ROLE OF ENDOGENOUS NITRIC-OXIDE IN THE BRAIN-STEM ON THE RAPID ADAPTATION OF BAROREFLEX

It has been shown that nitric oxide in the brain stern plays an import ant role in the control of sympathetic nerve activity. We examined the role of endogenous nitric oxide in the brain stem in the rapid centra l adaptation of baroreflex control of sympathetic nerve activity in an esthetized rabbits. Bilateral carotid sinuses were isolated, and a ste pwise increase in pressure of 25 or 50 mm Hg for 50 to 60 seconds was applied to the carotid sinuses while the arterial pressure and renal s ympathetic nerve activity were recorded, The renal sympathetic nerve a ctivity was inhibited by the stepwise increase in carotid sinus pressu re, but thereafter it gradually returned toward the baseline level des pite the fact that carotid sinus pressure was kept constant. This proc edure war performed after intracisternal injection of N-omega-nitro-L- arginine methyl ester (L-NAME, 8 mu mol), N-omega-nitro-D-arginine met hyl ester (D-NAME, 8 mu mol), L-arginine (40 mu mol), or the vehicle s olution. The magnitude of the immediate and maximal inhibition of rena l sympathetic nerve activity caused by a stepwise increase in carotid sinus pressure was similar between the vehicle and L-NAME treatment, b ut the rate of recovery of the renal sympathetic nerve activity after immediate inhibition was faster after L-NAME than after vehicle. L-Arg inine reversed the effects of L-NAME. However, D-NAME or L-arginine al one had no such effects on the rate of recovery of the nerve activity. These results thus suggest that endogenous nitric oxide in the brain stem attenuates rapid adaptation of the arterial baroreflex control of the sympathetic nerve activity in rabbits.