Submitted on February 20, 2009
Revised on May 5, 2009
Accepted on May 5, 2009
Hypotension- and Osmotically-Induced Thirst in Old Brown Norway Rats
Robert L. Thunhorst1*, Terry G. Beltz, and Alan Kim Johnson1
1 University of Iowa
* To whom correspondence should be addressed. E-mail: robert-thunhorst{at}uiowa.edu.
Compared to young cohorts, old rats drink less water in response to several thirst-inducing stimuli. In these experiments, we characterized water drinking in response to hypotension and cellular dehydration in young (4 mo), "middle-aged" adult (12 mo) and old (29-30 mo) male Brown Norway rats. We injected the vasodilator, minoxidil as an iv bolus in a range of doses (0 - 20 mg/kg) so that drinking responses could be compared at equivalent reductions of arterial pressure. Old rats had greatly diminished reflex tachycardia and became significantly more hypotensive after minoxidil compared to young and middle-aged rats. When compared at equivalent reductions of arterial pressure, old rats drank one-third as much as middle-aged rats, and one-fifth as much as young rats. In addition, there were age-related deficits in drinking in response to a range of administered loads of sodium (0.15 - 2 M NaCl, 2 ml/100 g body weight). Urinary excretion of water and sodium in response to the loads was equivalent across ages. Both middle-aged and old rats were less able than young rats to repair their water deficits after sodium loading, attributable almost entirely to their reduced drinking responses compared to young rats. Lastly, age-related declines in drinking appeared to be more severe in response to hypotension than in response to cellular dehydration.
