Thigh Muscles Grow, Get Stronger in Elderly from
Increasing Insulin Supply
A blood insulin level double that produced by a
typical meal seems to turn back the clock on elderly thigh muscle
Sept. 25, 2009 – Recently, scientists have
recognized that loss of responsiveness to insulin plays a major role in
the loss of physical strength that occurs as people grow older. Now,
University of Texas Medical Branch at Galveston researchers have
demonstrated that by increasing insulin levels above the normal range in
elderly test subjects, they can restore the impaired muscle-building
process responsible for age-related physical weakness.
When most people think of insulin, they think of
diabetes - a disease that arises when, for one reason or another,
insulin can't do the critical job of helping the body process sugar.
But
the hormone has another, less well-known function. It's also necessary
for muscle growth, increasing blood flow through muscle tissue,
encouraging nutrients to disperse from blood vessels and itself serving
as a biochemical signal to boost muscle protein synthesis and cell
proliferation.
"Insulin is normally secreted during food intake,"
said Dr. Elena Volpi, senior author of a paper on the study published in
the September issue of Diabetologia.
"When you give insulin intravenously and increase
the blood insulin levels to the same amount produced after a meal, you
see that in young people it stimulates protein synthesis and muscle
growth, while in older people it really doesn't. But when we gave
seniors double the insulin they would normally produce after eating,
their muscles were stimulated like those of young people."
Volpi and her co-authors - postdoctoral fellows
Satoshi Fujita and Kyle Timmerman, graduate student Erin Glynn and
Professor Blake B. Rasmussen - worked with 14 elderly volunteers to
examine the response of thigh muscle to the two different blood insulin
levels, established by infusion into the thigh's main artery.
Blood samples taken from catheters inserted in the
femoral artery and vein of each subject enabled the researchers to
calculate blood flow and muscle protein synthesis, and muscle biopsies
allowed them to measure levels of signaling molecules involved in muscle
protein growth.
All the data pointed in the same direction, showing
that a blood insulin level double that produced by a typical meal seems
to turn back the clock on elderly thigh muscle.
"While we had called this 'insulin resistance' in
the past, we didn't really have evidence that you can get an elderly
person's muscle to grow if you give it a lot more insulin, which is what
we needed to truly say this is insulin resistance," Volpi said.
At the same time, she said, the phenomenon is also
quite different from the insulin resistance seen in diabetes.
"These were older subjects with perfect glucose
tolerance," she said. "So what we have identified is a novel kind of
insulin resistance that's not related to sugar control,” she said.
Instead, Volpi said, the UTMB researchers attribute
this new kind of insulin resistance to age-related changes in the
vascular system — in particular, changes in the endothelium, the
single-cell-thick layer that lines blood vessels.
The endothelium controls blood flow by increasing
or decreasing the diameter of capillaries (the smallest blood vessels),
and regulates the release of oxygen, nutrients, water and other
blood-borne cargo through the capillary walls and into muscles and other
body tissues.
"Having a capillary dilation induced by insulin is
important, because it exposes more muscle to the nutrients and hormones
and everything flows better and gets stored away better," Volpi said.
"But in even healthy older people, this dilation
response doesn't work, because they have this endothelial dysfunction."
The UTMB researchers are now testing whether using
drugs to dilate muscle blood vessels during insulin exposure can improve
muscle growth in older people.
"Preliminary data suggest that this treatment may
be effective, but these data are not yet published," Volpi said.
"On the other hand, in a paper we published two
years ago in Diabetes, we showed that a single bout of aerobic exercise
— a staple of diabetes treatment — may also improve muscle growth in
response to insulin in older nondiabetic people."
Clinical trial needs senior citizens
Volpi's group is now conducting a larger,
NIH-funded clinical trial to determine if aerobic exercise and
nutritional supplementation for six months can also boost muscle size
and function in sedentary but otherwise healthy seniors. UTMB's Sealy
Center on Aging and Claude D. Pepper Older Americans Independence Center
are recruiting seniors from the Galveston-Houston area for the study.
For more information, call 800-298-7015.
The University of Texas Medical Branch at Galveston
www.utmb.edu
Keep up with the latest news for senior citizens, baby
boomers
Sept. 25, 2009 – Recently, scientists have
recognized that loss of responsiveness to insulin plays a major role in
the loss of physical strength that occurs as people grow older. Now,
University of Texas Medical Branch at Galveston researchers have
demonstrated that by increasing insulin levels above the normal range in
elderly test subjects, they can restore the impaired muscle-building
process responsible for age-related physical weakness. 