Effects of carotid chemoreflex stimulation on regional vascular resistances in conscious dogs

Chemoreflex stimulation (CS) in anesthetized animals induces constriction in mesenteric (M), renal (K), and iliac (I) beds. Since in conscious animals CS also increases depth of respiration, concomitant reflex effects from thoracic stretch receptors must also be considered. Accordingly, effects of carotid CS were studied in 7 healthy, conscious dogs 3 wks after implantation of aortic and intra-carotid catheters and electromagnetic flow (F) probes on M, K, and I arteries. Nicotine, 4 μgm intra-carotid, induced a biphasic response. The initial phase was characterized by bradycardia, a decrease in IF (-47%), an increase in I resistance (R) (153%) and insignificant increases in MR and KR. The second phase followed an increase in depth of respiration and involved tachycardia, an increase in IF (107%) and a decrease in IR (-52%), again without significant changes in M or K beds. The initial increases and later decreases in IR were blocked by phentolamine but not by propranolol, atropine or tripelennamine. With respiration controlled CS elicited only constriction. Thus, in conscious dogs with intact respiratory drive, carotid CS induces substantial constriction in the I but not in K or M beds. The impressive later period of I dilation appears to be mediated by the pulmonary inflation reflex, as this response was absent when respiration was controlled. The efferent limbs of these reflexes appear to be α adrenergically mediated.