Relationship between baseline blood pressure and blood pressure decrease after calcium channel blockers in conscious rats

Objective: To whether vasodilators produced a fall in means blood pressure (MBP) that is directly proportional to baseline MBP. Design: The effects on blood pressures of there calcium channel blockers, nitrendipine, nisoldipine, and verapamil, were compared with sodium nitroprusside, a drug acting through a different mechanism. The drugs were infused in conscious, unrestrained rats and their effect on MBP was measured. Setting: Primary experimental. Animals: Animals were divided into groups according to their baseline MBP: high (greater than 119 mmHg), moderate (91 to 119 mmHg) and normal (less than 91 mmHg). Interventions: Five successive infusion rates for each drug were tested in each animal. MBP readings were taken at the end of each infusion period (usually 10 mins). Main results: At the maximal dose infused, nitrendipine (0.57 mg/kg.min) lowered MBP 13 ± 6 mmHg in the low MBP rats, 51 ± 4 mmHg in the rats with moderate MBP and 83 ± 8 mmHg in the rats with high MBP (P<0.05 within groups). Similar results (ie, the greater the baseline MBP, the greater the fall in MBP) were obtained with nisoldipine and verapamil. In the same groups, sodium nitroprusside (0.096 mg/kg.min) produced a fall of 52 ± 3 mmHg and 54 ± 13 mmHg, respectively (not significant). Conclusions: Nitrendipine, nisoldipine and verapamil behave as antihypertensives since their effect increase as a function of the initial MBP, while sodium nitroprusside showed no effect related to baseline MBP.