Surgery depresses pulsatile growth hormone release in rats for up to 2 days

In order to study the pulsatile release of rat growth hormone in a stress-free environment, many investigators obtain sequential blood samples from individual rats bearing an indwelling, right atrial cannula. There has been little systematic study of the stressful effects of the cannulation procedure on the pulsatile release of rat growth hormone. We examined the acute and chronic effects of right atrial cannulation on growth hormone release in male rats by monitoring plasma growth hormone concentration at every 2 min. Right atrial cannulation was performed under ether anaesthesia. Blood was collected from the rats for a 2 h period (11.00-13.00 h), either immediately following, or 2, 3 or 7 days following the surgery. When blood was collected 7 days after surgery, growth hormone was released in large bursts. The amplitude of these bursts however, did not differ significantly from the bursts of growth hormone in rats cannulated 3 days prior to blood collection. On the other hand, the bursts of hormone release in rats cannulated immediately or 2 days before blood collection were significantly smaller in amplitude than those in rats cannulated 3 days before collection, but were not significantly different from each other. In many of the rats cannulated immediately before or 2 days before blood collection, pulsatile growth hormone release was completely suppressed. The results of the study suggest that blood sampling from right atrial cannulae to measure the plasma concentration of rat growth hormone should not be carried out until at least 3 days after the cannulation procedure.