A reflex arising from contracting hind limb skeletal muscles is an important neural mechanism that is responsible for causing the cardiovascular adjustments to exercise. These adjustments, which include increases in peripheral vascular resistance, cardiac contractility and rate, function to increase arterial blood flow and oxygen to the exercising muscles, and in turn support their ability to contract. This neural mechanism has been named the exercise pressor reflex and its afferent arm is comprised of group III and IV fibers whose endings are located in and near the muscle interstitium. In patients with peripheral artery disease (PAD) the exercise pressor reflex is exaggerated. The overall goal of the experiments proposed in this application is to shed light on the role played by lactic acid in evoking the exercise pressor reflex in both health and simulated peripheral artery disease. The proposed experiments will pay particular attention to Acid Sensing Ion Channels (ASIC) and will focus on the role that they play in evoking the exaggerated exercise pressor reflex in PAD. Lactic acid, produced by contracting skeletal muscles is believed to stimulate group III and IV afferents, signaling the spinal cord and brainstem that the arterial blood supply to working muscle is not meeting its metabolic demand. In the proposed experiments, we will pay particular attention to two important ASIC isoforms, namely ASIC1a and ASIC3. We will examine in decerebrated unanesthetized rats the responses to contraction of group III and IV muscle afferents both before and after either pharmacological blockade of the above receptors or after they have been ?knocked down? with siRNA. The proposed experiments will also examine the responses to contraction of these afferents before and after knockdown of myophosphorylase in the triceps surae muscles. In addition the responses of group III and IV afferents to contraction in mutant rats whose ASIC3 has been functionally inactivated by CRISPR Cas 9 technology will be determined. The proposed experiments will be performed both in rats with freely perfused femoral arteries and in rats with femoral arteries that have been ligated for 72 hours before the start of the experiment. The latter preparation simulates the arterial blood flow patterns seen in patients with PAD and therefore serves as a useful animal model of this disease. The proposed experiments are anticipated to provide new information about metabolic factors that cause the exercise pressor reflex to be exaggerated in PAD.
Exercise is well known to stimulate thin fiber sensory nerves in contracting muscles. Stimulation of these nerves reflexively activates the sympathetic nervous system, an effect which causes vasoconstriction in the heart, kidneys and the skeletal muscles. In hearts whose coronary arteries are narrowed by disease, this vasoconstriction can cause chest pain and fatal arrhythmias. Likewise, in muscles, whose arteries are narrowed by disease, this vasoconstriction can cause muscle ischemia and result in pain that is only relieved by the cessation of exercise (i.e., intermittent claudication). Over time, the exaggerated pressor responses to exercise in patients whose arteries perfusing the vascular of the legs can result in thickening of the walls of the heart, which is maladaptive, and irregular heart rhythms, some of which are not compatible with life.
