L-name, a nitric oxide synthase inhibitor, as a potential countermeasure to post-suspension hypotension in rats

A large number of astronauts returning from spaceflight experience orthosta tic hypotension. This hypotension may be due to overproduction of vasodilat ory mediators, such as nitric oxide (NO) and prostaglandins. To evaluate th e role of the NO synthase inhibitor N-G-nitro-L-arginine methyl ester (L-NA ME) as a countermeasure against the post-suspension reduction in mean arter ial pressure (MAP), we assessed the cardiovascular responses and vascular r eactivity to 7-day 30 degrees tail-suspension and a subsequent 6 hr post-su spension period in conscious rats. After a pre-suspension reading, direct M AP and heart rate (HR) were measured daily and every 2 hrs post-suspension. The NO synthase. inhibitor L-NAME (20 mg/kg, i.v.), or saline, were admini stered after the 7(th) day reading prior to release from suspension and at 2 and 4 hrs post-suspension. At 6 hrs post-suspension, vascular reactivity was assessed. While MAP did not change during the suspension period, it was reduced post-suspension. Heart rate was not significantly altered. L-NAME administration reversed the post-suspension reduction in MAP. In addition, the baroreflex sensitivity for heart rate was modified by L-NAME. Thus, the post-suspension reduction in MAP may be due to overproduction of NO and al tered baroreflex activity.