Adrenaline-sensitive adenylate cyclase of human fat cell ghosts: properties and hormone-sensitivity

Salts promote activation of fat cell adenylate cyclase by GTP: special role for sodium ion

The effects of GTP on adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] of human and rat fat cell membranes ("ghosts" and purified membranes) were examined in the absence and presence of added inorganic salts. With human ghosts GTP alone (0.1 mM) inhibited enzyme activity by 40% at 30 degrees C and had no significant effect at 37 degrees C. At both temperatures Na+ salts of Cl-, N3-, and SO2-(4) stimulated activity (up to 4-fold basal activity for 200 mM NaN3), with maximal effects at salt concentrations of 100-200 mM. Over the same concentration range these salts also allowed temperature-dependent stimulation by GTP. GTP increased the maximal activity produced by salt alone by about 2-fold at 30 degrees C and about 4-fold at 37 degrees C. Na+ (added as Cl-) was much more effective than other alkali metal cations in promoting activation by GTP. Na+ salts allowed activation of the human enzyme by the GTP analog 5'-guanylyl imidodiphosphate and also promoted stimulation of rat fat cell adenylate cyclase by both nucleotides. In time course studies of human and rat fat cell ghosts, GTP appeared to sustain an initial high rate of salt-stimulated activity, which in the absence of nucleotide subsequently fell to a lower rate, suggesting that salts might activate adenylate cyclase by promoting the stimulatory effect of endogenous membrane-bound GTP. However, with purified human fat cell membranes and a GTP-free system, salts were still stimulatory and promoted activation by added GTP. These results differ from those of previous reports in other systems in which Na+ has promoted only inhibitory effects in GTP regulation of adenylate cyclase.

Adrenoceptor of the alpha 2-subtype mediating inhibition of the human fat cell adenylate cyclase

In an attempt to characterize the adenylate cyclase-coupled alpha-adrenoceptors of human fat cells the effects of various alpha-adrenergic agonists and antagonists were examined in the presence of 0.05 mmol/l of propranolol. The order of agonist potencies with respect to alpha-adrenergic inhibition was (-)-adrenaline greater than alpha-methyl-(-)-noradrenaline greater than (-)-phenylephrine with half-maximal inhibition occurring at 2 mumol/l of (-)-adrenaline and 7 mumol/l of alpha-methyl-(-)-noradrenaline respectively. The inhibition of adenylate cyclase induced by 0.05 mmol/l of (-)-adrenaline was reversed by the alpha-blocking agents yohimbine and prazosin, with the alpha 2-site-directed antagonist yohimbine being more than 100-times more potent than prazosin which is more active at alpha 1-sites. The results show that the cyclase-coupled alpha-adrenoceptors of human fat cells, which probably represent the physiologically relevant target of antilipolytic catecholamine effects, display characteristic features of the alpha 2-adrenoceptor subtype.

Inhibition of human fat cell adenylate cyclase mediated via alpha-adrenoceptors

Human adipose tissue contains alpha- as well as beta-adrenoceptor sites mediating antagonistic catecholamine effects on lipolysis. To characterize the mechanisms of catecholamine action in biochemical terms we have studied the effects of the almost pure beta-adrenoceptor agonist isoproterenol and of the mixed adrenergic agonist adrenaline on human fat cell adenylate cyclase in the presence of alpha- and beta-adrenoceptor blocking drugs. In contrast to the almost pure beta-adrenergic agent isoproterenol, the mixed agonist adrenaline, besides its stimulatory action, also had inhibitory effects which became apparent upon complete beta-adrenoceptor blockade using 0.05 mmol/l propranolol. Under these conditions adrenaline caused a dose-dependent inhibition of basal and parathyroid hormone-activated enzymic activity with a maximum of 30-50%, which was dependent on GTP and could be reversed by simultaneous alpha-adrenergic blockade using 5 mumol/l dihydroergotamine or 10 mumol/l phentolamine. These results support the concept of antogonistic alpha- and beta-adrenoceptor sites coexisting as regulatory subunits of the human fat cell adenylate cyclase.

Effects of some naturally occurring prostaglandins of the D-, E-, and I-type and synthetic analogues on adenylate cyclase of human fat cell ghosts

The effects of various prostaglandins originating from arachidonic acid (prostaglandin E2, D2, I2) as well as of methylated analogues of prostaglandin E2 (16,16-dimethyl prostaglandin E2 and 15-S-methyl prostaglandin E2) on the human fat cell adenylate cyclase were studied. In addition, the stable breakdown product of prostaglandin I2, prostaglandin 6-keto-F1-alpha, was tested for activity. Prostaglandin E2 and its methylated analogues as well as prostaglandin D2 and I2 were active stimulators of the human fat cell enzyme. The stable breakdown product of prostaglandin I2 (prostaglandin 6-keto-F1-alpha) which is biologically inactive had no stimulatory effects, suggesting that stimulation of the human fat cell function. The stimulatory action of prostaglandin D2 is consistent with the elevation of cAMP-levels induced by this compound in intact fat cells. It is concluded that the stimulatory effects of prostaglandins on the human fat cell adenylate cyclase are related to their stimulatory effects upon lipolysis.

[New aspects of catecholamin-receptor interactions. Pathophysiological and clinical implications (author's transl)]

The molecular aspects of catecholamine-receptor interactions are reviewed with respect to their clinical implications. Beta- and in some tissues alpha-adrenergic receptors are coupled to the membrane-bound adenylate cyclase. The adrenergic receptor sites are distinct membrane constituents. Their number and the ratio alpha-to beta-receptor site depend on the plasma concentration of catecholamines and are affected by diet and endocrinological factors. Recent clinical studies suggest that long-term treatment with beta-adrenergic agonists may induces desensitization of target tissues due to changes in the adrenergic receptor moieties.

Human fat cell adenylate cyclase: regional differences in adrenaline responsiveness

The effect of adrenaline on adenylate cyclase activity in membranes prepared from human abdominal and gluteal adipose tissue was examined. Basal enzyme activity averaged 1.0 mmol cAMP/mg protein/15 min in abdominal tissue and 0.68 nmol cAMP/mg protein/15 min in ghosts from the buttock. Maximally effective concentrations of sodium fluoride (20 mmol/1) induced an eight-fold increase in both membrane preparations. The maximal response to adrenaline (1.0 mmol/1) averaged 170% in ghosts of abdominal tissue compared with 60% in gluteal membranes. The concentrations of adrenaline required to produce half-maximal stimulation were similar in membrane preparations of both regions (about 5 X 10(-5) mol/1).

beta-adrenergic receptor coupled-adenylate cyclase of human fat cell ghosts

The effects of beta-adrenergic agonists such as isoproterenol, norepinephrine and epinephrine upon the adenylate cyclase activity of human fat cell ghosts were tested, each alone and in combination with the beta-blocking agent propranolol. Saturating concentrations of these agents showed a 2-6.5-fold increase of enzyme activity without addition of any artificial cofactors. Isoproterenol was more potent in stimulating the enzyme system than epinephrine and nor-epinephrine. Propranolol caused a dose-dependent rightward shift of the log-dose response curve of these beta-adrenergic agonists. The assay of human fat cell adenylate cyclase in vitro may provide a simple anc convenient assay system for the screening of beta-adrenergic drugs of potential therapeutic importance.