Response of Muscular and Cutaneous Vessels to Physiologic Stimulation of Chemoreceptors

Activation of chemoreceptors produces an increase in total peripheral resistance (l), which indicates vasoconstriction, but the constriction may not occur in all vascular beds. The concept that reflex stimuli produce a uniform sympathetic discharge (2), which would tend to produce vascoconstriction in different vascular beds, has been modified by the demonstration that some reflex stimuli produce profoundly different responses in different vascular beds (3-6). For example, a recent study indicates that pharmacologic activation of arterial chemoreceptors by nicotine or cyanide causes reflex vasoconstriction in muscle but reflex vasodilatation in skin (6). The purpose of this study was to determine whether physiologic stimulation of chemoreceptors causes different responses in blood vessels of skin and muscle. Materials and Methods. Male mongrel dogs weighing 15-28 kg were anesthetized with chloralose (50 mg/kg) and urethane (500 mg/kg) and treated with decamethonium bromide (0.3 mg/kg). The animals were ventilated with room air and supplemental oxygen to maintain normal systemic arterial blood gases. A carotid bifurcation was exposed. The internal carotid and all branches of the external carotid arteries were ligated, except for the occipital artery, preserving the blood supply to the carotid chemoreceptor. The common carotid and external carotid arteries were cannulated. Blood to perfuse the carotid was obtained from a femoral artery and flowed into a bubble oxygenator (Travenol regional perfusion oxygenator). The oxygenator was primed with 500 cc of dextran (average mol wt = 70,000). After passing through the oxygenator the blood was rewarmed and pumped at constant flow (100 d= 8.1 ml/ min) past an oxygen electrode into the common carotid artery. Perfusion pressure in the carotid was 80 =t 5.5 mmHg (mean =t SE). Blood flowed out through a cannula in the external carotid artery and through a Starling resistor to a jugular vein. The Starling resistor maintained pressure constant in the carotid bifurcation. In most studies the contralateral carotid sinus was denervated and bilateral cervical vagotomy was done. A gas mixture containing 13 % oxygen, 5 % carbon dioxide, and 82% nitrogen was bubbled through the oxygenator to maintain normal gas tensions. A mixture of approximately 10% carbon dioxide and 90% nitrogen was used to make the blood perfusing the carotid hypoxic and hypercapnic. One hundred percent nitrogen was used to make the blood hypoxic. These gas mixtures altered gas tensions in the blood perfusing the carotidchemoreceptor without affecting systemic blood gases. Blood samples were analyzed for Poz, Pco2, and pH with an Instrumentation Laboratory ultramicro gas analyzer. Responses to stimulation of chemoreceptors were observed in the gracilis muscle and hindpaw. The gracilis muscle was dissected free from surrounding tissues except for the gracilis artery, vein, and nerve. The cranial tibial artery to the hindpaw (which has a predominantly cutaneous vascular bed) was exposed about 4 cm upstream from the tarsus and collateral arteries at that level were ligated. The gracilis and cranial tibial arteries were cannulated and perfused separately at constant flow with heparinized arterial blood while perfusion pressure was recorded. Flow to the muscle was 16 d= 1.8 ml/min (mean =t SE) and flow to the paw was 39 & 4.4 ml/min. With blood flow constant, changes in perfusion pressure reflected changes in vascular resistance in the muscle and paw. Systemic arterial pressure and heart rate also were measured. Statistical comparisons were made by paired t tests. Results. Blood gases. Bubbling 10 % carbon