acid, named GPR109A (also known as HM74a in man or PUMA-G in the mouse) has been identified (Lorenzen et al., 2001, Lorenzen et al., 2002, Soga et al., 2003, Tunaru et al., 2003, Wise et al., 2003). This receptor, a member of the 7 transmembrane G-protein coupled family of receptors, mediates both the antilipolytic and flushing effects of nicotinic acid, and is an attractive target for the development of antilipolytic drugs (Benyo et al., 2005, Tunaru et al., 2005, Offermanns, 2006). However, in order for these agonists to be successful they would require an improved therapeutic index, i.e., a significant separation of the doses required for the antilipolytic effect from those that induce flushing. In order to test these novel agonists in vivo, models are required to study both lipolysis and flushing. The ideal model would allow the measurement of both responses simultaneously, and the therapeutic index calculated by comparing the doses effectively inhibiting lipolysis with the doses lacking the ability to induce vasodilatation (as a surrogate for flushing). To date, rats and dogs are the most frequently used species for studies of nicotinic acid-mediated inhibition of lipolysis (Lipson et al., 1971a, Lipson et al., 1971b, Lovisolo et al., 1981, Lovisolo et al., 1981, Pereira, 1967, Pereira and Holland, 1967) although mice have also been used (Tunaru et al., 2003). Of these three species, only the mouse has been optimized as a model for nicotinic acid-induced vasodilatation (Benyo et al., 2005, Cheng et al., 2006, Carballo-Jane et al., 2007). Rats have been shown to display modest increases in ear temperature after treatment with nicotinic Fmoc-Ser(tBu)-OH (Turenne, Seeman, & Ross, 2001), but no comprehensive studies have been performed to evaluate this species as a model for the flushing response to nicotinic acid observed in man. There are anecdotal reports of flushing being observed in dogs after administration of nicotinic acid (Pereira, 1967), but again, no optimized model has been developed. Given the usefulness of the rat and dog to test the effects of GPR109A agonists on lipolysis, we decided to develop optimized models of vasodilatation in these two species.
Here, we show that both rats and dogs respond to nicotinic acid with a vasodilatation response that is predominantly mediated by the release of the vasodilator PGD2. Acipimox, another GPR109A agonist (Lorenzen et al., 2002, Soga et al., 2003, Tunaru et al., 2003), has also been tested in these models. Using simultaneous measurements of plasma free fatty acids and dermal vasodilatation, we were able to calculate therapeutic indices for nicotinic acid and acipimox.
Discussion Our goal in undertaking the studies presented here was to develop animal models for the calculation of the therapeutic index of GPR109A agonists, using inhibition of lipolysis as a surrogate for the lipid effects of GPR109A agonists, and exploring animal models of nicotinic acid-induced flushing. Rats and dogs are accepted models to study the antilipolytic effects of nicotinic acid (Lipson et al., 1971a, Lipson et al., 1971b, Lovisolo et al., 1981, Lovisolo et al., 1981, Pereira, 1967, Pereira and Holland, 1967), and their potential as models for nicotinic acid-induced flushing was evaluated. To our knowledge, there was only one model described in the literature that evaluated the effects of nicotinic acid on rat vasodilatation, measuring temperature changes in the rat ear. This model detected 1 °C changes in ear temperature in animals exposed to relatively high doses of nicotinic acid (30 mg/kg) (Turenne et al., 2001). We were able to replicate this observation (data not shown), but the limited window of response suggested that ear temperature might not be the optimal model for our purpose. Laser Doppler has proven to be a highly sensitive method in the detection of nicotinic acid-induced vasodilatation in the mouse ear (Benyo et al., 2005, Cheng et al., 2006, Carballo-Jane et al., 2007), and we adapted the mouse model to measure perfusion changes in the rat ear. The window of response utilizing laser Doppler is considerably wider than that observed with temperature measurements, with an average increase in perfusion at the peak of ∼100% over basal values. As described for the human flushing response, rat vasodilatation in response to nicotinic acid is dependent on COX activity and mainly mediated by PGD2.