Pressor and endocrine responses to lesions of canine rostral ventrolateral medulla

Abstract

To understand the mechanism(s) of reduction in arterial pressure (AP) after kainic acid cytotoxic lesions in the rostral ventrolateral medulla (RVLM), recordings of AP, cardiac output (CO), heart rate (HR), plasma catecholamines, and cortisol were obtained from awake relaxed dogs. Mongrels (n = 12) were instrumented with a solid-state pressure transducer in the descending aorta and a pulse transit-time ultrasonic flowmeter on the aortic arch. After the dogs recovered from thoracotomy, AP, CO, HR, and plasma levels of epinephrine, norepinephrine, and cortisol were recorded periodically or measured at rest over 2–3 wk followed by unilateral, cytotoxic lesion placement in the rostral C1 area of the RVLM. After the dogs recovered from lesion surgery, the measurements were repeated, and, compared with prelesion control values, they showed significantly decreased AP (-23.1 mmHg) and total peripheral resistance (-0.014 peripheral resistance units) but no significant changes in CO or HR. The reduction in AP may be caused partially by significant reductions in plasma epinephrine (-47%), norepinephrine (-34%), and cortisol (-66%) levels. Lesion sites corresponded to the rostral C1 area, immunocytochemically positive for phenylethanolamine-N-methyltransferase (PNMT). These observations support the concept that a discrete site in the RVLM, demarcated by a rostral subgroup of PNMT-immunoreactive cell bodies, contributes to direct sympathoexcitatory support of resting AP. The neurotransmitter responsible for vasomotor tone, however, remains unknown. This study also provides new information suggesting a role for catecholamine and cortisol regulation from this putative vasomotor center.