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Within the past decade an entire family of membrane proteins--aquaporins--which function as transmembrane water channels has been identified; they occur throughout the plant, animal, and bacterial kingdoms. Several family members permit glycerol and urea permeability. Most aquaporins are inhibited by mercury. Constitutively expressed aquaporin 1 is the major permeability channel of the proximal tubule, descending thin limb of the loop of Henle, and it is also found in vasa recta. Aquaporin 2 is expressed in the principal cells of the collecting duct where it shuttles between intracellular vesicles and the apical membrane in response to vasopressin. Aquaporin 2 mutations cause nephrogenic diabetes insipidus; increased aquaporin 2 activity is implicated in the pathophysiology of heart failure, cirrhosis, and nephrotic syndrome. Aquaporins 3 and 4 provide basolateral membrane water channels in the collecting duct. These 4 channels and 6 others are also found elsewhere throughout the body. The physiological importance of several of the channels remains unknown. Aquaporin 1 inhibitors might induce useful diuresis, but humans who lack aquaporin 1 have no significant clinical disease. Inhibition of aquaporin 2 activity by vasopressin receptor antagonists may be useful in heart failure, cirrhosis, nephrotic syndrome, and the syndrome of inappropriate antidiuretic hormone (ADH) release.

It is noted that while a wide variety of syndromes have been associated with hypernephroma in the clinical literature, there is clear understanding of the pathophysiology of these effects only in the cases of the endocrine disorders where direct tumor production of hormone can be demonstrated in vitro. Furthermore, this knowledge has done little to alter the care of patients with the disease, except for indications that indomethacin might be of benefit in some patients with hypercalcemia and that one might consider the use of converting enzyme inhibitors in patients with hypernephroma and hypertension. The overall approach to the disease is still surgical. Resection of the tumor also removes the paraneoplastic syndrome. Persistence or recurrence of a syndrome suggests the continued presence of the neoplasm, with the considerations for prognosis which that fact entails. To that degree, at least, these conditions are useful as tumor markers, but such use is limited because they are inconsistent. Further studies of pathophysiology of paraneoplastic syndromes will lead to better understanding of processes of cell differentiation and regulation, and possibly better ways to manage the patients in which they occur.