Renal norepinephrine overflow during graded renal nerve stimulation before and after beta-adrenoceptor blockade

Mechanism of resistance to alpha-adrenergic receptor antagonists of renal nerve stimulation-induced vasoconstriction at low frequencies

To determine why renal vasoconstriction elicited by periarterial nerve stimulation (RNS) at lower frequencies (< 4 Hz) is resistant to alpha-adrenergic receptor blockade in the rat kidney, we reevaluated the effect of alpha-receptor antagonists on the vasoconstrictor response to norepinephrine (NE) and to RNS and on the release of adrenergic transmitter. The alpha-receptor antagonist prazosin (PZ) at 0.2 and 7 nM reduced the vasoconstrictor response to NE, and 2.4 microM PZ abolished it. PZ (0.2 or 7 nM) reduced RNS-induced vasoconstriction without altering the fractional tritium overflow. PZ (2.4 microM) enhanced fractional tritium overflow and reduced the vasoconstrictor response to RNS at 2-10 Hz, but not at 0.5 or 1 Hz. The effect of 0.2 nM PZ to reduce RNS-induced vasoconstriction was reversed by increasing the concentration to 2.4 microM. Corynanthine (COR; 2.6 microM), a preferential alpha-receptor blocker, or phenoxybenzamine (PBZ; 30 nM) abolished the vasoconstrictor response to NE but only partially reduced response to RNS and enhanced the fractional tritium overflow. Rauwolscine (RW; 2.5 nM), a preferential alpha 2-receptor antagonist, did not alter the vasoconstrictor response to NE but potentiated RNS-induced vasoconstriction and fractional tritium overflow. RW (7.7 microM) inhibited NE-induced vasoconstriction but potentiated the vasoconstrictor response to RNS and fractional tritium overflow. PZ (7 nM) abolished the potentiation by RW and reduced the vasoconstrictor response to RNS. These data suggest that a component of RNS-induced vasoconstriction in the rat kidney is attributable to co-release of a nonadrenergic transmitter with NE. The diminished effect of alpha-receptor antagonists at higher concentrations (e.g., PZ 2.4 microM) to reduce RNS-induced vasoconstriction is caused by their prejunctional action to enhance co-release of the nonadrenergic transmitter.

[Modulation of renal transmitter release by presynaptic receptors]

Renal sympathetic nerve varicosities possess a variety of receptors which when activated by appropriate agonists can modulate noradrenaline release at the local level of the kidney. Thus, activation of prejunctional alpha 1- and alpha 2-adrenoceptors, prostaglandin (PG), dopamine, adenosine and serotonin receptors inhibits, whereas activation of prejunctional beta 2-adrenoceptors and angiotensin (A) II receptors enhances renal noradrenaline release. Moreover, neuronally released noradrenaline itself activates prejunctional inhibitory alpha 1- and alpha 2-adrenoceptors forming a "negative feedback loop" of its own release (autoinhibition). PGE2 and adenosine locally formed in the kidney by renal nerve stimulation inhibits noradrenaline release through activation of their specific prejunctional receptor system (transjunctional inhibition).

Abnormalities in systemic norepinephrine kinetics in human congestive heart failure

A high venous plasma norepinephrine (NE) level is a predictor of poor prognosis in congestive heart failure (CHF). To evaluate the mechanisms responsible for the high plasma NE in CHF, NE kinetics were studied in 19 patients with CHF and 18 normal subjects during a 90-min steady-state intravenous infusion of tracer [3H]NE of high specific activity. Venous plasma NE between 70 and 90 min of infusion was significantly higher in the CHF patients (CHF, 634, and normal, 247 pg/ml; P less than 0.001). The following equations were used: NE clearance = [3H]NE infusion rate (dpm/min)/plasma [3H]NE (dpm/l), and NE spillover = [3H]NE infusion rate (dpm/min)/[3H]NE specific activity (dpm/nmol). In CHF, a decreased clearance and an increased spillover contributed nearly equally to the high plasma NE (NE clearance: CHF, 0.99; normal, 1.48 l.min-1.m-2; P less than 0.001; NE spillover: CHF, 3.60; normal, 2.08 nmol.min-1.m-2; P less than 0.001). These data document that both NE clearance and NE spillover are abnormal in CHF, and they raise the new possibility that the factors responsible for the reduced NE clearance could be related to the factors linking a high plasma NE with early mortality.

Physical stress and catecholamine release

In both health and disease, noradrenaline and adrenaline concentrations in plasma increase with intensity and duration of exercise (Figure 1). These changes are only to a minor extent due to decreased catecholamine clearance (Figure 2). The increase in sympathoadrenal activity during exercise is primarily elicited by feed-forward stimulation from motor centres in the brain (Figure 3, Table 1), and by afferent impulses from working muscles (Figure 4). During continued exercise, changes in internal milieu may enhance the catecholamine response. Of particular interest from a metabolic point of view is the fact that during exercise a decrease in plasma glucose causes a relatively large increase in plasma adrenaline (Figure 5). Sympathoadrenal activity is of major importance for exercise capacity. By depressing insulin secretion, as well as by direct effects on target tissues, sympathoadrenal activity enhances mobilization of glycogen as well as triglyceride from both extra- and intramuscular depots. After training, noradrenaline responses to given absolute work loads are reduced, while responses to given relative loads, i.e. work load in percent of individual work capacity, VO2/VO2max%, are unchanged. Prolonged endurance training may increase the size and secretory capacity of the adrenal medulla (Figure 7, Table 2), an adaptation which may improve exercise capacity. Differences in catecholamine levels cannot explain the fact that physically-active individuals have a lower cardiac mortality than inactive ones.

Role of renal nerve activity, plasma catecholamines and plasma vasopressin in cardiovascular responses to intracisternal neurotoxins in the rabbit

We have examined the acute (0-3 h) effect of intracisternally administered 5,7-dihydroxytryptamine (DHT) and 6-hydroxydopamine (6-OHDA) on blood pressure, heart rate, renal nerve activity, plasma adrenaline, plasma noradrenaline and plasma vasopressin in conscious rabbits. The increase in blood pressure seen following 5,7-DHT treatment was associated with increases in adrenaline and vasopressin levels and renal nerve activity throughout the response. The increase in blood pressure which followed 6-OHDA administration was associated with an increase in renal nerve activity alone. These findings indicate that the rise in blood pressure elicited by these drugs involves an increase in sympathetic nerve activity. The absence of a rise in vasopressin levels during the response to 6-OHDA suggests that the rise in blood pressure seen in these animals is due entirely to a bulbospinal sympathoexcitatory pathway.

Renal overflow of noradrenaline and dopamine to plasma during hindquarter compression and thoracic inferior vena cava obstruction in the dog

Efferent renal nerve activity was assessed by measurements of the overflow of endogenous noradrenaline (NA) and dopamine (DA) to plasma in the kidney. To obtain correct estimates of the renal contribution to the renal venous outflow of NA and DA, corrections for the renal extraction of catecholamines in arterial plasma were performed by use of tracer amounts of [3H]NA. Hindquarter compression (previously known to cause a neurogenically mediated blood pressure elevation) increased the concentrations of NA, adrenaline and DA in arterial and renal venous plasma. The renal overflow of NA increased from 83.7 +/- 32.0 to 361.3 +/- 119.4 pmol min-1 (P less than 0.05) during hindquarter compression. When compared to the renal NA overflow during electrical renal nerve stimulation, this corresponds to an increase in average renal nerve impulse activity from approximately 0.4 to 1.6 Hz. Hindquarter compression also increased the renal overflow of DA to plasma. When venous return to the heart was reduced by obstruction of the thoracic inferior vena cava, the mean arterial blood pressure fell and all catecholamines in plasma increased gradually during the first 10 min of obstruction. The renal overflow of NA was only slightly increased, indicating a minor increase in renal nerve activity. The overflow of DA to plasma was not altered by obstruction of the thoracic inferior vena cava. Neither maneuver substantially altered the DA/NA ratio for renal overflow rates or for renal venous plasma concentrations indicating that there was no preferential activation of either noradrenergic or putative dopaminergic nerve fibres.