Mechanism of plasma cyclic AMP response to hypoglycemia in man

Extracellular cAMP-Adenosine Pathway Signaling: A Potential Therapeutic Target in Chronic Inflammatory Airway Diseases

Adenosine is a purine nucleoside that, via activation of distinct G protein-coupled receptors, modulates inflammation and immune responses. Under pathological conditions and in response to inflammatory stimuli, extracellular ATP is released from damaged cells and is metabolized to extracellular adenosine. However, studies over the past 30 years provide strong evidence for another source of extracellular adenosine, namely the “cAMP-adenosine pathway.” The cAMP-adenosine pathway is a biochemical mechanism mediated by ATP-binding cassette transporters that facilitate cAMP efflux and by specific ectoenzymes that convert cAMP to AMP (ecto-PDEs) and AMP to adenosine (ecto-nucleotidases such as CD73). Importantly, the cAMP-adenosine pathway is operative in many cell types, including those of the airways. In airways, β₂-adrenoceptor agonists, which are used as bronchodilators for treatment of asthma and chronic respiratory diseases, stimulate cAMP efflux and thus trigger the extracellular cAMP-adenosine pathway leading to increased concentrations of extracellular adenosine in airways. In the airways, extracellular adenosine exerts pro-inflammatory effects and induces bronchoconstriction in patients with asthma and chronic obstructive pulmonary diseases. These considerations lead to the hypothesis that the cAMP-adenosine pathway attenuates the efficacy of β₂-adrenoceptor agonists. Indeed, our recent findings support this view. In this mini-review, we will highlight the potential role of the extracellular cAMP-adenosine pathway in chronic respiratory inflammatory disorders, and we will explore how extracellular cAMP could interfere with the regulatory effects of intracellular cAMP on airway smooth muscle and innate immune cell function. Finally, we will discuss therapeutic possibilities targeting the extracellular cAMP-adenosine pathway for treatment of these respiratory diseases.

c-AMP and ANP levels in VVI and DDD pacing with different AV delays during daily activity and exercise

Nine patients (three males) mean age 68 +/- 8 years, having complete heart block, and paced in the DDD mode were examined in VVI and DDD pacing with 100 and 150 ms atrioventricular delays (AVD) during rest and exercise. Plasma atrial natriuretic peptide (ANP) and cyclic AMP (c-AMP) were measured at rest and at peak exercise test. ANP plasma levels at rest were significantly higher in VVI pacing compared to 150 AVD (P less than 0.03). On exercise, ANP release was statistically increased only in DDD with 150 ms AVD, while in VVI it remained in high levels at exercise but no significant change was found (p:ns). c-AMP during rest was unchanged in any pacing mode or AVD, but on exercise DDD pacing with short AVD (100 ms) released lower c-AMP plasma levels, than at rest (p:ns). DDD pacing with long AVD (150 ms) during exercise produced statistically higher c-AMP plasma levels (P less than 0.05) than at rest. Also in VVI pacing the c-AMP plasma levels were statistically higher than at rest (P less than 0.02). Adrenergic activity seems to be lower during exercise in DDD pacing with shorter AVD (100 ms) than in DDD with 150 ms AVD or VVI pacing. No difference was found in c-AMP plasma levels at rest. ANP release was also found to be lower at exercise in DDD pacing with short AVD (100 ms) than in DDD with 150 ms AVD. ANP plasma levels at rest were statistically higher in VVI pacing.

Blood hormone and metabolite levels during graded cycle ergometer exercise

To study the effect of the intensity of physical exercise on plasma hormone and metabolite levels, a group of 11 well-conditioned males participated in cycle ergometer exercise. The subjects pedalled at three different work loads, corresponding to 63% of VO2max (duration 10 min), 86% of VO2max (duration 10 min) and maximal work load (tolerated 5-7 min) The increases in blood adrenaline, noradrenaline, growth hormone, cyclic AMP, glycerol and lactate concentrations were remarkably similar and exponentially related to the work load. The concentrations of blood glucose, cortisol and glucagon increased only at maximal work load. Many of these changes in blood metabolite and hormone concentrations seem to be related to the increased sympathetic activity during graded exercise.

Increased plasma cyclic nucleotide concentrations in congestive heart failure

Plasma concentrations of cyclic nucleotides (adenosine monophosphate (AMP) and guanosine monophosphate (GMP) were measured by an ultrasensitive radioimmunoassay in 138 patients with heart failure due to various causes. Measurements were related to the New York Heart Association classification of symptoms, plasma noradrenaline concentrations, and mean pulmonary artery pressures. Serial concentrations of cyclic AMP and GMP were also measured daily in four patients treated for acute left ventricular failure. Plasma concentrations of cycle AMP were related to the severity of the heart failure, plasma noradrenaline concentrations, and pulmonary artery pressures. Cyclic AMP concentrations fell rapidly after treatment of acute left ventricular failure. Plasma concentrations of cyclic GMP also depended on the severity of heart failure and the pulmonary artery pressure, and decreased sharply with treatment although remaining at a high value. The cyclic GMP concentrations were significantly higher in patients with mitral stenosis than in those with other types of heart failure.

Insulin-induced hypoglycaemia in diabetic women during late pregnancy and one year post partum

Insulin (0.1 IW/kg) later followed by glucose was injected intravenously in nine diabetic women in the supine position both during pregnancy and one year post partum. C-peptide was present in five subjects, indicating some residual beta-cell function. Their mean basal C-peptide level, before insulin, was twice as high in the pregnant as inthe non-pregnant state. C-peptide decreased progressively after insulin. The mean basal plasma glucose level was lower during pregnancy (4.8 mmol/l) than after it (9.6 mmol/l), but decreased to the same level (2.2 mmol/l) after insulin. The rate of fall in glucose was thus lower during pregnancy (kt = 2.54) than after (kt = 4.08), but was unrelated to the basal glucose levels. Basal levels of free fatty acids (FFA), 3-hydroxybutyrate (3-HB), cyclic AMP, and lactate were similar, while glycerol was lower during pregnancy. Insulin-induced changes in FFA, glycerol, 3-HB, cyclic AMP, and lactate were similar during and after pregnancy. Plasma amino acid concentrations were generally lower in pregnancy, significantly so only for arginine and glycine. Amino acid levels were unaffected by insulin in pregnancy, whereas leucine, isoleucine and tyrosine decreased significantly in the non-pregnancy, whereas leucine, isoleucine and tyrosine decreased significantly in the non-pregnancy, whereas leucine, isoleucine and tyrosine decreased significantly in the non-pregnancy state. We conclude that there are differences in metabolic responses to insulin in diabetic women during and after pregnancy, indicating a decreased sensitivity to insulin during pregnancy in some tissues.

Autonomic neural control mechanisms of substrate and hormonal responses to acute hypoglycaemia in man

The contributions of adrenergic and cholinergic mechanisms to recovery from acute hypoglycaemia induced by insulin (0.15 units/kg i.v.) were examined in eleven normal subjects, six subjects with a pre-ganglionic sympathectomy (adrenergic denervation) and six sympathectomized subjects given atropine (combined adrenergic denervation and cholinergic blockade). Blood glucose recovery was impaired only in the sympathectomized subjects given atropine. The blood lactate response was reduced and the rise in free fatty acids was delayed in both groups of sympathectomized subjects, in whom the normal rises of plasma cyclic AMP and noradrenaline were absent. The plasma pancreatic glucagon response was appropriate to the prevailing blood glucose concentrations in all three groups. The cortisol response was impaired and the pattern of ACTH secretion was abnormal in sympathectomized subjects given atropine. Growth hormone levels were higher in both sympathectomized groups. Blood glucose homeostasis was impaired during combined adrenergic denervation and cholinergic blockade. Glucagon secretion was activated independently of vagal control. In the sympathectomized group given atropine, the rise in plasma cortisol was blunted despite a greater degree of hypoglycaemia. A blockade of central cholinergic receptors producing impaired activation of ACTH secretion at hypothalamic level may explain, at least in part, this delayed restoration of normoglycaemia.

Plasma cyclic nucleotide responses to insulin-induced hypoglycaemia and methacholine in patients with hyperthyroidism

The effect of insulin-induced hypoglycaemia and methacholine on plasma cAMP and cGMP levels was studied in normal volunteers, hyperthyroid and hypothyroid patients. A significant positive correlation existed between the maximal increase in plasma cAMP and the maximal decrease in plasma glucose in normals during insulin-induced hypoglycaemia. Therefore, the plasma cAMP response is considered to be dependent on the degree of hypoglycaemia, rather than the dose of insulin. The cAMP response to hypoglycaemia was significantly higher in hyperthyroid patient, and was lower in patients with hypothyroidism than in normals. The cAMP response of the hyperthyroid patients was normalized when their hyperthyroidism was controlled after 3 months of treatment. The plasma level of cGMP was slightly elevated during hypoglycaemia, but there was no significant difference between controls and hyperthyroid patients. The cGMP response to methacholine, which is probably mediated by cholinergic receptors, was significantly potentiated in hyperthyroid patients. The cAMP response, which is presumably dependent on endogenous catecholamines secreted during methacholine-induced hypotension, was also enhanced in hyperthyroid patients. It is likely that beta-adrenergic receptor responses and cholinergic receptor responses are both enhanced in hyperthyroidism.

Comparison of hormonal and metabolic effects of salbutamol infusion in normal subjects and insulin-requiring diabetics

A comparison of the metabolic effects of salbutamol in diabetic patients and normal subjects showed that salbutamol infused at 5 and 2 microgram/min (a) stimulated hepatic glucose production to a greater extent in diabetic patients than in normal subjects despite prior restoration of the diabetic patients' fasting blood glucose to normal by an overnight infusion of insulin; (b) caused a greater rise in plasma glucose, free fatty acids, glycerol, and ketone-body concentrations in the diabetic patients; and (c) produced a marked fall in plasma-potassium concentrations. The differences between the diabetic and normal groups were accounted for by an immediate (six-fold) stimulation of insulin secretion in the normal subjects.

Actions of salbutamol in late pregnancy: plasma cyclic AMP, insulin and C-peptide, carbohydrate and lipid metabolites in diabetic and non-diabetic women

Salbutamol was administered intravenously in dose increasing from 3.75 to 22.5 microgram/min to 5 non-diabetic and 7 diabetic women in the last trimester of pregnancy. In diabetic as well as non-diabetic women the diastolic blood pressure fell progressively with increasing doses, and the systolic BP and heart rate increased at doses above 7.5 microgram/min. The effect of fetal heart rate was less pronounced than the effect on maternal heart rate. Cyclic AMP levels in plasma were similar in non-diabetic and diabetic women before salbutamol. Twenty min following 3.75 microgram/min a significant increase was seen in both groups. The peak increase (3-5 fold) was higher in the diabetic than in the non-diabetic women. Plasma insulin and C-peptide levels rose in a dose-dependent manner in the non-diabetic and four of the diabetic women. However, in three of the diabetic women the insulin level was unaffected by salbutamol and C-peptide was almost undetectable. Plasma concentrations of glucose, glycerol, NEFA and 3-HB were higher in the diabetics than in the non-diabetics before salbutamol and the elevations induced by salbutamol were also significantly larger in the diabetic women. The present data show that salbutamol in doses employed clinically may cause pronounced metabolic effects, especially in diabetic women, and it is suggested that when intravenous infusion of salbutamol is given to pregnant diabetic women not only cardiovascular but also some metabolic variable such as glucose should be carefully monitored.

Increases in plasma cyclic AMP dependent on endogenous catecholamines

Administration of tyramine (with or without phentolamine) as well as induction of ether anesthesia or insulin hypoglycemia caused a sharp increase in plasma cyclic AMP in rats. Based on the findings that the treatment of rats with reserpine, 6-hydroxydopamine, cocaine or propranolol totally abolished tyramine-induced increases in plasma cyclic AMP, it was concluded that catecholamines released from sympathetic neuronal terminals by tyramine could activate adenylate cyclase via the stimulation of postsynaptic beta-adrenoceptors. In contrast, catecholamines secreted from adrenal medulla were largely responsible for the increase in plasma cyclic AMP induced by ether anesthesia; whereas glucagon, in addition to adrenal catecholamines, played a significant role in hypoglycemia-induced increases in plasma cyclic AMP. Assay of plasma cyclic AMP following these stimuli is very promising as a test for adrenergic activities in experimental and clinical studies.

Cyclic AMP, adrenaline and noradrenaline in plasma during surgery

Plasma concentrations of cyclic AMP, adrenaline and noradrenaline were measured in 6 patients undergoing hysterectomy from before induction of anaesthesia to 6 h after skin incision. Noradrenaline did not vary significantly during the observation period, whereas cyclic AMP and adrenaline increased after skin incision. A significant correlation was found between plasma concentrations of cyclic AMP and adrenaline (r = 0.84, P less than 0.01), suggesting that the latter is responsible for the increase in plasma cyclic AMP which is observed in relation to surgical procedures. Peak concentrations of cyclic AMP and adrenaline were seen in the early postoperative phase. This indicates that the most pronounced acute endocrine stress response to surgery of moderate severity occurs after termination of anaesthesia.

Inhibition of plasma cyclic AMP, glucose and cortisol response to surgery by epidural analgesia

Cyclic AMP, glucose and cortisol in plasma were measured in three groups of patients undergoing hysterectomy. The operations were performed under general anaesthesia, under general anaesthesia combined with epidural analgesia and under epidural analgesia alone. Surgery elicited a significant rise in plasma cyclic AMP, glucose and cortisol when performed under general anaesthesia alone. Epidural analgesia extending from T4-6 to S5 combined with general anaesthesia abolished the rise in cyclic AMP and reduced the increase in glucose and cortisol and epidural analgesia alone extending from T4 to S5 blocked the rise in glucose and cortisol as well as that in cyclic AMP. The results support the theory that afferent nerve impulses from the area of trauma are of major importance for the catabolic state induced by surgical procedures and indicate that anaesthetic management which includes blockade of afferent nerve impulses which includes blockade of afferent nerve impulses from the area of trauma can be reduce the catabolic response to surgery. These observations could be of value in the operative management of patients with diabetes mellitus and possibly in other groups by patients with a high surgical morbidity.