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SUMMARY Although evidence exists for exaggerated sympathetic nervous system activity in spontaneously hypertensive rats (SHR), there are no studies in conscious animals that directly demonstrate that this increased activity is functionally involved in the elevated vascular resistance of these animals. In our present study, 8-week-old and 13-week-old SHR and Wistar Kyoto controls (WKY) were chronically Instrumented with arterial and venous catheters and miniaturized pulsed Doppler flow probes on the renal and mesenteric arteries and lower abdominal aorta. While the rats were conscious and unrestrained, bexamethonium was administered intravenously to block sympathetic nervous system transmission. Prior to bexamethonlum, the mean arterial pressure of young SHR and WKY averaged 123 ± 5 and 109 ± 4 nun Hg respectively (p < 0.05), while adult SHR and WKY averaged 159 ± 7 and 128 ± 3 mm Hg respectively (p < 0.05). Hexamethonium produced an equivalent fall in arterial pressure of young SHR (-32%) and WKY (-30%) and adult SHR (-39%) and WKY (-41%). Vascular resistance was reduced by hexamethonium in the kidney, gut, and hindquarters, but the percent changes were not significant between SHR and WKY. These data suggest that, in both young and adult SHR, vascular resistance and arterial pressure are sustained at elevated levels by some other mechanism than aeurally-derived vasoconstrictor tone.

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