Study of the hemodynamic contributing factors of spontaneously hypertensive rats in the early stage of established hypertension.

The neural and nonneural factors contributing to maintain the hemodynamic status in 14 to 16 weeks old stroke prone spontaneously hypertensive rats (SHRSP) and spontaneously hypertensive rats (SHR) were compared with those of the age matched normotensive Wistar Kyoto (WKY) rats under chloralose-urethane anesthesia. The ascending aortic blood flow, arterial blood pressure and heart rate were measured under resting condition, ganglionic blockade by hexamethonium, 5 mg/kg i.v. and maximum vasodilation by hexamethonium and 0.07 mg/kg nitroprusside. The neural component (hexamethonium blockable component) was the major factor in the maintenance of resting blood pressure and total peripheral resistance index (TPRI) in the above three strains of rats. In contrast, the residual component after hexamethonium and nitroprusside administrations was the most important controlling factor for cardiac index (CI). In SHRSPs, the neural component of TPRI and residual component of CI were both significantly higher than those of WKYs by 75% and 47%, respectively. In SHRs, the residual component of CI was 52% higher than that of WKYs. These results suggest that both the increased neural component of TPRI and nonneural component of CI are important contributing factors for the blood pressure increase in the early stage of established hypertension in SHRSPs. On the other hand, in the age matched SHRs, the most important contributing factor for hypertension is the increased nonneural component of CI. Considering the differences between these two strains of hypertensive rats, the neural component of TPRI is the major factor responsible for the different levels of hypertension in SHRSPs and SHRs.