Hypertension through adrenaline activation of prejunctional beta-adrenoceptors

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.

Increased density of perivascular nerves to the major cerebral vessels of the spontaneously hypertensive rat: differential changes in noradrenaline and neuropeptide Y during development

Fluorescence and immunohistochemical techniques were used to study the pattern and density of perivascular nerves containing noradrenaline (NA) and neuropeptide Y (NPY) supplying the major cerebral arteries of 4-, 6-, 8- and 12-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar (WIS) controls. Levels of NA and NPY in the superior cervical ganglia were measured. The density of nerves containing NA and NPY was greater in the hypertensive animals at all ages studied. However, the developmental changes in the density of innervation showed similar trends in both SHR and WIS groups. With few exceptions, there was a significant increase in the density of nerves containing NA from 4 to 6 weeks and from 8 to 12 weeks of age. This was in contrast to a low expression, and in some vessels a significant decrease in the number of NPY-containing nerves from 4 to 6 weeks. The density of nerve fibres containing NPY increased significantly in almost all vessels between 6 and 8 weeks of age and then stabilized. Thus there is a differential time course for the appearance of NA and NPY during development. Furthermore, the hyperinnervation of cerebral vessels in SHR by nerves containing NA and NPY precedes the onset of hypertension and associated medial hypertrophy. High-performance liquid chromatography and enzyme-linked immunosorbant assays show that the NA and NPY contents of the superior cervical ganglion do not reflect the changes in innervation pattern seen in the terminal fibres in the cerebral arteries. This tends to support the view that a local neurovascular mechanism is involved in the maintenance of hypertension. The possibility that increase in NPY as well as NA in cerebral perivascular nerves of hypertensive animals is involved in the protection of the blood-brain barrier against oedema and cerebral haemorrhage is raised.

Factors contributing to the maintenance of arterial blood pressure in adult rats treated neonatally with guanethidine

Adult rats treated neonatally with guanethidine had normal arterial blood pressures, but these were more dependent on the renin-angiotensin system than in control animals. Antagonism of V1-receptors (with d(CH2)5DAVP) enhanced the depressor effects of captopril in guanethidine-treated animals, but blood pressure was unaffected by this manoeuvre in control animals. Although there is evidence that sympathetic efferent vasomotor function is abolished following the schedule of neonatal guanethidine treatment used, pentolinium had a hypotensive effect in the presence of d(CH2)5DAVP and captopril, indicating that the adrenal medulla could have been contributing to the maintenance of blood pressure. This is consistent with the finding of marked supersensitivity to exogenous adrenaline in guanethidine-treated rats.

Increased plasma noradrenaline during low dose adrenaline infusion in resting man and during sympathetic stimulation

Both resting and stimulated (straight-leg raising and head-up tilt) levels of arterial and venous plasma noradrenaline were significantly higher during low-dose adrenaline infusion in five mild hypertensive and four normotensive patients with one adrenal. Repeat adrenaline infusions in the five hypertensive patients while measuring noradrenaline clearance (3H-noradrenaline constant infusion) achieved similar levels of plasma adrenaline, and similar increases in plasma noradrenaline, within five min of achieving target infusion rate. Increased plasma noradrenaline was not explained by reduced clearance. These results are consistent with the hypothesis that physiological concentrations of adrenaline are capable of facilitating noradrenaline release in man.

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

Adrenaline activates prejunctional beta-adrenoceptors of the beta 2-subtype on sympathetic nerve terminals and enhances noradrenergic transmission. Adrenaline can be incorporated in transmitter stores of noradrenergic nerves and, when released as a cotransmitter, activates the prejunctional beta 2-adrenoceptors, thereby mediating autofacilitation of noradrenergic transmission. Adrenaline released from the adrenal medulla in stress may be incorporated in noradrenergic transmitter stores and reach a sufficient concentration as a cotransmitter to activate the autofacilitatory feedback loop involving prejunctional beta 2-adrenoceptors, resulting in prolongation of the increases in vasomotor tone and cardiac activity that occur acutely: with frequent repetition of stress, there may be progression into a hypertensive state. In accord with this hypothesis, adrenaline administration produces persistent increases in blood pressure in rats, and plasma levels of adrenaline are elevated in a proportion of hypertensive patients; furthermore, repeated stress produces prolonged increases in blood pressure in animals and man.

Minireview. Presynaptic receptors and alterations in norepinephrine release in spontaneously hypertensive rats

The ability of blood vessels to constrict to a given stimulus is significantly increased in spontaneously hypertensive rats (SHR). Such an increase in the vasoconstrictor responsiveness contributes to the elevated peripheral vascular resistance noted in SHR. The present review discusses evidence in support of the concept that an increased release of norepinephrine during sympathetic nerve stimulation may contribute to the increase in vasoconstrictor responsiveness and, subsequently, to an increase in vascular resistance in the SHR. Several studies suggest that the exocytotic release of norepinephrine from sympathetic nerves may be altered by endogenously occurring neurohumoral substances which produce their effects by interacting with presynaptic receptors located on postganglionic sympathetic nerves. Therefore, it is postulated that alterations in presynaptic regulation of norepinephrine release, resulting from changes in the functioning of one or more of these presynaptic receptors, may lead to a greater release of norepinephrine in the SHR. This review summarizes the results of studies evaluating presynaptic receptor mechanisms and norepinephrine release in the SHR. These studies suggest that norepinephrine release during sympathetic nerve stimulation is greater in the SHR and that alterations in some of the presynaptic receptor mechanisms may be responsible for this phenomenon.

Effect of epinephrine on norepinephrine release from rat kidney during sympathetic nerve stimulation

Experiments were performed to study presynaptic beta-adrenoceptor facilitation of sympathetic neurotransmitter release in the isolated perfused rat kidney and evaluate the effect of epinephrine on norepinephrine release during sympathetic nerve stimulation. The right kidney was isolated and perfused with Krebs-Ringer solution. Norepinephrine storage sites were labelled with [3H]norepinephrine. Increasing concentrations of isoproterenol and salbutamol when perfused through the kidney, caused an enhancement of the stimulus-induced release of [3H]norepinephrine at 0.5 and 2 Hz, with the maximum facilitatory effect being observed at 0.5 Hz. The effect of salbutamol on [3H]norepinephrine release was concentration-dependent and more pronounced than that of isoproterenol. While propranolol (10(-9)-10(-5)M) by itself did not cause any significant changes in the stimulus-induced release of [3H]norepinephrine, it antagonized the facilitatory action of salbutamol on [3H]norepinephrine release during periarterial nerve stimulation. When epinephrine (10(-10)-10(-7)M), was perfused through the kidney in the presence of cocaine, it caused a concentration-dependent inhibition of the stimulus-induced release of [3H]norepinephrine release elicited during periarterial nerve stimulation. However, when epinephrine was perfused in the presence of cocaine, phentolamine and corticosterone it caused a slight but significant increase in the stimulus-induced release of [3H]norepinephrine; the highest concentration (10(-7)M) still caused a decrease in the [3H]norepinephrine release. These results, while providing evidence for the existence of presynaptic facilitatory beta-adrenoceptors on renal sympathetic nerves, fail to support the hypothesis that these receptors have a physiological role in the regulation of sympathetic neurotransmitter release.

The influence of arterial baroreceptors in man on the variability of blood pressure and plasma catecholamines in man

The variability of intra-arterial blood pressure, recorded over the waking hours in free-ranging subjects away from the hospital, was expressed as the SD of the mean of all recorded beats. This variability was found to be related to age, the individual's level of arterial pressure, and his baroreflex sensitivity using the Oxford phenylephrine method. Multiple regression analysis showed that baroreflex sensitivity was the only independent influence on variability. Baroreflex sensitivity also strongly determined the level of pressure rise in response to a number of different stimuli, including bicycling exercise, isometric exercise, mental arithmetic, a reaction time test, and the response to injected phenylephrine or noradrenaline. Plasma noradrenaline proved a poor index of sympathetic nervous activity, except with bicycle exercise.

Plasma catecholamine concentrations in normotensive rats of different strains and in spontaneously hypertensive rats under basal conditions and during cold exposure

Under basal conditions, the levels of circulating norepinephrine (NE) and epinephrine (E) were higher in normotensive Wistar rats of different origins than in Sprague-Dawley rats. Since the decline of 3H-NE concentration in the plasma after i.v. injection was similar in Wistar and in Sprague-Dawley rats, the higher levels of endogenous NE in the former strain probably reflect greater NE release from sympathetic nerve terminals. In normotensive Sprague-Dawley and Wistar rats, plasma NE rose to various extents during cold exposure (4 degrees C), depending on the basal plasma NE levels. Compared with normotensive Wistar Kyoto rats (WKY), spontaneously hypertensive rats (SHR) had similar basal plasma E and NE concentrations, similar rates of 3H-NE disappearance, but more rapid increases to higher values of plasma NE during cold exposure. It is concluded that the basal rate of peripheral catecholamine release does not seem to be the main determining factor for arterial blood pressure in the various rat strains and that the sympathetic neuronal system of SHR is more responsive to cold exposure than that of WKY rats.