Normally your kidneys filter about 50 gallons of blood a day, remove such waste as salts and minerals, and concentrate it into urine. When you exercise or don't take in water, your blood "thickens" and instead of removing water, the kidney reverses the process, pulls in water from the body that would normally go into the urine, and puts it into the blood, thus maintaining water balance.
Various diseases disrupt this process and in one small population the kidneys keep dumping watered-down urine into the bladder, at the rate of about 5 gallons a day compared with the normal quart or so. Besides the obvious inconvenience, people with nephrogenic diabetes insipidus (NDI) are constantly thirsty and in danger of severe dehydration, electrolyte imbalance, and have other problems that may ultimately require a kidney transplant.
While the disease is rare, its cause is known, involving malfunctioning receptors (V2R) for the anti-diuretic hormone vasopressin. Based on their earlier work, researchers at Massachusetts General Hospital-Harvard Medical School, Boston, believed they might be able to bypass the V2R and alleviate NDI symptoms by activating the kidney's water channels, called aquaporins, through a different mechanism.
Their study, entitled "Stimulation of AQP2 membrane insertion in renal epithelial cells in vitro and in vivo by the cGMP phosphodiesterase inhibitor sildenafil citrate (Viagra)," is in the June issue of the American Journal of Physiology-Renal Physiology, published by the American Physiological Society. The research was conducted by Richard Bouley, Nuria Pastor-Soler, Ori Cohen, Margaret McLaughlin, Sylvie Breton and Dennis Brown at Massachusetts General Hospital (MGH) and Harvard Medical School, Boston.
Dennis Brown, director of the Program in Membrane Biology in the MGH Renal Unit, said "it's a target-trafficking problem. The aquaporins are in the kidney cells, but weren't getting the proper signals that begin the cascade of events leading to proper water balance." Some drugs could have the necessary effect, but their action would affect most cells in the body.
Richard Bouley, a postdoctoral student in Brown's lab, proposed bypassing the V2R which is activated through a signaling molecule called cyclic-AMP. Bouley thought the same effect might be achieved by increasing another signaling molecule, cyclic-GMP (cGMP) which would accumulate aquaporins on the cell membrane, thus stimulating water permeability.
In vitro tests showed that cGMP elevation in these cells "could be achieved by inhibition of cGMP phosphodiesterases (PDE), specifically PDE5" – a PDE isoform that is expressed in renal collecting ducts. The search then began for a PDE5 inhibitor that had been clinically tested. It didn't take long: Viagra (sildenafil citrate marketed by Pfizer Inc.) is a cGMP phosphodiesterase inhibitor with annual U.S. sales estimated at about $1 billion in the erectile dysfunction market.
Administering Viagra to kidney cells showed a rapid rise in cGMP levels, not because more was being produced, but because less was being broken down. When tested in Brattleboro rats, which don't produce vasopressin, Viagra "mimicked the effect of vasopressin," Brown said, "producing the cascade of events leading to aquaporin accumulation on the cell surface."
"This study provides proof-of-principle data that pharmacologically mediated PDE5 inhibition can potentially be used to bypass the V2R signaling cascade, leading to AQP2 appearance on the plasma membrane of epithelial cells," the paper said. Brown added later that there "are many diseases where there are problems in trafficking of proteins that don't get to the right place, and it's unlikely that this mechanism is restricted to the water channel." (Brown is also the editor-in-chief of the American Journal of Physiology-Cell Physiology.)
However, Brown warned that aquaporin accumulation is "only one step in the cascade, and we have shown that a clinically approved drug has a useful effect. However, now we need to understand the physiology of the whole kidney." Other "next steps" include:
- Since the enzyme inhibited by Viagra is in vascular cells as well as in kidney cells, administration of Viagra also increases blood flow, putting extra pressure on kidneys.
- Recent evidence indicates that PDE5 inhibition is associated with excretion of abnormal amounts of sodium in the urine.
- Because the present study was performed over a short time period, only a small effect on final urine concentration was found. Long-term studies need to now be carried out, and combination therapy as well as maintenance of subjects on a low-sodium diet need to be tested.
- The paper adds: "metabolism of sildenafil citrate between human and rat is different."
Although statistics are hard to come by, fewer than 700 Americans, mostly male, have inherited forms of NDI. Many more patients have acquired NDI through using certain prescription drugs, or other physical conditions or diseases, but the total is still small.
"Studying the vasopressin receptor's mutations that affect urine concentration was a chance to understand the role of some amino acids or sequence motifs on this protein in cell physiology," lead author Bouley said. "This knowledge may be applied to other proteins and other diseases involving protein trafficking, including cystic fibrosis, where abnormal proteins trapped in cells can't perform their normal physiological function," Bouley noted.