Cardiovascular Responses to Long-term Blockade of Nitric Oxide Synthesis

The goal of this study was to determine if there is a basal release of nitric oxide that affects long-term arterial pressure regulation in dogs. Studies were conducted over a 23-day period in eight conscious dogs with indwelling catheters. Nitric oxide synthesis was blocked by continuous intravenous infusion of nitro-L-arginine-methyl ester at 37.1 nmol/kg per minute for 11 days. Arterial pressure increased to 120±4% of control on the first day, decreased for a few days, and then increased to a maximum value of 122±6% of control on day 7. Bradycardia was sustained throughout the entire nitro-arginine period. Blockade of nitric oxide synthesis was evidenced by attenuated pressure and flow responses to systemic acetylcholine infusion. The pressor response to phenylephrine was increased for only 1 day, and the hypotensive effects of nitroprusside were enhanced. Also, the variability of arterial pressure was significantly increased during nitro-arginine. Sodium and water balances were positive the first day of nitro-arginine infusion but were unchanged for the entire nitro-arginine period. In conclusion, the data suggest that blockade of the basal release of nitric oxide in dogs causes an increase in the long-term level of arterial pressure without any sustained sodium or water retention.