Angiotensin II blockade during combined thiazide-beta-blocker treatment

Effect of metoprolol on serial measurements of exchangeable sodium, blood pressure, renin and pulse rate in spontaneously hypertensive rats

The cardioselective beta-blocker metoprolol was mixed with the diet (6 mg metoprolol/g food) and given to 9 spontaneously hypertensive rats (SHR) for four weeks, while 9 other SHR were given the plain diet only and served as controls. All rats were also given 0.5% drinking saline labeled with isotope 22Na (37 kBq/1) to drink. Measurements of total exchangeable sodium, blood pressure, pulse rate and weight were performed before and repeatedly during treatment. Plasma renin activity was measured at the end of the study. Before treatment exchangeable sodium, blood pressure, pulse rate and weight were no different between the groups. Metoprolol reduced the pulse rate and prevented the usual blood pressure increase in SHR. Both groups gained weight similarly. Exchangeable sodium increased similarly in both groups along with the weight increase. Plasma renin activity was not significantly different in the two groups. Thus, chronic metoprolol treatment of SHR reduced blood pressure and pulse rate in SHR but did not lower plasma renin or cause measurable sodium or fluid retention.