The V2 receptor antagonist tolvaptan raises cytosolic calcium and prevents AQP2 trafficking and function: an in vitro and in vivo assessment.

Tolvaptan, a selective vasopressin V2 receptor antagonist, is a new generation diuretic. Its clinical efficacy is in principle due to impaired vasopressin-regulated water reabsorption via aquaporin-2 (AQP2). Nevertheless, no direct in vitro evidence that tolvaptan prevents AQP2-mediated water transport, nor that this pathway is targeted in vivo in patients with syndrome of inappropriate antidiuresis (SIAD) has been provided. The effects of tolvaptan on the vasopressin-cAMP/PKA signalling cascade were investigated in MDCK cells expressing endogenous V2R and in mouse kidney. In MDCK, tolvaptan prevented dDAVP-induced increase in ser256-AQP2 and osmotic water permeability. A similar effect on ser256-AQP2 was found in V1aR -/- mice, thus confirming the V2R selectively. Of note, calcium calibration in MDCK showed that tolvaptan per se caused calcium mobilization from the endoplasmic reticulum resulting in a significant increase in basal intracellular calcium. This effect was only observed in cells expressing the V2R, indicating that it requires the tolvaptan-V2R interaction. Consistent with this finding, tolvaptan partially reduced the increase in ser256-AQP2 and the water permeability in response to forskolin, a direct activator of adenylyl cyclase (AC), suggesting that the increase in intracellular calcium is associated with an inhibition of the calcium-inhibitable AC type VI. Furthermore, tolvaptan treatment reduced AQP2 excretion in two SIAD patients and normalized plasma sodium concentration. These data represent the first detailed demonstration of the central role of AQP2 blockade in the aquaretic effect of tolvaptan and underscore a novel effect in raising intracellular calcium that can be of significant clinical relevance.

Impact of heart failure on the incidence of contrast-induced nephropathy in patients with chronic kidney disease.

We randomized patients with chronic kidney disease (serum creatinine ≥ 1.5 mg/dl or glomerular filtration rate (GFR) <60 ml/min/1.73 m²) in a double-blind fashion to receive saline or sodium bicarbonate prior to and after cardiac or vascular angiography. The primary endpoint was contrast-induced nephropathy (CIN), defined as an increase in serum creatinine by 25% or by 0.5 mg/dl from baseline. Patients with congestive heart failure (CHF), cardiac ejection fraction (EF) <30%, or GFR < 20 ml/min/1.73 m² were excluded. The study was discontinued (after 142 patients were randomized) due to a low incidence of CIN (1.5%). We retrospectively identified all cases of CIN (n = 30) at our institution during the same time period to see if these patients differed from our trial sample. There was no difference in serum creatinine (1.7 ± 0.4 vs. 1.7 ± 0.6 mg/dL), GFR (42.7 ± 9.7 vs. 45.3 ± 3.2 ml/min), incidence of diabetes (51.8% vs. 63.3%), contrast volume (121.7 ± 63.8 vs. 122.7 ± 68.3 ml), ACE inhibitor or angiotensin receptor blocker use (54.0% vs 63.3%), and periprocedure diuretic use (33.1% vs 26.7%). On multivariate analysis, only a cardiac ejection fraction (EF) of less than 40% was significantly associated with CIN (odds ratio, 4.52; 95% confidence interval, 1.30-15.71; P = 0.02). In all, 22/30 patients (73.3%) who developed CIN had at least one or more characteristics that would have excluded their enrollment in our randomized trial including evidence of congestive heart failure (17/30 patients), EF less than 30% (9 patients), age greater than 85 years (2 patients), or advanced renal failure with a baseline GFR of less than 20 cc/min (1 patient). In summary, patients with CKD without evidence of CHF who receive adequate hydration appear to have a very low risk of CIN associated with angiography. A low EF (less than 40%) appeared to be the most significant risk factor for CIN in our population.