Developmental key to overactive sympathetic system

Nov 20 2012

By Eleanor McDermid, Senior medwireNews Reporter

Being small for gestational age may be accompanied by an exaggerated sympathetic response to physical exercise, an animal study suggests.

This "is in agreement with a growing body of evidence, albeit predominately indirect, that there is an alteration in sympathetic nerve activity in human adults born small for gestational age," say the researchers, Scott Smith and team, from the University of Texas Southwestern Medical Center in Dallas, USA.

They add that the abnormal sympathetic response could contribute to these individuals' tendency to develop chronic hypertension.

In an editorial accompanying the study in Hypertension, Norma Ojeda (University of Mississippi-Medical Center, Jackson, USA) comments: "Nowadays, physical activity is closely associated with wellness and fitness, and it is included in programs to promote healthy lifestyle to prevent chronic diseases.

"Therefore, deeper assessment of individuals' health history, including birth weight, should be a requirement before any interventional strategy, such as regular exercise programs."

For the study, Smith et al induced prenatal programming of hypertension (PPH) by feeding pregnant rats a diet containing just 6% protein. The resultant offspring were, on average, significantly smaller than those whose mothers were fed a 20%-protein diet.

Systolic blood pressure in awake rats was significantly higher in the PPH than control rats, but contrary to the researchers' expectations, renal sympathetic nerve activity under general anesthesia or after decerebration was no higher in the PPH rats than in the control rats.

However, activation of the exercise pressor reflex by electrically induced static muscle contraction in decerebrated rats caused a significantly larger increase in renal sympathetic nerve activity in PPH than control rats, by an average of 198% versus 68%.

In response to the exercise pressor reflex, the PPH rats also had significantly larger increases in heart rate than the controls, at 19 versus five beats per minute, and in mean arterial pressure, at 40 versus 20 mmHg, which was caused by exaggerated increases in both systolic and diastolic pressure.

Ojeda notes that humans who were born small for gestational age reportedly have increased sympathetic responses to psychologic stress.

She explains: "An insult during development will initiate adaptive changes to allow fetal survival but will also modify the setting of several regulatory systems leading to abnormal responses to normal stimuli such as exercise, resulting in exacerbated increases in blood pressure."

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