[Effect of oxytocin on regional 3H-orotic acid uptake in rat brain]

The uptake of the RNA precursor [3H] orotic acid by 18 brain regions of male rats was measured after intracarotid injection and a survival time of 15 s. Ocytocin added to the injected solutions (final concentrations: 1 to 1000 mU/ml) caused a dose-dependent enhancement of tracer uptake by 11 areas. Some of these regions (e.g. hippocampus) are thought to be involved in the behavioral effects of vasopressin and other peptide hormones (Van Wimersma Greidanus et al., 1975 a). Results of experiments in which the net uptake of tritiated water was used as a measure of relative blood flow led us to the conclusion that the blood-brain barrier permeability to [3H] orotic acid rather than the cerebral blood flow is changed by ocytocin. The present results support the assumption that the enhanced precursor supply caused by ocytocin might contribute to the influence of this peptide hormone on memory consolidation.

Prolactin: a diabetogenic hormone

During an oral glucose tolerance test (OGTT) glucose and insulin levels were measured in 26 patients with prolactin-producing pituitary tumours without growth hormone excess. Basal glucose and insulin levels did not differ from the values of an age-matched control group. After glucose load the hyperprolactinaemic patients showed a decrease in glucose tolerance and a hyperinsulinaemia. Bromocriptine (CB 154), which suppressed PRL, improved glucose tolerance and decreased insulin towards normal in second OGTT. Human PRL or CB 154 had no significant influence on insulin release due to glucose in the perfused rat pancreas. These findings suggest a diabetogenic effect of PRL. CB 154 might be a useful drug in improving glucose utilization in hormone-active pituitary tumours.

[Influence of somatastatin on oral glucose tolerance in autonomous hypersecretion of growth hormone, prolactin or insulin (author's transl)]

Oral glucose tolerance tests (OGTT) were performed for two subsequent days in 4 patients with active acromegaly, 2 patients with prolactin-producing pituitary adenomas and one insulinoma patient. Thirty minutes before the second OGTT 250 mug of somatostatin were injected intravenously as a bolus followed by a somatostatin infusion (500 mug) over 21/2 hours. The OGTTs were pathologic due to the hGH- and hPRL-induced insulin antagonism; they could not be normalized or improved by somatostatin. Only the peak of the blood sugar curve was shifted from one to two and a half hours after glucose administration; insulin and hGH levels were regularly suppressed after somatostatin whereas hPRL remained unchanged in most instances. Gastrin levels increased in all patients during the OGTT, the increase was suppressed in 4 patients. These findings show that the pathologic glucose tolerance due to insulin antagonism could not be improved by somatostatin in contrast to the deteriorated glucose tolerance in insulinopenic states.

The stimulatory and inhibitory function of theophylline on amino acid-induced insulin release: studies with the perfused rat pancreas

The effects of 5 mM theophylline on L-phenylalanine, L-alanine, and L-leucine-induced insulin secretion were studied using the isolated perfused rat pancreas, varying the sequence of the stimuli. Theophylline in the absence or presence of 3 mM D-glucose caused a small amount of insulin release with a slow onset and a slight and steady increase. Theophylline had no effect on phenylalanine (5, 10, or 20 mM)-induced insulin release independent of the sequence of stimuli. Alanine (20 mM), in the absence of glucose, had no significant insulin stimulatory action. When theophylline was added during the alanine perfusion only a small insulin release, comparable to that given by theophylline alone, could be observed. However, superimposing alanine on a theophylline perfusion led to a potentiation of insulin release. The leucine-induced insulin secretion was significantly altered by the addition of theophylline. At a low concentration of leucine (5 mM) theophylline caused potentiation of leucine-induced insulin secretion. At 10 and 20 mM leucine, theophylline led to a rapid concentration-dependent inhibitory period, followed by a potentiation in the case of 10 mM leucine, and by a restoration of the secretion rate at 20 mM leucine which did not exceed the secretion rate of 20 mM leucine alone. Subsequent removal of theophylline caused a marked "off effect." When 5 or 20 mM leucine was superimposed on a theophylline perfusion, a marked dose-dependent potentiation of the biphasic leucine-induced insulin release and no inhibitory phase could be observed. From these data it must be concluded that the effect of theophylline on the insulin secretory reposure of the beta cell to theophylline and the stimulus. Possible explanations for these phenomena are discussed.

Glucoreceptor mechanisms in islets of Langerhans

A widely accepted concept has been that metabolites of glucose or factors associated with glucose metabolism, rather than glucose itself, trigger insulin release from the β cells. If this concept is correct, levels of metabolites or cofactors or of both should change within seconds of the glucose pulse. To examine the hypothesis, glucose, glucose-6-P, 6-P-gluconate, fructose-l,6-P2 plus triose-P, 3-P-glycerates plus P-enolpyruvate, ATP, ADP and P-creatine were measured in slices of individual rat islets after intravenous infusion of glucose in vivo or perfusion of the isolated pancreas with glucose in vitro. Tissue samples were obtained by quickfreezing methods as early as fifteen seconds and as late as sixty minutes following the glucose load. Insulin levels in serum and perfusate were measured concomitantly.

Insulin levels in peripheral blood rose within one minute of injection of glucose in vivo. Penetration of glucose, although very rapid, was carrier-mediated. Most of the metabolites and cofactors measured were unchanged during the first five minutes of glucose infusion. After one hour of hyperglycemia there was a marked rise in all metabolites and cofactors except ATP.

In the perfused pancreas, insulin secretion increased within 0.5 minute of the glucose stimulus and the typical biphasic response was observed. The glucose content of islets increased threefold within 0.25 minute and sixfold within 0.5 minute. The levels of all other metabolites measured were virtually unchanged during the early phase of the endocrine response, in spite of the eightfold increment in the glucose of the medium and the almost tenfold rise of insulin release.

The constancy of the islet's metabolite pattern when the islets were exposed to high glucose—particularly during the early transition phase—is remarkable, since glycolytic flux was supposedly increased severalfold.

The fact that the metabolite profile of the islets is so precisely maintained within a narrow range makes it unlikely that any of the metabolites measured might serve as triggers for insulin release. It is more likely that the glucose molecule itself exerts its releasing action by stimulating glucoreceptors located on the cell membrane.

Kinetics of insulin release from the perfused rat pancreas caused by glucose, glucosamine, and galactose

Under appropriate conditions, not only glucose but also glucosamine and galactose can serve as potent stimulants for insulin release from the isolated, perfused rat pancreas. Since galactose and, probably, glucosamine are not metabolized in the islets, and since these three compounds have in all likelihood common sites of action, it is postulated that a glucoreceptor of broad specificity is involved in the mechanism of insulin release, and that metabolism of glucose is not an essential part of the releasing action of this sugar.