Simultaneous parasympathetic and sympathetic activation reveals altered autonomic control of heart rate, vascular tension, and epinephrine release in anesthetized hypertensive rats

Abstract

Sympathetic hyperactivity and parasympathetic insufficiency characterize blood pressure (BP) control in genetic hypertension. This shift is difficult to investigate in anesthetized rats. Here we present a pharmacological approach to simultaneously provoke sympathetic and parasympathetic transmitter release, and identify their respective roles in the concomitant cardiovascular response. To stimulate transmitter release in anesthetized normotensive (WKY) and spontaneously hypertensive rats (SHR), we injected intravenously 4-aminopyridine (4-AP), a voltage-sensitive K(+) channel (K(V)) inhibitor. A femoral artery catheter monitored BP, an ascending aorta flow-probe recorded cardiac output and heart rate (HR). Total peripheral vascular resistance (TPVR) was calculated. 4-AP-induced an immediate, atropine (muscarinic antagonist)- and hexamethonium (ganglion blocker)-sensitive bradycardia in WKY, and in both strains, a subsequent, sustained tachycardia, and norepinephrine but not epinephrine release. Reserpine (sympatholytic), nadolol (β-adrenoceptor antagonist) or right vagal nerve stimulation eliminated the late tachycardia, adrenalectomy, scopolamine (central muscarinic antagonist) or hexamethonium did not. 4-AP increased TPVR, transiently in WKY but sustained in SHR. Yohimbine (α(2)-adrenoceptor antagonist) prevented the TPVR down-regulation in WKY. Reserpine and prazosin (α(1)-adrenoceptor antagonist) eliminated the late vasoconstriction in SHR. Plasma epinephrine overflow increased in nadolol-treated SHR. Through inhibition of K(V), 4-AP activated parasympathetic ganglion transmission and peripheral, neuronal norepinephrine release. The sympathetic component dominated the 4-AP-HR-response in SHR. α(2)-adrenoceptor-dependent vasodilatation opposed norepinephrine-induced α(1)-adrenergic vasoconstriction in WKY, but not SHR. A βAR-activated, probably vagal afferent mechanism, hampered epinephrine secretion in SHR. Thus, 4-AP activated the autonomic system and exposed mechanisms relevant to hypertensive disease.

Keywords: 4-aminopyridine; adrenal; epinephrine; heart rate; norepinephrine; parasympathetic nerves; sympathetic nerves; total peripheral vascular resistance.

Figure 1

The cardiovascular response to 4-AP in WKY and SHR. 4-AP was injected as a bolus injection (arrow). The SBP- and DBP-responses paralleled, MBP was therefore used to demonstrate the BP-response. Curve evaluation was done by Repeated Measures Analyses of Variance and Covariance with significant responses located as described in Materials and Methods, at HR nadir (brackets left of curves) and at 25 min (brackets right of curves). Please see Table 2 for cardiovascular baselines prior to 4-AP. * – P < 0.01.

Figure 2

The HR- and TPVR-response to 4-AP in WKY and SHR after pre-treatment with other K⁺ channel inhibitors. Rats were pre-treated with the BK_Ca inhibitor TEA or the K_ATP channel inhibitor glibenclamide as indicated by symbol legend. Significant responses during the initial response and at 25 min were subsequently located as indicated (* within symbol). Significant group differences were not detected. The effects of TEA and glibenclamide on baselines are shown in Table 2. * – P ≤ 0.025.

Figure 3

The HR-response to 4-AP after pre-treatment with the mAChR antagonist atropine or electrical stimulation of the right, efferent vagal nerve. After curve evaluation, significant responses (* within symbol) and group differences at HR nadir (brackets left of curves) and at 25 min (brackets right of curves) were located as indicated. Atropine had little effect on the HR baseline, whereas vagal stimulation greatly reduced HR in both strains (Table 2). * – P < 0.025.

Figure 4

The HR-response to 4-AP after pre-treatment with nAChR antagonist, i.e., the ganglion blocker hexamethonium. Significant responses (* within symbol) and group differences at the initial HR_nadir (brackets left of curves) and after 25 min (none detected) were as indicated. Hexamethonium reduced baseline HR in both strains (Table 2). * – P ≤ 0.025.

Figure 5

HR-response to 4-AP after removal of components of the adrenergic system. The rats were pre-treated to eliminate adrenal (AdrX) or neuronal (reserpine) catecholamines, peripheral β_1 + 2AR (peripherally restricted antagonist nadolol), or β₃AR-activity (β₃AR antagonist SR59230A). Reserpine and nadolol were also combined with mAChR antagonist (atropine), as indicated by symbol legends. Column graphs to the right shows HR_nadir during the initial response (1–2 min) and ΔHR at 25 min. Significant responses (* within column) and group differences (brackets) were located as indicated. Baseline HR was slightly reduced after reserpine and nadolol in WKY and clearly after nadolol in SHR (Table 2). * – P ≤ 0.025.

Figure 6

The effect of the norepinephrine reuptake inhibitor desipramine on the HR-response to 4-AP. Significant responses (* within symbol) at HR nadir and at 25 min were located as indicated. A difference between the SHR control and the desipramine-treated group was detected at 10 min (* between curves). Baseline HR was not different after desipramine (Table 2). * – P ≤ 0.025 and 0.0167 for significant responses and group differences, respectively.

Figure 7

The effect of scopolamine on the HR- (A) and TPVR- (B) response to 4-AP in SHR. Scopolamine had no effect on the MBP-response to 4-AP (not shown). We have previously shown that scopolamine did not significantly alter the response in WKY (Berg, 2002). Significant responses (* within symbol) during the initial response and after 25 min, as well as group differences at these times (brackets) were detected as indicated. Scopolamine caused a minor reduction in baseline HR (Table 2). * – P ≤ 0.025.

Figure 8

The TPVR- (A) and MBP- (B) response to 4-AP after blockade of components of the adrenergic system. Adrenal catecholamines were eliminated by AdrX, neuronal norepinephrine by reserpine, β_1 + 2AR activity with the peripherally restricted antagonist nadolol, and β₃AR-activity with the β₃AR antagonist SR59230A. Reserpine and nadolol were given alone or combined with mAChR antagonist (atropine), as indicated by symbol legends. Significant responses (* within symbol) and group differences (brackets) were located as indicated. Baseline TPVR was higher in AdrX SHR, and reduced after reserpine, reserpine + atropine, and nadolol + atropine in SHR. MBP was lower after reserpine in WKY (Table 2). * – P ≤ 0.025.

Figure 9

The TPVR- (A), MBP- (B), and HR- (C) response to 4-AP in WKY and SHR after pre-treatment with αAR antagonists. Significant responses (* within symbol) and group differences during the immediate TPVR_max and at 25 min (brackets) were located as indicated. All antagonists reduced baseline TPVR in WKY, but not significantly in SHR (Table 2). Baseline MBP was lower after all three antagonists in WKY and after phentolamine and prazosin in SHR. Baseline HR was lower after phentolamine in WKY and after phentolamine and prazosin in SHR (Table 2). * – P ≤ 0.025.

Figure 10

The TPVR- (A) and MBP- (B) response to 4-AP after pre-treatment with the peripherally restricted nAChR antagonist, ganglion blocker hexamethonium. Significant responses (* within symbol) during the initial response and after 25 min, were detected as indicated. Significant group differences were not detected. Hexamethonium reduced baseline MBP in both strains and also TPVR in SHR (Table 2). * – P ≤ 0.025.

Figure 11

The TPVR- (A) and MBP- (B) response to 4-AP after atropine and vagal nerve stimulation. Significant responses (* within symbols) at the initial peak response and after 25 min were detected as indicated. Significant group differences were not detected. Atropine and vagal nerve stimulation did not alter MBP and TPVR baselines (Table 2). * – P ≤ 0.025.

Figure 12

The TPVR- (A) and MBP- (B) response to 4-AP after inhibition of norepinephrine reuptake with desipramine. Significant responses (* within symbol) and group differences during the immediate TPVR_max (none detected) and after 25 min (brackets right of curve) were located as indicated. Desipramine reduced baseline TPVR in both strains and also MBP in WKY (Table 2). * – P ≤ 0.025.