Whole-body venous capacity and effective total tissue compliance in SHR

Whole-body venous capacity was examined in conscious 4-month-old male spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive (WKY) rats by determining mean circulatory filling pressure (MCFP) and blood volume. The MCFP was determined in conscious animals after briefly arresting the circulation by inflating an indwelling balloon in the right atrium. Blood volume was determined by dilution of 125I-albumin and 51Cr-red cells. Although blood volume was not significantly different between SHR (60.9 +/- 0.7 ml/kg, SE) and WKY (59.6 +/- 0.8 ml/kg), MCFP was slightly, but significantly elevated in the SHR (9.5 +/- 0.3 vs 8.5 +/- 0.2 mm Hg, mean +/- SE, p less than 0.05). Increased MCFP with normal blood volume suggests decreased venous capacity in the SHR. In addition, effective total tissue compliance (ETTC) was measured in conscious 5-month-old female SHR and WKY. A decrease in tissue fluid volume was induced by i.v. infusion of hyperoncotic albumin solution. Changes in interstitial fluid pressure were monitored continuously with implanted tissue capsules. Changes in tissue fluid volume were estimated from changes in plasma volume and urine volume. In SHR 3 hours postinfusion, tissue fluid volume decreased by 38.7 +/- 2.7 ml/kg and interstitial fluid pressure decreased from -1.4 +/- 0.3 to -6.6 +/- 1.5 mm Hg. In WKY, tissue fluid volume decreased by 32.5 +/- 2.7 ml/kg and interstitial fluid pressure decreased from -1.4 +/- 0.4 to -3.9 +/- 0.5 mm Hg. The linear regression line for delta interstitial fluid pressure and delta fluid volume was estimated for each rat and the inverse of this slope represented ETTC, which averaged 7.4 +/- 1.0 and 9.6 +/- 2.1 ml/kg/mm Hg (p greater than 0.3) in SHR and WKY respectively. Thus, there were no significant differences in interstitial fluid pressure or ETTC between female SHR and WKY. The results of this study confirm a decreased venous capacity in male SHR with established hypertension and provide new information indicating no measurable abnormalities in interstitial fluid pressure or effective total tissue compliance in adult female SHR as compared with WKY.