Central and regional hemodynamics and plasma volume distribution during the development of spontaneous hypertension in rats.

In this study, we investigated the central as well as the regional hemodynamics of spontaneous hypertension in different phases using the reference sample microsphere method in conscious, unrestrained spontaneously hypertensive rats (SHR) using age-matched progenitor Wistar Kyoto rats (WKy) as controls. Moreover, in order to study the role of intravascular fluid volumes and their regional distribution in the early hemodynamics of spontaneous hypertension, we measured albumin distribution spaces at different ages in SHR and WKy. Microsphere studies showed an increase of cardiac output in 5 weeks old SHR when compared to age-matched WKy. This increased flow was mainly transmitted to the muscular and skin vascular bed. The kidney vascular resistance was increased significantly already in 5 weeks old SHR. In 7-8 weeks old animals, cardiac output was not significantly different in SHR and WKy. The increase in blood pressure was then characterized by a generalized increase in vascular resistance in almost all tissues. Plasma volume (PV) measurements indicate a significantly higher PV in 5 weeks old SHR when compared to age-matched WKy. At later ages (7-16 weeks), PV was lower in SHR than in WKy. Regional measurements show a consistently lower PV in gut, muscle, testes, and skin in SHR. It is concluded that the early increase in cardiac output in SHR is related to an increased central intravascular volume, possibly because of renal fluid retention. Moreover, the later further rise in blood pressure and increase in total peripheral resistance seem to be caused by an independent secondary rise in vascular resistance of all vascular beds.