The mammalian kidney is a complex organ composed of an heterogeneous arrangement of blood vessels and tubules. In 1965 Maurice Burg and the principal investigator devised a new method to study specific segments of renal tubules in vitro. The method allows researchers to take a closer and more specific look at transport mechanisms in renal tissue than is possible otherwise. Several new functions of the kidney have been revealed by studies of isolated tubules in vitro. The proposed experiments are designed to determine some mechanisms of salt and water transport in various tubule segments, with sufficient flexibility in each protocol to permit the followup of interesting new leads as they come along. Segments of kidney tubules isolated by microdissection, and key tubule cells isolated from specific nephron segments will be studied in vitro. The mechanism of fluid secretion, a newly recognized function in proximal straight tubules, will be determined. The secretion of prostaglandins by the pars recta portion of the proximal tubule will be quantified and the mechanism of secretion of these fatty acids will be determined. The mode of action of vasopressin on the urinary plasma membrane of collecting duct cells will be assessed using direct methods of inquiry. Studies of the effects of hormones on tubule cell electrolyte content will made. The cellular mechanisms for the adjustment and control of renal tubule cell volume will be determined, specifically in respect to volume control in hypotonic and isotonic media. These studies will provide unique information about the function of specific portions of the nephron which can be inserted into an integrated concept of the mechanisms of urine formation.