NMDA receptor blockade in cat dorsal horn blunts reflex pressor response to muscle contraction and stretch

The role of N-methyl-D-aspartate (NMDA) receptors in the reflex presser response to static muscle contraction and passive stretch was examined by microdialyzing the NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (AP-5) into the L(7) or L(6) and S-1 levels of the dorsal horn of anesthetized cats. Contraction, elicited by electrical stimulation of the cut L(7) and S-1 ventral roots, increased mean arterial pressure (MAP) and heart rate (HR). Passive stretch at tensions similar to those generated by contraction also increased these variables. These cardiovascular changes were unaffected by dialyzing AP-5 (10 mM) into the dorsal horn at L(7) Increasing the syringe concentration of AP-5 to 100 mM attenuated the presser and HR responses from 62 +/- 8 to 31 +/- 6 mmHg and 18 +/- 4 to 12 +/- 4 beats/min, respectively. AP-5 blunted the increase in MAP (59 +/- 10 vs. 41 +/- 10 mmHg) evoked by muscle stretch. Simultaneously microdialyzing AP-5 (10 or 100 mM) into the dorsal horn at the L(6) and S-1 spinal levels also blunted the MAP and HR responses to contraction and stretch. These results suggest that NMDA receptors play a role in mediating the MAP and HR responses to static muscle contraction at the spinal level of the central nervous system. Furthermore, these data demonstrate that collaterals from muscle afferents partially mediate the reflex cardiovascular responses evoked by muscle contraction and stretch.