Chemoreflex inactivation, arterial stiffness and exercising muscle blood flow in human hypertension

This is a non-therapeutic mechanistic physiological study. All the following measurements and tests described will be conducted by a trained human physiologist staff member. At an initial visit to the laboratory (~1hr), written informed consent will be obtained from participants. Anthropometric (height, weight, hip-to-waist ratio), demographic, clinical history information will be obtained, and health questionnaires and 7-day physical activity recall data will be collected. Following, participants will attend 2 experimental visits (~2.5hr each), including set-up time. The first experimental visit will be scheduled ~2-7 days after the initial familiarisation visit and the second experimental visit will be scheduled ~2 weeks after the first experimental visit. In the experimental visit 1 participants will be asked to perform a lung function test. The participants will be asked to empty their lungs by gently breathing out as much air as they can. Then they will breathe in a quick (but deep breath), hold your breath for 10 seconds, and then breathe out as instructed. This test will be undertaken to test how well their lungs are working. Following this, participants will be asked to lie in a semi-recumbent position on a medical examination couch and to remain in that position throughout the session. Participants will then be instrumented for continuous monitoring of blood pressure (BP), heart rate (HR), ventilation and arterial stiffness recordings. More specifically, brachial BP will be measured with a clinically validated automated sphygmomanometer (Omron), using a cuff wrapped around the upper arm. In addition, beat-to-beat BP will be measured using finger photoplethysmography, using a small lightweight cuff wrapped around the finger. Heart rate will be measured using a standard electrocardiogram involving the placement of sticky patch electrodes on the collarbones and chest (standard 3 lead ECG). For breathing monitoring, participants will wear a mouthpiece and a nose clip. Arterial stiffness will be recorded using two different ways: 1) using the BP+ device (Uscom) which involves a blood pressure cuff placed around the upper arm; 2) using the Complior device (Artech Medical) which involves lightweight probes that are positioned at three specific locations (neck, wrist, and top of the thigh/groin) and held lightly in place with a probe holder. After a resting period of 15 min (last 5 min used for analysis), peripheral chemoreflex testing will be undertaken. Peripheral chemoreflex testing will involve two tests. Each test lasts 5 min and is separated by a 15-minute recovery period. Each participant will receive both tests in a randomised order. The two tests are: 1) isocapnic hypoxia (to evaluate peripheral chemoreflex stimulation, using a gas concentration of 10% O2 and 90% N2); and 2) isocapnic hyperoxia (to evaluate a decrease in the peripheral chemoreflex activity, using a gas concentration of 50% O2 and 50% N2). After each trial participants will be asked to rate the difficulty of their breathing. On the second visit, participants will be asked to lie in a semi-recumbent position on a medical examination couch and to remain in that position throughout the session again. A thin, flexible tube will be inserted into the hand of the participant by a clinically qualified investigator. Participants will be instrumented again for BP, heart rate, and ventilation. Brachial artery blood flow will be measured using duplex Doppler ultrasound. This ultrasound examination is similar to the scan done for pregnant women, but a large artery is examined. This is a simple and safe procedure and involves a probe being put on the patients’ skin over the region of interest with the help of a ‘water jelly’. Sympathetic nerve activity will also be measured using the microneurography technique. This involves the insertion of a small, sterile wire (unipolar tungsten microelectrodes, tip measuring 1-5 um) near the fibular head on the outside of the leg, to obtain a multiunit recording of postganglionic muscle sympathetic nerve activity from the peroneal nerve. After a resting baseline of 15 min (last 5 min used for analysis), two trials will then be performed to test the peripheral chemoreflex deactivation. Trials will be undertaken with either intravenous 0.9% saline (control) or intravenous low-dose dopamine (2 mcg·Kg-1·min-1) to deactivate the peripheral chemoreflex. Following the drug infusion participants will be asked to breathe an isocapnic hypoxia mixture (the same gas concentration as the one from the first visit - 10% O2 and 90% N2) for 5 minutes to evaluate peripheral chemoreflex activity. After that, a 10-minute recovery will be undertaken. Following the recovery, a rhythmic handgrip exercise (1-second contraction, 1-second relaxation) will be performed at 50% of the participant’s maximal voluntary contraction for 3 minutes. After the breathing trial participants will be asked to rate the difficulty of their breathing and after the exercise participants will be asked to rate their perceived exertion.