Modulation of carotid sinus baroreceptor reflex by hypothalamic defense response

In anesthetized and vagotomized dogs cardiovascular changes associated with the hypothalamic defense response were compared at different intrasinus pressures (ISP). This response was elicited by electrically stimulating the lateral hypothalamic area and the ventral portion of the H fields of Forel. Relationships between ISP and the following cardiovascular variables were compared before and during hypothalamic stimulation: arterial pressure (AP), heart rate (HR), cardiac output (CO), total peripheral resistance (TRP) and flows, and resistances of renal, superior mesenteric and femoral beds. Hypothalamic stimulation raised equilibrium pressure (value of AP at which AP equaled ISP) and increased maximum gain of the ISP AP relation. Gain was significantly increased by stimulation at ISP between 150 and 240 mmHg, but not between 60 and 150 mmHg. Maximum gain was attained at or near the elevated equilibrium pressure during hypothalamic stimulation. Similar modulation of gain was observed in the ISP HR relation, but modulation was less in extent and limited to a narrower ISP range. Although both TRP and CO contributed to the rise in equilibrium pressure, modulation of the gain of ISP AP relation was ascribable to that to TRP only. Among vascular beds investigated, modulation of the ISP regional resistance relation by the hypothalamic defense response was greater in renal and superior mesenteric beds than in the femoral bed. The hypothalamic defense response modulated the carotid sinus baroreceptor reflex by modifying nonuniformly the reflex responses of cardiovascular system components.