Respiratory and vasomotor effects of excitatory amino acid on ventral medullary surface

Three glutamic acid analogues, N-methyl-D-aspartic (NMDA), quisqualic (QQ), and kainic (KAI) acids were applied topically to the ventral surface of the medulla (VMS) in paralyzed, vagotomized and carotid sinus denervated cats hyperventilated to apnea. Respiratory and vasomotor effects were assessed by changes in phrenic nerve activity and systemic arterial blood pressure. All three agents to varying degrees raised systemic blood pressure, but only NMDA consistently initiated phrenic nerve activity at pCO2 levels below that observed in control trials. KAI and QQ raised blood pressure even in those animals in which they had little effect on initiating phrenic nerve activity. Furthermore, respiratory responses were obtained from localized areas on VMS, namely the intermedio-caudal zone (I-C areas); whereas blood pressure elevations could be obtained from wider VMS areas including the rostral zone (R areas). In addition, the effects of the three amino acids on blood pressure were quantitatively different with KAI causing much greater increases in blood pressure than QQ or NMDA. The respiratory and vasomotor effects of NMDA and QQ were blocked by the use of 2-amino-5-phosphonovaleric acid and L-glutamic acid diethylester, their respective antagonists. The results suggest that neurons in the VMS which cause respiratory and vasomotor responses are not identical. Cells containing receptors stimulated by NMDA predominantly increase respiration, whereas cells containing receptors excited by KAI are more effective in eliciting vasomotor responses.