The role of skeletal muscle beta-adrenoreceptors in the regulation of plasma potassium

Catecholamine-dependent hyperpolarization of the junctional membrane via β2- adrenoreceptor/Gi-protein/α2-Na-K-ATPase pathway

We investigated the effects of catecholamines, adrenaline and noradrenaline, as well as β-adrenoceptor (AR) modulators on a resting membrane potential at the junctional and extrajunctional regions of mouse fast-twitch Levator auris longus muscle. The aim of the study was to find which AR subtypes, signaling molecules and Na,K-ATPase isoforms are involved in the hyperpolarizing action of catecholamines and whether this action could be accompanied by changes in the pump abundance on the sarcolemma. Adrenaline, noradrenaline and specific β2-AR agonist induced hyperpolarization of both junctional and extrajunctional membrane, but the underlying mechanisms were different. In the junctional membrane the hyperpolarization depended on α2 isoform of the Na,K-ATPase and Gi-protein, whereas in the extrajunctional regions the hyperpolarization mainly relied on α1 isoform of Na,K-ATPase and adenylyl cyclase activities. In both junctional and extrajunctional regions, AR activation caused an increase in Na,K-ATPase abundance in the plasmalemma in a protein kinase A-dependent manner. Thus, the compartment-specific mechanisms are responsible for catecholamine-mediated hyperpolarization in the skeletal muscle.

Metabolic effects of antihypertensive agents: role of sympathoadrenal and renin-angiotensin systems

Reports of beneficial, neutral and adverse impacts of antihypertensive drug classes on glucose and lipid metabolism can be found in human data. Furthermore, mechanisms for these diverse effects are often speculative and controversial. Clinical trial data on the metabolic effects of antihypertensive agents are highly contradictory. Comparisons of clinical trials involving different agents are complicated by differences in the spectrum of metabolic disturbances that accompany hypertension in different groups of patients. Two physiological systems are predominant at the interface between metabolic and cardiovascular regulation: the sympathetic nervous system (SNS) and the renin-angiotensin system (RAS). These two systems are major targets of antihypertensive drug actions, and also mediate many of the beneficial and adverse effects of antihypertensive agents on glucose and lipid metabolism. Thiazides and beta-adrenergic antagonists can adversely affect glucose and lipid metabolism, which are frequently compromised in human essential hypertension, and increase the incidence of new cases of diabetes. Laboratory studies confirm these effects, and suggest that compensatory activation of the SNS and RAS may be one mechanism. Other antihypertensives directly targeting the SNS and RAS may have beneficial effects on glucose and lipid metabolism, and may prevent diabetes. Resolution of the controversies surrounding the metabolic effects of antihypertensive agents can only be resolved by further laboratory studies, in addition to controlled clinical trials.

The importance of beta-adrenergic receptors in immune regulation: a link between neuroendocrine and immune system

Our knowledge of autoimmunity and autoimmune diseases has been advanced in the past decades. Receptors present on the immune cells may potentially regulate the immune system, among them, beta-adrenergic receptors are of special interest. As neurotransmitter receptors which are also present on lymphocytes, beta-adrenergic receptors play an important role as the linkage of two important systems, neuroendocrine and immune systems. Here I summarize several lines of evidence of the importance of the beta-adrenergic receptors in immune regulation.

Decreased beta2-adrenergic receptor density on peripheral blood mononuclear cells in myasthenia gravis

Beta2-adrenergic receptors (beta2AR) are present on both lymphocytes and skeletal muscle cells. Antibodies and T cells that react with these receptors are present in patients with myasthenia gravis (MG). Immune reactivity against the beta2AR may thus modify both the immune and the muscle functions in MG. In this study, we analysed the density and affinity of beta2AR on peripheral blood mononuclear cells using a radioligand binding assay. The density (Bmax) of the receptor on cells from patients with MG was significantly lower than that on cells from patients with other neurological disorders and healthy individuals. The affinity (Kd) of the receptor and the concentration of the second messenger, cAMP, in the cells did not differ between the groups. Serum antibodies against beta2AR were demonstrated in 22% of 27 MG patients vs. 0% of 26 healthy controls. Incubation of cells with serum or purified IgG containing antibodies against the beta2AR resulted in a decline in ligand binding of the receptor in samples from three out of five patients. Thus, this study suggests that a downregulation of the beta2AR may occur in MG. This downregulation might be of importance in the patho-genesis of the disease and its symptoms.

Coexistence of beta 2- and beta 3-adrenoceptors in plasma potassium control in conscious rabbits

1. In conscious rabbits the intravenous infusion of adrenaline (0.3 microgram kg-1 min-1), noradrenaline (1 microgram kg-1 min-1) or isoprenaline (1.25 micrograms kg-1 min-1) caused a significant decrease in plasma potassium levels. Propranolol (9 mg kg-1, s.c.) and ICI 118551 (30 micrograms kg-1, s.c.) reversed adrenaline-induced hypokalaemia and revealed a sustained hyperkalaemia. 2. Salbutamol (0.5 microgram kg-1 min-1, i.v.), beta 2-adrenoceptor agonist, evoked a biphasic response: an initial hyperkalaemia which was followed by a hypokalaemia; a higher dose (3 micrograms kg-1 min-1, i.v.) solely induced hypokalaemia. ICI 118551 blocked the salbutamol-mediated response. 3. Noradrenaline evoked hypokalaemia was blunted completely in the presence of bupranolol (0.1 mg kg-1, s.c.), a beta 1-, beta 2- and beta 3-adrenoceptor antagonist, but not in the presence of the beta 1-adrenoceptor antagonist CGP 207 12A (1 mg kg-1, s.c.). 4. BRL 37344 (0.15 microgram kg-1 min-1, i.v.), SR 58611A (0.26 microgram kg-1 min-1, i.v.), both full beta 3-agonists, and CGP 12177 (0,25 micrograms kg-1 min-1, i.v.), a partial agonist which also acting as a non-selective beta 1- and beta 2-antagonist, induced a significant hypokalaemia. Bupranolol, but not ICI 118551 or CGP 20712A, blocked the BRL 37344-mediated hypokalaemia. 5. Ouabain (1.7 micrograms kg-1 min-1, i.v.), an inhibitor of the Na,K-pumps, inhibited both salbutamol-and BRL 37344-mediated hypokalaemia. 6. These data suggest the coexistence of beta 2- and beta 3-adrenoceptor control of extrarenal potassium disposal; moreover both beta 2 and beta 3 hypokalaemic effects would be mediated by activation of Na,K-pumps.

Effects of catecholamines on plasma potassium: the role of alpha- and beta-adrenoceptors

The sympathetic nervous system plays an important role in the control of plasma potassium levels. Administration of adrenaline or noradrenaline evokes, in the majority of mammal species a dual response: first a short transient hyperkalaemia, followed by a maintained hypokalaemia. Alpha 1- and alpha 2-adrenoceptors mediate the initial hyperkalaemia through the activation of hepatic Ca(2+)-dependent-K(+)-channels. Stimulation of beta 1- and beta 2-adrenoceptors induces the late hypokalaemia by stimulation of skeletal muscle Na(+)-K(+)-ATPase. Beta 3-adrenoceptor stimulation may also have an effect on plasma potassium control since administration of selective beta 3-adrenoceptor agonists induces a decrease in plasma potassium. The simultaneous infusion of phenyleprine (alpha-adrenoceptor agonist) and isoprenaline (beta-adrenoceptor agonist) increases plasma potassium levels: this effect is several times larger than the algebric summation of the changes in plasma potassium when each agent is infused separately, thus suggesting potentiation. The physiological (changes in cell volume and function secondary to changes in ion fluxes) and clinical implications (pathophysiological conditions with hypo or hyperkalaemia, hyperkalaemic periodic paralysis, ventricular arrythmias) of these findings are discussed.

Skeletal muscle beta 2-adrenoreceptors and the effect of adrenergic drugs on plasma potassium in perinephritis hypertension in rabbits

1. It has been suggested that beta 2-adrenoreceptors in skeletal muscle regulate plasma potassium. The possibility that alterations in the function and/or density of these receptors occurs in perinephritis hypertension in rabbits was studied. 2. Intravenous infusion of adrenaline (0.2 micrograms kg-1 min-1) caused a fall in potassium while intravenous bolus injection of propranolol (0.75 mg kg-1) resulted in an increase in serum potassium which was of similar magnitude in both perinephritis hypertensive and sham-operated normotensive rabbits. 3. Binding studies with the radioligand [125I] cyanopindolol (ICYP) showed that there were no significant differences between the hypertensive and normotensive rabbits in the density (Bmax) or affinity (KD) of the skeletal muscle beta 2-adrenoreceptor. 4. The results suggest that function and density of skeletal muscle beta 2-adrenoreceptors are not altered in rabbits with perinephritis hypertension.

Beta-adrenoceptors and human skeletal muscle characterisation of receptor subtype and effect of age

1. Rectus abdominis muscle biopsies were obtained from 28 patients undergoing abdominal surgery. In membranes prepared from these biopsies beta-adrenoceptor binding was examined. The apparent affinity (KD) and the density (Bmax) of the receptors for the radioligand (-)-[125I]cyanopindolol were 28.5 +/- 2.7 (pM) and 25.9 +/- 2.1 (fmol mg-1 protein) (mean +/- s.e. mean) respectively. In forceps biopsies from vastus lateralis muscle from four healthy volunteers the values for KD and Bmax were 22.5 +/- 4.4 (pM) and 16.4 +/- 2.2 (fmol mg-1 protein). The binding characteristics for the radioligand were similar in the biopsies from the two muscle sites. 2. Inhibition of the radioligand binding by the selective beta 2-adrenoceptor antagonist ICI 118551 (KI = 117 +/- 45 nM) and selective beta 1-adrenoceptor antagonist metoprolol (KI = 15229 +/- 5046 nM) suggests the dominance of beta 2-adrenoceptor subtype in human skeletal muscle. 3. There were no significant differences in the skeletal muscle beta-adrenoceptor densities or affinities between the young and older patients.

Regulation of beta-adrenoceptors by catecholamines in the rabbit skeletal muscle

(-)-[125I]Cyanopindolol (ICYP) was used to characterise beta-adrenoceptors in the gastrocnemius muscle of the rabbit. The binding of ICYP was saturable. The KI value for the selective beta 2-adrenoceptor antagonist, ICI 118551, was 0.16 +/- 0.01 nM (mean +/- S.E.M) and for the selective beta 1-adrenoceptor antagonist, metoprolol, was 300.1 +/- 33.3 nM. Hofstee plots for both antagonists were linear indicating the presence of homogeneous beta 2-adrenoceptors in the skeletal muscle of the rabbit. In gastrocnemius muscles obtained from rabbits chronically pretreated in vivo with vehicle (0.1% ascorbic acid), adrenaline (40-50 nmol.kg-1.h-1) or noradrenaline (80-100 nmol.kg-1.h-1) via osmotic mini-pumps implanted for 10 days, the Bmax values were 44 +/- 3, 25 +/- 4 (P less than 0.05) and 41 +/- 7 (fmol.mg-1 protein) and KD values were 7.3 +/- 0.9, 6.3 +/- 0.8 and 9.3 +/- 2.5 (pM), respectively. Thus pretreatment with the circulating hormone adrenaline but not noradrenaline down-regulated the number of skeletal muscle beta 2-adrenoceptors but did not influence the affinity.