Direct effects of vasoactive substances on blood pressure can be examined in individuals with
tetraplegia due to disruption of descending spinal pathways to sympathetic preganglionic neurons, as cervical lesions interfere with baroreceptor reflex buffering of sympathetic outflow. In this study, we assessed effects of the
nitric oxide synthase inhibitor nitro-
L-arginine methyl ester (
L-NAME) on mean arterial pressure, heart rate, and plasma
norepinephrine concentrations in individuals with
tetraplegia vs. effects shown in a neurologically intact control group. Seven individuals with
tetraplegia and seven age-matched controls received, on separate visits and in the following order, placebo (30 ml
normal saline) and 0.5, 1, 2, and 4 mg/kg
L-NAME intravenously over 60 min. Supine hemodynamic data were collected, and blood was sampled at the end of each infusion and at 120, 180, and 240 min thereafter.
L-NAME increased mean arterial pressure, and the relative increase was greater in the
tetraplegia group than in the control group. Heart rate was reduced after
L-NAME administration in both groups.
L-NAME decreased plasma
norepinephrine in the control group but not in the group with
tetraplegia. These findings suggest that reflexive sympathoinhibition normally
buffers the pressor response to
nitric oxide synthase inhibition, an effect that is not evident in individuals with
tetraplegia as a result of decentralized sympathetic vasomotor control.