Hormonal Control

• Substrate utilization during exercise:

  • significant quantity of substrate comes from blood
  • low intensity = blood
  • high intensity = muscle
• Blood glucose homeostasis:

  • glucose is a major substrate for metabolism
  • glucose glucose is the only fuel acceptable to the brain and CNS

  normal resting blood glucose concentration:

  • 4.0 — 5.5 mmol/L
  • 90 — 100 mg/dL
• Exercise increases blood glucose uptake:

  • uptake is initiated by muscle contraction
  • sends signals that cause translocation of glut-4 to sarcolemma
  • contraction changes metabolism concentration
  • glut-4 accepts glucose and transports it into cytosol
• Another reason why maintaining blood glucose is important:

  • to prevent hypoglycemia
• We do not store a lot of glucose in our body, therefore:

  • if we only relied on glucose, we would deplete it rapidly
• Blood glucose and exercise:

  • at moderate intensity, energy is produced via oxidative phosphorylation
  • glucose actually rises then plateaus
  • suggests glucose is entering blood from another source at rate equal to its uptake
• How do we replenish glucose?

  liver is a big source of sugar

  2 processes:

  • anabolic rxns: gluconeogenesis; building glucose; uses lactate, pyruvate, and glycerol (some amino acids)
  • catabolic rxns: breakdown glycogen (stored in liver)
• Lipid mobilization:

  • we eventually limit glucose use by relying on fats instead
  • indirect way to maintain glucose
• Glucose regulation during exercise:

  requires physiological regulation:

  • endocrine system = chemical communication
  • nervous system = electrical communication
• Endocrine:

  • controls physiological processes that support exercise and maintain homeostasis
  • releases hormones
  • hormones = produced and released by glands, transported through blood
• Hormones:

  • secretion = released in pulsatile bursts (plasma concentration fluctuates); triggered by negative feedback
  • receptors = bind to specific receptor
  • action = exert effects after binding; initiate reactions
• Endocrine glands:

  • pancreas
  • adrenal glands
  • thyroid gland
  • anterior pituitary gland
• Pancreas:

  • insulin = lowers blood glucose; counters hyperglycemia
  • glucagon = raises blood glucose; counters hypoglycemia
• Blood glucose control:

  • type 1 diabetes = insulin deficiency
  • type 2 diabetes = impaired glucose control
• Insulin-stimulated glucose uptake:

  • insulin binds to receptors to signal glut-4 transporters from vesicles
• Glucose regulation by pancreatic hormones:

  • during moderate to heavy, blood glucose does not change much because…
  • insulin falls
  • glucagon rises
• Prolonged exercise:

  • glucagon rises quickly at exercise onset to prevent hypoglycemia
  • insulin falls so so that there is less glucose uptake
• Adrenal medulla:

  • above kidneys
  • releases catecholamines (epinephrine and norepinephrine)
  • stimulate glycogenolysis
  • stimulate lipolysis (Epi)
  • suppress insulin secretion
  • increase glucose and free fatty acids to active muscle
• EPI and NE:

  • increase intensity, Epi and NE rise
  • shift of use from carbs to fats
• Summary:

  • insulin = cellular glucose uptake
  • insulin = glycogen synthesis
  • insulin = triglyceride synthesis
  • insulin = decrease blood glucose
  • glucagon = liver glycogenolysis
  • glucagon = liver gluconeogenesis
  • Epi = liver glycogenolysis
  • Epi = muscle glycogenolysis
  • Epi = lipolysis
  • NE = liver glycogenolysis
  • NE = muscle glycogenolysis
• Hormones…?

  regulate shift in substrate utilization and mobilization of fat and carb stores