Mechanisms of respiration-induced changes in vasomotor control exerted by rostral ventrolateral medulla

Efferent and afferent mechanisms involved in the differential depressor response to inactivating rostral ventrolateral medulla (RVLM) under conditions of normal (2.5 ml) and reduced (1.5 ml) tidal volume were examined in urethan-anesthetized rats. Under conditions of reduced tidal volume, bilateral microinjection of lidocaine (200 nl, 4%) into RVLM produced an attenuated fall in both mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) despite the fact that base-line levels of both are unchanged. The specificity of the differential response was tested by examination of regional hemodynamics. Microinjecting lidocaine into RVLM produced an attenuated fall in renal, mesenteric, and hindquarter vascular resistances in parallel with the attenuated fall in MAP produced by reducing tidal volume. Microelectrode stimulation of RVLM produced an increase in resistance in all three vascular beds. Of the neural sensory systems examined, both vagal and chest wall afferents appear to be involved in mediating the differential depressor response. These results demonstrate the importance of afferent sensory input to the medulla in determining sites of generation and maintenance of sympathetic vasomotor tone.