DIFFERENTIAL CONTROL OF VASCULAR TONE AND HEART-RATE BY DIFFERENT AMINO-ACID NEUROTRANSMITTERS IN THE ROSTRAL VENTROLATERAL MEDULLA OF THE RAT

1. To test the hypothesis that a central mechanism may play a role in the minimal reflex tachycardia noted in response to peripheral convert ing enzyme inhibition, we compared the effects of intravenous (i.v.) c eronapril (CER) with nitroglycerin (NTG) on neurotransmitter release i n the rostral ventrolateral medulla (RVLM), using an in vivo microdial ysis method in pentobarbital anaesthetized rats. 2. CER (0.1 mg/kg, i. v.) caused a progressive decrease in glutamate (GLU) release (CER 65 /- 7% vs NTG 83 +/- 3% of each baseline at 140 min, P<0.05) and attenu ated the increase in glycine (GLY) release (CER 100 +/- 8% vs NTG 122 +/- 9%, P<0.05). 3. Prevention of blood pressure reduction due to i.v. CER by concomitant infusion of a subpressor dose of angiotensin II (A II) attenuated the progressive reduction of GLU release (87 +/- 4%, P< 0.05 compared with NTG group), whereas GLY release was not affected (1 06 +/- 5%, NS compared with NTG group). 4. Perfusion of GLU into this area at approximately physiological concentrations resulted in a susta ined tachycardia with an attenuation of the depressor effect of i.v. C ER and perfusion of GLY solely lowered blood pressure. 5. These result s demonstrate that i.v. converting enzyme inhibitor reduces the releas e of GLU in the RVLM, which was specifically caused by reducing circul ating AII, without any effect on GLY release, thus resulting in the re duction of blood pressure with minimal effect on the heart rate.