Differential sensitivity to guanine nucleotides of basal and insulin-stimulated glucose transporter activity reconstituted from adipocyte membrane fractions

The effects of GTP gamma S on glucose transport activity reconstituted from adipocyte membrane fractions were studied in order to test the hypothesis that intrinsic activity changes of the insulin-sensitive glucose transporter may be mediated by guanine nucleotide-dependent mechanisms. GTP gamma S and GTP inhibited reconstituted glucose transport activity by 50% in membrane fractions from insulin-treated cells in a concentration-dependent manner; no inhibitory effect was observed in membrane fractions obtained from basal cells. GDP, GMP and guanosine were less effective than GTP, whereas the adenine nucleotides ATP gamma S and AMP failed to reduce the reconstituted transport activity. The data indicate that guanine nucleotides may modulate the activity of the adipocyte glucose transporter. Since the effect is dependent on treatment of cells with insulin, the hormone appears to induce a specific functional alteration of the glucose transporter.

Quantitative dissociation of glucose transport stimulation and insulin receptor tyrosine kinase activation in isolated adipocytes with a covalent insulin dimer (B29,B29'-suberoyl-insulin)

The covalent insulin dimer B29,B29'-suberoyl-insulin was investigated for its effects on insulin receptor binding, insulin receptor tyrosine kinase activity and glucose transport in isolated adipose cells. The dimer stimulated glucose transport (initial 3-O-methylglucose uptake rate) to the same extent as insulin did (basal rate, 35 +/- 3 pmol/sec/microliter lipid; insulin, 380 +/- 27; B29,B29'-suberoyl-insulin, 369 +/- 24, means +/- S.E.), although at higher concentrations (EC50 1.94 +/- 0.64 nM versus 0.1 +/- 0.02 with insulin). In contrast, the dimer only partially (23%) mimicked insulin's effect on phosphate incorporation into insulin receptors immunoprecipitated after equilibration of cells with [32P]phosphate. Similarly, insulin receptor tyrosine kinase as assessed by receptor autophosphorylation and phosphorylation of the substrate poly-(Glu/Tyr) was not fully activated by treatment of cells with the insulin dimer (31 and 42% of the effect of insulin, respectively) in concentrations which maximally activate glucose transport and give rise to full insulin receptor occupancy (5 X 10(-7) M). Further, the dimer activated the receptor tyrosine kinase in solubilized purified insulin receptor preparations from adipose cells to only 25% of the effect of insulin (EC50 32.0 +/- 16 versus 1.9 +/- 1.0 nM with insulin) in spite of full receptor occupancy. Binding of the dimer to insulin receptors followed single site binding kinetics, indicating that the derivative is unable to induce negative cooperativity of the insulin receptor. It is concluded that a partial phosphorylation of insulin receptors and a submaximal tyrosine kinase activation are sufficient for full stimulation of glucose transport in the adipocyte. Further, it is suggested that negative cooperativity of the insulin receptor and activation of its tyrosine kinase require a similar conformational change of the receptor protein.

Adipocyte plasma membranes contain two Gi subtypes but are devoid of Go

Antisera generated against synthetic peptides were used to identify G-protein alpha-subunits in plasma membranes from rat adipocytes. Applying the immunoblot technique, we detected two Gs alpha-subunits of 42 and 43 kDa, corresponding to the two cholera toxin substrates, and two Gi alpha-subunits of 40 and 41 kDa, corresponding to the two pertussis toxin substrates present in these membranes. The 40 kDa protein was tentatively identified as the Gi2 alpha-subunit. A serum specific for the Go alpha-subunit failed to detect any immunoreactive protein. Thus plasma membranes of adipocytes possess two forms of Gi but not Go.

Activation of adenylate cyclase and inhibition of glucose transport in rat adipocytes by forskolin analogues: structural determinants for distinct sites of action

Forskolin and four analogues of forskolin, 7-beta-[gamma-(N'-methylpiperazino)-butyryloxy]-7-desacet ylforskolin, 7-desacetylforskolin, 7-tosyl-7-desacetylforskolin, and 1,9-dideoxyforskolin, were tested for their ability to activate adenylate cyclase, inhibit glucose transport, and inhibit cytochalasin B binding in rat adipocyte membranes. Forskolin was the most potent analogue in activating adenylate cyclase with an EC50 of 2 microM, whereas 7-beta-[gamma-(N'-methylpiperazino)butyryloxy]-7-desacety lforskolin and 7-desacetylforskolin were less potent, with EC50 values of 3 microM and 20 microM, respectively. The 7-tosyl-7-desacetylforskolin and 1,9-dideoxyforskolin did not stimulate adenylate cyclase even at the highest concentrations tested (100 microM). In contrast, forskolin and all of the analogues were able to fully inhibit glucose transport in adipocyte plasma membranes. The order of potency for the inhibition was forskolin greater than 7-beta-[gamma-(N'-methylpiperazino)butyryloxy]-7-desacety lforskolin greater than 7-desacetylforskolin greater than 7-tosyl-7-desacetylforskolin greater than 1,9-dideoxyforskolin, and the EC50 values were 0.24 microM, 1.8 microM, 7.1 microM, 8.8 microM, and 12.8 microM, respectively. Cytochalasin B binding to rat adipocyte membranes was inhibited by forskolin and the four analogues with the same order of potency as observed for the inhibition of glucose transport. Thus, the site of action of forskolin which is responsible for the inhibition of glucose transport and cytochasin B binding exhibits structural requirements for forskolin and its analogues that are different from those of the site responsible for the activation of adenylate cyclase.

Inhibition of insulin-stimulated glucose transport in rat adipocytes by nucleoside transport inhibitors

In isolated rat adipocytes, basal as well as insulin-stimulated 3-O-methylglucose transport was inhibited nearly completely (maximal inhibition: 95%) by the nucleoside transport inhibitors dipyridamole (IC50 = 5 microM), nitrobenzylthioguanosine (20 microM), nitrobenzylthioinosine (35 microM) and papaverine (130 microM). Transport kinetics in the presence of 10 microM dipyridamole revealed a significant increase in the transport Km value of 3-O-methylglucose (3.45 +/- 0.6 vs 2.36 +/- 0.29 mM in the controls) as well as a decrease in the Vmax value (4.84 +/- 0.95 vs 9.03 +/- 1.19 pmol/s per microliter lipid in the controls). Half-maximally inhibiting concentrations of dipyridamole were one order of magnitude higher than those inhibiting nucleoside (thymidine) uptake (0.48 microM). The inhibitory effect of dipyridamole (5 microM) reached its maximum within 30 s. The agent failed to affect insulin's half-maximally stimulating concentration (0.075 nM) indicating that it did not interfere with the mechanism by which insulin stimulates glucose transport. Further, dipyridamole fully suppressed the glucose-inhibitable cytochalasin B binding (IC50 = 1.65 +/- 0.05 microM). The data indicate that nucleoside transport inhibitors reduce glucose transport by a direct interaction with the transporter or a closely related protein. It is suggested that glucose and nucleoside transporters share structural, and possibly functional, features.

Qualitative and quantitative comparison of glucose transport activity and glucose transporter concentration in plasma membranes from basal and insulin-stimulated rat adipose cells

Conditions are described which allow the isolation of rat adipose-cell plasma membranes retaining a large part of the stimulatory effect of insulin in intact cells. In these membranes, the magnitude of glucose-transport stimulation in response to insulin was compared with the concentration of transporters as measured with the cytochalasin-B-binding assay or by immunoblotting with an antiserum against the human erythrocyte glucose transporter. Further, the substrate- and temperature-dependencies of the basal and insulin-stimulated states were compared. Under carefully controlled homogenization conditions, insulin-treated adipose cells yielded plasma membranes with a glucose transport activity 10-15-fold higher than that in membranes from basal cells. Insulin increased the transport Vmax. (from 1,400 +/- 300 to 15,300 +/- 3,400 pmol/s per mg of protein; means +/- S.E.M.; assayed at 22 degrees C) without any significant change in Km (from 17.8 +/- 4.4 to 18.9 +/- 1.4 nM). Arrhenius plots of plasma-membrane transport exhibited a break at 21 degrees C, with a higher activation energy over the lower temperature range. The activation energy over the higher temperature range was significantly lower in membranes from basal than from insulin-stimulated cells [27.7 +/- 5.0 kJ/mol (6.6 +/- 1.2 kcal/mol) and 45.3 +/- 2.1 kJ/mol (10.8 +/- 0.5 kcal/mol) respectively], giving rise to a larger relative response to insulin when transport was assayed at 37 degrees C as compared with 22 degrees C. The stimulation of transport activity at 22 degrees C was fully accounted for by an increase in the concentration of transporters measured by cytochalasin B binding, if a 5% contamination of plasma membranes with low-density microsomes was assumed. However, this 10-fold stimulation of transport activity contrasted with an only 2-fold increase in transporter immunoreactivity in membranes from insulin-stimulated cells. These data suggest that, in addition to stimulating the translocation of glucose transporters to the plasma membrane, insulin appears to induce a structural or conformational change in the transporter, manifested in an altered activation energy for plasma-membrane transport and possibly in an altered immunoreactivity as assessed by Western blotting.

Activity and phosphorylation state of glucose transporters in plasma membranes from insulin-, isoproterenol-, and phorbol ester-treated rat adipose cells

The counterregulatory action of catecholamines on insulin-stimulated glucose transport and its relation to glucose transporter phosphorylation were studied in isolated rat adipose cells. Plasma membranes exhibiting reduced glucose transport activity were prepared as described previously (Joost, H. G., Weber, T. M., Cushman, S. W., and Simpson, I. A. (1986) J. Biol. Chem. 261, 10033-10036) from cells treated with insulin, and subsequently with isoproterenol and adenosine deaminase. In these membranes, transporter affinity for cytochalasin B binding was significantly reduced (KD = 133.5 +/- 14 versus 89.8 +/- 11 nM, means +/- S.E.) with no change in number of sites or immunoreactivity of the transporter on Western blots. Reconstituted plasma membrane transport was significantly lower with isoproterenol treatment (0.50 +/- 0.12 versus 0.97 +/- 0.27 nmol/mg protein/10 s). In contrast, transport activity reconstituted from corresponding intracellular transporters (from low density microsomes) was unchanged (5.4 +/- 2.2 versus 6.9 +/- 1.2 nmol/mg protein/10 s). Thus, the intrinsic activity change of the transporter produced by catecholamines appears to reflect a structural modification that is confined to the plasma membrane and not recycled into the intracellular compartment. In cells equilibrated with [32P]phosphate, neither insulin nor isoproterenol induced [32P]phosphate incorporation into the glucose transporter immunoprecipitated from plasma membranes. Conversely, phorbol 12-myristate 13-acetate stimulated significant incorporation of [32P]phosphate into the glucose transporter in insulin-stimulated cells without any change in plasma membrane transport activity or transporter concentration. Thus, the phosphorylation state of the glucose transporter does not seem to be involved in either signaling transporter translocation or triggering changes in transporter intrinsic activity.

Prostaglandin E2 differentiates between two forms of glucose transport inhibition by lipolytic agents

The inhibition of insulin-stimulated glucose transport by lipolytic agents was studied in isolated rat adipose cells. Two different mechanisms for the inhibition of glucose transport by lipolytic hormones and agents were distinguished by use of the antilipolytic agent prostaglandin E2 (PGE2). The inhibition of glucose transport induced by lipolytic hormones such as glucagon, catecholamines or ACTH in the presence of adenosine deaminase was antagonized by PGE2. In contrast, inhibition of hexose transport by alkylxanthines was only partially (20-30%) attenuated by PGE2, although the eicosanoid had antagonized cyclic AMP accumulation and stimulation of lipolysis in response to all tested lipolytic agents. The inhibition of glucose transport by IBMX was immediate, whereas the lipolytic hormones (isoprenaline and ACTH) exhibited a latency of 2-3 min. In addition, the inhibition induced by the lypolytic hormones disappeared after cooling of the cells to 22 degrees C, at which temperature IBMX was still inhibitory. Thus, the PGE2-sensitive component of the effect of lipolytic agents on glucose transport appears to be mediated by adenylate cyclase or its subunits Ns/Ni. The PGE2-insensitive effect of alkylxanthines probably reflects a direct interaction of the agents with a regulatory site at the transporter or a related protein.

Forskolin inhibits insulin-stimulated glucose transport in rat adipose cells by a direct interaction with the glucose transporter

The mechanism of the inhibitory action of forskolin, a plant-derived stimulator of adenylate cyclase, on glucose transport in rat adipose cells was studied. Lipolysis (glycerol release) and glucose transport activity (initial 3-O-methylglucose uptake rate) were measured after treatment of intact cells. In isolated plasma membranes, D-glucose transport and glucose-inhibitable binding of cytochalasin B, a specific labeling agent for the glucose transporter, were assayed. Forskolin inhibited insulin-stimulated glucose transport in intact cells at low concentrations which failed to stimulate lipolysis. Furthermore, the adenylate cyclase inhibitor prostaglandin E2 reduced forskolin-stimulated lipolysis but failed to reverse the transport inhibition. Therefore, the effects of the agent on lipolysis appeared to be dissociable from those on glucose transport. In plasma membrane vesicles, forskolin inhibited D-glucose transport in a competitive manner by an increase in the apparent transport Km without any detectable change in Vmax. In parallel to the transport inhibition, the agent inhibited the specific binding of cytochalasin B in both plasma membranes and low density microsomes, which contain the intracellular pool of glucose transporters in insulin-sensitive cells. The Kl of this inhibition (205 nM) was very similar to that of the inhibition of glucose transport in the membrane vesicles (203 nM). It is concluded that forskolin inhibits glucose transport by a direct interaction with the transporter (or a closely related protein) rather than through activation of adenylate cyclase.

Insulin-stimulated glucose transport in rat adipose cells. Modulation of transporter intrinsic activity by isoproterenol and adenosine

The mechanism of modulation of insulin-stimulated glucose transport activity in isolated rat adipose cells by lipolytic and antilipolytic agents has been examined. We have measured glucose transport activity in intact cells with 3-O-methylglucose and in plasma membranes with D-glucose, and the concentration of glucose transporters in plasma membranes using a cytochalasin B binding assay. In intact cells, isoproterenol reduced insulin-stimulated transport activity by 60%. This effect was lost after cooling and washing the cells with homogenization buffer, and neither the concentration of glucose transporters nor transport activity in the plasma membranes differed from control. However, treatment of cells with KCN prior to homogenization preserved the isoproterenol effect through the fractionation procedure. Plasma membranes from these cells contained an unchanged number of transporters (31 +/- 7, mean +/- S.E., versus 31 +/- 4 pmol/mg of protein in controls) but transported glucose at a reduced rate (19 +/- 6 versus 48 +/- 9 pmol/mg of protein/s). Conversely, incubation of intact cells in the presence of adenosine stimulated plasma membrane glucose transport activity compared to that in the absence of adenosine (44 +/- 6 versus 36 +/- 6 pmol/mg of protein/s). Kinetic studies of isoproterenol-inhibited glucose transport in plasma membranes revealed a 60% decrease in Vmax (2900 +/- 350 versus 7200 +/- 1000 pmol/mg of protein/s) and a small increase in Km (15.1 +/- 1 versus 13.0 +/- 0.6 mM). These data indicate that modifications of glucose transport activity produced by lipolytic and antilipolytic agents in intact adipose cells can be fully retained in plasma membranes isolated under appropriate conditions. Furthermore, the effects of these agents occur through a modification of the glucose transporter intrinsic activity.

Modulation of glucose transport in hamster adipocytes by insulin and by beta- and alpha 2-adrenoceptor agonists

Glucose transport in hamster adipocytes and its modulation by insulin and isoprenaline was characterized with the aid of the non-metabolizable hexose 3-0-methylglucose. Insulin stimulated the initial uptake rates by an increase in Vmax of the transport without any detectable change in Km. The hormone concentration producing half maximal stimulation was identical to that required in rat adipocytes. However, hamster adipocytes were much less responsive to insulin (3-fold stimulation as compared to a 12-fold stimulation in rat fat cells), and maximal transport rates were 10-fold lower than that observed in rat adipocytes. Accordingly, the number of glucose transporters, as assessed by glucose-inhibitable cytochalasin-B binding, was considerably lower in plasma membranes of hamster adipocytes. Moreover, no transporters were detected in the low-density microsomes which in insulin-sensitive cell types represent the intracellular pool of recruitable glucose transporters. The relative insulin resistance of the hamster fat cells may therefore be due to a depleted pool of intracellular glucose transporters. In the presence of adenosine, the beta-adrenoceptor agonist isoprenaline produced a moderate stimulation of the basal transport rate which was antagonized by the alpha 2-agonist clonidine. If adenosine deaminase was added in order to remove endogenous adenosine, isoprenaline inhibited the insulin-stimulated transport by 50%. In contrast to the stimulatory effects of insulin and isoproterenol, the inhibitory effect of the catecholamine was reversed by cooling the cells to 22 degrees. Glucagon produced a comparable inhibition, suggesting that the inhibitory effect was mediated by adenylate cyclase or its regulatory subunits.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosine kinase activity of insulin receptors from human placenta. Effects of autophosphorylation and cyclic AMP-dependent protein kinase

The kinase activity of partially purified insulin receptor obtained from human placenta was studied. When autophosphorylation of the beta-subunit of the receptor was initiated by ATP prior to the addition of the exogenous substrate, both basal and insulin-stimulated kinase activity was increased. However, half-maximum effective insulin concentrations were unchanged. Insulin receptor autophosphorylation as stimulated by ATP and insulin failed to affect significantly 125I-insulin binding to partially purified insulin receptor from human placenta. It is concluded that autophosphorylation of the insulin receptors regulates its kinase activity but not its affinity for insulin. The catalytic subunit of cyclic AMP-dependent protein kinase failed to phosphorylate either subunit of the insulin receptor, and each kinase failed to affect the affinity of the other one. Thus no functional interaction between cyclic AMP-dependent protein kinase and insulin receptors was observed in the in vitro system.

Insulin-like stimulation of glucose transport in isolated adipocytes by fatty acids

Glucose transport rate as assessed with the 3-0-Methylglucose method was significantly increased in adipocytes preincubated with aliphatic carboxylic acids. The magnitude of stimulation apparently depended on the chain length of the carboxylic acid, and was highest with palmitic acid (130%). The stimulation was additive to the effect of insulin, and reflected a decrease of km rather than an increase in vmax of the transport rate. The results suggest that fatty acids may modulate the activity of the glucose transporter, providing an insulin-dependent supply of adipose tissue with glycerol-phosphate during lipolysis for reesterification of excess fatty acids. Further, it is suggested that fatty acids mediate the stimulatory effect of catecholamines on glucose transport as observed in isolated fat cells.

Effects of chemical sympathectomy on insulin receptors and insulin action in isolated rat adipocytes

Insulin receptors and effects of insulin on lipolysis, lipogenesis and glucose transport were investigated in fat cells obtained from rats chemically sympathectomized with 6-hydroxydopamine. Four days after a single injection of 6-hydroxydopamine (50 mg/kg), the norepinephrine content of the epididymal adipose tissue was reduced by 97.5%. The number of high-affinity insulin binding sites was increased moderately (16%). In parallel, the sensitivity to insulin of the isoprenaline-stimulated lipolysis was increased as judged from insulin concentrations yielding half-maximal inhibition which were lower (40%) in the treated group. Glucose metabolism, however, was inhibited by chemical sympathectomy: the glucose transport rate was significantly reduced and fatty acid synthesis was nearly totally abolished. Insulin was still effective in stimulating both parameters but failed to restore normal levels. The results suggest that the sympathetic innervation of adipose tissue may exert an inhibitory effect on the number of high-affinity insulin receptors as well as on the sensitivity of the lipolysis to insulin, as both parameters were increased by sympathectomy. To explain the inhibitory effect of 6-hydroxydopamine treatment on glucose transport and fatty acid synthesis, a possible trophic effect of the sympathetic innervation is discussed as well as indirect mechanism counteracting the effects of the chemical sympathectomy.

Effects of islet-activating protein on insulin- and isoprenaline-stimulated glucose transport in isolated rat adipocytes

The effects of islet-activating protein (IAP), a Bordetella pertussis toxin, on insulin- and isoprenaline-stimulated glucose transport were studied in isolated rat adipocytes. Basal as well as insulin-stimulated glucose transport were not affected when cells were pretreated with IAP. In contrast, IAP pretreatment abolished the stimulatory effect of isoprenaline. When IAP-pretreated cells were exposed to a combination of insulin and isoprenaline, the catecholamine significantly reduced the stimulatory effect of insulin. Since IAP is supposed to specifically block the inhibitory component Ni of adenylate cyclase, the results suggest that: (a) the effect of insulin is unrelated to the regulation of adenylate cyclase; (b) isoprenaline may exert both stimulatory and inhibitory effects depending on activation of Ni. The inhibitory regulation of adenylate cyclase may thus be a pivotal link in the regulation of glucose transport.

Effects of theophylline on insulin receptors and insulin action in the adipocyte

The effects of theophylline on insulin receptors and insulin action in isolated rat adipocytes were studied. Theophylline reduced insulin binding by a decrease of receptor affinity. As concentration-response curves revealed, the effect was paralleled by a reduction of the cellular ATP content. Basal as well as insulin-stimulated glucose transport (2-deoxyglucose and 3-O-methylglucose uptake) were inhibited by much smaller theophylline concentrations (0.15-0.6 mM) than those necessary to reduce insulin binding and to lower ATP levels (1-4.8 mM), or to stimulate lipolysis (0.3-2.4 mM). Insulin fully antagonized the effect of theophylline on lipolysis but failed to reverse the inhibition of glucose transport completely. The results suggest that (a) theophylline impairs insulin action at a post-receptor level and, at higher concentrations, by a decrease of receptor binding, (b) the reduction of insulin receptor affinity probably reflects ATP depletion of the adipocyte, and (c) the xanthine inhibits glucose transport independently from its effects on lipolysis.

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.

Effects of pimozide and apomorphine on insulin secretion from the perfused rat pancreas

The effects of apomorphine, a partial agonist of dopamine, and of pimozide, an agent commonly used as an antagonist of dopamine, on insulin secretion from the perfused rat pancreas were studied. Apomorphine (10 and 100 microM) significantly reduced the insulin released in response to a stimulating glucose load. As judged from the perfusion pressure vasoconstriction could not account for the inhibitory action. The effect of apomorphine was not reversed by phentolamine infused to block alpha-receptors. Pimozide (1 microM) slightly weakened the inhibitory effect of apomorphine (10 microM) but failed to completely restore a normal secretion. An increase of the pimozide concentration (10 microM) revealed, however, that the agent itself reduced the glucose-induced insulin release. Thus a differentiation of dopamine agonistic and antagonistic effects could not be achieved with the aid of pimozide.

Modulation of insulin sensitivity by adenosine. Effects on glucose transport, lipid synthesis, and insulin receptors of the adipocyte

Insulin action and insulin binding in isolated rat fat cells incubated with adenosine or adenosine deaminase were studied. Adenosine enhanced the effects of insulin on glucose transport and glucose metabolism. The nucleoside shifted the concentration-response curves of insulin-stimulated D-[3-3H]glucose incorporation into total lipids, and of D-[U-14C]glucose conversion to fatty acids to smaller insulin concentrations. In addition, the maximal response of the fatty acid synthesis was increased. Insulin sensitivity and maximal response to insulin of the glucose transport system, as assessed by the rate of uptake of 2-deoxyglucose and 3-O-methylglucose, were increased by adenosine. The adenosine derivative N6-phenylisopropyladenosine similarly enhanced deoxyglucose transport in the presence of insulin. However, insulin binding was not affected by adenosine. The results suggest that adenosine modulates insulin action at a step distal from the insulin receptor, and before, or at, the glucose transport system.

Effects of tolbutamide on insulin binding to isolated fat cells of the rat

In isolated fat cells of the rat the in vitro and in vivo effects of tolbutamide on insulin binding and insulin response were studied. 450 mg tolbutamide/kg/day given for 7 days significantly increased the binding of insulin to isolated adipocytes. The binding curves reflected an increase in the number of receptor sites rater than in the affinity. The effect was associated with an enhanced response to insulin of the adipose tissue, since the fat cells obtained from animals treated with tolbutamide converted significantly more glucose to lipids in the presence of insulin than those obtained from the control group. However, the augmentation of insulin binding sites was observed only at a large tolbutamide dosage, which reduced the pancreatic insulin content, the secretory response of the isolated pancreas, and the serum insulin levels. Smaller doses, sufficient to produce metabolic effects via a stimulation of insulin secretion, did not provide additional insulin binding sites. When added in vitro to the binding assay or to adipose tissue incubated for 16 h, tolbutamide failed to increase insulin binding of the fat cells. It is suggested, therefore, that the effects produced by tolbutamide after in vivo treatment reflect an indirect rather than a direct action of the sulphonylurea.

Insulin binding and response to insulin of adipocytes from thyroxine-treated rats

Insulin binding to isolated fat cells from rats rendered hyperthyroid by daily injections of T4 (1 mg/kg) for 5 days was approximately doubled. The Scatchard curves reflected a large increase in receptor number, as well as an elevation in affinity of the high affinity binding sites. The response to insulin of the fat cells, however, was not increased accordingly: glucose incorporation into lipid in the presence of insulin did not differ significantly from that observed in the control group, whereas the effect of insulin on the lipolytic response to isoprenaline (isoproterenol) was even reduced in the T4-treated animals. T4 treatment had thus dissociated insulin binding from the metabolic effects of insulin, since the increase in membrane receptors was not paralleled by an enhanced effect of the hormone. Since levels of serum insulin were increased in the treated animals, the increase in number of insulin receptors was not mediated by reduced exposure to insulin. Propranolol failed to fully antagonize the effect of T4 on insulin binding, and reserpine treatment even enhanced it. It seems unlikely, therefore, that the increase in insulin receptors of adipocytes results from an augmented response to endogenous catecholamines in T4-treated rats.