Disturbed vasopressin release in 4 dogs with so-called primary polydipsia

Primary polydipsia is characterized by a marked increase in water intake an d secondary polyuria, and in dogs often is described as a behavioral proble m or a psychological disorder. We describe 4 dogs with primary polydipsia, diagnosed on the basis of a modified water deprivation test, in which furth er examination included serial measurements of urine osmolality (UOsm) and plasma vasopressin (VP) measurements during water deprivation and hypertoni c saline infusion. The dogs, ranging in age from 4 months to 4 years, all w ere presented for evaluation of polyuria and polydipsia. Physical examinati on, routine blood chemistry, and urinalysis disclosed no specific cause for the polyuria and polydipsia. During serial measurements UOsm spontaneously reached high concentrations in 2 dogs, whereas in the other 2 dogs UOsm al so fluctuated but on no occasion exceeded 1,000 mosm/kg. Primary polydipsia was diagnosed when UOsm exceeded 1,000 mosm/kg at the end of the modified water deprivation test and plasma osmolality did not exceed the upper limit of the reference range during testing. During water deprivation, plasma VP concentrations remained relatively low. The VP response to hypertonic sali ne infusion was abnormal, with an increased threshold value in 3 dogs, an i ncreased sensitivity in 2 dogs, and an exaggerated response in 1 dog. It is concluded that some dogs fulfilling current criteria for primary polydipsi a produce concentrated urine spontaneously throughout the day in a pattern similar to what has been observed in healthy pet dogs. This finding can be regarded as diagnostic and precludes the need for a water deprivation test. During water deprivation testing, all 4 dogs produced highly concentrated urine in the face of low basal plasma VP concentrations. The observed abnor mal VP release in response to hypertonic stimulation may be interpreted as a primary disturbance in the regulation of VP secretion, although it might also be the result of overhydration caused by a primary abnormality in drin king behavior.