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Sympathetic nerve activity (SNA) and high pressure baroreceptor regulation of SNA were studied in the Okamoto strain of spontaneously hypertensive rat (SHR). Mean arterial pressure (MAP) and SNA were not significantly affected by anesthesia with low doses of pentobarbital (20-25 mg/kg). Thus, most of these studies were performed in anesthetized rats. SNA in visceral sympathetic nerves increased rapidly with age up to 24 weeks and slowly thereafter. MAP increased with SNA, following the same time course. Both SNA and MAP in SHR were significantly greater than that found in normotensive Wistar control rats of comparable ages. Abolition of ganglionic transmission with hexamethonium in both SHR and normotensive controls reduced postganglionic SNA and MAP to comparable levels. In SHR less than 16 weeks old, increased baroreceptor stimulation effectively inhibited SNA with the same sensitivity as found in Wistar control rats. However, older SHR appeared to lose their ability to completely inhibit SNA during induced hypertension, whereas in Wistar control rats as old as 52 weeks, elevation of blood pressure to 165.3 +/- 2.3 mm Hg completely suppressed SNA. These results suggest that SNA may play an important role in the development and maintenance of hypertension in SHR, and that central sympathetic centers, uninhibited by baroreceptor afferents, become active during the development of hypertension in the SHR.

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