Studies on the mechanism of reduced urinary osmolality after exposure of renal papilla.

Studies were performed in Munich-Wistar rats to determine whether changes in papillary plasma flow might be responsible for the concentrating defect which occurs after exposure of the extrarenal papilla. Papillary plasma flow was measured by (125)I-albumin accumulation. Initial studies in hydropenic animals revealed that papillary plasma flow was 40% higher in the kidney with the exposured papilla, 41 vs. 29 ml/min per 100 g of papilla (P < 0.001). This increase in papillary plasma flow was detectable 15 or 45 min after removing the ureter. Because it was unclear whether the rise in papillary plasma flow was a cause or the result of the fall in urine osmolality, similar studies were performed in animals undergoing a water diuresis. In this setting, papillary plasma flow still increased on the exposed side compared to the control side, 81 vs. 60 ml/min per 100 g, despite similarly low urine osmolalities of 155 and 174 mosmol/kg, respectively. This finding is compatible with the possibility that papillary exposure per se causes an increase in papillary plasma flow and that this hemodynamic alteration may lead to a reduction in urinary osmolality secondary to washout of the medullary interstitium. A final group of hydropenic rats was given either indomethacin or meclofenamate before removing the ureter. In these studies, there was no difference in either the papillary plasma flow or the urine osmolality between control and exposed kidneys. It is therefore suggested that opening the ureter induces an increase in papillary plasma flow by some mechanism which may involve an alteration in prostaglandin synthesis.