Why does the spontaneously hypertensive rat have an exaggerated preference for sweet and salty solutions? An hypothesis

Objective We hypothesised that the elevated saline preference of the spontaneously hypertensive rat (SHR) might actually represent an `appetite' for volume expanding osmolyte rather than an exaggerated hedonic drive. Further, we propose that this drive is due to an inappropriately perceived hypovolemia in the SHR. In order to test this hypothesis, we examined the preference of the spontaneously hypertensive rat for other isotonic solutions, namely, glucose and urea. We predicted that glucose, by exerting an osmotic force, would be over-ingested, while urea, being freely permeable across cell membranes, would not. Design We used two-bottle preference tests, whereby individually housed SHR and Brown Norway (BN) control rats were given a choice of water and either isotonic saline, glucose or urea to drink over a period of 6-7 days. During this period, the total fluid intake (water volume plus solute solution volume) and saline preference (proportion of total fluid intake taken as solute solution) were calculated daily. Results We found that while the spontaneously hypertensive rat did in fact exhibit an exaggerated preference for isotonic glucose solutions when compared with normotensive Brown Norway rats, this was not the case for urea solutions possessing the same particle density. Conclusions These findings support the notion that the spontaneously hypertensive rat might actually be seeking out solutions that provide a volume stimulus to the circulation. Thus, these findings, in combination with the other known disturbances in the spontaneously hypertensive rat (sympathetic overdrive, altered activity of the hypothalamo-pituitary-adrenal axis), leads us to propose that the spontaneously hypertensive rat might over-ingest 'tonic' osmolytes as a result of some disturbance in central volume sensing or regulation. (C) 2004 Lippincott Williams Wilkins.