Marked reduction in intramembranous particle clusters in the terminal portion of inner medullary collecting ducts of antidiuretic rats.

Antidiuretic hormone (ADH) induces an aggregation of intramembranous particles (IMP) into discrete clusters in the luminal plasma membrane of rat renal papillary collecting duct cells (Harmanci et al. 1978). The correlation between an elevated dose of ADH, increased urine osmolality, and greater IMP cluster frequency has led to speculation that the water permeability of the luminal plasma membrane is reflected by the IMP cluster density (Harmanci et al. 1980). The present study indirectly evaluated this water permeability by quantitating collecting duct IMP cluster frequency from freeze-fracture replicas in two regions of the renal papilla, at its base and at its tip, in antidiuretic and in water diuretic rats. During antidiuresis there was a high frequency of IMP clusters (189/100 micron2 of luminal plasma membrane) in cells from the papilla base but not at the papilla tip (9.0/100 micron2). During water diuresis there were few IMP clusters in either cells from the papilla base (5.9/100 micron2) or at the papilla tip (1.4/100 micron2). Most significantly these results suggest that the water permeability of the terminal portion of the inner medullary collecting duct of antidiuretic rats is significantly lower than that of the collecting duct epithelium higher in the papilla.