Arginine vasopressin (AVP or antidiuretic hormone) released from the pituitary gland targets vasopressin receptors (VRs) 1a/1b/2 located on the apical epithelial membrane of the collecting ducts in the kidney to induce transport of water and solutes and regulate plasma osmolarity. However, recent studies also confirm the localization of VR1a/1b/2 on bladder urothelial cells, blood vessels and detrusor smooth muscle, which can be activated by locally or systemically released AVP for altering urine/plasma osmolarity and stored urine volumes. AVP synthesis outside of pituitary gland is reported in collecting ducts of kidney, and in human and rodent bladder, whose functional significance was recently demonstrated by increased solute-free water reabsorption in the mouse bladder following intravesical instillation of AVP and desmopressin. In this study, we have further examined the role of paracrine AVP signaling, by assessing the localized effects of VR2 agonist, fedovapagon in mouse bladder.

Adult male and female C57Bl6 mice were used in two sets of experiments:

Isoflurane (1.5-5%) anesthetized mice (glomerular filtration rate; GFR, ~63% of the unanesthetized value) had one ureter diverted to a tube for collecting fresh urine from the renal pelvis while other ureter was left intact for delivering urine to the bladder, for collection 2 hrs later, both before and after administration of fedovapagon (1 mg/kg, IP). Collected urine samples were analyzed for volume and electrolyte content.

Mice were also implanted with bladder catheters routed under the skin to the back of the animal and connected to an externalized port. After recovery, catheters were connected to a pressure transducer and syringe pump through a tether and bladders filled with 0.01 ml/min saline or soybean oil with or without novel VR2 agonist (fedovapagon, 100 nM) and VR2 antagonist (lixivaptan, 10 mg/kg, IP) for awake cystometry.

The urine volume pooling in the bladder over the 2 hour period was ~20% lower than urine collected from the transected ureter. There was also a significantly higher Na+ concentration (108 ± 5 vs. 74 ± 20 mM) in urine pooling in the bladder (Fig. 1). Fedovapagon administration significantly decreased urine production and increased urine reabsorption from the bladder to reduce kidney output by ~55%.

Intravesical administration of VR2 agonist, fedovapagon, decreased baseline pressure as well as contractile activity in both saline and soybean oil cystometries, which were abolished by administration of lixivaptan prior to fedovapagon instillation (Fig. 2).

Significantly lower volume coupled with higher Na+ concentration in urine flowing in the intact ureter to bladder compared to the volume and Na+ in urine collected from transected ureter of mouse recapitulated the Na+ and osmolality differences reported for urine collected from the renal pelvis and bladder of patients undergoing percutaneous nephrolithotomy [1]. Findings also confirm that a solute-free water reabsorption of urine stored in the bladder dilutes the hyperosmolality of the plasma resulting from a GFR decline under surgical anesthesia. The anti-diuretic effect of fedovapagon reported in a randomized clinical trial [2] is mirrored by fedovapagon-treated mice exhibiting  increased water reabsorption from the kidney and from stored urine in the bladder.

Unlike saline, soyabean oil is not absorbed from the bladder and therefore VR2 activation in the bladder by localized administration of fedovapagon during soyabean oil cystometry isolates the effect of fedovapagon on detrusor smooth muscle relaxation from the additive effect of accentuated water reabsorption seen only during saline cystometry.

AVP signaling in the bladder is involved in the regulation of plasma osmolarity in addition to the renal collecting ducts, which is relevant in pathophysiology and treatment of nocturia.

1.	Cahill DJ, Fry CH, Foxall PJ. Variation in urine composition in the human urinary tract: evidence of urothelial function in situ? J Urol. 2003;169(3):871-4.
2.	McElwaine-Johnn H, Golor G, Handy R, Yea C, Lambert J... Antidiuretic effect of fedovapagon in older males with benign prostatic hyperplasia and nocturia. ICS 2013.