Regulation of the synthesis of the major surfactant apoprotein in fetal rabbit lung tissue

Antibodies directed against the major apoprotein of rabbit lung surfactant, a 29-36-kDa glycoprotein, were used to study changes in the levels of translatable surfactant apoprotein mRNA in rabbit lung tissue during development, as well as the effects of cortisol and cyclic AMP analogues on the levels of surfactant apoprotein and its mRNA in fetal rabbit lung tissue in organ culture. The major surfactant apoprotein and its mRNA were undetectable in lung tissues of 21-day gestational age fetal rabbits. Translatable mRNA specific for the major surfactant apoprotein was first detectable in lung tissues of 26-day fetuses, increased 25-fold on day 28, reached peak levels at day 31, and declined after birth. Incubation of 21-day fetal rabbit lung explants with cortisol in serum-free medium resulted in an increase in the specific content of the 29-36-kDa apoprotein. Cyclic AMP analogues and forskolin, an activator of adenylate cyclase, also caused a marked increase in the accumulation of surfactant apoprotein. When fetal lung explants were incubated with cortisol and dibutyryl cyclic AMP in combination, the specific content of the surfactant apoprotein was increased to levels greater than that of explants treated with either cortisol or dibutyryl cyclic AMP alone. These effects of dibutyryl cyclic AMP and cortisol on surfactant apoprotein accumulation were associated with comparable changes in the levels of translatable surfactant apoprotein mRNA. Thus, we have shown for the first time that the induction of pulmonary surfactant apoprotein synthesis during differentiation in vitro and in vivo is associated with an increase in the level of translatable mRNA and that cortisol and cyclic AMP increase both the accumulation of the major surfactant apoprotein and the corresponding mRNA in fetal rabbit lung tissue in vitro.     

Glucose metabolism in the newborn rat. Hormonal effects in vivo

1. The concentrations of liver glycogen and plasma d-glucose were measured in caesarian-delivered newborn rats at time-intervals up to 3h after delivery after treatment of the neonatal rats with glucagon, dibutyryl cyclic AMP, cortisol or cortisol+dibutyryl cyclic AMP. Glycogenolysis was promoted by glucagon or dibutyryl cyclic AMP in the third hour after birth but not at earlier times. Cortisol and dibutyryl cyclic AMP together (but neither agent alone) promoted glycogenolysis in the second hour after birth, but no hormone combination was effective in the first postnatal hour. 2. The specific radioactivity of plasma d-glucose was measured as a function of time for up to 75 min after the intraperitoneal injection of d-[6-(14)C]glucose and d-[6-(3)H]glucose into newborn rats at delivery and after treatment with glucagon or actinomycin D. Glucagon-mediated hyperglycaemia at this time was due to an increased rate of glucose formation and a decreased rate of glucose utilization. Actinomycin D prevented glucose formation and accelerated the rate of postnatal hypoglycaemia. 3. The specific radioactivity of plasma l-lactate and the incorporation of (14)C into plasma d-glucose was measured as a function of time after the intraperitoneal injection of l-[U-(14)C]lactate into glucagon- or actinomycin D-treated rats immediately after delivery. The calculated rates of lactate formation were unchanged by either treatment, but lactate utilization was stimulated by glucagon administration. Glucagon stimulated and actinomycin D diminished (14)C incorporation into plasma d-glucose. 4. The factors involved in the initiation of glycogenolysis and gluconeogenesis in the rat immediately after birth are discussed.     

Urinary and plasma cyclic AMP levels during short term starvation in obese man: response to glucagon stimulation.

Glucagon is known to elevate the intracellular concentration of cyclic AMP in the hepatocyte. The increase in intracellular cyclic AMP is reflected by an increase in the plasma concentration of the nucleotide. Intravenous glucagon stimulation was performed on six obese non-diabetic human subjects before and after a three day fast. All patients responded to starvation by a lowering of plasma immunoreactive insulin and blood glucose. Whereas the plasma immunoreactive glucagon concentration increased over the three day period, the plasma and urinary cyclic AMP did not significantly change. Intravenous glucagon promoted qualitatively similar increases in the blood glucose and plasma concentrations of insulin and cyclic AMP before and after three days starvation.     

Plasma adenosine 3′,5′ -cyclic monophosphate in human hypertension

In a previous study we observed an increase in urinary cyclic AMP in labile hypertension in the upright position and during isoproterenol infusion, in contrast to a decrease in control subjects. In the present study we measured the plasma level of cyclic AMP in control subjects and patients with various types of hypertension. We obtained the following results: (1) plasma cyclic AMP increases in response to upright posture in control subjects and hypertensive patients; (2) values of cyclic AMP in the recumbent and upright positions are comparable in control subjects and patients with essential hypertension, but are significantly higher in those with true renovascular hypertension due to bilateral renal artery stenosis; (3) propranolol inhibits the increase of plasma cyclic AMP in response to posture in control subjects, but has an opposite effect in labile hypertension where there is a further increase; (4) the rise in blood pressure in pheochromocytoma is associated with a considerable increase in plasma cyclic AMP.Present and previous data suggest that kidney handling of cyclic AMP is abnormal in hypertension, and that the specific defect may be related to the type of hypertension.     

Studies on the mechanism of the effects of experimental inflammation on adaptive synthesis of rat liver fatty acid synthetase

The mechanism of suppression, by experimental inflammation of the usual increase in hepatic fatty acid synthetase activity resulting from fat-free feeding following starvation (adaptive synthesis), was investigated immunochemically. That suppression results from changes in amount of hepatic fatty acid synthetase was shown by the observation that fatty acid synthetase preparations from inflamed and uninflamed animals, exhibiting a wide variety of specific enzyme activities, had identical immunochemical equivalence points. In confirmation of this, the amounts of fatty acid synthetase, determined by radial immunodiffusion in gels containing anti-fatty acid synthetase serum, varied concomitantly with changes in enzyme activity regardless of the relative times of inflammation and onset of adaptive synthesis. Serum insulin levels were not dramatically elevated during the first 48 h of fat-free feeding, but rose markedly thereafter. Inflammation, either alone or combined with fat-free feeding, resulted in increased serum glucose levels, followed by a similar pattern of increased serum insulin levels some 12 h later. Fat-free feeding did not affect serum cortisol levels, but depressed liver cyclic AMP. Inflammation invariably resulted in a marked increase in serum cortisol within 12 h and a concomitant elevation of hepatic cyclic AMP, indicating possible roles for cortisol and cyclic AMP in suppression of hepatic fatty acid synthetase synthesis.     

Intraoperative protein sparing with glucose.

We examined the hypothesis that glucose infusion inhibits amino acid oxidation during colorectal surgery. We randomly allocated 14 patients to receive intravenous glucose at 2 mg x kg(-1) x min(-1) (glucose group) starting with the surgical incision or an equivalent amount of normal saline 0.9% (control group). The primary endpoint was whole body leucine oxidation; secondary endpoints were leucine rate of appearance and nonoxidative leucine disposal as determined by a stable isotope tracer technique (L-[1-(13)C]leucine). Circulating concentrations of glucose, lactate, insulin, glucagon, and cortisol were measured before and after 2 h of surgery. Leucine rate of appearance, an estimate of protein breakdown, and nonoxidative leucine disposal, an estimate of protein synthesis, decreased in both groups during surgery (P < 0.05). Leucine oxidation intraoperatively decreased from 13 +/- 3 to 4 +/- 3 micromol x kg(-1) x h(-1) in the glucose group (P < 0.05 vs. control group) whereas it remained unchanged in the control group. Hyperglycemia during surgery was more pronounced in patients receiving glucose (9.7 +/- 0.5 mmol/l, P < 0.05 vs. control group) than in patients receiving normal saline (7.1 +/- 1.0 mmol/l). The administration of glucose caused an increase in the circulating concentration of insulin (P < 0.05) resulting in a lower glucagon/insulin quotient than in the control group (P < 0.05). Intraoperative plasma cortisol concentrations increased in both groups (P < 0.05), whereas plasma concentrations of lactate and glucagon did not change. The provision of small amounts of glucose was associated with a decrease in amino acid oxidation during colorectal surgery.     

Effect of salmon calcitonin on renal excretion of adenosine 3', 5' monophosphate in man.

Intravenous infusion of salmon calcitonin in six healthy subjects produced an increase in the plasma levels and urinary excretion of cyclic AMP. Cyclic AMP clearance diminished but remained higher than inulin clearance. Salmon calcitonin was also infused in six hypertensive patients with normal glomerular filtration rate. Arterial and renal venous plasma concentration of cyclic AMP were clearly raised. The difference between both these concentrations was not significant in the control periods but became marked during the treatment and post treatment periods demonstrating a net extraction of cyclic AMP from plasma by the kidneys. Renal extraction of cyclic AMP was lower than its urinary excretion in the control periods whereas it was clearly higher after salmon calcitonin was given. This shows that salmon calcitonin stimulates the production of cyclic AMP in extra-renal tissues and that the excess of cyclic AMP formed is catabolized by the kidneys.     

Rates of cortisol increase and decrease in channel catfish and sunshine bass exposed to an acute confinement stressor

Channel catfish and sunshine bass were exposed to a low-water stress event and allowed to recover in fresh water or a solution of metomidate (dl-1-(1-phenylethyl)-5-(metoxycarbonyl) imidazole hydrochloride), which inhibits the synthesis of cortisol. Change in time of plasma cortisol was used as an index of cortisol secretion and clearance. Plasma cortisol and glucose increased during the exposure to low-water stress in both fish, but the changes of both plasma components were more dramatic in sunshine bass. Exposure to metomidate during recovery resulted in a short-term increase in plasma glucose but differences between controls and metomidate-exposed fish were relatively minor thereafter. Cortisol began to decrease in catfish immediately after the removal of the stress but continued to increase for 15 min in sunshine bass recovering in fresh water and for 5 min in bass recovering in metomidate. Catfish recovering in fresh water had a cortisol elimination rate of -1.28 ng/mL/min compared with -2.45 ng/mL/min for fish recovering in metomidate (P>0.05) while sunshine bass recovering in fresh water had an elimination rate of -6.96 ng/mL/min compared with -4.50 ng/mL/min for fish recovering in metomidate (P>0.05). These data indicate that the rapid decrease of plasma cortisol after removal of the stressor is due to an almost immediate decrease of secretion, tissue uptake and a rapid renal loss due to the absence of a plasma binding protein.     

Regulation of Glycogenolysis in Transformed Astrocytes In Vitro

Cultured astrocytes, transformed by Herpesvirus, were used as a model system to study several aspects of the control of glycogenolysis. Adrenergic agonists such as norepinephrine and isoproterenol caused an immediate and dose-dependent increase in the intracellular levels of cyclic AMP. Concomitant with the initial phase of cyclic AMP increase, conversion of phosphorylase b to a and glycogenolysis were observed. The elevation of cyclic AMP, phosphorylase conversion, and glycogenolysis were simultaneously blocked by beta-adrenergic blockers, but not by alpha-adrenergic blocking agents. Repeated administration of norepinephrine caused an attenuated response in both cyclic AMP accumulation and glycogenolysis. Glycogen degradation is also partially regulated by glucose availability. In the presence of glucose, norepinephrine-induced glycogenolysis is blocked, despite elevations in cyclic AMP. The direct role of glucose is postulated, since glucose analogs mimic the effects of glucose.     

Changes in plasma cyclic nucleotides levels during various acute physical stresses

Studies were performed with healthy volunteers to determine the effects of various stresses known to increase sympathetic nerve activity on the plasma concentration of cyclic AMP and cyclic GMP. Plasma cyclic AMP rose promptly in response to exercise; the elevation was completely abolished by a simultaneous injection of propranolol. Plasma cyclic GMP rose slightly after exercise; the elevation was completely abolished by a simultaneous injection of atropine. No significant changes occurred during tilting and cold pressor stress. The increase in plasma concentrations of cyclic AMP and cyclic GMP may serve as putative indices for beta-adrenergic, and cholinergic functions respectively during exercise of humans.     

A possible cyclic AMP-mediated regulation of microsomal fatty acyl-CoA desaturation system in Tetrahymena microsomes

Profound alterations in the microsomal fatty acyl-CoA desaturase activities and cyclic AMP production of a unicellular eukaryote, Tetrahymena pyriformis NT-1, originally grown in the glucose-deficient medium, were observed, following the administration of glucose or beta-adrenergic agonists such as epinephrine and isoproterenol. There was a great increase of stearoyl-CoA (delta 8) desaturase activity coincident with a 2-fold decrease of oleoyl-CoA (delta 12) desaturase activity over the first 2 h after administration of these compounds. During this period of time, it was found that the production in vivo of labeled oleic acid from [14C]acetic or [3H]palmitic acid increases 2-fold and the formation in vivo of each labeled linoleic and gamma-linolenic acids drastically decreases. Glucose or beta-adrenergic agonists caused an increase of stearoyl-CoA-stimulated reoxidation rate of NADH-reduced cytochrome b5 but depressed oleoyl-CoA-stimulated reoxidation rate of b5, indicating that both desaturase activities are controlled by the respective terminal components of the desaturase system. A significant and reproducible increase of adenylate cyclase activity and a slight decrease of cyclic AMP phosphodiesterase activity were observed to occur within the first 2 h after the addition of these compounds, when cyclic AMP content in Tetrahymena cell rose by 3-4-fold. Propranolol, a beta-adrenergic blocker, abolished the effects of glucose or beta-adrenergic agonists on the activities of fatty acyl-CoA desaturases and the terminal components as well as cyclic AMP production of cells. These results suggest that glucose and beta-adrenergic agonists may modulate the microsomal fatty acyl-CoA desaturase system in Tetrahymena by acting through the increase of intracellular cyclic AMP content.     

Cyclic AMP and steroidogenesis in superovulated rat ovaries

During the gonadotropin stimulated differentiation of ovarian follicles into corpora lutea, the concentrations of total cyclic AMP and protein bound cyclic AMP increased only marginally. In contrast, there was a 10 fold increase in progesterone production. After acute stimulation of luteinized ovaries with choriogonadotropin total cyclic AMP levels increase more than 6 times while bound cyclic AMP and plasma progesterone rose by 80 and 60% respectively. Our results question the role of cyclic AMP in the basal production of progesterone by the ovary, and suggest a relationship between bound cyclic AMP and progesterone synthesis only exists after acute gonadotropin stimulation.     

Integrated analysis of protein and glucose metabolism during surgery: effects of anesthesia.

The aim of this study was to assess dynamic changes in protein and glucose metabolism during surgery. Twelve patients undergoing colorectal surgery received either intravenous propofol anesthesia (n = 6) or inhalational anesthesia with desflurane (n = 6). Pre- and intraoperative protein and glucose kinetics were analyzed by an isotope dilution technique using L-[1-(13)C]leucine and [6,6-(2)H(2)]glucose. Plasma concentrations of glucose, lactate, free fatty acids, insulin, glucagon, and cortisol were measured before and after 2 h of surgery. The rates of appearance of leucine and glucose, leucine oxidation, protein synthesis, and glucose clearance decreased during surgery, independent of the type of anesthesia (P < 0.05). A correlation between the rate of appearance of leucine and glucose was observed (r = 0.755, P < 0.001). Intraoperative plasma cortisol and glucose concentrations increased (P < 0.05), whereas plasma concentrations of lactate, free fatty acids, insulin, and glucagon did not change. Surgery causes a depression of whole body protein and glucose metabolism, independent of the anesthetic technique. There is a correlation between perioperative glucose production and protein breakdown.     

Role of cortisol on the glycogenolytic effect of glucagon and on the glycogenic response to insulin in fetal hepatocyte culture.

The effects of insulin and glucagon on glycogen metabolism were studied in cultured fetal hepatocytes transplanted from 15-day-old fetuses. The effects of these hormones were examined just after transplantation, when the cells contained only minute amounts of glycogen, and during the 3 to 4 day culture period, when the hepatocytes were exposed to 10 muM cortisol and actively accumulated glycogen. At all stages of the culture, glucagon addition (10 nM) was followed by a rapid depletion of labeled glycogen, previously synthesized during a pulse labeling with [14C]glucose: this effect was mimicked by N6, O2'-dibutyryl adenosine 3':5'-monophosphate (dibutyryl cyclic AMP) (0.3 to 1 nM). Such a glycogenolytic effect of glucagon was observed even 6 hours after transplantation, i.e. at a time when cortisol was not present. In addition, glucagon clearly induced cyclic adenosine 3':5'-monosphosphate (cyclic AMP) accumulation in cells grown for 18 hours in the absence of cortisol. With cells grown for 3 days in the presence of cortisol, glucagon-dependent glycogenolysis was also obtained when cortisol was removed from the medium 20 hours before hormone addition. Thus the presence of cortisol is not necessary either to maintain a response to glucagon or for the onset of the glycogenolytic effect of glucagon. Insulin addition (10 nM) stimulated [14C]glucose incorporation into glycogen at all stages of the culture when grown in the presence of cortisol; no glycogenic response to insulin was observed 6 hours after transplantation where cortisol was not previously introduced. In addition, if the hepatocytes were grown in the presence of insulin alone (i.e. in the absence of cortisol) no significant storage of glycogen occurred. Maximal storage (or labeling) of glycogen was observed when hepatocytes were grown in the presence of both cortisol and insulin. The presence of cortisol was therefore necessary for the expression of the glycogenic effect of insulin. These data show that marked difference exist between the onset of developmental responses towards glucagon and insulin. The glucagon-dependent regulatory pathway should be present very early in fetal development and should not depend on cortisol. On the contrary, the onset of the insulin-dependent regulatory pathway seems to be induced during culture, and it is likely that this is caused by cortisol.     

Effects of naloxone on the breathing, electrocortical, heart rate, glucose and cortisol responses to hypoxia in the sheep fetus

Continuous infusions of naloxone HC1 (0.5 mg/kg or 3.8 mg/kg) or saline were given intravenously to fetal sheep at 119 to 137 days of gestation during a one hour period of air administration and a one hour period of hypoxia induced by having ewes breathe 9% O2, 3% CO2 and 88% N2. Fetal carotid PaO2 fell to 13.0 +/- 0.5 mmHg during hypoxia with no change in pH. During hypoxia, plasma cortisol concentration increased significantly more in naloxone-infused fetuses than controls. Ewes, whose fetuses received naloxone, showed a significant increase in cortisol during hypoxia whereas no increase was observed in controls. There were no significant differences between saline and naloxone-infused fetuses during hypoxia in fetal breathing incidence, amplitude, frequency, number of deep inspiratory efforts per hour, heart rate, electrocortical activity or in the rise in plasma glucose caused by hypoxia. Results suggest that endogenous opiates may have a role in modulating cortisol production in the ewe and fetus during hypoxia but do not have a role in mediating the decrease in incidence of breathing activity or rise in plasma glucose. During air administration, naloxone significantly increased fetal breath amplitude, fetal and maternal plasma glucose, fetal heart rate, and the number of electrocortical changes per hour. This suggests endogenous opiates may have a more important role in the normoxic pregnant ewe and fetus.     

Characterization of regulation of thyrotropin (TSH) receptor function in thyroid plasma membrane: interaction between TSH receptor function and activation of adenosine 3',5'-monophosphate-dependent protein kinase

The changes in the characteristics of thyrotropin (TSH) binding to thyroid plasma membranes during the activation of cyclic AMP-dependent protein kinase in the membranes were studied. Preincubation of thyroid plasma membranes with TSH or cyclic AMP reduced the maximal binding capacity but increased the association rate for TSH binding. In double reciprocal analysis, a marked reduction of the total number of binding sites and association constant was observed in the membranes treated with cyclic AMP. These reductions were also observed in the membranes preincubated with buffer alone. The degree of these reductions, however, was greater in the membranes pretreated with cyclic AMP. During incubation of the membranes with buffer alone, cyclic AMP formation (activation of adenylate cyclase) was observed though the degree of the formation was lower than that induced by TSH. The results suggested that not only TSH receptor release from thyroid plasma membrane but also the modification of TSH binding activity in the membrane is produced by cyclic AMP-dependent protein kinase.     

Effects of electric stress on glucose metabolism, glucose-stimulated cyclic adenosine 3',5'-monophosphate accumulation and 45 Ca++ efflux in isolated pancreatic islets from rats fed with a high fat diet.

The effects of the electric stress on glucose oxidation, cyclic adenosine 3', 5'-monophosphate (AMP) accumulation and 45Ca++ efflux in response to glucose were studied in pancreatic islets isolated from rats fed on a control (C) or a high fat diet (F) for 12 weeks. The half of rats on each diet were subjected to electrical shocks in the random time schedule for 1 hr per day for the last 3 weeks of the feeding period (group C-S and F-S). The remaining rats were not given any shocks (group C-NS and F-NS). The rats in F-S group had the high levels of plasma epinephrine, dopamine and blood glucose. The basal content of cyclic AMP after 20 min of incubation with 2.8 mM glucose was decreased in islets from F-S group without affecting insulin release. After 20 min of incubation with 25 mM glucose, the cyclic AMP content in islets from F-S group, which was identical with that in F-NS group, was only 50% of that in C-S group. Insulin release in response to high glucose was significantly inhibited in islets from F-S group. In spite of a remarkable increase of cyclic AMP content in islets from C-S group, insulin release did not differ from that in C-NS group. Glucose (16.7 mM)-stimulated 45Ca++ efflux from the perfused islets was greatly inhibited by the high fat diet rather than by stress. The rate of glucose oxidation with 16.7 mM glucose was decreased in islets from F-S group. It is suggested that the decreased insulin release in response to glucose provoked by the combined effects of the feeding of a high fat diet and electric stress may be mediated by changes of the adenylate cyclase-cyclic AMP system on the plasma membrane of the B-cell or be related to changes in glucose metabolism in islets.     

Renal vein plasma adenosine 3',5'-cyclic monophosphate in renovascular hypertension.

The concentration of plasma adenosine 3'',5''-cyclic monophosphate (cyclic AMP) and plasma renin activity (PRA) were measured concomitantly in blood from both renal veins and in arterial blood in 22 hypertensive patients. In the nine patients with true renovascular hypertension the concentration of plasma cyclic AMP was greater in the venous effluent of the kidney affected by the renal artery stenosis than in that of the unaffected or less affected kidney. The arteriovenous difference in cyclic AMP concentration was less on the affected side in all but one patient. The arteriovenous differences in PRA identified the affected kidney as the source of hyper-reninemia and showed that renin release from the other kidney was suppressed. In the 13 patients with hypertension associated with but unrelated to renal artery stenosis there were no consistent patterns of cyclic AMP concentration or PRA in the venous effluent of the kidneys or of their arteriovenous differences. In renovascular hypertension the venous effluent of the kidney affected by renal artery stenosis contains not only more renin but also more cyclic AMP, owing to either increased cyclic AMP production or decreased excretion or extraction of cyclic AMP by the affected kidney. This unilateral increase in cyclic AMP concentration may become a complementary diagnostic feature of true renovascular hypertension.     

Hexose transport stimulation and membrane redistribution of glucose transporter isoforms in response to cholera toxin, dibutyryl cyclic AMP, and insulin in 3T3-L1 adipocytes

Exposure of 3T3-L1 adipocytes to 100 ng/ml of cholera toxin or 1 mM dibutyryl cyclic AMP caused a marked stimulation of deoxyglucose transport. A maximal increase of 10- to 15-fold was observed after 12-24 h of exposure, while 100 nM insulin elicited an increase of similar magnitude within 30 min. A short term exposure (4 h) of cells to cholera toxin or dibutyryl cyclic AMP resulted in a 3- to 4-fold increase in deoxyglucose transport which was associated with significant redistribution of both the HepG2/erythrocyte (GLUT1) and muscle/adipocyte (GLUT4) glucose transporters from low density microsomes to the plasma membrane fraction. Total cellular amounts of both transporter proteins remained constant. In contrast, cells exposed to cholera toxin or dibutyryl cyclic AMP for 12 h exhibited elevations in total cellular contents of GLUT1 (but not GLUT4) protein to about 1.5- and 2.5-fold above controls, respectively. Although such treatments of cells with cholera toxin (12 h) versus insulin (30 min) caused similar 10-fold enhancements of deoxyglucose transport, a striking discrepancy was observed with respect to the content of glucose transporter proteins in the plasma membrane fraction. While insulin elicited a 2.6-fold increase in the levels of GLUT4 protein in the plasma membrane fraction, cholera toxin increased the amount of this transporter by only 30%. Insulin or cholera toxin increased the levels of GLUT1 protein in the plasma membrane fraction equally (1.6-fold). Thus, a greater number of glucose transporters in the plasma membrane fraction is associated with transport stimulation by insulin compared to cholera toxin. We conclude that: 1) at early times (4 h) after the addition of cholera toxin or dibutyryl cyclic AMP to 3T3-L1 adipocytes, redistribution of glucose transporters to the plasma membrane appears to contribute to elevated deoxyglucose uptake rates, and 2) the stimulation of hexose uptake after prolonged treatment (12-18 h) of cells with cholera toxin may involve an additional increase in the intrinsic activity of one or both glucose transporter isoforms.     

The role of adenosine 3′:5′-cyclic monophosphate in the regulation of insulin release by isolated rat islets of Langerhans

1. Concentrations of cyclic AMP (adenosine 3':5'-cyclic monophosphate) and rates of insulin release were measured in islets of Langerhans isolated from rat pancreas and incubated for various times in the presence of glucose, 3-isobutyl-1-methylxanthine, caffeine, theophylline, adrenaline and diazoxide. 2. Caffeine and theophylline produced small but significant increases in both cyclic AMP and release of insulin when they were incubated in the presence of 10mm-glucose. 3. 3-Isobutyl-1-methylxanthine produced a marked increase in the intracellular concentration of cyclic AMP in the presence of 5mm- and 10mm-glucose. However, insulin release was stimulated only in the presence of 10mm-glucose. 4. In response to rising concentrations of extracellular glucose (5-20mm) there was no detectable increase in the intracellular concentration of cyclic AMP even though there was a marked increase in the rate of insulin release. 5. In response to 10mm-glucose insulin release occurred in two phases and 3-isobutyl-1-methylxanthine potentiated the effect of glucose on both phases. The intracellular concentration of cyclic AMP remained constant with glucose and rose within 10min to its maximum value with 3-isobutyl-1-methylxanthine. 6. Adrenaline and diazoxide inhibited insulin release and lowered the intracellular concentration of cyclic AMP when islets were incubated with glucose or 3-isobutyl-1-methylxanthine. 7. It is suggested that glucose does not stimulate insulin release by increasing the concentration of cyclic AMP in islet cells. However, the concentration of cyclic AMP in islet cells may modulate the effect of glucose on the release process.