Characterizing the Incretin Effect of Amino Acids and Defining GLP-1 Role on Skeletal Muscle

This study has two protocols the aims of which are:

1. To identify age-related effects of AA on incretin secretion and whether and to what extent AA exhibit a true incretin effect (gut- mediated increases in plasma insulin) in younger individuals. (Protocol 1)
2. To define the extra-pancreatic ''novel'', insulin independent effects of glucagon like peptide-1 (GLP-1) on postprandial muscle protein and glucose metabolism and microvascular blood flow. (Protocol 2)

Protocol 1:

This will explore the first aim. 8 Healthy younger volunteers will be recruited to under go 3 arms cross over studies. Interventions will include oral and intravenous amino acids, in addition to intravenous GLP-1 and glucose dependent insulinotropic polypeptide (GIP).

8 older subjects also will be recruited for comparison of the response of GI hormones to amino acids oral feed between young and older men.

Therefore the total number will be recruited to perform this protocol is 16.

Post intervention in all visits, measurements will be taken for:

Insulin, Amino acids, GLP-1, GIP, Ghrelin and peptide YY (PYY).

The measurable end points for this protocol are:

1. Gut hormones levels in response to the 2 methods of AA delivery (I.V and oral)
2. Differences in gut hormones levels between young and older subjects when AA's are delivered orally

Protocol 2:

This will explore the second aim. 16 healthy older subjects will be recruited and subdivided randomly into two groups to receive either post absorptive or postprandial insulin concentrations with or without GLP-1 at physiological ranges in a cross over fashion . During acute study parameters of muscle glucose and amino acids metabolism will be tested together with muscle microvascular recruitment and macro vascular flow in the tested leg.

The measurable end points for this protocol are:

1. Muscle Glucose uptake, assessed by measuring 2-deoxyglucose (2-DOG) phosphate in muscle biopsies
2. Myofibrillar protein synthesis, assessed via muscle biopsy fractional synthesis rate (FSR)
3. Whole Leg Muscle Protein Synthesis, assessed via Arterial-Venous difference (AV method)
4. Whole Leg Muscle Protein Breakdown, assessed via AV method
5. Whole Leg Net Protein Balance, assessed via AV method
6. Muscle microvascular recruitment, assessed via microvascular contrast bubbles filling and refilling post destruction by ultrasound waves.

Eligibility Criteria:
Inclusion Criteria:

- For protocol 1: i. Aged between 18-40 or 65-75 years ii. A body mass index (BMI) \>18 and \<28 kg/m2

- For Protocol 2: i. Age 65-75 years ii. A body mass index (BMI) \>18 and \<28 kg/m2

Exclusion Criteria:

* For protocol 1:

  i. A BMI \< 18 or \> 28 kg·m2 ii. Active cardiovascular disease: uncontrolled hypertension (BP \> 160/100), angina, heart failure (class III/IV), arrhythmia, right to left cardiac shunt or recent cardiac event iii. Cerebrovascular disease: previous stroke, aneurysm (large vessel or intracranial) iv. Respiratory disease including pulmonary hypertension, chronic obstructive pulmonary disease (COPD), asthma or an forced expiratory volume in 1 minute (FEV1) less than 1.5 litre.

  v. Metabolic disease: hyper and hypo-parathyroidism, untreated hyper and hypothyroidism, Cushing's disease, types 1 or 2 diabetes vi. Active inflammatory bowel or renal disease vii. Malignancy viii. Recent steroid treatment (within 6 month), or hormone replacement therapy ix. Clotting dysfunction x. Musculoskeletal or neurological disorders xi. Family history of early (\<55y) death from cardiovascular disease
* For protocol 2:

Same as protocol 1 in addition to:

i. Overt muscle wasting i.e. muscle mass is more than 1 standard deviation below normal muscle or fat-free mass for age.

ii. Taking beta-adrenergic blocking agents or non-steroidal anti-inflammatory drugs iii. Known sensitivity to SONOVUE or any other drug used in the study. iv. Subject deemed unsuitable for femoral cannulation at screening visit.