Ellis Samols

Interrelationship of Glucagon, Insulin and Glucose: The Insulinogenic Effect of Glucagon

Glucagon in doses of 0.25 or 1.0 mg. was injected rapidly, intravenously, into healthy normal subjects. There was a striking rise in arterial immunoreactive insulin levels within one minute of the start ot the injection before any change in arterial glucose concentration occurred Plasma insulin levels reached their peak within ten minutes and began to fall thereafter. In contrast, blood glucose levels were highest twenty to thirty minutes after the injection. A second glucagon injection during the hyperglycemic phase produced a larger rise in plasma insulin than the first. During constant infusions of smaller amounts of glucagon, the insulinogenic effect of glucagon was usually only apparent if blood glucose levels were raised. The effect of consecutive forty-minute intravenous infusions of glucagon (10μg./min.), glucose (0.5 gm./min.) and a glucagon-glucose (5μg. and 250 mg./min., respectively) on plasma insulin and arterio-venous blood glucose differences in six healthy normal subjects was studied. Plasma insulin levels were two to five times higher during glucose infusions alone, despite similar peak arterial blood glucose levels during the infusions. Similar results were obtained in other subjects during separate infusions of glucose and glucagon. Total body glucose clearance, judged by the rate of fall in arterial blood glucose levels, was faster after glucagon- than after glucose-induced hyperglycemia. Forearm A-V blood glucose differences were not obviously greater during glucagon, in spite of the higher plasma insulin levels, than during glucose infusions. Possible reasons for the failure of forearm A-V blood glucose differences to reflect accurately total peripheral glucose assimilation under these experimental conditions are discussed, and it is suggested that the co-existence of hyperglucagonemia and hyperinsulinemia may tend to favor incorporation of glucose into adipose tissue rather than muscle. Plasma glucagon levels were measured by immunoassay and the half-life was about ten minutes. The results provide evidence of a direct effect of glucagon upon insulin secretion. The suggestion is made that the insulinogenic effect of glucagon is physiological importance and is due to accelerated intra-β-cell glycogenolysis.

The Vascular Order of Islet Cellular Perfusion in the Human Pancreas

The vascular order of pancreatic islet cellular perfusion is important in the intraislet regulation of hormone secretion. Establishment of the sequence of interaction is fundamental to understanding the physiology and pathophysiology of the human islet. Intraislet insulin from the β-cell regulates both net hormone secretion and pulsatile secretion from α- and δ-cells. In terms of vascular perfusion, the δ-cell is perfused last and does not directly affect α- or β-cells in humans.

α-Adrenergic Blockade Improves Glucose-Potentiated Insulin Secretion in Non-Insulin-Dependent Diabetes Mellitus

The impairment of glucose-potentiated insulin secretion present in non-insulin-dependent diabetes mel I it us (NIDDM) can be approximated in normal subjects by an epinephrine infusion. Therefore, we sought to determine the role of the endogenous sympathetic nervous system in glucose-potentiated insulin secretion in both NIDDM (n = 6) and normal (n = 6) subjects. Glucose-potentiated insulin secretion was calculated as the slope of the curve relating increasing ambient glucose levels to the acute insulin response to an intravenous pulse of 5 g of L-arginine. Glucose-potentiated insulin secretion was determined on separate days during α-, β-, and combined α- plus β-adrenergic blockade and compared with a saline control. In. normal subjects, there was no effect of α-, β-, or α- plus β-blockade on the slope of glucose potentiation. In NIDDM, the initially decreased slope of glucose potentiation (0.25 ± 0.06 μU · ml−1 · mg−1 · dl, mean ± SE; P < .01) was not affected by β-blockade but increased during α-blockade (0.91 ± 0.22 μU · ml−1 · mg−1 · dl; P < .05). However, this improvement was abolished by combined α- plus β-blockade (0.32 ± 0.07 μU · ml−1 · mg−1 dl). Plasma norepinephrine was increased above basal levels in both normal (+ 260 ± 89 pg/ml) and NIDDM (+ 438 ± 162 pg/ml) subjects during α-blockade (P < .05 for both). This increase in plasma norepinephrine strongly suggests that there is an increase in synaptic cleft norepinephrine concentration. These results suggest that the increase in glucose-potentiated insulin secretion during α-adrenergic blockade was a result of both a decrease in an endogenous overactive α-adrenergic stimulation and an increase in synaptic cleft norepinephrine levels, which resulted in an increase in islet β-adrenergic stimulation. We suggest that a chronic decrease in islet α-adrenergic stimulation may prove to be a useful adjunct to NIDDM management.

Insulin Assay in Insulinomas

Non-functioning Kidney and Renal Agenesis (Cases 26 to 28) The recognition of a non-functioning or absent kidney may be of importance-for example, when emergency nephrectomy is contemplated. In this situation renography is the simplest investigation for detecting the presence of functioning renal tissue. Should the renogram suggest an absent or nonfunctioning kidney, intravenous pyelography is indicated to exclude an ectopic kidney. Fig. 6 shows the renogram from a patient with left renal agenesis. These tracings should be compared with those illustrated in Fig. 7; the intravenous pyelogram from this patient showed a calcified tuberculous pyonephrosis of the left kidney without evidence of renal function; although the renogram showed gross depression of function on the left side, comparison with a simultaneous record obtained over the heart failed to show the progressive downward gradient of the heart curve, thus indicating persistence of some function. In Case 25, a prospective donor for renal transplantation, the finding of normal symmetrical renogram tracings in addition to normal total renal clearances and normal nephrograms and pyelograms was further evidence for the presence of two normally functioning kidneys.

A Comparison of Insulin Immunoassays.

Summary Radio-immunoassay of insulin gives the same results whether immunoprecipitation or chromatography is used. The effects on the assay systems of interference with equilibrium, and the influence of human anti-insulin sera on immunoprecipitation are described. It is suggested that present radio-immunoassays should give similar plasma insulin levels in man if similar reference standards insulins are used. The immunological potency of several human insulins in general use is compared.

β → α → δ Pancreatic Islet Cellular Perfusion in Dogs

Intraislet communication between β-, α-, and δ-cells and their secretory products may theoretically occur via the paracrine (interstitial) and/or vascular routes. Recently, we have shown that there is a directed microvascular circulation in the rat islet with a cellular order of perfusion of β → α → δ. The direction of microvascular perfusion of cells within the dog islet has been controversial. Anterograde (arterial) perfusion and retrograde (reversed or venous) perfusion of a segment of isolated dog pancreas with potent insulin antibodies yielded results similar to those found in the rat pancreas (anterograde, 158 ±44% increase in glucagon and 65 ± 20% increase in somatostatin; retrograde, no change in glucagon or somatostatin). Anterograde infusion of glucagon antibody (no change in insulin, −33.5 ± 3% decrease in somatostatin) or somatostatin antibody (no change in insulin or glucagon) also yielded the same results as in the rat pancreas. Anterograde infusion of 500 pg/ml glucagon caused a larger increase in insulin secretion (245 ± 10%) than retrograde infusion (45 ± 4%), whereas somatostatin was stimulated more retrogradely (339 ± 17%) than anterogradely (121 ± 9%). Anterograde infusion of somatostatin produced a larger decrease in insulin and glucagon than did retrograde perfusion (P < .0001 for both comparisons). The retrograde infusion of 0.3 mU/ml insulin caused a decrease in glucagon but was without effect anterogradely. The results from the infusion of exogenous hormones suggest that the sensitivity of the α-, β-, and δ-cells to insulin, glucagon, and somatostatin is determined by the β → α → δ order of perfusion. The antibody studies indicate that directed microvascular perfusion is central to intraislet regulation of insular secretions in dogs.

beta----alpha----delta pancreatic islet cellular perfusion in dogs.

Intraislet communication between alpha-, beta-, and delta-cells and their secretory products may theoretically occur via the paracrine (interstitial) and/or vascular routes. Recently, we have shown that there is a directed microvascular circulation in the rat islet with a cellular order of perfusion of beta----alpha----delta. The direction of microvascular perfusion of cells within the dog islet has been controversial. Anterograde (arterial) perfusion and retrograde (reversed or venous) perfusion of a segment of isolated dog pancreas with potent insulin antibodies yielded results similar to those found in the rat pancreas (anterograde, 158 +/- 44% increase in glucagon and 65 +/- 20% increase in somatostatin; retrograde, no change in glucagon or somatostatin). Anterograde infusion of glucagon antibody (no change in insulin, -33.5 +/- 3% decrease in somatostatin) or somatostatin antibody (no change in insulin or glucagon) also yielded the same results as in the rat pancreas. Anterograde infusion of 500 pg/ml glucagon caused a larger increase in insulin secretion (245 +/- 10%) than retrograde infusion (45 +/- 4%), whereas somatostatin was stimulated more retrogradely (339 +/- 17%) than anterogradely (121 +/- 9%). Anterograde infusion of somatostatin produced a larger decrease in insulin and glucagon than did retrograde perfusion (P less than .0001 for both comparisons). The retrograde infusion of 0.3 mU/ml insulin caused a decrease in glucagon but was without effect anterogradely. The results from the infusion of exogenous hormones suggest that the sensitivity of the alpha-, beta-, and delta-cells to insulin, glucagon, and somatostatin is determined by the beta----alpha----delta order of perfusion.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucagon test for insulinoma: a chemical study in 25 cases

The capillary blood glucose response to lmg of intramuscular glucagon was determined in 13 patients with insulinoma and in 33 normal controls; the insulinoma patients showed a normal initial rise, but this was followed by an abnormally large fall, reaching hypoglycaemic levels between 90 and 180 minutes in every case. In 14 insulinoma patients the response of venous blood glucose and also plasma insulin to lmg of intravenous glucagon was compared with 10 normal controls; there was an abnormally large rise of plasma insulin in 10 of the 14 patients, and in the majority the venous blood glucose was below normal throughout the test. In these 14 patients the plasma insulin response was also determined after oral and intravenous glucose, after oral leucine, and after intravenous tolbutamide, and the value of these tests in the recognition and differential diagnosis of insulinoma was compared with that of the intravenous glucagon test.

Modulation of Insulin Secretion by Pancreatic Ganglionic Nicotinic Receptors

Autonomie ganglia may be regulated, in part, by nicotinic receptors. To test whether basal insulin secretion may be modulated by an endogenous pancreatic ganglionic mechanism, the effects of ganglionic pre- and postsynaptic nicotinic receptor antagonism were studied in the in vitro canine pancreas. Combined infusion of atropine, phentolamine, and propranolol had no affect on insulin secretion (P < .30). Presynaptic nicotinic receptor blockade by β-bungarotoxin (β-BuTX) in combination with atropine and phentolamine reduced mean insulin secretion (78 ± 18 U/ml, P < .0025) from preinfusion concentrations (287 ± 43 U/ml). The decrease in insulin secretion resulting from BuTX, atropine, and phentolamine was prevented by the addition of either specific postsynaptic nicotinic receptor blockade by α-bungarotoxin (P < .05) or propranolol (P < .005). Because it is known that postsynaptic nicotinic receptor agonism may stimulate the intragan-glionic release of norepinephrine, these results suggest that nicotinic receptors are present at the ganglionic level in the pancreas and modulate insulin secretion by a complex intraganglionic mechanism. The postulated ganglionic nicotinic receptor-mediated mechanism may operate by the interaction of a β-adrenergic inhibitory component, which may be activated by intraganglionic norepinephrine, and a stimulatory nonmuscar-inic nonadrenergic (possibly peptidergic) component, which may be activated in the absence of intraganglionic norepinephrine.

Factitious hypoglycemia in a diabetic: Metabolic studies and diagnosis with radioactive isotopes

A case of “spontaneous hypoglycemia” in a diabetic, due to misuse of insulin, is described. Differentiation from insulinoma was difficult. Some of the anomalous results obtained during standard provocative tests are regarded in retrospect, as due to self-administration of exogenous insulin even though the patient was in hospital and apparently under strict supervision at the time of the tests. This may also explain some previously published reports with anomalous results. Distal pancreatectomy was carried out because of persistent unexplained hypoglycemia with temporary relief of symptoms but subsequent relapse. Correct diagnosis was made when an illicit store of insulin was discovered in the false bottom of a jewel box. This was labeled surreptitiously with 131I which was later demonstrated in the urine, blood, and at the site of injection. Though rare in diabetics, factitious hypoglycemia presents difficult problems in diagnosis.