Data collection methods and equipment
Glucose clamps
During glucose clamps, insulin is infused in research volunteers at physiologic dosages which causes suppression of insulin release by the body. Glucose enters cells and can stimulate glucose counterregulatory mechanisms if blood sugar is allowed to decrease. A dextrose solution is variably infused to maintain blood glucose within certain specified parameters for each experiment. Potassium chloride is infused to counteract the physiologic effect of insulin that causes potassium to enter cells. Without this infusion blood potassium would decrease. Small blood samples are obtained frequently to monitor blood glucose levels whenever insulin is infused. This procedure is tightly controlled and performed by highly trained professionals to ensure safety of the volunteers.
Ideally, blood glucose concentration would be obtained from an artery, since this is blood that is transported to tissues that will metabolize glucose. In order to circumvent sampling blood from an artery, we sample "arterialized" blood with a method that allows us to approximate arterial blood when sampled from a vein. Blood samples are taken through a catheter inserted retrograde in a vein on the back of a hand. This hand is placed inside a hot box regulated at 125 degrees Fahrenheit (the white box with subject's left hand placed inside). This causes arterio-venous anastomoses in the hand to vasodilate. The blood in these hand vessels is shunted to them for thermoregulation. since the hand tissues are not metabolically active during these studies.
In addition to blood travelling preferentially in the hand through these arterio-venous anastomoses, blood that does circulate to other hand tissues is blood that is in contact with capillaries of a resting hand, not travelling through highly metabolizing tissue. Blood that returns to the hand veins approximates arterial blood, thus "arterialized". Arterialized blood gives analyzed values similar to arterial blood, without sampling blood from an artery.
Glucose analyzers are used to analyze blood samples every 5 minutes in order to maintain specified blood glucose levels during glucose clamping with less than 5% variabilty.Tracers to measure glucose kinetics
Very small amounts of tracers are infused to give us information on the kinetics of selected parameters of metabolism. Tritiated radioactive, or deuterated non-radioactive tracers are infused to enable the calculation of glucose kinetics (e.g. endogenous glucose production), and glycerol metabolism.
Sympathetic microneurography
Sympathetic microneurography is a direct recording of the brain's neural output directed to muscle or skin arterioles.
In this picture, the muscle sympathetic nerve activity recording is being taken from the mixed fiber peroneal nerve at the level of the popliteal fossa.
A 200 micron tungsen microelectrode with 4-5 microns exposed at the tip is placed in a muscle fascicle to make an extracellular measurement of sympathetic C-fiber activity. This is a measure of brain neural activity directed to control vasoconstriction of arterioles in the muscle bed. It represents activation of the sympathetic nervous system.
The signal is amplified 100,000 times, filtered at a bandwidth of 700-2000 Hz, rectified, integrated at a 0.1 sec time constant, sampled at 1000Hz, with auditory feedback, and collected and displayed online, real time by computer.
This is the computer screen output of real-time muscle sympathetic nerve activity (MSNA, bottom graph) and the electrocardiogram (top graph) from a human experiment. Since MSNA bursts are linked to heart rate (R-R intervals), they must be collected and analyzed concurrently.
Metabolic Carts
Indirect calorimetry is used to determine rates of oxygen consumption, carbon dioxide production, and respiratory rate for both exercise and resting metabolic rate. Fuel mixture utilized by the body is inferred from the respiratory exchange ratio (carbon dioxide production/oxygen consumption) during resting periods. Subjects breathe through a mouthpiece and one-way valve while expired respiratory gases are collected and analyzed using metabolic carts.
Electronically braked ergometers are used for subjects performing maximal oxygen consumption testing. The results are used for characterization and to predict workloads where exercise is used to provide an exercise stimulus for experimental protocols.
