In common with other steroid hormones, aldosterone exerts rapid effects via non-genomic mechanisms in addition to its classical genomic actions. We have previously reported that, contrary to its well known genomic action on urinary sodium reabsorption and potassium secretion, aldosterone stimulates a rapid increase in sodium excretion in the anaesthetised rat (Rad et al. 2002). HCO₃ excretion and pH were not affected (Rad et al. 2003). The site of action in the nephron is unknown, but aldosterone has been shown to rapidly stimulate cAMP generation rapidly in rat inner medullary collecting ducts (Sheader et al. 2002) and human proximal tubule cells (Koppel et al. 2003). As the latter represents a major site of renal sodium transport, this study sought to determine whether aldosterone stimulates rapid changes in cAMP production in rat proximal convoluted tubules (PCT) in vitro.Male Sprague Dawley rats (n = 6) were anaesthetised (Intraval 100 mg kg^-1 sodium thiopentone) and the left kidney perfused with collagenase before animals were humanely killed. PCT segments were dissected from cortico-medullary slices and incubated at 37°C for 4 min with either vehicle or aldosterone at 10^-10 to 10^-6M. To study the potential involvement of the mineralocorticoid receptor (MR) in this procedure, a further group of tubules were preincubated with 1 mM spironolactone for 10 min at 37°C prior to incubation with aldosterone. cAMP concentrations were measured by radioimmunoassay (Sheader et al. 2002). Data are presented as mean ± SEM and are expressed as fmol cAMP mm tubule^-1 4 min^-1. Statistical analysis was by one-way ANOVA or unpaired t-test. Aldosterone at 10^-10 to 10^-6M had no effect on cAMP generation in PCT segments (Table 1). Spironolactone had no effect on basal cAMP generation and did not alter the lack of response to aldosterone (10^-10 to 10^-6M). Forskolin incubation, as a positive control, increased cAMP generation by 55% over the basal rate (basal, n = 7, 100 ± 8 vs. forskolin, n = 8, 155 ± 21 %, P< 0.05).Table 1. cAMP generation by isolated proximal tubules in the presence of aldosterone with or without 1 mM spironolactone (n = 5 tubules per group). This investigation has shown that aldosterone does not exert a rapid effect on cAMP generation in PCT cells under the current conditions. However, we cannot rule out an effect on PCT transport via an alternative signalling pathway.