Adrenaline mediates a positive feedback loop in noradrenergic transmission: its possible role in development of hypertension

Aerobic training increases renal antioxidant defence and reduces angiotensin II levels, mitigating the high mortality in SHR-STZ model

OBJECTVE

The purpose of the research was to investigate the effects of aerobic training on renal function, oxidative stress, intrarenal renin-angiotensin system, and mortality of hypertensive and diabetic (SHR-STZ) rats.

MATERIALS AND METHODS

Blood pressure, creatinine, urea levels, urinary glucose, urine volume, and protein excretion were reduced in trained SHR-STZ rats.

RESULTS

Aerobic training not only attenuated oxidative stress but also elevated the activity of antioxidant enzymes in the kid'ney of SHR-STZ rats. Training increased intrarenal levels of angiotensin-converting enzymes (ACE and ACE2) as well as the neprilysin (NEP) activity, along with decreased intrarenal angiotensin II (Ang II) levels. Aerobic training significantly improved the survival of STZ-SHR rats.

CONCLUSION

The protective role of aerobic training was associated with improvements in the renal antioxidative capacity, reduced urinary protein excretion along with reduced intrarenal Ang II and increased NEP activity. These findings might reflect a better survival under the combined pathological conditions, hypertension, and diabetes.

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

Age-related memory impairments due to reduced blood glucose responses to epinephrine

Increases in blood glucose levels are an important component of the mechanisms by which epinephrine enhances memory formation. The present experiments addressed the hypothesis that a dysfunction in the blood glucose response to circulating epinephrine contributes to age-related memory impairments. Doses of epinephrine and glucagon that significantly increased blood glucose levels in young adult rats were far less effective at doing so in 2-year-old rats. In young rats, epinephrine and glucose were about equally effective in enhancing memory and in prolonging post-training release of acetylcholine in the hippocampus. However, glucose was more effective than epinephrine in enhancing both memory and acetylcholine release in aged rats. These results suggest that an uncoupling between circulating epinephrine and glucose levels in old rats may lead to an age-related reduction in the provision of glucose to the brain during training. This in turn may contribute to age-related changes in memory and neural plasticity.

Plasma epinephrine levels in hypertension and across gender and ethnicity

Epinephrine (E) infusions raise blood pressure and there is an excess incidence of hypertension among males and blacks. However, reports of E levels by ethnicity, gender, and blood pressure status are inconsistent. Insensitive assays, variability in plasma E levels within individuals, and the small size of most studies have contributed to these conflicting reports. We measured plasma E levels in a large diverse sample of subjects, using a highly sensitive assay. A total of 361 individuals participated in the study: 61% were men and 39% women, 74% were normotensive and 26% hypertensive, 59% were white and 41% were black. Except for difference in blood pressure and body mass index between the normotensives and hypertensives, subjects had similar baseline characteristics and took no antihypertensive medications for at least five days prior to sampling. All blood samples were collected after resting for a least 30 minutes following the insertion of an indwelling i.v. catheter. Catecholamine levels were determined using a radioenzymatic assay (assay sensitivities for E and norepinephrine were 6 pg/ml and 10 pg/ml, respectively). An ethnicity by gender interaction was found (F(1,315) = 5.126, p = .024). Subsequent analysis revealed that white women had significantly lower basal plasma E levels than white men (p <0.001) and black women (p = 0.036). There were no significant differences in E levels between black men and women or between white men and black men. Uncorrected E levels were lower in normotensive than hypertensive subjects (p = .009) but this difference was not significant when corrected for body mass index (BMI). Uncorrected norepinephrine levels were higher in women than men (p = .03) but the difference was no longer significant when corrected for BMI. Plasma E levels were significantly lower among white women than men or black women. In contrast to prior studies, E levels were lower in hypertensives, but this may reflect obesity among hypertensives.

A test of the "epinephrine hypothesis" in humans

-According to the "epinephrine hypothesis," circulating epinephrine taken up by sympathetic nerves is coreleased with norepinephrine during sympathetic stimulation and binding of coreleased epinephrine to presynaptic beta-adrenoceptors augments exocytotic release of norepinephrine, contributing to high blood pressure. This study examined whether infusion of a physiologically active amount of epinephrine affects subsequent vascular responses and the estimated rate of entry of norepinephrine into regional venous plasma (norepinephrine spillover). Each of 3 experiments included intravenous infusion of 3H-norepinephrine, measurements of forearm vascular resistance, and intra-arterial infusion of epinephrine (3 ng/min per deciliter forearm volume). In experiment 1, subjects underwent lower body negative pressure (LBNP-25 mm Hg) before and after intra-arterial epinephrine; in experiment 2, LBNP and intra-arterial yohimbine before and after intra-arterial epinephrine; and in experiment 3, intravenous nitroprusside before and after intra-arterial epinephrine. In all subjects, intra-arterial epinephrine produced ipsilateral pallor and decreased forearm vascular resistance. Ipsilateral venous epinephrine increased by 10-fold. Epinephrine did not affect forearm vasoconstrictor responses to LBNP or vasodilator responses to intra-arterial yohimbine or intravenous nitroprusside; did not affect venous norepinephrine levels or norepinephrine spillover during LBNP, yohimbine, LBNP during yohimbine, or nitroprusside; and did not increase venous epinephrine levels during any of these manipulations. Loading of forearm sympathetic terminals with epinephrine therefore does not augment subsequent neurogenic vasoconstriction or norepinephrine release in the human forearm in response to sympathetic stimulation. The findings are inconsistent with the epinephrine hypothesis.

Basal and stimulated sympathetic responses after epinephrine: no evidence of augmented responses

Delayed facilitation of norepinephrine release through the action of epinephrine (NE) at presynaptic beta-adrenoceptors has been postulated to account for the delayed hemodynamic effects of epinephrine and to be a mechanism causally related to the development of hypertension. To determine whether a short-term increase in epinephrine concentrations resulted in subsequent facilitation of sympathetic responses, 9 healthy subjects (age, 21+/-0.9 years) were studied at rest and during physiological stress on 2 occasions when they received an infusion of either saline or epinephrine (20 ng/kg per minute) in random order. Heart rate, blood pressure, forearm blood flow, epinephrine concentrations, and NE spillover were measured at rest, during mental stress (Stroop test), and during a cold pressor test. Measurements were performed before, during the 1-hour infusion of epinephrine or placebo, and 1 hour after the infusion. A radioisotope dilution method was used to measure NE spillover. Hemodynamic measurements and NE spillover were increased during the infusion of epinephrine, but 1 hour after discontinuation of epinephrine there was no significant augmentation of hemodynamic or sympathetic responses. NE spillover 1 hour after saline or epinephrine infusion was similar (0.85+/-0.2 versus 0. 87+/-0.2 microg/min; P=0.92). In addition, there was no delayed facilitation of stress-induced hemodynamic or NE responses after epinephrine. These findings do not support the hypothesis that epinephrine results in delayed facilitation of NE release.

Disposition of endogenous adrenaline compared to noradrenaline released by cardiac sympathetic nerves in the anaesthetized dog

The fate of adrenaline released from cardiac sympathetic nerves was compared with that of noradrenaline before and during two periods of electrical stimulation of the left ansa subclavia in eight anaesthetized dogs. Cardiac spillovers and extractions of both catecholamines were estimated simultaneously using infusions of 3H-labelled adrenaline and noradrenaline. Animals were studied before and after neuronal uptake blockade with desipramine. Cardiac spillover of adrenaline, detectable at rest at 1.4 +/- 0.3 pmol/min, increased to 4.0 +/- 1.1 and 5.3 +/- 1.2 pmol/min during sympathetic stimulation. Cardiac noradrenaline spillover increased from 49 +/- 12 to 205 +/- 40 and 451 +/- 118 pmol/min. After desipramine, cardiac spillovers of adrenaline were decreased, whereas those of noradrenaline were increased so that the ratio of adrenaline to noradrenaline spillover, meaned before and during stimulation, decreased substantially from 1:42 to 1:166. The desipramine-induced decrease in cardiac extractions of 3H-labelled catecholamines indicated adrenaline was removed 60% less efficiently than noradrenaline by neuronal uptake, whereas the extractions remaining indicated adrenaline was removed 50% more efficiently by extraneuronal uptake. The differences in removal processes indicated that 35% of the adrenaline released by cardiac sympathetic nerves was recaptured compared to 88% for noradrenaline, leaving 53% to be removed extra-neuronally compared to 6.6% for noradrenaline, so that proportionally more released adrenaline than noradrenaline escaped to spillover into plasma (12% versus 5.4%). Since extra-neuronal uptake was more efficient for adrenaline than noradrenaline, proportionally less released adrenaline than noradrenaline escaped local removal to spillover into plasma when neuronal uptake was blocked (17% versus 45%). This reversed the situation before blockade so that desipramine substantially decreased the ratio of adrenaline to noradrenaline spillover. Thus, differences in the efficiencies of neuronal or extraneuronal uptake are important determinants of the amounts of locally released adrenaline and noradrenaline that escape removal processes to act at neuroeffector sites or spillover into plasma.

Cardiovascular dystonia in recovered panic patients

Resting heart rate, heart rate variability and blood pressure; the heart rate and blood pressure response to standing; and the heart rate response to Valsalva's manoeuvre, have been measured in a group of 12 patients with panic disorder and a group of 12 age- and sex-matched normal subjects. The patients had undergone treatment for their panic attacks with cognitive therapy; all had responded and all had been panic-free for a minimum of 4 months (mean 7.6 months). Mood ratings (BDI, BAI and SSAI) were comparable with established norms. The patients nevertheless had a raised resting systolic blood pressure, a reduced resting heart rate and an abnormal orthostatic response.

Single doses of catecholamines in the rat: catecholaminotropic, but not hyperglycemic

1. As in two "lower" vertebrates, the lamprey and the eel, single intravascular injections of physiological doses (2.5 micrograms/kg) of epinephrine (E) into the rat immediately increased levels of plasma dopamine (DA) and norepinephrine (NE). 2. Single doses of DA (5 micrograms/kg) enhanced circulating NE and E, while NE (5 micrograms/kg) had no clear impact on the plasma levels of the other two catecholamines (CAs). 3. These data are at variance with findings in the eel, where all three CAs are mutually stimulatory; and in the lamprey, where only E stimulates release of the other two CAs. 4. It appears that E-stimulated CA release is widespread or ubiquitous among vertebrates, and that complex interactions between circulating CAs must be considered under experimental, physiological, and clinical conditions. 5. None of the injections had a significant hyperglycemic effect.

Sensitivity changes of the perfused hindquarters' vasculature in rats with alloxan-induced diabetes mellitus

1. Arachidonic acid (AA, 0.125-1.0 mg/kg) injected via the aorta into the autoperfused hindquarters caused dose-dependent increases in perfusion pressure. This effect was reduced after intravenous administration of the thromboxane receptor antagonist AH23848 (5 mg/kg) or indomethacin (5 mg/kg). 2. Responses to AA (0.125-1.0 mg/kg) were reduced markedly in the Krebs-perfused hindquarters when compared with those occurring in the blood-perfused preparation. 3. Doses of guanethidine (1 mg/kg) and pentacynium (1 mg/kg) blocking pressor responses to intravenous administration of the ganglion stimulants McN-A-343 and DMPP, respectively, did not affect responses to AA. 4. Constrictor responses to AA (0.5-1.0 mg/kg) in blood-perfused hindquarters were increased in 14 day alloxan-diabetic rats but those to the thromboxane A2-mimetic U46619 (0.5-8.0 micrograms/kg, i.a.) were reduced when compared with non-diabetic controls. 5. In 14 day alloxan-diabetic rats vasoconstrictor responses to noradrenaline and methoxamine were potentiated but those to 5-hydroxytryptamine were reduced compared with non-diabetic animals. 6. It is concluded that AA causes constriction in the blood-perfused hindquarters by release of a product of cyclo-oxygenase acting on thromboxane A2-receptors. A constituent of blood, perhaps the platelet, appears necessary for this effect. Conversion of AA to the constrictor metabolite is augmented during experimentally induced diabetes.

Subendothelial beta 2-adrenoceptors in the rat vena cava: facilitation of noradrenaline release via local stimulation of angiotensin II synthesis

Preparations of the cranial segment of the rat inferior vena cava preincubated with 3H-noradrenaline were superfused in the presence of desipramine and corticosterone. Tritium overflow was stimulated electrically (2 Hz). The experiments were carried out in spirally cut strips with or without intima or in segments ligated at both ends and superfused either on the adventitial side ("conventionally") or "inside out". In spirally cut strips electrically evoked 3H overflow was increased by isoprenaline and procaterol, but much less so by prenalterol. Adrenaline 1 nmol/l increased overflow, but at high concentrations it reduced it, just as noradrenaline did at all concentrations. The concentration-response curve for isoprenaline was shifted to the right by propranolol (apparent pA2:8.29) and even more so by ICI 118-551, whereas atenolol was less potent (apparent pA2:6.42). Rauwolscine which, given alone, increased the evoked 3H overflow antagonized the inhibitory effect of noradrenaline (apparent pA2:7.58). These findings indicate that beta 2- and alpha 2-adrenoceptors mediating facilitation and inhibition of noradrenaline release, respectively, are present in the vena cava. The response to isoprenaline (at all concentrations) was considerably lower in segments superfused "conventionally" than in spirally cut strips, but no difference was observed with respect to the effects of noradrenaline, rauwolscine and angiotensin II. The effect of isoprenaline was clearly more pronounced in segments superfused "inside out" than in segments superfused "conventionally". In spirally cut strips angiotensin II increased 3H overflow. This effect was antagonized by saralasin, suggesting the involvement of facilitatory angiotensin receptors. In spirally cut strips or segments superfused "inside out", saralasin or captopril considerably attenuated the facilitatory effect of isoprenaline on 3H overflow. Conversely, in the presence of isoprenaline, captopril inhibited the electrically evoked 3H overflow in spirally cut strips, whereas in the absence of isoprenaline, captopril was ineffective. In conclusion, angiotensin receptors and alpha 2-adrenoceptors appear to be located on the sympathetic nerve endings, but a major part of the beta 2-adrenoceptors probably is subendothelial (most likely on smooth muscle cells). Angiotensin II, synthesized in response to beta 2-adrenoceptor activation, probably stimulates angiotensin receptors on the noradrenergic nerves, leading to an increase in noradrenaline release.