ACTH receptors in the adrenal: specific binding of ACTH-125I and its relation to adenyl cyclase

Bovine adrenal cortex adenylate cyclase: properties of the particulate enzyme and effects of guanyl nucleotides

The preparation of a partially purified plasma membrane fraction from bovine adrenal cortex is described. Adenylate cyclase in this particulate preparation retained high sensitivity to ACTH and is also stimulated by 5'-guanylyl-imidodiphosphate [Gpp(NH)p]. GTP, in contrast to Gpp(NH)p, had very little intrinsic activity to stimulate activity to stimulate adenylate cyclase. GTP could however, with high affinity, inhibit the Gpp(NH)p effects on adenylate cyclase. When the concentration of creatine phosphate, a component of the ATP-regenerating system in the adenylate cyclase assay mixture, was lowered from 20 to 2 mM (at 0.1 mM ATP, 5 MM MG2+) GTP, dGTP and other nucleotides like ITP and much less UTP or CTP gained considerable intrinsic activity in the presence of ACTH to stimulate adenylate cyclase. The apparent affinities of the nucleotides for ACTH-stimulated adenylate cyclase from bovine adrenal cortex (at 2 mM creatine phosphate) were, GTP = dGTP greater than Gpp(NH)p greater than Gpp(CH2)p (5'-guanylyl-beta, gamma-methylene-diphosphonate) greater than ITP greater than UTP greater than CTP. These findings indicate that regulatory nucleotide binding sites exist for bovine adrenal cortex adenylate cyclase. Their specificity is similar to the nucleotide sites modulating angiotensin binding in bovine adrenal cortex plasma membranes (Glossmann et al., 1974a). The regulatory nucleotide binding sites for the adrenal cortex adenylate cyclase complex can also be identified under conditions where only Gpp(NH)p has high intrinsic activity (e.g. at 20 mM creatine phosphate) but other nucleotides like GTP act as antagonists. Both stimulants, ACTH and Gpp(NH)p, appear to remain firmly bound to the particulate membrane preparation, as suggested by preincubation experiments.

The effects of potassium, 5-hydrocytryptamine, adrenocorticotrophin and angiotensin II on the concentration of adenosine 3':5'-cyclic monophosphate in suspensions of dispersed rat adrenal zona glomerulosa and zona fasciculata cells

Dispersed rat adrenal cells prepared from both the capsule and the decapsulated gland were used to investigate the effects on cyclic AMP accumulation of known stimuli of steroidogenesis [ACTH (adrenocorticotrophin), angiotensin II, K(+) ions and 5-hydroxytryptamine]. Since glomerulosa-cell preparations from capsular strippings are normally contaminated with a proportion of fasciculata cells, cells purified by fractionation on a bovine serum albumin gradient were also used. The results showed that: (1) ACTH and angiotensin II stimulated cyclic AMP accumulation in both fractionated and unfractionated zona fasciculata cells; (2) 5-hydroxytryptamine and an increased extracellular K(+) concentration (from 3.6 to 8.4mm) had no effect on cyclic AMP concentrations in fasciculata cell preparations; (3) the addition of ACTH, angiotensin II, 5-hydroxytryptamine or K(+) to the incubation medium resulted in increased cyclic AMP concentrations in unpurified zona glomerulosa cell preparations; (4) fractionation and hence the virtual elimination of fasciculata contamination, did not affect the response to 5-hydroxytryptamine and increased K(+) concentration. However, the responses to ACTH and angiotensin II were markedly lowered but not abolished. These results strongly suggest a link between cyclic AMP production and steroidogenesis in the zone of the adrenal gland that specifically secretes aldosterone. All four agents used stimulated both steroid output and cyclic AMP accumulation. However, at certain doses of 5-hydroxytryptamine, K(+) and angiotensin II the significant increases in corticosterone output were not accompanied by measurable increases in cyclic AMP accumulation.

Properties of a prolactin receptor from the rabbit mammary gland

Receptors for human, simian, ovine, bovine and murine prolactin, human growth hormone and human placental lactogen have been identified in plasma-membrane-containing subcellular particles isolated from rabbit mammary glands. The association and dissociation of (125)I-labelled prolactin are time- and temperature-dependent processes, both being maximal at 37 degrees C. (125)I-labelled prolactin prepared by the enzymic iodination procedure with lactoperoxidase binds better to receptors than does the preparation obtained by using chloramine-t as the oxidizing agent. The binding of (125)I-labelled prolactin to receptors is strongly influenced by pH and ionic composition but not by many low-molecular-weight compounds tested, e.g. steroids, nucleotides and several drugs. Receptor activity is sensitive to trypsin and phospholipase C digestion, suggesting that protein and phospholipid moieties are essential for the binding of (125)I-labelled prolactin. The binding of (125)I-labelled prolactin to receptors is a saturable and reversible process. Scatchard and Lineweaver-Burk analyses suggest that (125)I-labelled prolactin has a high affinity for its receptor. Binding of (125)I-labelled prolactin to receptors does not result in the destruction of the hormone. Considerable prolactin-binding activity is also observed in subcellular fractions isolated from the adrenal gland, liver, ovary and kidney of the pregnant rabbit, a finding that is consistent with other reported actions of prolactin in these organs.

Insulin-dependent regulation of insulin receptor concentrations: a direct demonstration in cell culture

Chronic (5-16 hr) exposure of cultured human lymphocytes to 10(-8) M insulin at 37 degrees in vitro produced a decrease in insulin receptor concentrations unaccounted for by simple occupancy of sites; acute exposure (0-2 hr) was without effect. These results reproduced observations in vivo where chronic hyperinsulinemia (e.g., 10(-8) M insulin in the circulation of obese insulinresistant hyperglycemic mice) is associated with a substantial reduction in the concentration of insulin receptors per cell, while acute hyperinsulinemia in vivo has no effect on receptor concentration. These data suggest a reciprocal relationship between insulin in the extracellular fluid and the concentration of insulin receptors per cell, which is mediated at the target cell itself by intracellular insulin-sensitive regulatory processes and directly affects target-cell sensitivity to hormone.

Mitogenic properties of lectin and its chemical dervatives

A mitogenic lectin that has a carbohydrate-binding specificity similar to that of concanavalin A (Con A) can be isolated from pea seeds. Chemical modification (succinylation, acetylation, or treatment with the diazonium salt of sulfanilic acid), changes its biological properties. In mitogenic stimulation of mouse spleen cells and in hemagglutination, the differences between the chemically modified pea lectin and the native molecule are similar to those observed between succinyl-Con A and native Con A. However, whereas chemical modification converts tetrameric Con A to a dimeric molecule, similar treatments of the pea lectin do not change its quaternary structure. The results of binding studies of the pea lectin and its derivatives to mouse spleen cells suggest that the differences in biological activities may be explained by a reduction in binding affinity of the pea lectin for glycoproteins on the spleen cell surface after chemical modification.

"Big" growth hormone components from human plasma: decreased reactivity demonstrated by radioreceptor assay

Plasma as well as pituitary immunoreactive human growth hormone (HGH) comprises at least two discrete components which have been designated as "big" HGH and "little" HGH. Using a newly developed radioreceptor assay, which depends on the ability of a substance to compete with labeled HGH for binding sites on cultured human lymphocytes, we find that the big HGH component from both normal and acromegalic subjects has much less activity in the radioreceptor assay than in the radioimmunoassay, whereas the little HGH component has simnilar activity in both assays.

Binding of parathyroid hormone to bovine kidney-cortex plasma membranes

1. Plasma membranes were purified from bovine kidney cortex, with a fourfold increase in specific activity of parathyroid hormone-sensitive adenylate cyclase over that in the crude homogenate. The membranes were characterized by enzyme studies. 2. Parathyroid hormone was labelled with (125)I by an enzymic method and the labelled hormone shown to bind to the plasma membranes and to be specifically displaced by unlabelled hormone. Parathyroid hormone labelled by the chloramine-t procedure showed no specific binding. (75)Se-labelled human parathyroid hormone, prepared in cell culture, also bound to the membranes. 3. Parathyroid hormone was shown to retain biological activity after iodination by the enzymic method, but no detectable activity remained after chloramine-t treatment. 4. High concentration of pig insulin inhibited binding of labelled parathyroid hormone to plasma membranes and partially inhibited the hormone-sensitive adenylate cyclase activity in a crude kidney-cortex preparation. 5. EDTA enhanced and Ca(2+) inhibited binding of labelled parathyroid hormone to plasma membranes. 6. Whereas rat kidney homogenates were capable of degrading labelled parathyroid hormone to trichloroacetic acid-soluble fragments, neither crude homogenates nor purified membranes from bovine kidney showed this property. 7. Binding of parathyroid hormone is discussed in relation to metabolism and initial events in hormone action.

Identification of the cardiac beta-adrenergic receptor protein: solubilization and purification by affinity chromatography

A protein that binds catecholamines with a specificity parallel to that of their in vivo effects on cardiac contractility (isoproterenol > epinephrine or norepinephrine > dopamine > dihydroxyphenylalanine) was solubilized from a microsomal fraction of canine ventricular myocardium. The binding protein was purified 500 to 800-fold by solubilization and subsequent affinity chromatography with conjugates of norepinephrine linked to agarose beads. Purified beta-adrenergic binding protein exists in two forms, corresponding to molecular weights of 40,000 and 160,000. The purified material has a single association constant, 2.3 x 10(5) liters/mol (as compared to two association constants, 10(7) and 10(6) liters/mol, for the binding protein in particulate form) but retains the identical binding specificity for beta-adrenergic drugs and antagonists.

Insulin receptors in human circulating cells and fibroblasts

Human lymphocytes obtained from fasted adult subjects and cultured human tumor lymphocytes were investigated for specific insulin receptors. By use of monoiodoinsulin, specific insulin binding sites were demonstrated in peripheral human lymphocytes, cultured human lymphocytes, and in other types of human circulating cells. Insulins and insulin derivatives that varied in their potency to stimulate glucose oxidation in the fat cell and to inhibit binding of [(125)I]insulin to purified plasma membranes, varied in an analogous fashion in their ability to inhibit the binding of labeled insulin to human lymphocytes. Hormones that had no effect on the binding of insulin to fat cells or liver membranes also had no effect on the binding of insulin to lymphocytes. Binding was time and temperature dependent; dissociation of [(125)I]insulin was rapid upon addition of 10 muM insulin. These findings afford a direct approach to the study of endocrine disorders in man.

Epinephrine binding to the catecholamine receptor and activation of the adenylate cyclase in erythrocyte membranes (hormone receptor- -adrenergic receptor-cyclic AMP-turkey)

Turkey erythrocyte membranes showed specific binding of [(3)H]epinephrine. The concentration of hormone required for half-maximal binding (30 muM) was the same as that required for half-maximal activation of the adenylate cyclase located in the same membrane preparation. The binding reaction at 37 degrees C reached completion during the first minute of incubation, which agrees well with the known rapidity of the biological response to catecholamines. Specific binding was abolished by heating the membranes 1 min at 100 degrees C. Chromatography of the bound (3)H, after its extraction from the membranes, indicated that the hormone had fully retained its chemical structure. Epinephrine binding was inhibited by the beta-adrenergic blocking agent propranolol, which also inhibited the activation of adenylate cyclase by the hormone. The specificity of phenethylamine derivatives in displacing [(3)H]epinephrine from the binding sites showed that a typical catecholamine receptor was responsible for the binding. Displacement of the bound hormone by analogs lacking the catechol group was more extensive at 37 degrees C than at 0 degrees C. Some of the analogs that displaced epinephrine from the binding site caused only a feeble activation of the adenylate cyclase, but were able to inhibit the activation of the enzyme by epinephrine. Thus, binding to a catecholamine receptor on a membrane preparation is an essential, but insufficient, condition to elicit a response.

Binding of thyrotropin-releasing hormone to plasma membranes of bovine anterior pituitary gland (hormone receptor-adenylate cyclase-equilibrium constant-( 3 H)thyrotropin)

An assay for the binding of [(3)H]thyrotropin-releasing hormone ([(3)H]TRH) is described. Plasma membranes isolated from bovine anterior pituitary gland bind about 600 femtomoles of this hormone per mg of protein, as compared to 15 femtomoles per mg of protein in the total adenohypophyseal homogenate (40-fold purification). The equilibrium constant of membrane receptor-[(3)H]TRH binding at 0 degrees C is 4.3 x 10(7) L.M(-1), or a half-maximal binding of this hormone at 23 nM. The binding is time-dependent; addition of unlabeled hormone induces dissociation of the receptor-[(3)H]TRH complex with a half-life of 14 min. The binding of TRH is not altered by 10 muM melanocyte-stimulating hormone-release inhibiting hormone, lysine-vasopressin, adrenocorticotropin, growth hormone, prolactin, luteinizing hormone, insulin, glucagon, L-thyroxine, or L-triiodothyronine. K(+) and Mg(++) increase formation of the receptor-TRH complex at optimal concentrations of 5-25 mM and 0.5-2.5 mM, respectively, with inhibition at higher concentrations. Ca(++) inhibits binding of TRH at all concentrations tested.