Sympathetic nerves and adrenal medulla: contributions to cardiovascular-conditioned emotional responses in spontaneously hypertensive rats

This report investigates the contributions of the sympathetic nerves and adrenal medulla to resting mean arterial pressure (MAP) and to emotionally conditioned MAP and heart rate (HR) responses in unrestrained spontaneously hypertensive rats (SHR) and Wistar-Kyoto normotensive control rats (WKY). Resting MAP (in mm Hg), which was higher in SHR (WKY = 120 ± 4; SHR = 163 ± 4; p < 0.01), did not differ in the two strains following chemosympathectomy (WKY = 105 ± 2; SHR = 101 ± 2; n.s.). Adrenal medullectomy did not affect resting MAP in WKY (125 ± 6; n.s.) but lowered it in SHR (146 ± 5; p < 0.05), relative to controls (see above). The conditioned pressor response (in mm Hg) in controls consisted of two peaks (I, II) in both strains, but was exaggerated in SHR (I = WKY, 13 ± 1; SHR, 25 ± 2; p < 0.01; II = WKY 10 ± 2; SHR 20 ± 2; p < 0.01). Chemosympathectomy suppressed (relative to controls) the first peak, but not the second, in both strains (WKY: I = 4 ± 1, p < 0.01; II = 12 ± 2, n.s.; SHR: I = 6 ± 1, p < 0.01; II = 15 ± 2, n.s.). Adrenal medullectomy alone had little effect on the pressor response, but when combined with chemosympathectomy both peaks were largely eliminated (WKY: I = 2 ± 1 ; I I = 5 ± 1 ; SHR: I = 1 ± 0; II = 2 ± 0). These data indicate that: 1) hypertension in conscious, freely behaving SHR is largely sustained by the sympathetic vasomotor nerves but that the adrenal medulla contributes to the magnitude of the elevation; 2) the early component of the exaggerated pressor response during aversive stimulation is mediated by sympathetic vasomotor excitation; and 3) the later component of the exaggerated pressor response reflects coactivation of the sympathetic vasomotor nerves and the adrenal medulla.