Brainstem PNMT neurons and experimental hypertension in the rat

The number of phenylethanolamine-N-methyl transferase (PNMT) cells visualised with immunohistochemical techniques in the medulla oblongata is increased by 20% in 4 week old spontaneously hypertensive rats (SHR) and stroke prone spontaneously hypertensive rats (SHR-SP). This is associated with a 50% increase in the activity of PNMT and a significant rise in the amount of PNMT enzyme protein present in the medulla and spinal cord of both 4 weeks old and 4 months old SHR and SHR-SP. Since previous experiments had demonstrated that sinoaortic denervation also increased spinal cord PNMT activity we subjected normotensive Wistar Kyoto control rats (WKY) and hypertensive SHR and SHR-SP to denervation and measured the changes in blood pressure and in PNMT activity. Mean arterial pressure rose immediately after denervation in all 3 strains of rats, with much greater rises in the SHR and SHR-SP than in WKY, but the increase in pressure was only sustained in the normotensive WKY, in which it remained elevated throughout the one week observation period. In a similar way, denervation of the arterial baroreceptors increased the activity of PNMT in the medulla and spinal cord of normotensive WKY controls, confirming the results of previous studies but was not able to increase the already elevated PNMT levels in the SHR and SHR-SP any further in these two tissues. We suggest that there is good evidence that PNMT neurons contribute to the maintenance and elevation of arterial pressure in both the neurogenic and genetic models of hypertension. It also seems likely that the activity of descending spinal PNMT neurons is more important in the maintenance of a sustained increase in pressure than in the induction of a transient rise.