Tyramine Reveals Failing α2-Adrenoceptor Control of Catecholamine Release and Total Peripheral Vascular Resistance in Hypertensive Rats

Abstract

α2-Adrenoceptor-activation lowers central sympathetic output, peripheral, vesicular norepinephrine release, epinephrine secretion, and modulates vascular tension. We previously demonstrated that α2-adrenoceptor-mediated inhibition of basal norepinephrine release was not reflected in plasma unless re-uptake through the norepinephrine transporter (NET) was blocked. Tyramine activates reverse norepinephrine transport through NET. Here we tested the hypothesis that tyramine, by engaging NET in release, also blocks re-uptake, and therefore allows manipulation of pre-junctional α2-adrenoceptors to directly regulate norepinephrine overflow to plasma. We compared in anesthetized spontaneously hypertensive rats (SHRs) and normotensive controls (WKYs), the effect of α2-adrenoreceptor antagonist (L-659,066) and/or agonist (clonidine) on norepinephrine overflow and increase in total peripheral vascular resistance (TPR) evoked by tyramine-infusion (1.26 μmol/min/kg, 15 min) and epinephrine secretion activated by the surgical stress. TPR was computed as blood pressure divided by cardiac output, recorded as ascending aortic flow. Plasma catecholamine concentrations after tyramine were higher in SHRs than WKYs. Pre-treatment with L-659,066 increased the catecholamine concentrations in WKYs, but only if combined with clonidine in SHRs. Clonidine alone reduced tyramine-induced norepinephrine overflow in SHRs, and epinephrine in both strains. Tyramine-induced increase in TPR was not different after clonidine, eliminated after L-659,066 and L-659,066 + clonidine in WKYs, but only after L-659,066 + clonidine in SHRs. We conclude that tyramine-infusion does allow presynaptic regulation of vesicular release to be accurately assessed by measuring differences in plasma norepinephrine concentration. Our results indicate that presynaptic α2-adrenoceptor regulation of norepinephrine release from nerve vesicles and epinephrine secretion is dysfunctional in SHRs, but can be restored by clonidine.

Keywords: epinephrine; hypertension; norepinephrine; release control; sympathetic nervous system; total peripheral vascular resistance; α2-adrenoceptors.

Figure 1

Norepinephrine release in the sympathetic synapse. Vesicular release is stimulated by Ca²⁺ and inhibited by α₂AR. The re-uptake of norepinephrine through NET can be reversed to release by tyramine. Then NET will be engaged in release and re-uptake is hampered, and overflow to plasma is increased. However, vesicular norepinephrine release is not directly influenced by this process, and will still be subjected to presynaptic control by the α₂AR or other presynaptic receptors. Clonidine will lower the sympathetic tone through a central action, as well as stimulate presynaptic and postsynaptic α₂AR. L-659,066, which does not cross the blood-brain barrier, will inhibit the peripheral pre- and postsynaptic α₂AR. In the VSMC, L-659,066 will allow βAR-G_s-signaling to dominate tension control when norepinephrine release is stimulated. Blunted arrows indicate inhibition.

Figure 2

The effect of the NET inhibitor desipramine and the muscarinic antagonist atropine on the cardiovascular response to tyramine. After curve evaluation using Repeated Measures Analyses of Variance and Covariance (please see Materials and Methods), significant responses (* within symbol) were detected at 3 and 15 min for ΔMBP, ΔTPR, and ΔSV, and at 15 min for ΔCO and ΔHR, as indicated. Significant group differences at 3 min (* in left brackets) or 15 min (* in right brackets) were detected as indicated. Baselines prior to tyramine are shown in Table 1. *within symbols: P ≤ 0.025 for ΔMBP, ΔTPR and ΔSV, and P ≤ 0.05 for ΔCO and ΔHR. * in brackets: P ≤ 0.001.

Figure 3

The changes in MBP, TPR, CO, and HR in WKYs and SHRs during tyramine-induced norepinephrine release after pre-treatment with the centrally active, non-selective α₂AR-agonist clonidine and the peripherally restricted α₂AR antagonist L-659,066, separately or combined. After curve evaluation using Repeated Measures Analyses of Variance and Covariance (please see Materials and Methods), significant responses (* within symbol) and group differences (* in brackets) were detected at 3 and 15 min for ΔMBP and ΔTPR, and at 15 min for ΔCO and ΔHR, as indicated. Baselines prior to tyramine are shown in Table 1. * within symbols and * in brackets: P ≤ 0.025 for ΔMBP and ΔTPR, and P ≤ 0.05 for ΔCO and ΔHR.