Plasma Norepinephrine in Hypertensive Rats Reflects α(2)-Adrenoceptor Release Control Only When Re-Uptake is Inhibited

Abstract

α(2)-adrenoceptors (AR) lower central sympathetic output and peripheral catecholamine release, thereby protecting against sympathetic hyperactivity and hypertension. Norepinephrine re-uptake-transporter effectively (NET) removes norepinephrine from the synapse. Overflow to plasma will therefore not reflect release. Here we tested if inhibition of re-uptake allowed presynaptic α(2)AR release control to be reflected as differences in norepinephrine overflow in anesthetized hypertensive spontaneously hypertensive rats (SHR) and normotensive rats (WKY). We also tested if α(2)AR modulated the experiment-induced epinephrine secretion, and a phenylephrine-induced, α(1)-adrenergic vasoconstriction. Blood pressure was recorded through a femoral artery catheter, and cardiac output by ascending aorta flow. After pre-treatment with NET inhibitor (desipramine), and/or α(2)AR antagonist (yohimbine, L-659,066) or agonist (clonidine, ST-91), we injected phenylephrine. Arterial blood was sampled 15 min later. Plasma catecholamine concentrations were not influenced by phenylephrine, and therefore reflected effects of pre-treatment. Desipramine and α(2)AR antagonist separately had little effect on norepinephrine overflow. Combined, they increased norepinephrine overflow, particularly in SHR. Clonidine, but not ST-91, reduced, and pertussis toxin increased norepinephrine overflow in SHR and epinephrine secretion in both strains. L-659,066 + clonidine (central α(2)AR-stimulation) normalized the high blood pressure, heart rate, and vascular tension in SHR. α(2)AR antagonists reduced phenylephrine-induced vasoconstriction equally in WKY and SHR.

Conclusions: α(2A)AR inhibition increased norepinephrine overflow only when re-uptake was blocked, and then with particular efficacy in SHR, possibly due to their high sympathetic tone. α(2A)AR inhibited epinephrine secretion, particularly in SHR. α(2A)AR supported α(1)AR-induced vasoconstriction equally in the two strains. α(2)AR malfunctions were therefore not detected in SHR under this basal condition.

Keywords: catecholamine release; epinephrine; hypertension; norepinephrine; norepinephrine re-uptake transporter; plasma catecholamine concentrations; sympathetic nervous system activity; α2-adrenoceptors.

Figure 1

The changes in MBP, TPVR, and HR in response to the centrally active, non-selective α₂AR agonist clonidine, without or with prior administration of the peripherally restricted, non-selective α₂AR antagonist L-659,066. After curve evaluations, significant responses (within symbols) and group differences at peak response (*in brackets left of curves) and at 15 min (*in brackets right of curves) were detected as indicated. Cardiovascular baselines prior to clonidine are shown in Table 2. ,*P ≤ 0.025.

Figure 2

The influence of α₂AR on the TPVR-peak-response to the α₁AR agonist phenylephrine. The rats were pre-treated as indicated by symbol legends. TPVR prior to phenylephrine is shown in Table 2. Within columns; significant responses. *In brackets; significant group differences. P ≤ 0.005; *P ≤ 0.0042.