Hemodynamics and monoamine oxidase activity in spontaneously hypertensive rats (SHR)

The relationship between the onset of hypertension and changes in monoamine oxidase (MAO) activity in the brains and hearts of spontaneously hypertensive rats (SHR) were studied. After 7-weeks-old, blood pressure of SHR increased rapidly and reached a level of 170 to 180 mmHg; but following 4 weeks of propranolol treatment (10 mg/kg/day), blood pressure decreased significantly compared to that of untreated SHR. Heart/body weights ratio of SHR was higher than that of normotensive Wistar Kyoto rats (WKY). MAO activities in the brain stem, the medulla oblongata and pons of the SHR were significantly higher than those in WKY at 7 weeks of age, and MAO activity in the brain stem of the propranolol-treated SHR was significantly lower than that in the untreated SHR. Propranolol inhibited MAO activity in brain tissue in vitro, and the I50 values of propranolol were identical (1 X 10(-4) M) in SHR and WKY. In both the WKY strain and the SHR, the Vmax values of heart MAO increased with age, and the Vmax values of SHR were twice those of WKY. Km values for tyramine of heart MAO in WKY and SHR were approx. 100 microM and 140 microM, respectively; however, these values were not age-dependent. It was concluded that an increase in MAO activity in SHR brain stem may trigger a reduction in noradrenaline content and that propranolol may be responsible for its restoration, thus reducing peripheral sympathetic activity; moreover, the increase in MAO activity in the hearts of SHR may be of genetic origin.