Cell metabolism affects the density of beta-adrenergic receptors in intact rat reticulocytes

Changes in beta-adrenoceptor binding properties and receptor-cyclase coupling during in vitro maturation of rat reticulocytes

The loss of beta-adrenergic responsiveness during reticulocyte maturation was studied under tissue culture conditions in a defined cell population from rats. Initial beta-receptor density and receptor-mediated cAMP formation in culture medium (RPMI 1640) exceeded the values obtained in isotonic KC1-buffer by 56 and 120% respectively. During cell cultivation receptor density and hormonal responsiveness decreased rapidly by 40-50% of their initial values within the first 20 h. In the following 3 days the rate of loss varied between 10 and 15%/d. The same time course was observed for the maturation-dependent decrease in forskolin-stimulated cAMP formation. ATP depletion of cultivated cells caused a complete and irreversible loss of cAMP response within 90 min. Our results indicate that cell metabolism regulates the strength of the hormonal response. A defect in adenylate cyclase or in cyclase N-protein interaction seems to be rate-limiting for the functional inactivation of the beta-adrenergic system during reticulocyte maturation.

[Beta-adrenergic receptors and plasma catecholamine behavior in trained and untrained athletes]

6 sports students (VO2 max. 54.0 +/- 2.6 ml/kg . min) and 6 marathoners (VO2 max. 65.7 +/- 2.0 ml/kg . min) performed graded treadmill tests. Free plasma catecholamines (noradrenaline and adrenaline), heart rate, lactate were assessed at rest and during exercise. The behaviour of beta-adrenergic receptors of polymorphonuclear leukocytes was estimated additionally before the treadmill test. The maximum running velocity was 14.6 +/- 0.8 km/h (sports students) and 17.3 +/- 0.6 km/h (marathoners). Noradrenaline was approximately 46% (v = 12 km/h) to 67% (v = 14 km/h) lower in the group of marathoners than in sports students; adrenaline showed no or smaller differences between both groups. No significant differences in the plasma catecholamine behaviour occurred between the groups at rest and during maximum exercise. Specific binding of 3H-Dihydroalprenolol to intact cells was higher in the marathoners than in sports students (p greater than 0.01). Scatchard analysis revealed a maximum binding of 21.1 fmol/10(7) cells (sports students) and 35.3 fmol/10(7) cells (marathoners), which indicated approximately 1,300 (sports students) and 2,150 binding sites cell (marathoners). Inverse correlations between noradrenaline (r = -0.63), VO2max. (r = -0.79) and the specific binding of 3H-DHA were observed. The reduction of plasma catecholamines (approximately 46-67%) and the increase in specific binding sites (approximately 67%) were in the same range.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversible reduction of insulin receptor affinity by ATP depletion in rat adipocytes

The effects of the metabolic inhibitor NaN3 on insulin receptors in isolated rat fat-cells were investigated. The agent reduced insulin binding in parallel to a decrease of the ATP content of cells. Both effects were observed in the same concentration range of NaN3, and were fully reversible. According to the binding curves the affinity rather than the number of receptors was reduced. Kinetic experiments revealed an increased dissociation rate of the insulin-receptor complex. The effects outlasted cell disruption, since the receptor affinity was still lowered in plasma membranes obtained from NaN3-treated cells. Thus an inhibition of insulin internalization could not account for the observed effects. It is suggested that the observed ATP-dependence of insulin receptor affinity reflects a reversible structural alteration of the receptor, or of some closely related membrane protein.

beta-Adrenergic receptors of brain cells. Membrane integrity implies apparent positive cooperativity and higher affinity

Beta-Adrenergic receptors were studied in intact cells of chick, rat and mouse embryo brain in primary cultures, by the specific binding of [3H]dihydro-L-alprenolol ([3H]DHA). The results were compared to the receptor binding of broken cell preparations derived from the cell cultures or from the forebrain tissues used for the preparation of the cultures. Detailed analysis of [3H]DHA binding to living chick brain cells revealed a high-affinity, stereoselective, beta-adrenergic-type binding site. Equilibrium measurements indicated the apparent positive cooperativity of the binding reaction. By direct fitting of the Hill equation to the measured data, values of Bmax = 12.01 fmol/10(6) cells (7200 sites/cell), Kd = 60.23 pM and the Hill coefficient n = 2.78 were found. The apparent cooperative character of the binding was confirmed by the kinetics of competition with L-alprenolol, resulting in maximum curves at low ligand concentrations. The rate constants of the binding reaction were estimated as k+ = 8.31 X 10(7) M-1 X min-1 and k- = 0.28 min-1 from the association results, and k- = 0.24 min-1 from the dissociation data. The association kinetics supported the cooperativity of the binding, providing a Hill coefficient n = 1.76; Kd, as (k-/k+)1/n was found to be 101 pM. Analysis of the equilibrium binding of [3H]DHA to rat and mouse living brain cells resulted in values of Bmax = 13.04 fmol/10(6) cells (7800 sites/cell), Kd = 43.85 pM and n = 2.52, and Bmax = 8.08 fmol/10(6) cells (4800 sites/cell), Kd = 46.70 pM and n = 1.63, respectively, confirming the apparent cooperativity of the beta-receptor in mammalian objects, too. The [3H]DHA equilibrium binding to broken cell preparations of either chick, rat or mouse brain cultures or forebrain tissues was found to be non-cooperative, with a Hill coefficient n = 1, Kd in the range 1-2 nM, and a Bmax of 10(3) - 10(4) sites/cell. Our findings demonstrate that cell disruption causes marked changes in the kinetics of the beta-receptor binding and in the affinity of the binding site, although the number of receptors remains unchanged.

Adrenoceptors: molecular nature and role in atopic diseases

Since Szentivanyi proposed the idea that asthma and other atopic diseases are due to a beta adrenergic defect there has been much interest in the role of the adrenergic receptors in allergy. The radioactive ligand binding techniques developed within the last few years have greatly increased our knowledge concerning the molecular nature of the adrenoceptors and the events following receptor stimulation. The adrenoceptors have shown to be very dynamic structures. Their number and affinity may be altered due to various physiological and pharmacological stimuli. Their role in the pathogenesis of atopic diseases has not been definitely settled, but there seem to be a true beta adrenergic hyporesponsiveness and alpha hyperresponsiveness in asthma. This article briefly describes the radioligand binding technique and summarizes our present knowledge of the nature of the alpha and beta adrenoceptors and their possible role in atopic diseases.