1470 Aldosterone and pressor responses to torcetrapib MJ Forrest et al effects of torcetrapib were not selective for aldosterone release as plasma corticosterone levels were also increased in animals that received torcetrapib (Figure 4b) but not following administration of anacetrapib. Pressor responses to torcetrapib are absent in adrenalectomized rats Having observed an acute increase in plasma aldosterone and corticosterone in response to administration of torcetrapib, we sought to explore further the relationship between adrenal steroids and the acute hypertensive response to torcetrapib. In acutely adrenalectomized, anaesthetized rats, torcetrapib (5 mg kg 1, i.v. infused over 30 min) did not increase blood pressure, whereas administration of torcetrapib to sham- operated animals evoked a robust pressor response (Figure 5). Pressor responses to acute angiotensin II challenge (50 ng kg 1, i.v.) were equivalent in both adrenalectomized and sham-operated animals (data not shown). At the conclu- sion of torcetrapib infusion, plasma aldosterone levels were 488±101 pg mL 1 in sham-operated animals and 126±17 pg mL 1 (mean±s.e.mean, n ¼ 7) in acutely adrena- lectomized animals. The relationship between the acute pressor effect of torcetrapib and the release of aldosterone and corticosterone was explored by administration of the 3b- hydroxysteroid dehydrogenase inhibitor, trilostane. Torcetra- pib (5 mg kg 1, i.v.) evoked equivalent increases in blood pressure (Figure 6a) in animals administered trilostane or control animals administered vehicle (50% DMA:50% PEG200). However, trilostane completely inhibited the torce- trapib-induced increase in plasma aldosterone (Figure 6b) and plasma corticosterone (Figure 6c). Furthermore, administra- tion of exogenous aldosterone (1 and 10 mg kg 1) to rats did not increase blood pressure, whereas administration of torcetrapib (5 mg kg 1) to these same animals evoked a robust increase in blood pressure of 10–15 mm Hg (Figure 7). Figure 5 Adrenalectomy prevented the pressor response to torcetrapib. The effects of torcetrapib infusion (5 mg kg 1 20 min) or vehicle (2% DMSO:4% Cremophor EL:94% saline) on mean arterial pressure were evaluated in pentobarbital (55 mg kg 1, i.p.)- anaesthetized rats following acute adrenalectomy or in animals that were sham-operated. Data are expressed as the change in blood pressure (D mean BP) from the baseline period 20 min prior to giving test compounds. Each point represents the mean and vertical lines represent the s.e. for groups of six to nine animals. DMSO, dimethyl sulphoxide. British Journal of Pharmacology (2008) 154 1465–1473 Effects on isolated primary adrenocortical cells The mechanism of torcetrapib-evoked aldosterone release in vivo is not known. Possibilities include a direct secretago- gue action of torcetrapib or an indirect effect via known stimuli such as angiotensin II, adrenocorticotrophic hor- mone or potassium. To determine if torcetrapib has a direct effect on the adrenal gland, primary adrenocortical cells were isolated from rat adrenal glands and exposed in vitro to either torcetrapib (Figure 8a) or anacetrapib (Figure 8b). Angio- tensin II (0.1 mM) served as a positive control and produced a robust increase in aldosterone release into the medium. Torcetrapib also stimulated aldosterone release, whereas Figure 6 3b-hydroxysteroid dehydrogenase inhibition does not block the pressor effect of torcetrapib. (a) The effects of torcetrapib infusion (5 mg kg 1 30 min) or vehicle (2% DMSO:4% Cremophor EL:94% saline) on mean arterial pressure were evaluated in conscious rats 60 min after the administration of trilostane (10 mg kg 1, i.v.) or an equivalent volume (1 mL kg 1) of vehicle (50% dimethylaceta- mide:50% polyethylene glycol 200). Data are expressed as the change in blood pressure (D mean BP) from the baseline period 15– 30 min prior to giving test compounds. Each point represents the mean and vertical lines represent the s.e. for groups of eight animals. (b) Plasma levels of aldosterone were measured in a subgroup (n ¼ 4) of the animals in (a). Plasma samples were prepared from blood obtained at the conclusion of the infusion of torcetrapib or vehicle. (c) Plasma levels of corticosterone were measured in a subgroup (n ¼ 4) of the animals in (a). Plasma samples were prepared from blood obtained at the conclusion of the infusion of torcetrapib or vehicle. DMSO, dimethyl sulphoxide.