β1-Blockers Lower Norepinephrine Release by Inhibiting Presynaptic, Facilitating β1-Adrenoceptors in Normotensive and Hypertensive Rats

Abstract

Peripheral norepinephrine release is facilitated by presynaptic β-adrenoceptors, believed to involve the β2-subtype exclusively. However, β1-selective blockers are the most commonly used β-blockers in hypertension. Here the author tested the hypothesis that β1AR may function as presynaptic, release-facilitating auto-receptors. Since β1AR-blockers are injected during myocardial infarction, their influence on the cardiovascular response to acute norepinephrine release was also studied. By a newly established method, using tyramine-stimulated release through the norepinephrine transporter (NET), presynaptic control of catecholamine release was studied in normotensive and spontaneously hypertensive rats. β1AR-selective antagonists (CGP20712A, atenolol, metoprolol) reduced norepinephrine overflow to plasma equally efficient as β2AR-selective (ICI-118551) and β1+2AR (nadolol) antagonists in both strains. Neither antagonist lowered epinephrine secretion. Atenolol, which does not cross the blood-brain barrier, reduced norepinephrine overflow after adrenalectomy (AdrX), AdrX + ganglion blockade, losartan, or nephrectomy. Atenolol and metoprolol reduced resting cardiac work load. During tyramine-stimulated norepinephrine release, they had little effect on work load, and increased the transient rise in total peripheral vascular resistance, particularly atenolol when combined with losartan. In conclusion, β1AR, like β2AR, stimulated norepinephrine but not epinephrine release, independent of adrenal catecholamines, ganglion transmission, or renal renin release/angiotensin AT1 receptor activation. β1AR therefore functioned as a peripheral, presynaptic, facilitating auto-receptor. Like tyramine, hypoxia may induce NET-mediated release. Augmented tyramine-induced vasoconstriction, as observed after injection of β1AR-blocker, particularly atenolol combined with losartan, may hamper organ perfusion, and may have clinical relevance in hypoxic conditions such as myocardial infarction.

Keywords: adrenal glands; angiotensin II, hypertension; atenolol; metoprolol; norepinephrine release; β-adrenoceptors.

Figure 1

Presynaptic control of norepinephrine release from peripheral sympathetic nerve endings. Presynaptic modulation of vesicular release is reflected as differences in overflow to plasma only when re-uptake through NET is blocked (9). Tyramine stimulates norepinephrine release from peripheral sympathetic nerve terminals by reverse transport through NET. Consequently, re-uptake through NET is prevented. The influence of presynaptic release control can therefore be studied by differences in the tyramine-induced norepinephrine overflow to plasma (10). The presynaptic receptors will be activated by the released transmitters, or by other agonists present in their vicinity. The release of norepinephrine from secretory granules is activated by adenylyl cyclase, which is stimulated and inhibited, respectively, by stimulatory (Gs) and inhibitory (Gi) G proteins. Gs- and Gi-activation is mediated by β₁₊₂AR and α₂AR presynaptic receptors, respectively. The AT1R augment adenylyl cyclase activity by inhibiting Gi-signaling. β₁AR and α₂AR also modulate renin release from the kidneys. The nicotinic receptor antagonist hexamethonium will inhibit ganglion transmission as well as nerve-stimulated epinephrine release from the adrenals. Dotted arrows indicate nerve signals, curved arrows indicate action of tyramine-released norepinephrine, thick arrows indicate positive effects, whereas blunted arrows indicate inhibitory actions. Modified from Ref. (11).

Figure 2

Outline of the experimental design. During pre-treatment, the rats were subjected to (arrows) (1) none or surgical intervention with NX or AdrX, (2) no injection or injection of PBS or hexamethonium or losartan, and (3) injection of PBS or βAR antagonist, in combinations as outlined in Table 1. All rats were subsequently infused with tyramine. Changes in MBP, CO, HR TPR, and cardiac work load (CWL) were recorded from before drug pre-treatment to before tyramine, and every min throughout the tyramine infusion for MBP, CO, HR, and TPR, and from before to the end of the 15-min tyramine infusion for CWL. The effect of NX and AdrX was analyzed by comparing the baselines prior to drug pre-treatment with that in rats not subjected to surgical intervention. Blood for measurement of plasma catecholamines was collected from the femoral artery after the 15-min tyramine-observation period but without discontinuing the infusion.

Figure 3

The effect of atenolol (A) or metoprolol (B) on the HR-response to tyramine in WKY and SHR. The rats were pre-treated as outlined in the legends. After overall and step-by-step curve evaluations with Repeated Measures Analyses of Variance and Covariance (please see Statistical Analyses), group differences were located at 15 min (* in brackets right of curves), as indicated. Comparisons were made in (A) between the controls and losartan-treated groups, between the losartan- and AdrX + losartan-pre-treated groups and between corresponding groups without and with pre-treatment with atenolol; and in (B) between the controls and the metoprolol-treated groups, and between the metoprolol- and the losartan + metoprolol-treated groups. In the same order as in the legends, HR baselines prior to tyramine was in (A) 340 ± 9, 338 ± 9, 333 ± 13, 312 ± 10, 310 ± 8, 304 ± 10, 307 ± 16 bpm in WKY and 394 ± 5, 360 ± 10^†, 343 ± 13^†, 325 ± 9^†, 372 ± 6^†, 328 ± 21^†, 297 ± 5^† bpm in SHR (^†; P  ≤ 0.0083 compared to the controls), and in (B) 340 ± 9, 300 ± 8^†, 318 ± 14 bpm in WKY and 394 ± 5, 333 ± 12^†, 292 ± 7^† bpm in SHR (^†P  ≤ 0.004). * In brackets – P  ≤ 0.05 after curve evaluations with Bonferroni-adjusted P-values.

Figure 4

The effect of atenolol (A, B) or metoprolol (C) on the TPR-response to tyramine in WKY and SHR. The rats were pre-treated as outlined in the legends. After overall and step-by-step curve evaluations with Repeated Measures Analyses of Variance and Covariance (please see Statistical Analyses), significant responses were located at peak-response (all significant except in the WKY AdrX + losartan + tyramine group, not indicated) and at 15 min (* within symbol, as indicated). Group differences were located at the same times (* in brackets left and right of curves, respectively), as indicated. Comparisons were made in (A) between the PBS- and losartan-treated groups in not AdrX and AdrX rats, between the losartan- and AdrX + losartan-pre-treated groups, and between corresponding groups without and with pre-treatment with atenolol; in (B) between the controls and the NX groups, between corresponding groups without and with pre-treatment with atenolol; and in (C) between the controls and the metoprolol-treated groups, and between the metoprolol- and the losartan + metoprolol-treated groups. In the same order as in the legends, TPR baselines prior to tyramine was in mm Hg/mL/min in (A) 2.1 ± 0.1, 1.7 ± 0.1, 1.7 ± 0.1, 2.2 ± 0.1, 2.3 ± 0.2, 2.2 ± 0.2 in WKY and 4.8 ± 0.2, 4.0 ± 0.5, 4.1 ± 0.4, 6.3 ± 0.5, 7.6 ± 1.1, 5.1 ± 0.9 in SHR, in (B) 2.1 ± 0.1, 2.4 ± 0.1, 1.4 ± 0.1^†, 1.9 ± 0.2 in WKY and 4.8 ± 0.2, 5.7 ± 0.4, 2.2 ± 0.3^†, 4.1 ± 0.5 in SHR, and in (C) 2.1 ± 0.1, 2.0 ± 0.2, 1.6 ± 0.1^† in WKY and 4.8 ± 0.2, 4.3 ± 0.2, 4.1 ± 0.3 in SHR (^†P  ≤ Bonferroni-adjusted P-values). * In brackets – P  ≤ 0.025 after curve evaluations with Bonferroni-adjusted P-values.