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N Freinkel

Publications and source records attributed to N Freinkel.

At least 73 records · Page 4Linked to original sources
PubMed1978

Localization of inorganic phosphate in the pancreatic B cell and its loss on glucose stimulation.

Perifusion experiments have shown that there is a discharge of inorganic phosphate into the medium when insulin secretion from isolated islets is stimulated by glucose. Histochemical and microprobe examination of resting pancreatic islets in the electron microscope shows a specific accumulation of inorganic phosphate adjacent to the plasmalemma and nucleolus of the B (beta) cells. This phossphate is lost from the cells during secretory stimulation of islets with high concentrations of glucose.

Animals↗
PubMed1978

32P-labeling patterns in rat pancreatic islets: tissue source of the radiophosphate released after glucose stimulation.

Immediately after stimulation with glucose in vitro, isolated rat pancreatic islets prelabeled with [32P]orthophosphate release a pulse of [32P]orthophosphate into the media (the "phosphate flush"). Islets have been rapidly frozen before, during, and after this pulse to assess the concurrent changes in the distribution of tissue radioactivity. Under the present experimental conditions, approximately 90% of the islet radioactivity was soluble in perchloric acid (PCA-soluble) immediately before stimulation and slightly more than half of that was present as [32P]orthophosphate. The tissue pool of [32P]orthophosphate declined 55% and 62% after 7 and 14 min of stimulation which, respectively, incorporated the peak and the end of the heightened efflux of radioactivity. The net decrementa in tissue orthophosphate could account for all of the radioactivity which was released during the "phosphate flush." During the 14-min period of stimulation, labeled ATP and GTP (which had accounted for 13% and 4% of total PCA-soluble radioactivity before stimulation) increased 51% and 35%, respectively, and labeled ADP and AMP (which had accounted for 5.4% and 1.5% of PCA-soluble counts) fell 36% and 77%, respectively. Certain other PCA-soluble components, such as phosphorylcholine and phosphorylethanolamine, and total PCA-insoluble radioactivity were not demonstrably altered. The findings indicate that the "phosphate flush" originates from a labile pool of tissue orthophosphate. It remains to be established whether the simultaneous changes in the turnover of selected nucleotides are coupled to the translocation of orthophosphate or are mediated separately.

Animals↗
PubMed1978

Phosphate metabolism and glucose-initiated efflux of phosphate ions in islets of fetal pancreas.

Elsewhere we have proposed that the rapid transient efflux of 32P orthophosphate that occurs when prelabeled pancreatic islets are exposed to nutrient secretagogues (the "phosphate flush") reflects one of the earliest steps in islet stimulus-secretion coupling. We have now shown that the "phosphate flush" is much smaller with islets from fetal rats 21 1/2 days old. This could be attributed to decreased cellular stores of radioactivity, especially inorganic orthophosphate (32P), at the onset of stimulation, which may have been due, in part, to the diminished ability of fetal islets to retain the radiophosphorus accumulated during the labeling period. Certain other differences in phosphate metabolism were also observed. With prelabeled islets from adult rats, exposure to 3.0 mg. per milliliter glucose effected acute increases in the tissue content of AT32P and GT32P. Comparable stimulation of prelabeled fetal islets with 3.0 mg. per milliliter glucose did not elicit detectable changes in the labeling of ATP or GTP. The findings indicate that selected aspects of phosphate metabolism may be immature in fetal islets and, perhaps, implicated in their obtunded secretion of insulin in response to stimulation with glucose.

Adenosine Triphosphate↗
PubMed1977

Carbohydrate metabolism in pregnancy. XIV. Relationships between circulating glucagon, insulin, glucose and amino acids in response to a "mixed meal" in late pregnancy.

Gestational influences upon the changes in circulating glucose, amino acids, insulin, and glucagon after the ingestion of a "mixed meal" containing carbohydrate (50 g), protein (25 g), and fat (10 g) were examined. Nine subjects were tested during weeks 30-40 of gestation and again 6--8 wk postpartum. The "mixed meal" elicited greater and more prolonged increases in plasma glucose anterpartum, whereas the increments in total serum amino acids were blunted at all time points. In the face of greater glycemic but lesser aminogenic stimulation, the integrated increase in plasma insulin was 60% greater antepartum than post partum, whereas the increment in glucagon was not significantly altered. Thus, integrated insulin/glucagon response was increased during antepartum studies. The insulin preponderance following alimentary challenge with mixed nutrients would suggest that the anabolism of ingested amino acids is "facilitated" during late human pregnancy.

Amino Acids↗
PubMed1977

Fetal fuels. I. Utilization of ketones by isolated tissues at various stages of maturation and maternal nutrition during late gestation.

The availability and utilization of B-hydroxybutyrate as an alternate oxidative fuel during fasting hypoglycemia has been examined in the rat conceptus at 18 and 20 days gestation. A 48-hr maternal fast between days 16 and 18 or 18 and 20 resulted in a 50% fall in fetal glucose levels and a marked rise in B-hydroxybutyrate, i.e., 30-fold at 18 and 60-fold at 20 days. Tissue concentrations of B-hydroxybutyrate or acetoacetate did not exceed extracellular levels. Placenta, fetal brain, carcass, and liver all oxidized 14C-labeled B-hydroxybutyrate to 14CO2 when incubated in vitro in the presence of B-hydroxybutyrate. Highest rates of oxidation were apparent in the placenta, followed by brain, liver, and carcass. The D isomer of B-hydroxybutyrate appeared to be oxidized preferentially by all tissues studied. Despite levels of 3-ketoacid CoA transferase and acetoacetyl CoA thiolase lower at 18 than at 20 days, rates of oxidation in individual tissues incubated under identical concentrations of substrate were similar at both times. In liver and brain, increasing rates of 14CO2 generation proportionate to graded concentrations of B-hydroxybutyrate in vitro indicated that such rates were probably determined by substrate availability. B-hydroxybutyrate oxidation in extrahepatic fetal tissues was unaffected by maternal fasting. By contrast, fetal liver derived from fasted mothers generated significantly less 14CO2 from B-hydroxybutyrate than livers from fed mothers. It has been suggested that capabilities for ketone utilization are widespread in tissues of the conceptus, and that such utilization may fulfill in part the oxidative demands for continued anabolic growth during fasting hypoglycemia in the mother.

Acetyl-CoA C-Acetyltransferase↗
PubMed1977

Fetal fuels II: contributions of selected carbon fuels to oxidative metabolism in rat conceptus.

The effects of fasting on the oxidative disposition of selective fuels in tissues of the rat conceptus were examined on day 20 of gestation. Placentas and portions of fetal liver and brain from fed and 48-h fasted mothers were incubated in vitro with artificial mixtures containing glucose, lactate, and beta-hydroxybutyrate in concentrations simulating those that obtain in fed or 48-h fasted animals in vivo. Oxidative contributions from individual components were evaluated by separate incubations in which only one of the three fuels was 14C-labeled. As judged by the evolution of 14CO2, rates of oxidation of individual fuels by tissues of the conceptus appeared to be conditioned by ambient fuel concentrations rather than the dietary status of the mother. Additional studies indicated that evolution of 14CO2 from glucose or lactate may be depressed directly by adding beta-hydroxybutyrate to the incubation medium. This substitutive property of beta-hydroxybutyrate may "spare" glucose and lactate within the placenta for transfer to the fetus and preserve glucose and lactate availability for biosynthetic rather than oxidative disposition within the fetus.

Animals↗
PubMed1976

Relationship between efflux of ionic calcium and phosphorus during excitation of pancreatic islets with glucose.

Simultaneous rates of [32P]orthophosphate and 45Ca2+ efflux from prelabeled rat pancreatic islets have been evaluated to assess whether these ions move in concert throughout all phases of "stimulus-secretion coupling". Perifusion with stimulatory concentrations of glucose elicited immediate but transitory increases in 32P outflow accompanied by initial retardations and subsequent augmentations in net 45Ca2+ outflows. These monophasic 32P and biphasic 45Ca2+ responses to secretory stimulation were abolished completely by membrane stabilization with tetracaine. However, certain manipulations enabled individual components to be modified separately. During stimulation with glucose, inhibition of insulin release by Ni2+ abolished the late increases in 45Ca2+ outflow without affecting the initial retentions of 45Ca2+ or the increased releases of 32P. Under basal conditions, the ionophore A23187 "triggered" increased releases of 45Ca2+ and insulin without prior retentions of 45Ca2+ or enhancements of 32P efflux. Thus, the immediate retardations of 45Ca2+ outflow and heightened efflux of 32P may reflect early events in stimulus-secretion coupling which can be dissociated from the augmented release of 45Ca2+ accompanying activated emiocytosis.

Animals↗
PubMed1976

[32P] Orthophosphate efflux from pancreatic islets: graded response to glucose stimulation.

The dose-response relationships of the glucose-induced rapid transient efflux of [32P] orthophosphate from prelabeled pancreatic islets ("phosphate flush") have been investigated. Threshold levels for eliciting a "phosphate flush" were between 0.5 and 1.0 mg/ml glucose, the apparent "Km" for this event was 1.0-1.5 mg per ml and the apparent "Vmax" was reached at ambient glucose concentrations between 1.5 and 2.0 mg per ml. This dose-response curve is somewhat shifted to the left in comparison with previously published data for glucose-induced insulin release. Thus, the "phosphate flush" appears to display a more narrow dose-response curve to ambient glucose concentrations than the actual release of insulin. It is proposed that this may constitute further evidence that the "phosphate flush" reflects an early step in stimulus-secretion coupling and that there may be some loss of sensitivity as the glucose signal is transmitted from the site of recognition to the final site of hormone release.

Animals↗
PubMed1976

Insulin release and phosphate ion efflux from rat pancreatic islets induced by L-leucine and its nonmetabolizable analogue, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid.

When isolated rat pancreatic islets that had been labeled with 32P were exposed to 10mM L-leucine in a microperifusion system, there was a transitory, immediate heightened efflux of [32P]phosphate ions. Commencement of the phosphate flush coincided with the first release of insulin, and it occurred in the absence or presence of nonstimulatory levels of glucose (0.5 mg/ml). The effects of leucine upon phosphate efflux were not inhibited by D-mannoheptulose, whereas glucose-induced stimulations were suppressed. The phosphate flush could be induced also by the nonmetabolizable analogue of leucine, 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid. Like insulin release, the effect was stereospecific, with only one of the four stereoisomers, (-)-b-aminobicycloheptane carboxylic acid, being active. Analogue-induced phosphate and insulin release were totally suppressed in a medium containing D2O, but on reversion to H2O the efflux of both hormone and anion occured. It is concluded that insulin secretion from islet beta cells and the release of phosphate ions showed the same specificity for nutrient secretagogues and that both can be triggered even in the absence of exogenous oxidizable fuels. However, the partial dissociation of the dose-response curves for the two phenomena lends support to the contention that the phosphate flush reflects an earlier event in the sequence of stimulus-secretion coupling.

Amino Acids↗