Sodium appetite and 24-hour variations in fluid balance, mean arterial pressure and heart rate in SHR and WKY on various sodium diets.

Young spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) were compared with respect to firstly, sodium (Na) appetite during 'rest', mild social stress and adrenocorticotrophic hormone (ACTH) injections and secondly, diurnal patterns of water intake, urine output, mean arterial pressure (MAP) and heart rate (HR) while on various Na diets: 0.5 mmol Na (LNa), 5 or 12-13 mmol Na (CNa), 50 (HNa) or 120 mmol Na (vHNa) per 100 g food. Sodium appetite and water intake, being 50% higher in SHR than in WKY (4-4.5 versus 2.5-3 mmol Na/100 g body weight per day), was modestly increased by both social stress and ACTH, more so in WKY. With regard to influences of the various Na diets, daytime resting MAP was modestly lowered in LNaSHR and slightly increased in vHNaSHR compared with CNaSHR, but about equal in all WKY groups. Food and water consumption was concentrated to the active night period, but even high Na water intakes caused no signs of sustained hypervolaemia, as intake bouts were eliminated by urine within 30-40 min in both SHR and WKY. However, the vHNa diet in particular in SHR increased drinking frequency; each drinking bout induced such rapid, 5-10 min long neurogenic MAP and HR increases that they could not be ascribed to blood volume expansion per se. The diurnal MAP-HR patterns in SHR therefore varied markedly with the Na diets, with a considerably raised average MAP in the vHNa group, even though resting daytime MAP was nearly the same as in CNaSHR.(ABSTRACT TRUNCATED AT 250 WORDS)