Hormone-receptor interactions. Adrenocorticotrophin-(7-24)-octadecapeptide stimulates adipocyte membrane adenylate cyclase without causing lipolysis in fat cells

Increase in cytosolic free Ca2+ in corticotropin-stimulated white adipocytes

The mechanism of the adrenal corticotropin hormone (ACTH)-stimulated increase in cytosolic free Ca2+ concentration ([Ca2+]i) was investigated in rat white adipocytes. ACTH at concentrations > 10 mU/ml caused a rapid and transient increase in [Ca2+]i followed by a small but sustained elevation of [Ca2+]i. A similar phenomenon was also induced by alpha-adrenergic or synthetic ACTH stimulation. The effect of norepinephrine (NE) plus ACTH on [Ca2+]i was nearly additive. Pertussis toxin completely blocked the ability of ACTH or NE to increase [Ca2+]i. NE but not ACTH caused a significant increase in inositol 1,4,5-trisphosphate levels. ACTH caused a rapid and transient accumulation of [3H]arachidonic acid (AA) and a marked loss of [3H]AA from phosphatidylinositol (PI) and phosphatidylcholine (PC) 10 s after stimulation. Neither a lipoxygenase inhibitor nor a dual inhibitor of cyclooxygenase and lipoxygenase blocked the increases in [Ca2+]i and the accumulation of [3H]AA in response to ACTH. On the other hand, either pertussis toxin or phospholipase A2 inhibitor drastically blocked both parameters in response to ACTH. These results indicate that ACTH stimulates AA release from PC and PI via the activation of phospholipase A2 coupled with pertussis toxin-sensitive GTP-binding protein(s), which leads to an increase in [Ca2+]i in rat white adipocytes.

A comparative study of adenylate cyclase activity and progesterone synthesis in ovine corpora lutea stimulated by different chemical derivatives or natural analogs of ovine LH

The adenylate cyclase activation by ovine native LH, natural analogs (porcine LH, hCG) and chemical derivatives of LH (methylated, ethylated, isopropylated, guanidinated) was studied in purified plasma membranes of ovine corpora lutea, including the regulatory effects of guanyl 5'-yl imidophosphate (Gpp(NH)p) and Mg2+. The Ka app. for native LH (about 15 nM) was independent of Gpp(NH)p and Mg2+. Similar maximal activation of the enzyme was obtained by using ovine LH or natural analogs, but differences were remarked concerning the Ka app. values of these hormones. Porcine LH was equipotent with ovine LH; on the contrary, hCG exhibited a lower Ka app. value (3 nM). All chemical derivatives (Me-LH, Et-LH, Iso-LH and Gu-LH) exhibited Ka app. higher than native (about 2- to 4-fold), but similar maximal activation. No modification was observed in the regulatory effects of Gpp(NH)p and Mg2+ on the adenylate cyclase activation as a consequence of structural modifications of the hormone. A comparison of the steroidogenic activity on intact luteal cells and the adenylate cyclase activation ability on purified plasma membranes of the derivatives mentioned above evidenced some interesting discrepancies. The drop in adenylate cyclase activation potency of Me-LH was not reflected in its steroidogenic activity (Me-LH was equipotent with native LH); on the contrary, the capacity of Gu-LH to stimulate adenylate cyclase was not so much decreased as was its steroidogenic potency which was almost abolished.

Role of calcium ion in hormone-stimulated lipolysis

Using the flask-incubated fat cell system, the effects of Ca2+ removal from the incubation medium on the lipolytic system were studied. The removal of Ca2+ resulted in a total abolition of the lipolytic response and the increased cyclic AMP accumulation produced by ACTH. The lipolytic response to isoproterenol and forskolin were reduced approximately 40% by Ca2+ removal, but cyclic AMP accumulation was not altered in the presence of either of these agents using a Ca2+-free medium. The lipolytic response to the dibutyryl analog of cyclic AMP was also reduced by omission of Ca2+ from the incubation medium. It is concluded the Ca2+ is required for the interaction of ACTH with its receptor and the resultant activation of adenylate cyclase. Ca2+ also is required at some step in the lipolytic process distal to cyclic AMP.

Comparison of basal metabolic activities and hormonal sensitivities between mesenteric and epididymal adipocytes

The mesenteric adipose tissue, which is present in a considerable amount in the rat but had not been used for the experiments, contained a larger number of adipocytes than the epididymal adipose tissue per wet weight of the tissue. The basal metabolic activities and the responses to epinephrine and insulin in the mesenteric adipocytes were essentially equal to those in the epididymal adipocytes. The mesenteric adipocytes are useful for elucidation of metabolism in white adipose tissue.

'Molecular sandwiches' as a basis for structural and functional similarities for interferons, MSH, ACTH, LHRH, myelin basic protein, and albumins

Sequential similarities between the tryptophan peptide of myelin basic protein (residues 111-121), luteinizing hormone releasing hormone, melanotropin, adrenocorticotropin (residues 1-13), human leukocyte interferon (residues 28-40), and various segments of human and bovine serum albumin and hen ovalbumin are presented. It is suggested that these structural similarities may explain observations concerning common functional characteristics such as serotonin modulation, immunological activity with the adjuvant muramyl dipeptide, immunological cross-reactivity, and the possible MSH-ACTH-like activity of a pepsin-derived peptide of interferon.

ACTH-like neurotropic peptides: possible regulators of rat brain cyclic AMP

The influence of behaviorally active, N-terminal fragments of ACTH on the accumulation of cAMP in rat brain investigated in broken cell preparations of subcortical tissue, in slices of neostriatum and in vivo. ACTH1--24 has a biphasic effect on the activity of adenylate cyclase in broken cell preparations of rat brain subcortical tissue: concentrations below 25 micrometer stimulated, whereas concentrations of 0.1 mM and higher inhibited adenylate cyclase activity. The magnitude of the stimulation was dependent on the concentrations of ATP and Mg2+ in the incubation medium. Structure activity studies revealed that at a concentration of 10(-4) M ACTH1--16-NH2 and ACTH4--7 also inhibited the activity of adenylate cyclase, whereas ACTH11--24, ACTH1--10, ACTH4--10, [D-Phe7]ACTH1--10 and [D-Phe7]ACTH4--10 were inactive in this respect. Addition of 0.8 mM EGTA but not of 0.25 mM Ca2+ prevented the inhibition by 10(-4) M ACTH1--24. GMP-N-P (10(-5) M), naltrexone (10(-3) M) and ergometrine (10(-3) M) did not influence the inhibitory effect. ACTH1--24 enhanced the accumulation of cAMP in slices from rat brain neostriatum in a dose-dependent manner. This effect was already maximal 7.5 min after the addition of the peptide and was potentiated by isobutylmethylxanthine, a potent inhibitor or phosphodiesterase. Intraventricular injection of 1 microgram ACTH1--16-NH2 in rats significantly elevated (+ 27%) the concentration of cAMP in the septal region 60 min after the injection of the peptide. The results are discussed in terms of a possible involvement of cAMP as a second messenger in the central nervous system for N-terminal fragments of ACTH.

Purification of bovine adrenal-cortex plasma-membrane vesicles containing a highly corticotropin-sensitive adenylate-cyclase system and angiotensin-II-binding sites

The purpose of this experimental investigation was to provide a purified plasma membrane fraction containing a highly hormone-responsive adenylate cyclase system. Bovine adrenal cortex was homogenised and a washed pellet (450 000 X g - min) was fractionated by zonal centrifugation in a sucrose and dextran gradient. Adenylate cyclase activity was purified up to 60-fold to a specific activity of 55, 340 and 210 pmol of adenosine 3':5'-monophosphate (cyclic AMP) produced/minute per mg of protein at 38 degrees C for the basal, adrenocorticotrophin and fluoride-activated states, respectively. The time course of the adenylate cyclase activity is linear. The concentration necessary for half-maximal stimulation by adrenocorticotrophin-(1-24)-tetracosipeptide is 0.5 muM. The high hormone-responsiveness of the membrane preparation allows one to demonstrate activation of adenylate cyclase by very weakly agonistic adrenocorticotrophin fragments. The F- activated state can be detergent-dispersed by Lubrol and shows a Km (ATP) different from that of either the basal or adrenocorticotrophin-stimulated state. Other marked enzymes such as 5'-nucleotidase, glucose-6-phosphatase and cytochrome oxidase were followed during purification. The plasma membrane fraction shows rather homogeneous, relatively large vesicles (mean diameter 0.5 mum). It contains high-affinity binding sites for angiotensin II (about 2 pmol per mg protein) with an apparent association constant of 2 X 10(7) (1/mol) at 12 degrees C. The yield, 20 mg of membrane protein per preparation, may make it a tool in either affinity-labelling studies with the peptide hormones mentioned or the starting point for solubilisation and purification of adenylate cyclase.