Regulation of 86Rb+ outflow from pancreatic islets. I. Reciprocal changes in the response to glucose, tetraethylammonium and quinine

Stimulus-secretion coupling of arginine-induced insulin release: comparison between the cationic amino acid and its methyl ester

The role currently ascribed to the accumulation of L-arginine in the pancreatic islet B-cell as a determinant of its insulinotropic action was reevaluated by comparing the uptake and the metabolic, ionic, electric, and secretory effects of the cationic amino acid with those of its more positively charged methyl ester in rat pancreatic islets. The response to L-arginine methyl ester differed from that evoked by the unesterified amino acid by a lower uptake and oxidation, lack of inhibitory action on D-glucose metabolism, more severe inhibition of the catabolism of endogenous L-glutamine, inhibition of 45Ca net uptake, decrease in both 86Rb outflow from prelabeled islets perifused at normal extracellular Ca2+ concentration and 45Ca efflux from prelabeled islets perifused in the absence of extracellular Ca2+, and delayed and lesser insulinotropic action. These findings reinforce the view that the carrier-mediated entry of L-arginine into the islet B-cells, with resulting depolarization of the plasma membrane, represents the essential mechanism for stimulation of insulin release by this cationic amino acid.

Effect of the meglitinide analog S21403 on cationic fluxes and insulin release in perifused rat pancreatic islets exposed to a high concentration of D-glucose

The effect of the meglitinide analog S21403 (10 microm) upon(86)Rb and(45)Ca outflow and insulin release was investigated in perifused rat islets exposed to a high concentration of D-glucose (16.7 mm) in order to simulate the situation found in diabetic patients. Under these conditions, S21403 provoked a rapid, sustained and rapidly reversible increase in(86)Rb outflow, (45)Ca efflux and insulin release. These effects were suppressed or reversed when the experiments were conducted in the absence of extracellular Ca2+. They support the view that S21043 could be used as a novel insulinotropic tool in the treatment of non-insulin-dependent diabetes mellitus, the cationic and secretory responses to the drug displaying a favourable time course for prompt and not unduly prolonged activation of islet B-cells.

Effect of D-mannoheptulose upon the cationic and secretory responses to alpha-D-glucose pentaacetate in perifused rat pancreatic islets

The metabolism of alpha-D-glucose pentaacetate and its positive insulinotropic action in isolated rat pancreatic islets are both unexpectedly resistant to D-mannoheptulose, as judged from experiments conducted over 90-120 min incubation. In the present study, the possible effects of the heptose upon the immediate cationic and secretory response to the ester were investigated in perifused islets prelabeled with either 86Rb or 45Ca. At a 10 mM concentration, sufficient to abolish the inhibitory action of unesterified D-glucose upon 86Rb outflow, D-mannoheptulose failed to suppress the decrease in 86Rb outflow and increase in 45Ca efflux caused by alpha-D-glucose pentaacetate at normal extracellular Ca2+ concentration and also failed to prevent the decrease in both 45Ca and insulin release provoked by the ester in the absence of extracellular Ca2+. The sole obvious effect of the heptose was to change the early peak-shaped positive secretory response to alpha-D-glucose pentaacetate to a transient inhibition of insulin release. This change was observed in islets either deprived of any other exogenous nutrient or exposed to L-leucine throughout the experiments. These findings support the view that the islet functional response to alpha-D-glucose pentaacetate is largely resistant to D-mannoheptulose. They also reinforce the concept that the insulinotropic action of this and other monosaccharide esters involves a dual modality of B-cell activation, linked to both the catabolism of their carbohydrate moieties and a direct effect of the esters themselves upon a specific receptor system.

Inhibition of glucose-induced insulin release by 3-O-methyl-D-glucose: enzymatic, metabolic and cationic determinants

The analog of D-glucose, 3-O-methyl-D-glucose, is thought to delay the equilibration of D-glucose concentration across the plasma membrane of pancreatic islet B-cells, but not to exert any marked inhibitory action upon the late phase of glucose-stimulated insulin release. In this study, however, 3-O-methyl-D-glucose, when tested in high concentrations (30-80 mM) was found to cause a rapid, sustained and not rapidly reversible inhibition of glucose-induced insulin release in rat pancreatic islets. In relative terms, the inhibitory action of 3-O-methyl-D-glucose was more marked at low than high concentrations of D-glucose. It could not be attributed to hyperosmolarity and appeared specific for the insulinotropic action of D-glucose, as distinct from non-glucidic nutrient secretagogues. Although 3-O-methyl-D-glucose and D-glucose failed to exert any reciprocal effect upon the steady-state value for the net uptake of these monosaccharides by the islets, the glucose analog inhibited D-[5-3H]glucose utilization and D-[U-14C]glucose oxidation. This coincided with increased 86Rb outflow and decreased 45Ca outflow from prelabelled islets, as well as decreased 45Ca net uptake. A preferential effect of 3-O-methyl-D-glucose upon the first phase of glucose-stimulated insulin release was judged compatible with an altered initial rate of D-glucose entry into islet B-cells. The long-term inhibitory action of the glucose analog upon the metabolic and secretory response to D-glucose, however, may be due, in part at least, to an impaired rate of D-glucose phosphorylation. The phosphorylation of the hexose by beef heart hexokinase and human B-cell glucokinase, as well as by parotid and islet homogenates, was indeed inhibited by 3-O-methyl-D-glucose. The relationship between insulin release and D-glucose utilization or oxidation in the presence of 3-O-methyl-D-glucose was not different from that otherwise observed at increasing concentrations of either D-glucose or D-mannoheptulose. It is concluded, therefore, that 3-O-methyl-D-glucose adversely affects the metabolism and insulinotropic action of D-glucose by a mechanism largely unrelated to changes in the intracellular concentration of the latter hexose.

Insulinotropic action of beta-L-glucose pentaacetate

The metabolism of beta-L-glucose pentaacetate and its interference with the catabolism of L-[U-14C]glutamine, [U-14C]palmitate, D-[U-14C]glucose, and D-[5-3H]glucose were examined in rat pancreatic islets. Likewise, attention was paid to the effects of this ester on the biosynthesis of islet peptides, the release of insulin from incubated or perifused islets, the functional behavior of individual B cells examined in a reverse hemolytic plaque assay of insulin secretion, adenylate cyclase activity in a membrane-enriched islet subcellular fraction, cAMP production by intact islets, tritiated inositol phosphate production by islets preincubated with myo-[2-3H]inositol, islet cell intracellular pH, 86Rb and 45Ca efflux from prelabeled perifused islets, and electrical activity in single isolated B cells. The results of these experiments were interpreted to indicate that the insulinotropic action of beta-L-glucose pentaacetate is not attributable to any nutritional value of the ester but, instead, appears to result from a direct effect of the ester itself on a yet unidentified receptor system, resulting in a decrease in K+ conductance, plasma membrane depolarization, and induction of electrical activity.

Effects of artificial sweeteners on insulin release and cationic fluxes in rat pancreatic islets

Beta-L-glucose pentaacetate, but not alpha-D-galactose pentaacetate, was recently reported to taste bitter and to stimulate insulin release. This finding led, in the present study, to the investigation of the effects of both bitter and non-bitter artificial sweeteners on insulin release and cationic fluxes in isolated rat pancreatic islets. Sodium saccharin (1.0-10.0 mM), sodium cyclamate (5.0-10.0 mM), stevioside (1.0 mM) and acesulfame-K (1.0-15.0 mM), all of which display a bitter taste, augmented insulin release from islets incubated in the presence of 7.0 mM D-glucose. In contrast, aspartame (1.0-10.0 mM), which is devoid of bitter taste, failed to affect insulin secretion. A positive secretory response to acesulfame-K was still observed when the extracellular K+ concentration was adjusted to the same value as that in control media. No major changes in 86Rb and 45Ca outflow from pre-labelled perifused islets could be attributed to the saccharin, cyclamic or acesulfame anions. It is proposed that the insulinotropic action of some artificial sweeteners and, possibly, that of selected hexose pentaacetate esters may require G-protein-coupled receptors similar to those operative in the recognition of bitter compounds by taste buds.

Insulinotropic action of the monoethyl ester of succinic acid

1. Selected esters of succinic acid are currently under investigation as potential insulinotropic tools in the treatment of non-insulin-dependent diabetes. At variance with the methyl esters of succinic acid used in most of the work so far conducted from this perspective, the monoethyl ester of succinic acid (EMS) offers the advantage of avoiding the undesirable generation of methanol by intracellular hydrolysis. In the present study, the metabolism and functional effects of EMS were investigated, therefore, in rat pancreatic islets. 2. At a 10 mM concentration, EMS enhanced insulin release from islets stimulated by 7-17 mM D-glucose but failed to do so at lower concentrations of the hexose. EMS was efficiently metabolized, as judged from the generation of 14CO2 by islets exposed to the monoethyl ester of either [1,4-14C] or [2, 3-14C]succinic acid. D-Glucose (6 mM) failed to affect the metabolism of EMS (10 mM), which itself failed to affect the metabolism of D-[5-3H]glucose or D-[U-14C]glucose. EMS also stimulated biosynthetic activity in the islets. It inhibited 86Rb and 45Ca outflow from prelabeled islets perfused in the absence of D-glucose but enhanced the efflux of the two cationic tracers in the presence of the hexose (7 mM). 3. It is concluded that the insulinotropic action of EMS is attributable, to a large extent, to its capacity to act as a nutrient in islet cells.

Effect of 1,1-dimethyl-2-[2-morpholinophenyl]guanidine fumarate on pancreatic islet function

The modality of the insulinotropic action of 1,1-dimethyl-2-[2-morpholinophenyl]guanidine fumarate (BTS 67 582), a new antidiabetic agent, was investigated in rat pancreatic islets. At a 0.1 mM concentration, which was sufficient to cause a close-to-maximal secretory response, BTS 67 582 failed to affect the utilization and oxidation of exogenous D-glucose, but slightly augmented 14CO2 production from islets prelabelled with either L-[U-14C]glutamine or [U-14C]palmitate. BTS 67 582 (0.1 mM) also failed to affect biosynthetic activity in islets incubated with L-[4-3H]phenylalanine. It augmented insulin release from islets incubated for 90 min in the absence or presence of D-glucose (2.8 to 16.7 mM), this coinciding with stimulation of 45Ca net uptake. In perifused islets deprived of extracellular D-glucose for 45 min, BTS 67 582 (0.1 mM) decreased 86Rb outflow from prelabelled islets, but failed to increase 45Ca efflux and insulin release. In the presence of D-glucose (7.0 mM), BTS 67 582, whilst failing to decrease 86Rb+ outflow, provoked rapid, sustained and rapidly reversible increases of both 45Ca2+ efflux and insulin output. The latter increases were attenuated, but not totally suppressed, in the absence of extracellular Ca2+. BTS 67 582 (0.1 mM) suppressed the inhibitory action of diazoxide (0.25 mM) upon glucose-stimulated insulin release, but nevertheless augmented insulin output from islets incubated in the presence of 90 mM K+. These findings support the view that the insulinotropic action of BTS 67 582 is mainly attributable to the inactivation of ATP-sensitive K+ channels. An intracellular redistribution of Ca2+ ions may also participate, however, to the islet functional response to BTS 67 582.

Insulinotropic action of alpha-D-glucose pentaacetate: functional aspects

The functional determinants of the insulinotropic action of alpha-D-glucose pentaacetate were investigated in rat pancreatic islets. The ester mimicked the effect of nutrient secretagogues by recruiting individual B cells into an active secretory state, stimulating proinsulin biosynthesis, inhibiting 86Rb outflow, and augmenting 45Ca efflux from prelabeled islets. The secretory response to the ester was suppressed in the absence of Ca2+ and potentiated by theophylline or cytochalasin B. The generation of acetate from the ester apparently played a small role in its insulinotropic action. Thus acetate, methyl acetate, ethyl acetate, alpha-D-galactose pentaacetate, and beta-D-galactose pentaacetate all failed to stimulate insulin release. The secretory response to alpha-D-glucose pentaacetate was reproduced by beta-D-glucose pentaacetate and, to a lesser extent, by beta-L-glucose pentaacetate. It differed from that evoked by unesterified D-glucose by its resistance to 3-O-methyl-D-glucose, D-mannoheptulose, and 2-deoxy-D-glucose. It is concluded that the insulinotropic action of alpha-D-glucose pentaacetate, although linked to the generation of the hexose from its ester, entails a coupling mechanism that is not identical to that currently implied in the process of glucose-induced insulin release.

Insulinotropic action of 1,2,3-Tri(methylsuccinyl)glycerol ester

A novel ester of succinic acid, 1,2,3-tri(methylsuccinyl)glycerol ester (3SMG), was found to stimulate insulin release from rat pancreatic islets. In the presence of 7 mM d-glucose, a 10 microM concentration of 3SMG was sufficient to cause a significant increase in insulin output. The ester mimicked the effect of other nutrient secretagogues in enhancing the synthesis of islet peptides, with a preferential action on proinsulin as distinct from nonhormonal peptides, in decreasing 86Rb outflow from prelabeled islets, and in stimulating Ca2+ inflow into the islet cells. It is proposed, therefore, that 3SMG displays the attributes suitable for stimulation or potentiation of insulin release in noninsulin-dependent diabetes, without requiring administration in large amounts and, hence, without the risk of excessive hepatic gluconeogenesis.

Cationic events stimulated by D-glucose in depolarized islet cells

Pancreatic islets prelabelled with either 86Rb or 45Ca were exposed to a rise in D-glucose concentration from 2.8 to 16.7 mM whilst perifused in the presence of 2 microM glibenclamide, 30 mM extracellular K+ and both 30 mM K+ and 250 microM diazoxide. In all three situations, the rise in glucose concentration provoked a dramatic increase in insulin output, despite unchanged or even increased efflux of 86Rb from the prelabelled islets. Also in all three situations, glucose sharply decreased effluent radioactivity from islets prelabelled with 45Ca but perifused in the absence of extracellular Ca2+, while augmenting 45Ca efflux, to a variable extent, from islets perifused at normal extracellular Ca2+ concentration (1.0 mM). It is proposed, therefore, that the insulinotropic action of D-glucose in depolarized islets, and presumably also under normal conditions, may involve the gating of voltage-insensitive Ca2+ channels.

Dynamics of the cationic and secretory responses to A-4166 in perifused pancreatic islets

The dynamics of the cationic and secretory response to A-4166, a hypoglycemic meglitinide analogue, was investigated in rat islets prelabelled with either 36Rb or 45Ca and placed in a perifusion system. In the absence of D-glucose, A-4166 (10 microM) provoked an immediate, sustained and rapidly reversible inhibition of 36Rb outflow, this contrasting with a short-lived stimulation of insulin release. In the presence of 6 mM D-glucose, A-4166 provoked a rapid, sustained and rapidly reversible stimulation of both insulin release and 45Ca efflux. The latter cationic response was suppressed in the absence of extracellular Ca2+, in which case A-4166 even caused a modest decrease in effluent radioactivity. These findings support the view that A-4166 acts mainly in the islet B-cell by closing ATP-responsive K+ channels, leading to subsequent depolarization of the plasma membrane and gating of voltage-sensitive Ca2+ channels. Independently of the latter effect, A-4166 may also affect the intracellular distribution of Ca2+ ions. The present data further indicate that A-4166 belongs to those hypoglycemic agents that cause rapidly reversible changes in cationic and secretory events, at variance with highly potent sulfonylureas such as glibenclamide or glimepiride.

Insulinotropic action of (2S)-2-benzyl-3-(cis-hexahydro-2-isoindolinylcarbonyl) propionate. I. Secretory and cationic aspects

1. Sodium (2S)-2-benzyl-3-(cis-hexahydro-2-isoindolinylcarbonyl) propionate (KAD-1229) is a newly introduced non-sulphonylurea insulinotropic agent. 2. It failed to affect insulin release by rat islets incubated in the absence of D-glucose, slightly increased insulin output at 2.8 mM D-glucose and markedly enhanced secretion at 6.0 and 11.1 mM D-glucose. At the latter hexose concentration, the threshold concentration for the insulinotropic action of KAD-1229 was below 0.1 microM and a close-to-maximal response recorded with 1.0 microM KAD-1229. Even at 16.7 mM D-glucose, KAD-1229 (10 microM) still augmented insulin output. 3. At 6.0 mM D-glucose, KAD-1229 (0.1-1.0 microM) caused a concentration-related increase in 45Ca uptake. This coincided, in prelabelled islets, with a rapid and dual change in 86Rb outflow and dramatic increase in 45Ca outflow. 4. KAD-1229 also increased insulin release evoked by 2-ketoisocaproate (10 mM), albeit to a lesser extent than observed at a D-glucose concentration of comparable insulinotropic efficiency. 14C-labelled KAD-1229 was poorly oxidized by the islets. 5. These findings support the view that the mode of action of KAD-1229 displays analogy with that of hypoglycemic sulphonylurea.

Respiratory, ionic, and functional effects of succinate esters in pancreatic islets

The methyl esters of succinic acid were introduced a few years ago as new potent insulin secretagogues. In the present study, they were found to increase O2 uptake by rat islets incubated in the absence or presence of D-glucose; to decrease 86Rb outflow from prelabeled islets; to stimulate biosynthetic activity in the islets, with a preferential effect on the synthesis of proinsulin; to inhibit 45Ca efflux from prelabeled islets perifused in the absence of extracellular Ca2+ but to augment 45Ca net uptake and to cause a biphasic stimulation of 45Ca outflow in islets incubated or perifused in the presence of extracellular Ca2+; and to evoke a biphasic stimulation of insulin release. The insulinotropic action of these methyl esters coincided with a shift to the left of the sigmoidal relationship between insulin output and D-glucose concentration, was concentration related in the 2-10 mM range, failed to be duplicated by succinic acid, displayed both Ca2+ dependency and resistance to a lowering of extracellular pH, and was operative in the absence of D-glucose whether or not the islets were stimulated by non-nutrient secretagogues. It is concluded that the respiratory, cationic, biosynthetic, and secretory responses of the islets to succinate methyl esters display the characteristic features usually encountered in the process of nutrient-stimulated insulin release.

Stimulation of insulin secretion by glucose in the absence of diminished potassium (86Rb+) permeability

Two inhibitors of the nucleotide-sensitive K+ (KATP) channel, tolbutamide and quinine, were utilized in order to assess the role of this channel in glucose-stimulated insulin release from perifused rat islets. In the absence of these drugs, the addition of 15 mM glucose elicited a marked biphasic stimulation of insulin secretion concomitant with a reduction in the rate of 86Rb+ efflux. In the presence of either 500 microM tolbutamide or 100 microM quinine, a reduced rate of efflux of 86Rb+ was observed together with an elevated rate of insulin release. Under such conditions, the addition of 15 mM glucose retained the ability to stimulate insulin secretion though this was associated with a marked increase in 86Rb+ efflux. It is concluded that a net reduction in beta-cell K+ permeability is not an obligatory step in glucose-stimulated insulin release. Thus, glucose is likely to exert depolarizing actions on the beta-cell in addition to the closure of K+ channels.

Stimulus-secretion coupling of arginine-induced insulin release: significance of changes in extracellular and intracellular pH

The possible relevance of changes in extracellular and/or intracellular pH to the insulinotropic action of L-arginine and L-homoarginine was investigated in rat pancreatic islets. A rise in extracellular pH from 7.0 to 7.4 and 7.8 augmented the secretory response to these cationic amino acids whilst failing to affect the uptake of L-arginine by islet cells and whilst decreasing the release of insulin evoked by D-glucose. Under these conditions, a qualified dissociation was also observed between secretory data and 45Ca net uptake. Moreover, at high extracellular pH, the homoarginine-induced increase in 86Rb outflow from prelabelled islets rapidly faded out, despite sustained stimulation of insulin release. The cationic amino acids failed to affect the intracellular pH of islet cells, whether in the absence or presence of D-glucose and whether at normal or abnormal extracellular pH. These findings argue against the view that the secretory response to L-arginine would be related to either a change in cytosolic pH or the accumulation of this positively charged amino acid in the beta-cell. Nevertheless, they suggest that the yet unidentified target for L-arginine and its non-metabolized analogue in islet cells displays pH-dependency with optimal responsiveness at alkaline pH.

Stimulus-secretion coupling of arginine-induced insulin release. Functional response of islets to L-arginine and L-ornithine

L-Arginine and L-ornithine stimulate insulin release from pancreatic islets exposed to D-glucose. This coincides with an increased outflow of 86Rb and 45Ca from prelabelled islets and an increased net uptake of 45Ca by the islets. In the presence of D-glucose, L-lysine stimulates insulin secretion to the same extent as L-arginine or L-ornithine, but the hormonal release is not further enhanced by combinations of these cationic amino acids. L-Arginine or L-ornithine failed to enhance insulin release evoked by either L-leucine or 2-ketoisocaproate. The inhibitor of ornithine decarboxylase D,L-alpha-difluoromethyl ornithine failed to affect the metabolism and insulinotropic action of D-glucose in pancreatic islets, and only caused a partial inhibition of the secretory response to either L-arginine or L-ornithine. The latter amino acids inhibited modestly but significantly D-glucose utilization and oxidation by pancreatic islets. These and complementary findings suggest that the secretory response to L-arginine and L-ornithine is not attributable to any major change in the overall oxidative catabolism of nutrients, but involves mainly a biophysical component, such as the depolarization of the plasma membrane by these cationic amino acids.

Vitamin D and pancreatic islet function. II. Dynamics of insulin release and cationic fluxes

Pancreatic islets were prepared from control and vitamin D-deprived rats 2 or 5 weeks after weaning and, in the latter case, after 3 or 6 days treatment with exogenous vitamin D3 (60 nmol per day). The islets were prelabelled with both 86Rb and 45Ca and placed in a perfusion chamber. Vitamin D deprivation or administration failed to affect 86Rb outflow whether prior or after stimulation of the islets by a rise in either extracellular D-glucose or Ca2+ concentration. However, vitamin D deprivation decreased and vitamin D administration enhanced the basal 45Ca fractional outflow rate, as well as the magnitude of changes in both 45Ca and insulin release evoked by the rise in either D-glucose or extracellular Ca2+. It is proposed that the alteration in 45Ca fluxes and insulin release attributable to changes in the supply of vitamin D are, to a large extent, independent of the changes in nutrient catabolism conceivably associated with vitamin D deprivation and administration.

Effect of exogenous ATP upon inositol phosphate production, cationic fluxes and insulin release in pancreatic islet cells

Endogenous ATP is thought to play a key regulatory role in nutrient-stimulated insulin release. The present study deals with the effect of exogenous ATP and its stable analog alpha, beta-methylene ATP upon pancreatic islet function. Both alpha, beta-methylene ATP (5.0 microM to 0.2 mM) and ATP (0.3-3.0 mM) caused a rapid and concentration-related increase in insulin output by rat islets incubated or perfused at an intermediate concentration of D-glucose (8.3 mM). The effect of the ATP analog faded out at both lower and higher D-glucose concentrations. In the presence of 8.3 mM D-glucose, ATP also increased both 86Rb and 45Ca outflow from prelabelled islets. The cationic response to ATP persisted in the absence of extracellular Ca2+ and, hence, was reminiscent of that evoked by cholinergic agents. Like carbamylcholine, ATP caused a dose-related increase in the production of [3H]inositol phosphates from prelabelled islets or tumoral islet cells (RINm5F line). The latter effect was duplicated by alpha, beta-methylene ATP and unaffected by atropine. It is speculated that ATP, liberated together with insulin at the exocytotic site, might participate in a positive feedback control of insulin release.

The coupling of metabolic to secretory events in pancreatic islets: inhibition by 2-cyclohexene-1-one of the secretory response to cyclic AMP and cytochalasin B

In rat pancreatic islets perifused in the presence of 2-cyclohexene-1-one (CHX; 1.0 mM), the secretory response to either D-glucose or 2-ketoisocaproate, but not that evoked by the association of L-leucine and L-glutamine, was severely decreased. This coincided with a decreased stimulation of [45Ca] efflux from prelabelled islets, whereas the inhibitory action of D-glucose or 2-ketoisocaproate upon both [86Rb] and [45Ca] efflux appeared little or not affected. In the presence of D-glucose, the islets exposed to CHX were virtually unresponsive to either forskolin, theophylline or cytochalasin B. A severe decrease in the secretory response to forskolin was also observed in CHX-treated islets exposed to L-leucine and L-glutamine. Except for a somewhat lower sensitivity to NaF, no major change in adenylate cyclase activity or cyclic AMP production was observed in CHX-treated islets. The activity of protein kinase A was decreased in such islets but its responsiveness to cyclic AMP appeared unaltered. Transglutaminase activity was severely decreased in homogenates derived from CHX-treated islets. These findings suggest that CHX, possibly by lowering the GSH content of islet cells, impairs the functional capacity of the effector system for insulin release, in addition to and independently of any effect that it may exert upon nutrient catabolism and cationic fluxes in the islet cells.

Fasting-induced dissociation of cationic and secretory events in pancreatic islets

In pancreatic islets removed from 48 h-fasted rats, as distinct from fed animals, the release of insulin evoked by D-glucose is more severely impaired than that evoked by 2-ketoisocaproate. This decreased secretory response to D-glucose contrasts with an unimpaired cationic response to the sugar in terms of the glucose-induced decrease in both 86Rb and 45Ca outflow from pre-labelled islets. Likewise, fasting only causes a modest decrease of the secondary rise in 45Ca outflow evoked by D-glucose in islets perifused at normal Ca2+ concentration. The latter decrease appears more marked, however, if the cationic response to glucose is expressed relative to that evoked by 2-ketoisocaproate in islets removed from rats in the same nutritional state. It is concluded that, in the process of nutrient-stimulated insulin release, neither the decrease in K+ conductance (inhibition of 86Rb outflow) nor the sequestration of Ca2+ by intracellular organelles and/or direct inhibition of Ca2+ outward transport (decrease in 45Ca outflow) represent the sole determinant(s) of the subsequent gating of Ca2+ channels (secondary rise in 45Ca efflux).

Cholinergic stimulation of ion fluxes in pancreatic islets

Cholinergic agents are known to stimulate the hydrolysis of polyphosphoinositides in pancreatic islets. The effect of carbamylcholine upon ion fluxes in the islet cells was investigated. Carbamylcholine provoked a rapid but poorly sustained increase in 45Ca and 86Rb outflow from perifused islets. Such a cationic response was observed at different glucose concentrations (zero to 16.7 mM), at three concentrations of carbamylcholine (10 microM, 100 microM and 1.0 mM), and in the absence or presence of extracellular Ca2+. It coincided with a biphasic stimulation of insulin release, both the cationic and secretory responses being abolished in the presence of atropine (10 microM). At variance with nutrient secretagogues, carbamylcholine failed to affect the net production of cyclic AMP and caused a transient decrease in 32P outflow from islets prelabelled with [32P]phosphate. It is proposed that cholinergic agents mobilize Ca2+ from intracellular sites, possibly through generation of inositol, 1,4,5-triphosphate from phosphatidylinositol 4,5-bisphosphate. The intracellular redistribution of Ca2+ does not appear sufficient, however, to account fully for the secretory response, which may also involve activation of protein kinase C by diacylglycerol.

Stimulation of protein kinase C and insulin release by 1-oleoyl-2-acetyl-glycerol

The membrane-accessible diacylglycerol 1-oleoyl-2-acetyl-sn-glycerol (OAG, 5-500 microM) caused a dose-related activation of protein kinase C in rat islet homogenates. In islet cell membranes exposed to [gamma-32P]ATP, OAG (100 microM) stimulated the net production of labelled phosphatidate and inhibited that of labelled phosphatidylinositol 4-phosphate. In intact islets exposed to 5.6 mM D-glucose, OAG (100 microM) decreased the outflow of 86Rb, increased that of 45Ca and caused a rapid stimulation of insulin release. The secretory response to OAG was dose-related in the 50-500 microM range, being most marked, in relative terms, at a glucose concentration close to the threshold value for stimulation of insulin release by this hexose. It was decreased but not abolished in the absence of CaCl2 and presence of EGTA. At variance with tumor-promoting phorbol esters, OAG failed to potentiate insulin release stimulated by a hypoglycaemic sulphonylurea. Although these findings support the view that activation of protein kinase C by diacylglycerol represents an efficient modality for stimulation of insulin release, they suggest that the effect of OAG upon islet function may not be solely attributable to such an activation.

The coupling of metabolic to secretory events in pancreatic islets. The possible role of glutathione reductase

The participation of glutathione reductase in the process of nutrient-stimulated insulin release was investigated in rat pancreatic islets exposed to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). BCNU caused a time-and dose-related, irreversible inhibition of glutathione reductase activity. This coincided with a fall in both GSH/GSSG ratio and the thiol content of the islets. Pretreatment of the islets with BCNU inhibited the oxidation of glucose and its stimulant action upon both 45Ca net uptake and insulin release. Although BCNU (up to 0.5 mM) failed to affect the oxidation of L-leucine and L-glutamine, it also caused a dose-related inhibition of insulin release evoked by the combination of these two amino acids. The latter inhibition was apparently not fully accounted for by the modest to negligible effects of BCNU upon 45Ca uptake, 45Ca efflux, 86Rb efflux and cyclic AMP production. Since BCNU failed to inhibit insulin release evoked by the association of Ba2+ and theophylline, these results support the view that glutathione reductase participates in the coupling of metabolic to secretory events in the process of nutrient-stimulated insulin release. However, the precise modality of such a participation, for example the control of intracellular Ca2+ distribution, remains to be elucidated.

Efflux of 86Rb from rat and mouse pancreatic islets: the role of membrane depolarization

The efflux of 86Rb from rat or mouse perifused islets preloaded with the isotope has been used as an index of the potassium permeability of the islet beta-cell membrane. Cellular transmembrane potentials were altered by changing [K]O or by direct electrical stimulation and the effects on potassium permeability examined. Omission of KCl from the medium perifusing rat islets induced a biphasic change in 86Rb efflux, a brief decline being superseded by a pronounced increase in efflux. Re-introduction of KCl, 4.7 mM, caused a further increase in 86Rb efflux preceding a return to control values. Increasing [K]O from 4.7 mM to 10 mM, 20 mM or 47 mM caused a phasic increase in 86Rb efflux with the magnitude of both the peak and average rate of efflux being dependent upon the extent of the change in [K]O. The increase in 86Rb efflux produced by [K]O, 47 mM, was attenuated by Co2+, 2.56 mM (51% inhibition) or quinine, 10 microM (47% inhibition), but efflux remained significantly (P less than 0.001) above control values. Electrical stimulation of single microdissected mouse pancreatic islets by currents of 0.1 to 0.5 mA evoked a rapid, phasic increase in 86Rb efflux. The magnitude of the response was unaffected by EGTA, 2 mM, or nupercaine, 100 microM. These observations are discussed in relation to the mechanisms controlling the potassium permeability, membrane potential and insulin secretion of the pancreatic islet beta-cell. It is concluded that beta-cell depolarization by a raised [K]0 increases potassium permeability and efflux by at least two mechanisms: (i) a calcium-dependent potassium efflux triggered by an increase in [Ca]i and (ii) an activation of voltage-sensitive potassium channels which occurs even when the calcium-dependent potassium permeability is blocked.

Inhibition by mepacrine and p-bromophenacylbromide of phosphoinositide hydrolysis, glucose oxidation, calcium uptake and insulin release in rat pancreatic islets

Mepacrine and p-bromophenacylbromide were both found to impair 3H-inositol phosphate production in response to both nutrient and hormone-neurotransmitter stimuli in islets prelabelled with 3H-inositol. Both drugs also inhibited net 45Ca uptake in response to glucose or glibenclamide and considerably modified the patterns of 45Ca and 86Rb efflux from perifused islets under both basal and glucose-stimulated conditions. In addition, the oxidation of [U-14C] glucose in islets was impaired by either mepacrine or p-bromophenacylbromide. These inhibitory effects were found to be concentration-related for both mepacrine (0.01-1.0 mM) and p-bromophenacylbromide (0.03-0.3 mM) and were accompanied, in general, by a similar degree of inhibition of insulin secretion. These results suggest that both mepacrine and p-bromophenacylbromide can inhibit phospholipase C activity in intact islets, but also impair 45Ca and 86Rb fluxes and oxidation of nutrients. The diversity of these drugs' inhibitory actions makes them unsuitable tools for examining the role of specific cellular processes in the regulation of islet function.

Methylamines and islet function: cationic aspects

Methylamine (2 to 10 mM) caused a dose-related inhibition of insulin release evoked in rat pancreatic islets by nutrient or non nutrient secretagogues. Trimethylamine exerted comparable effects upon insulin release. Methylamine (2 mM) inhibited insulin secretion but failed to affect 45Ca uptake and efflux in response to a rise in extracellular K+ concentration, suggesting that methylamine acts, to a certain extent at least, at a distal site in the secretory sequence. Methylamine, however, also exerted untoward ionic effects. First, methylamine (2 to 10 mM) apparently caused a dose-related increase in cellular pH. Second, methylamine (2mM) augmented 86Rb outflow from islets perifused either in the absence or presence of glucose or gliclazide, and inhibited Ca2+ inflow (as judged from the net uptake or efflux of 45Ca) in islets stimulated by D-glucose, L-leucine or 2-ketoisocaproate. This multiplicity of ionic and other effects may account for the fact that, in the presence of distinct secretagogues, the secretory process appeared more or less sensitive towards methylamine, depending on the relative importance of changes in cellular pH, K+ permeability and intracellular Ca2+ distribution as determinants of the secretory response.

The control of 86Rb efflux from rat isolated pancreatic islets by the sulphonylureas tolbutamide and glibenclamide

The efflux of 86Rb from rat isolated pancreatic islets preloaded with the isotope and perifused in vitro, has been used to monitor the effects of sulphonylureas on the potassium permeability, Pk, of pancreatic beta-cells. Tolbutamide (5 microM to 5 mM) had a dual effect, causing initially a decrease in 86Rb efflux (the 'on' response) which was rapidly superseded on drug removal by a large phasic increase in 86Rb efflux (the 'off' response). Each kinetic response had a different dose-dependency: the 'on' response was half-maximal at tolbutamide concentrations of 0.02 mM, maximal at 0.2 mM and decreased by concentrations greater than 0.2 mM whereas the 'off' response was half-maximal at 0.07 mM, maximal at 0.7 mM, with further increases in concentration (up to 5 mM) causing no further change in magnitude. Analysis of the time- and concentration-dependency of tolbutamide action, by presenting increasing concentrations (0 to 1.4 mM) of tolbutamide as a ramp or step function, established a critical dependence of the kinetics of 86Rb efflux during and after exposure to tolbutamide upon the initial rate of increase of the tolbutamide concentration rather than its final steady state. In the presence of quinine (10 microM), D600 (50 microM), or tetraethylammonium (20 mM), the secondary increase in 86Rb following tolbutamide (0.7 mM) removal was totally inhibited. Co2+ (2.56 mM) not only blocked the secondary 'off' response but also potentiated the initial 'on' response of tolbutamide. Glibenclamide produced a rapid decrease in 86Rb efflux but at a much lower concentration (10 microM) than tolbutamide and with no 'off' response apparent over a wide range of concentration (1 to 100 microM); moreover the decrease in 86Rb efflux was sustained and only slowly reversible. It is concluded that tolbutamide has two opposing actions on islet beta-cell 86Rb efflux, and therefore PK: (i) a tendency to increase a calcium-sensitive PK by stimulating calcium entry into the cell and (ii) a decrease in PK that may be due to a direct effect on the calcium-sensitive PK itself. The more sustained pharmacological action of glibenclamide is explained by the longer-lasting decrease in PK that it produces.

Impairment of insulin release by methylation inhibitors

The possible participation of enzymatic methylation reactions in the process of insulin release was investigated in rat pancreatic islets. The combination of 3-deazaadenosine and DL-homocysteine impaired the incorporation of 3H-methyl from L-[methyl-3H]methionine into endogenous islet proteins and phospholipids, but failed to affect turnover in the phosphatidylinositol cycle. The inhibitors of methylation decreased insulin release evoked by D-glucose or the combinations of D-glucose and gliclazide, L-leucine and L-glutamine, or Ba2+ and theophylline. The inhibitors of methylation did not impair either the oxidation of D-glucose or affect its capacity to decrease K+ conductance, stimulate Ca2+ inflow and provoke 45Ca accumulation in pancreatic islets. It is proposed that, in the process of insulin secretion, a methyl acceptor protein and/or phospholipid play(s) a limited modulatory role in the coupling of cytosolic Ca2+ accumulation to exocytosis.

Impairment by aminooxyacetate of ionic response to nutrients in pancreatic islets

Aminooxyacetate, an inhibitor of transamination reactions, was recently reported to prevent the induction of a more reduced state in the cytosolic redox couple, as normally evoked by glucose or 2-ketoisocaproate in rat pancreatic islets. The cationic effects of aminooxyacetate were examined, therefore, in perifused islets prelabeled with either 86Rb or 45Ca. Aminooxyacetate augmented the outflow of 86Rb from the islets whether in the absence or presence of an exogenous nutrient and slightly impaired the capacity of nutrients, especially 2-ketoisocaproate, to decrease 86Rb outflow. Aminooxyacetate abolished the nutrient-induced rise in 45Ca efflux that normally results from the stimulation by glucose or 2-ketoisocaproate of 40Ca influx into the islet cells. Aminooxyacetate, however, failed to suppress the early inhibitory effect of nutrients on 45Ca efflux. The alteration by aminooxyacetate of the cationic response to nutrient secretagogues coincided with a suppression of insulin release, whereas the cationic response to a nonnutrient stimulation by a high concentration of extracellular K+ was unaffected by aminooxyacetate. These findings suggest that the induction of a more reduced cytosolic redox state represents an essential link between metabolic events and both the decrease in K+ conductance and stimulation of Ca2+ inflow in the process of nutrient-induced insulin release.

Mechanism of 3-phenylpyruvate-induced insulin release. Secretory, ionic and oxidative aspects

1. 3-Phenylpyruvate caused a dose-related stimulation of insulin release from rat pancreatic islets deprived of exogenous nutrient or incubated in the presence of 5.6 or 8.3 mM-D-glucose. 2. 3-Phenylpyruvate inhibited insulin release evoked by high concentrations of D-glucose (16.7 or 27.8 mM) or 4-methyl-2-oxopentanoate (10.0 mM). This inhibitory effect appeared to be attributable to impairment of 2-oxo-acid transport into the mitochondria, with resulting inhibition of D-glucose, pyruvate or 4-methyl-2-oxopentanoate oxidation. 3. 3-Phenylpyruvate failed to affect the oxidation of, and secretory response to, L-leucine, and did not augment insulin release evoked by a non-metabolized analogue of the latter amino acid. 4. L-Glutamine augmented 3-phenylpyruvate-induced insulin release. The release of insulin evoked by the combination of 3-phenylpyruvate and L-glutamine represented a sustained phenomenon, abolished in the absence of extracellular Ca2+ or the presence of menadione and potentiated by theophylline. 5. Whether in the presence or in the absence of L-glutamine, the secretory response to 3-phenylpyruvate coincided with an increase in O2 uptake, a decrease in K+ conductance, a stimulation of both Ca2+ inflow and 45Ca2+ net uptake and an increase in cyclic AMP content. 6. It is concluded that the release of insulin induced by 3-phenylpyruvate displays features classically encountered when the B-cell is stimulated by nutrient secretagogues, and is indeed attributable to an increase in nutrient catabolism.

Ionic determinants of bioelectrical spiking activity in the pancreatic B-cell

A rise in extracellular glucose concentration from 8.3 to 16.7 mM stimulates both Ca2+ inflow and K+ exit in perfused rat pancreatic islets. These ionic changes are associated with an increase in bioelectrical spiking activity. From a quantitative analysis of 45Ca and 86Rb outflow from prelabelled islets, it is proposed that each electrical spike coincides, approximately, with the entry of 0.8 fmol of Ca2+ and exit of 3.6 fmol of K+ per mm2 of cell surface.

Stimulus-secretion coupling of amino acid-induced insulin release VII. The B-cell memory for L-glutamine

In the absence of another exogenous nutrient, L-glutamine does not stimulate insulin release from rat pancreatic islets or isolated perfused pancreases. L-glutamine, however, augments insulin release evoked by L-leucine. These two amino acids could interact by providing both the substrate (L-glutamate) and an activator (L-leucine) for the reaction catalyzed by glutamate dehydrogenase. Under suitable experimental conditions, as little as 0.5 mM L-glutamine is sufficient to enhance leucine-stimulated insulin release. When the pancreases or islets are first exposed to L-glutamine and then stimulated with L-leucine, the rate of secretion is much higher than that evoked by L-leucine in tissue not first exposed to L-glutamine. The memory of a prior exposure to L-glutamine persists for at least 25 min after removal of the latter amino acid from the extracellular fluid. This memory phenomenon is not dependent on the presence of Ca2+ in the extracellular fluid during the first exposure to L-glutamine, but is suppressed when such a prior exposure is performed in the absence of extracellular K+. The memory phenomenon could be due, in part at least, to inhibition by L-glutamine of K+ conductance in the B-cell plasma membrane. Moreover, the amount of L-glutamate which accumulates in islets exposed to L-glutamine is sufficient to maintain, for a much longer period than with other nutrient secretagogues, a sustained increase in catabolic fluxes after removal of the amino acid from the extracellular fluid.

The stimulus-secretion coupling of amino acid-induced insulin release. IV. Ionic response to L-Leucine and L-Glutamine

L-Glutamine enhances insulin release evoked by L-leucine in isolated rat pancreatic islets. The enhancing action of L-glutamine, which is a rapid but steadily increasing and not rapidly reversible phenomenon is not attributable to any major change in either K+ or Ca2+ outflow from the islet cells. It coincides with an apparent increase in Ca2+ inflow rate and, hence, with Ca accumulation in the islets. The initial ionic response to L-leucine is not qualitatively altered by the presence of L-glutamine. In their combined capacity to stimulate 45Ca net uptake in the islets, L-glutamine can be replaced by L-asparagine but not by L-glutamate, whereas L-leucine can be replaced by L-norvaline or L-isoleucine, but not by L-valine, glycine or L-lysine. Such a specificity is identical to that characterizing the effect of these various amino acids upon insulin release. It is postulated that the release of insulin evoked by the combination of L-leucine and L-glutamine involves essentially the same remodelling of ionic fluxes as that evoked by other nutrient secretagogues with, however, an unusual time course for the functional response to L-glutamine.

Regulation of 86Rb outflow from pancreatic islets: the dual effect of nutrient secretagogues

1. An increase in the concentration of extracellular D-glucose from zero to 1.7 mM or more (up to 16.7 mM) causes a rapid and sustained decrease in 86Rb fractional outflow rate (FOR) from prelabelled and perifused pancreatic rat islets. The 86Rb FOR also decreases when the concentration of D-glucose is raised from 1.7 mM or more (up to 5.6 mM) to higher values not exceeding 8.3 mM. 2. However, when the glucose concentration is raised from 8.3 mM (or 11.1 mM) to higher values, no decrease in 86Rb FOR is observed and, instead, a transient increase in 86Rb FOR now takes place. 3. Such a dual effect on 86Rb FOR is also observed when alpha-ketoisocaproic acid is used as the nutrient secretagogue or when the latter keto acid is used in combination with D-glucose. 4. The transient increase in 86Rb FOR evoked by D-glucose in islets already exposed to alpha-ketoisocaproate is abolished by mannoheptulose, suggesting that it depends on the integrity of glucose metabolism. 5. The transient increase in 86Rb FOR evoked, under suitable experimental conditions, by D-glucose of alpha-ketoisocaproate is abolished in the absence of extracellular Ca2+ and mimicked by theophylline and tolbutamide, suggesting that it is attributable to an increase in cytosolic Ca2+ concentration. The latter view is supported by the fact that the increase in 86Rb FOR coincides with an increase in 45Ca FOR, provided that Ca2+ is not removed from the extracellular medium. 6. It is concluded that, in contrast to the situation found when the concentration of the nutrient secretagogue is increased from a non-insulinotropic to a higher value, the stimulation of Ca2+ entry into islet cells and the subsequent increase in insulin secretion evoked by D-glucose or alpha-ketoisocaproate when the concentration of these nutrients is increased from intermediate (8.3-10.0 mM) to higher values is not attributable to a decrease in K+ conductance.

Stimulus-secretion coupling of glucose-induced insulin release. Timing of early metabolic, ionic, and secretory events

The timing of the early metabolic, ionic, and secretory responses to glucose in rat pancreatic islets was monitored by measuring, at 12 sec intervals, the concentrations of glucose, lactic, and pyruvic acids, 32P, 86Rb, 45ca, and insulin in the effluent of perifused prelabeled islets. The increase in glucose concentration from zero to 16.7 mM was complete within 133 sec. The output of organic acids increased after 24 sec of exposure to glucose and, in the case of lactic acid, fell slightly after the initial elevation. The phosphate flush was initiated only after 96 sec of exposure to glucose, whereas the decreases in 86Rb and 45Ca outflow were both detectable within 72 sec of stimulation. The secondary rise in 45Ca efflux was first seen after 157 sec of stimulation and its time course was not vastly different from that of insulin release. These data indicate that, in the secretory sequence, metabolic changes precede both the remodelling of ionic fluxes and the stimulation of insulin release. The results are compatible with the view that the secondary rise in 45Ca outflow is attributable, in part at least, to the glucose-induced decrease in K conductance (but not to the increase in phosphate outflow), with resulting membrane depolarization and gating of voltage-dependent Ca channels.

Regulation of calcium fluxes in rat pancreatic islets. Quinine mimics the dual effect of glucose on calcium movements

The effects of quinine and 9-aminoacridine, two blockers of potassium conductance in islet cells, on 45Ca efflux and insulin release from perifused islets were investigated in order to elucidate the mechanisms by which glucose initially reduces 45Ca efflux and later stimulates calcium inflow in islet cells. In the absence of glucose, 100 microM quinine stimulated 45Ca net uptake, 45Ca outflow rate and insulin release. Quinine also dramatically enhanced the cationic and the secretory response to intermediate concentrations of glucose, but had little effect on 45Ca net uptake, 45Ca fractional outflow rate and insulin release at a high glucose concentration (16.7 mM). The ability of quinine to stimulate 45Ca efflux depended on the presence of extracellular calcium, suggesting that it reflects a stimulation of calcium entry in the islet cells. In the absence of extracellular calcium, quinine provoked a sustained decrease in 45Ca efflux. Such an inhibitory effect was not additive to that of glucose, and was reduced at low extracellular Na+ concentration. At a low concentration (5 microM), quinine, although reducing 86Rb efflux from the islets to the same extent as a non-insulinotropic glucose concentration (4.4 mM), failed to inhibit 45Ca efflux. In the presence of extracellular calcium, 9-aminoacridine produced an important but transient increase in 45Ca outflow rate and insulin release from islets perifused in the absence of glucose. In the absence of extracellular calcium, 9-aminoacridine, however, failed to reduced 45Ca efflux from perifused islets. It is concluded that quinine, by reducing K+ conductance, reproduces the effect of glucose to activate voltage-sensitive calcium channels and to stimulate the entry of calcium into the B-cell. However, the glucose-induced inhibition of calcium outflow rate, which may also participate in the intracellular accumulation of calcium, does not appear to be mediated by changes in K+ conductance.

The stimulus-secretion coupling of glucose-induced insulin release. XLVI. Physiological role of L-glutamine as a fuel for pancreatic islets

Exogenous L-glutamine is actively metabolized in rat pancreatic islets. The rate of L-glutamine deamidation largely exceeds the rate of glutamate conversion to gamma-aminobutyrate and alpha-ketoglutarate. The latter conversion occurs in part by oxidative deamination, and in part by transamination reactions coupled with the conversion of 2-keto acids (pyruvate, oxaloacetate), themselves derived from the metabolism of glutamine, to their corresponding amino acids (alanine, aspartate). An important fraction of malate formed from alpha-ketoglutarate leaves the Krebs cycle and is converted to pyruvate, the process being apparently associated with the induction of a more reduced state in cytosolic redox couples. L-Glutamine abolishes the oxidation of endogenous nutrients is documented by the fact that the glutamine-induced increase in O2 consumption is much lower than expected from the rate of 14CO2 output from islets exposed to L-[U-14C]glutamine, L-Glutamine, although decreasing K+ conductance, fails to stimulate insulin release both in the absence and presence of D-glucose. It is proposed that L-glutamine represents a major fuel for pancreatic islets under physiological conditions.

The stimulus-secretion coupling in glucose-induced insulin release xliv. A possible link between glucose metabolism and phosphate flush

Above a threshold of 3.0-4.2 mmol/l, D-glucose provoked a transient increase in 32P fractional outflow rate from rat pancreatic islets prelabelled with 32P-orthophosphate. Nutrients which stimulate insulin release in the absence of glucose, alpha-ketoisocaproate and L-leucine, also provoked a phosphate flush. No flush occurred in islets exposed to non-insulinotropic nutrients (L-glutamine and and L-lactate) or non-nutriet secretagogues (arginine, tolbutamide, theophylline). A late increase in 32P fractional outflow rate was observed in Ca2+ deprived islets stimulated with BaCl2 and theophylline. The occurrence of a phosphate flush did not appear to be attributable to changes in insulin release, cyclic AMP content, membrane polarisation, K+ conductance, or reduced pyridine nucleotide content. The 32P response to glucose was slightly decreased in the absence of extracellular Ca2+ or HCO3-, markedly impaired in the absence of K4, and virtually abolished in the presence of menadione (10 mumol/l). It is proposed that the occurrence of a phosphate flush is linked to the metabolism of nutrient secretagogues, possibly via an increase in O2 uptake and the production rate of NAD(P)H and ATP.

Regulation of 86Rb+ outflow from pancreatic islets III. Possible significance of ATP

Antimycin A, oligomycin or anoxia augmented 86Rb fractional outflow rate (FOR) from prelabelled pancreatic islets, whether in the absence or presence of glucose. This effect could be due to activation of a Ca2+-sensitive modality of K+ extrusion, being little affected by tetraethylammonium and almost completely suppressed by quinine. In the presence of the latter drug, antimycin A still provoked a modest increase in 86Rb FOR, the magnitude of which was positively related to the extracellular glucose concentration and which could correspond to a discharge of mitochondrial 86Rb. It is proposed that the decrease in 86Rb FOR evoked by insulinotropic nutrients (e.g. glucose) may depend, to a limited extent, on an increase in ATP generation rate.

Regulation of 86Rb outflow from pancreatic islets. IV. Effect of cyclic AMP, dibutyryl-cyclic AMP and theophylline

Theophylline (1.4 mM), cyclic AMP (1.0 mM) and dibutyryl-cyclic AMP (0.5 mM) decreased 86Rb fractional outflow rate from pancreatic islets perifused in the absence of glucose. In the presence of glucose (16.7 mM), however, the same drugs provoked a modest increase in 86Rb fractional outflow rate. The increase in 86Rb outflow evoked by theophylline in the presence of glucose was suppressed by quinine, suggesting that it may result from an increase in cytosolic Ca2+ concentration. It is proposed that changes in the cyclic AMP content of islet cells may participate in the regulation of K+ conductance by insulin secretagogues.