VASCULAR DISTENSION IN MUSCLES CONTRIBUTES TO RESPIRATORY CONTROL IN SHEEP

It has recently been proposed that afferent fibers from skeletal muscl e could sense the state of the microvascular circulation, linking vent ilation to the degree of peripheral perfusion or vascular distension ( Huszczuk et al., Respir. Physiol., 91: 207-226, 1993). Ventilatory and circulatory responses to manipulation of peripheral vascular pressure s in the hind limbs of anaestetized (sodium thiopental) sheep were exa mined. Inflatable balloons were placed at the caudal ends of the abdom inal aorta and the vena cava (Vc). Aortic (Ao) occlusion induced a con sistent normocapnic decrease in minute ventilation (VE). In contrast, VE increased significantly during vena cava obstruction, leading to hy pocapnia. Small changes in systemic blood pressure were observed (+7 m mHg for Ao occlusion and -12 mmHg during Vc obstruction). Moreover, in flation of the caval balloon superimposed on a previously established Ao occlusion, preventing venous drainage of anastomotic inflow, result ed in a significant rise in distal vascular pressures with trivial cha nges in systolic blood pressure. This led to a gradual rise of VE, des pite further reduction of the CO2 flux to the lungs. The subsequent de flation of the aortic balloon, exposing the hindlimb vasculature to ao rtic pressure, resulted in an even more profound hypocapnic hyperpnea. The concurrent arterial blood pressure changes were too small to poss ibly involve the ventilatory component of the arterial baroreflex. We therefore hypothesize, that perfusion-related afferent signals within the muscles could contribute to respiratory homeostasis by maintaining ventilation of the lungs commensurate with the circulatory state of t he muscular apparatus.