Nicotine is implicated in smoking-related renovascular impairment and worsening of existing nephropathies. In the present study, we investigated whether nicotine aggravates the deleterious effect of the immunosuppressant drug cyclosporine A (CsA) on renal vasodilation induced by the beta-adrenoceptor agonist isoprenaline. Bolus isoprenaline (0.03-8.0 micromol) elicited dose-dependent vasodilation of phenylephrine-preconstricted perfused kidneys that was attenuated by infusion at 5 mL/min of nicotine (5 x 10(-4) mol/L) or CsA (2 micromol/L). Further, chronic administration of nicotine (0.4 mg/kg per day) or CsA (20 mg/kg per day) for 3 weeks reduced isoprenaline-induced vasodilation and elevated plasma urea and creatinine concentrations, effects that were magnified when both nicotine and CsA were administered concurrently. The role of endothelial and smooth muscle signalling in the acute nicotine/CsA renovascular interaction was investigated. Vasodilation caused by 0.25 micromol isoprenaline was attenuated by 6 micromol/L propranolol and 10 mmol/L tetraethylammonium (TEA), potentiated by 100 micromol/L hexamethonium and 7 micromol/L diclophenac, and virtually abolished in 80 mmol/L KCl-preconstricted tissues. N(G)-Nitro-L-arginine (L-NNA; 200 micromol/L), methylene blue (10 micromol/L), 3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propane-sulphonate (CHAPS; 0.2% for 30 s), nifedipine (750 nmol/L), atropine (1 micromol/L) and SQ22536 (an adenylyl cyclase inhibitor; 3 x 10(-5) mol/L) had no effect on isoprenaline responses. Nicotine (5 x 10(-4) mol/L) reduced isoprenaline-induced vasodilation and this effect was potentiated by concurrent CsA (2 micromol/L) infusion. Nicotine-induced impairment of the vasodilator response to isoprenaline was reduced by hexamethonium and potentiated by L-NNA, methylene blue, CHAPS and nifedipine. Alternatively, CsA exacerbation of the nicotine-isoprenaline interaction was abolished by propranolol, L-NNA, methylene blue, CHAPS, L-arginine, TEA and nifedipine. 5. In summary, nicotine and CsA produce additive impairment of kidney function and beta-adrenoceptor-mediated renovascular control, nitric oxide (NO)-cGMP signalling tonically restrains nicotine-induced impairment of isoprenaline vasodilation and the endothelial NO-K+ pathway modulates the aggravating effect of CsA on nicotine-isoprenaline interactions.