| The purpose of this dissertation was to begin to examine how the central nervous system may be involved in mediating the acute and chronic effects of exercise on resting arterial blood pressure in spontaneously hypertensive rats. SHR that were run on a treadmill for a period of 10 weeks or, alternatively, allowed access to running wheels for a similar period of time showed acute cardiovascular responses to microinjection of the GABA synthesis inhibitor 3-mercaptopropionic acid into the caudal hypothalamus. Further, the treadmill animals showed a reduced resting arterial pressure. The exercised animals, therefore, demonstrated responses more similar to normotensive animals. These findings are consistent with the hypothesis that exercise can upregulate the caudal hypothalamic GABAergic system, contributing to the reduced cardiovascular tone of exercised hypertensive animals.; In a second set of experiments, I found that neurons in the ventrolateral medulla demonstrate increased sensitivity to muscle contraction in hypertensive versus normotensive rats. This increase in neuronal sensitivity is accompanied by an increased cardiovascular response to muscle contraction. We did not observe differences in basal firing rates of neurons in the VLM between SHR and WKY. Therefore, muscle contraction may have unmasked differences in neuronal function and regulation of cardiovascular function.; In a third set of experiments, hypertensive rats were run on a treadmill and demonstrated this effect as soon as 15 minutes following exercise. Acute bouts of muscle contractions in anesthetized hypertensive rats also induced an acute post-exertional hypotensive state. Further, increases in arterial pressure induced by microinjections of the broad band excitatory amino acid receptor agonist D,L-homocysteic acid into the ventrolateral medulla were suppressed following muscle contractions. These findings suggest that a decreased ability of the ventrolateral medulla to drive cardiovascular function may contribute to post-exertional hypotensive effects following exercise.; In summary, these studies suggest that both acute and chronic exercise have the ability to affect central neural regulation of cardiovascular function in hypertensive rats. These findings suggest that the central nervous system may play a larger role in mediating the positive cardiovascular effects of exercise than previously thought. |
