Endocrine Responses to Resistance Exercise

Hormones

Chemical messengers that are synthesized, stored and released into blood by endocrine glands and certain other cells

Endocrine glands

Body structures specialized for releasing hormones into the blood

The principal endocrine glands of the body along with other glands that secrete hormones

Hypothalamus, pituitary gland, thyroid gland, parathyroid glands, heart, adrenal glands, liver, pancreas, kidneys, ovaries, testes

Muscle as the target for hormone interactions

Hormones are intimately involved with protein synthesis and degradation mechanisms that are part of muscle adaptations to resistance exercise, this includes both anabolic (promote tissue building) and catabolic (degrade cell proteins) hormones.

Roles of receptors in mediating hormonal changes

A schematic representation of the classic lock-and-key theory for hormonal action at the cell receptor level

Downregulation

The inability of a hormone to interact with a receptor

Roles of receptors in mediating hormonal changes

Alterations to a receptor’s binding characteristics or the number of receptors can be as dramatic in adaptation as the release of increased amounts of hormone from an endocrine gland.

Steroid hormone interactions

Fat soluble and passively diffuse across the cell membrane

Typical steroid migration into a target cell by either testosterone in skeletal muscle or dihydrotestosterone in sex-linked tissues. Only one hormone pathway (testosterone or dihydrotestosterone) is targeted for one cell, but the two are shown together in this diagram. Each has different physiological outcomes.

Polypeptide hormone interactions

Made of chains of amino acids, ex: growth hormone and insulin

Typical polypeptide hormone interaction with a receptor via cytokine-activated JAK/STAT signaling pathway, although the hormone binds to an external receptor, a secondary messenger (STAT) is activated that can enter the cell nucleus

Amino hormone interactions

Synthesized from the amino acids tyrosine (e.g. epinephrine, dopamine) or tryptophan (serotonin), bind to membranes via secondary messengers, not regulated by negative feedback

Heavy resistance exercise and hormonal increases

Hormones are secreted before, during, and after resistance exercise bout due to the physiological stress of resistance exercise, as few as one or two heavy resistance exercise sessions can increase the number of androgen receptors in the muscle

Key point: the specific force produces in activated fibers stimulates receptor and membrane sensitivities to anabolic factors, including hormones, which leads to muscle growth and strength changes

Mechanisms of hormonal interactions

The combination of many different mechanisms is thought to stimulate exercise-induced hypertrophy, molecular signaling including hormones is involved with this process, this signaling is influenced by neural factors that provide important signals to the skeletal muscle and thus can augment anabolic processes

Hormonal changes in peripheral blood

Peripheral concentrations of hormones in the blood do not indicate the status of the various receptor populations or the effects of a hormone within the cell, it is typically assumed that large increases in hormone concentration indicate higher probabilities for interactions with receptors

Physiological mechanisms that contribute to changes in peripheral blood concentrations of hormones with exercise:

-circadian pattern

-fluid volume shifts

-tissue clearance rates

-venous pooling of blood

-hormone interactions with binding proteins

Key point: hormone responses are tightly linked to the characteristics of the resistance exercise protocol

Adaptations in the endocrine system

Examples of the types of adaptations that are possible:

-amount of synthesis and storage of hormones

-time needed for clearance of hormones through liver and other tissues

-how many receptors are in the tissue

-changes in the contents of the secretory cells in a gland

-degree of interaction with the cell nucleus

Testosterone

Heavy resistance training using one or two repetitions in low volume, which may not cause any changes in testosterone concentrations after a workout, could potentially still increase the absolute number of receptors and thus binding sites available to testosterone

This effect on receptors has yet to be fully confirmed

Key point: large muscle group exercises using an adequate volume of total work result in acute increased total testosterone concentrations in men

Primary anabolic hormones testosterone

Exercise variables that can increase serum testosterone concentrations:

-large muscle group exercises (deadlift, squats)

-heavy resistance (85-95% of 1 RM)

-moderate to high volume of exercises

-short rest intervals (30 seconds to 1 minute)

-two years or more of resistance training experience

Free testosterone and sex hormone-binding globulin, free testosterone accounts for only 0.5%-2% of total testosterone; thus higher total testosterone concentration allows for more free testosterone. Heavy resistance exercise (e.g., six sets of 10 repetitions at 80% of 1RM) can acutely increase free testosterone in men and women, although the increase is much smaller for women.

Responses in women:

-women have about 15- to 20-fold lower concentrations of circulating testosterone than men do.

-the testosterone concentration can vary substantially between individual women, as some women secrete higher concentrations of adrenal androgens

Responses to six sets of squats at 80% of 1RM with 2 minutes rest between sets

-(a) total testosterone

-(b) free testosterone in response

-indicates a significant increase from preexisting values

-# indicates a significant difference from women at the corresponding time point

Training adaptations

-testosterone increases in response to the demands of an exercise protocol, and then the receptors either increase binding to use the testosterone or they do not due to a lack of need for the signal to increase muscle-related metabolism.

-other receptors on other target tissues (e.g. nervous, satellite cells) may be more affected at certain time points in training, depending on the window of adaptation available in the target tissues.

Growth hormone

Important for the normal development of a child, appears to play a vital role in adapting to the stress of resistance training

Main physiological roles of GH and its superfamily:

-decreases glucose utilization

-increase protein synthesis

-increases collagen synthesis

-stimulates cartilage growth

-enhances immune cell function

-increases lipolysis (fat breakdown)

Key point: growth hormone release is affected by the type of resistance training protocol used, including the duration of rest period. Short rest period types of workouts result in greater serum concentrations compared to long rest periods of similar total work; however, at present it is not clear how the various molecular forms (e.g. aggregates and splice variants) or types of GH are affected by rest period duration

Responses in women:

-hormone concentrations and hormone responses to exercise vary with menstrual phase

-mechanisms of this variation are unclear

Training adaptations:

-it appears that GH concentrations need to be measured over longer time period (2-24 hours) to show whether changes occur with resistance training.

-the area under the time curve, which includes an array of pulsatile effects, tells whether changes in release have occurred.

Cortisol

Exerts its major catabolic effects by stimulating the conversion of amino acids to carbohydrates, increasing the level of proteolytic enzymes (enzymes that break down proteins), inhibiting protein synthesis, and suppressing many glucose-dependent processes such as glycogenesis and immune cell function. Had a greater effect on type II fibers than type I fibers

Resistance exercise responses

-responds to resistance exercise protocols that create a dramatic stimulus to anaerobic metabolism

-increases in cortisol might not have negative effects in men after a period of training to which the body has adapted; adaptations “disinhibits” cortisol at the level of the testis, thereby maintaining testosterone’s primary influence on its nuclear receptors

Resistance exercise protocols that use high volume, large muscle groups, and short rest periods result in increased serum cortisol values. Though chronic high concentrations of cortisol may have adverse catabolic effects, acute increases still contribute to the remodeling of muscle tissue and maintenance of blood glucose.

Catecholamines

Roles

-increase force production via central mechanisms and increased metabolic enzyme activity

-Increase muscle contraction rate

-Increase blood pressure

-increase energy availability

-increase muscle blood flow (via vasodilation)

-augment secretion rates of other hormones, such as testosterone

Training adaptations

-heavy resistance training has been shown to increase the ability of an athlete to secrete greater amounts of epinephrine during maximal exercise

-because epinephrine is involved in metabolic control, force production, and the response mechanisms of other hormones (such as testosterone, GHs, and IGFs), stimulations of catecholamines is probably one of the first endocrine mechanisms to occur in response to resistance exercise.

Other hormonal considerations

Hormones such as insulin, thyroid hormones, and beta-endorphin have been implicated in growth, repair, and exercise stress mechanisms; unfortunately, few data are available concerning their responses and adaptations to resistance exercise or training.