Interactions between lithium, inositol and mono-oleoylglycerol in the regulation of insulin secretion from isolated perifused rat islets

Uptake and retention of 65Zn in lithium-treated rat liver: role of zinc

AIM

To evaluate the effects of zinc on the biokinetics of (65)Zn in rat and its distribution in various organs and in subcellular compartment following lithium therapy.

METHODS

Female wistar rats received either lithium treatment at a dose of 1.1g/kg in diet, zinc alone at a dose of 227 mg/L in drinking water, and combined lithium plus zinc for duration of four months.

RESULTS

After four months of lithium treatment, liver enzymes increased significantly (glutamic oxaloacetic transaminase, +66.73%; glutamic pyruvic transaminase, +63.70%; alkaline phosphatase, +40.28%; p< or =0.001); zinc supplementation to lithium-treated rats significantly reduced liver enzymes (glutamic oxaloacetic transaminase, -13.11%; glutamic pyruvic transaminase, -21.78%; alkaline phosphatase, -11.77%; p< or =0.001). The biological half-lives of (65)Zn showed an initial fast component (Tb(1)) and a slower component (Tb(2)). A significant increase in Tb(2) (38.82%, p< or =0.001) in liver was observed following lithium treatment, which significantly decreased following zinc treatment (21.71%, p< or =0.001). A significant decrease in the uptake of (65)Zn (53.93%, p< or =0.01) in liver was observed and in nuclear (p< or =0.01), mitochondrial (p< or =0.01), and microsomal (52.67%, p< or =0.001) fractions. A significant increase in the uptake of (65)Zn (82.92%, p< or =0.05) in liver microsomal fraction (34.09%, p< or =0.001) was observed in lithium-treated rats receiving zinc supplementation.

CONCLUSION

The study suggests that zinc has the potential to regulate the biokinetics of (65)Zn and its subcellular distribution in rat liver following lithium therapy.

Intrathecal lithium reduces neuropathic pain responses in a rat model of peripheral neuropathy

We tested the ability of lithium (Li(+)) to block heat hyperalgesia, cold allodynia, mechanical allodynia and mechanical hyperalgesia in rats experimentally subjected to painful peripheral neuropathy. Chronic constrictive injury (CCI) to the sciatic nerve induced persistent hyperalgesia and allodynia. Intrathecal injection of Li(+) (2.5-40 micromol) into the region of lumbar enlargement dose-dependently reduced heat hyperalgesia, cold allodynia and mechanical allodynia for 2-6 h after injection, but had no effect on mechanical hyperalgesia. Li(+) had no significant effect on responses from control and sham-operated animals. Intrathecal injection of myo-inositol (2.5 mg) significantly reversed both the anti-hyperalgesic and anti-allodynic effect of Li(+). These findings suggest that intrathecal Li(+) suppresses neuropathic pain response in CCI rats through the intracellular phosphatidylinositol (PI) second messenger system in spinal cord neurons. Lithium (Li(+)) has already found widespread clinical application; these results suggest that its therapeutic utility may be extended to include treatment of neuropathic pain syndromes resulting from peripheral nerve injury.

Glucose-induced desensitization of the pancreatic beta-cell is species dependent

The capacity of 20 mM glucose to desensitize insulin release was determined. A prior exposure to 20 mM glucose impaired the response of rat islets to subsequent restimulation. Compared with control islets, insulin secretory rates measured 25-30 min after the onset of 20 mM glucose stimulation were reduced by 75%. Restimulation of glucose-desensitized islets with 20 mM glucose plus 500 nM forskolin resulted in a dramatic enhancement of both phases of secretion. In contrast to the desensitization of rat islets induced by prior 20 mM glucose exposure, mouse islets were immune to this adverse effect of the hexose. Prior exposure to 20 mM glucose had no adverse effect on glucose usage rates. The activation of phospholipase C in glucose-desensitized rat islets was compromised when compared with control islets. The impairment could not be accounted for by a decrease in immunoreactive content of several major phospholipase C isozymes (beta1 or delta1) or their partitioning between the membrane and cytosolic compartments. In contrast to rat islets, prior exposure of mouse islets to 20 mM glucose for 180 min had no effect on inositol phosphate accumulation. These observations document an additional difference between rat and mouse islets and suggest that the evolution of desensitization is a consequence of the impaired activation of phospholipase C in rat islets.

Chronic exposure to TPA depletes PKC alpha and augments Ca-dependent insulin secretion from cultured rat islets

The insulin secretory responses of rat islets to glucose (15 mM), 12-O-tetradecanoylphorbol 13-acetate (TPA; 500 nM), and potassium (30 mM) were determined from perifused islets cultured for 22-24 h in CMRL-1066 medium (control cultured) or islets cultured in the additional presence of 500 nM TPA. Islet content of protein kinase C alpha (PKC alpha) and serine and threonine phosphoprotein patterns were also monitored after the culture period. Compared with freshly isolated islets, culturing alone had no adverse effect on the capacity of TPA or 30 mM potassium to stimulate secretion or on the islet content of PKC alpha. In agreement with previous studies, culturing in TPA reduced the islet content of immunoreactive PKC alpha by > 95% and abolished the capacity of the phorbol ester to stimulate secretion during a subsequent dynamic perifusion. Culturing in TPA slightly improved the insulin secretory response to 15 mM glucose compared with control-cultured islets; however, sustained rates of 15 mM glucose-induced secretion from these islets were significantly less than the responses of freshly isolated islets. Islets cultured in TPA responded to 30 mM potassium with a markedly amplified insulin secretory response that was abolished by nitrendipine. Enhanced phosphorylation of several islet proteins was also observed in TPA-cultured islets compared with control-cultured islets. These findings demonstrate that culturing alone impairs glucose-induced secretion, a response that is improved but still subnormal compared with freshly isolated islet responses, if TPA is included in the culture medium. Sustained phosphorylation of several islet proteins in TPA-cultured islets may account, at least in part, for augmented calcium-dependent secretion.

Regulation of insulin secretion via ATP-sensitive K+ channel independent mechanisms: role of phospholipase C

Groups of rat or mouse islets were isolated and perifused with 20 mM glucose plus 200 microM diazoxide. The further addition of 30 mM K+ resulted in a rapid and sustained biphasic insulin secretory response. The onset of secretion in response to the addition of K+ was comparable in both species, but the magnitude of the response was significantly greater from rat islets. After the labeling of islet phosphoinositide pools with 2-[3H]inositol, the accumulation of labeled inositol phosphates (IP) in response to 30 mM K+ addition in the simultaneous presence of 20 mM glucose plus diazoxide was assessed. The addition of 30 mM K+ significantly increased IP accumulation approximately 300% in rat islets, whereas only an insignificant 25-30% increase was observed in mouse islets. The protein kinase C inhibitor staurosporine (50 nM) dramatically reduced the sustained secretory response from rat islets in the presence of 30 mM K+, 20 mM glucose, and diazoxide. Its effect was minimal on mouse islets and a significant inhibitory effect on insulin secretion was observed only during the final 5 min of the perifusion. The further addition of carbachol, an agonist that activates an isozyme of phospholipase C distinct from that activated by glucose, together with K+, 20 mM glucose, plus diazoxide resulted in a sustained amplification of insulin secretion from mouse but not rat islets. K+ (30 mM)-induced insulin secretion in the presence of 3 mM glucose was similar from perifused rat or mouse islets, a finding that would seem to preclude the activation of voltage-regulated Ca2+ channels as the pertinent difference. These results confirm previous observations with these species and document another anomaly that exists between the responses of rat islets compared with mouse islets. The inability to activate a nutrient- and calcium-regulated phospholipase C isozyme in mouse islets to the same extent as in rat islets appears to account, at least in part, for these different insulin secretory responses under these unique conditions.

Influence of pyruvic acid methyl ester on rat pancreatic islets. Effects on insulin secretion, phosphoinositide hydrolysis, and sensitization of the beta cell

The methyl ester of pyruvic acid (methyl pyruvate) stimulated a dose-dependent increase in insulin secretion from isolated perifused rat islets. The threshold level for release was about 10 mM, and at 20 mM the addition of MP to perifused islets resulted in a large first phase of secretion followed by an insulin-secretory response that was sustained for at least 40 min. When compared to the effects of 20 mM glucose, peak first-phase release rates in response to 20 mM methyl pyruvate were comparable, but the second phase of release was only about 10-15% of that observed with an equimolar level of the hexose. The stimulatory effects of 20 mM methyl pyruvate on secretion were abolished by the K1+-ATP channel blocker diazoxide (200 microM) and by the calcium channel antagonist nitrendipine (500 nM). The glucokinase inhibitor mannoheptulose (20 mM) had no adverse effect on the secretory response to 20 mM methyl pyruvate, whereas 10 microM forskolin amplified the insulinotropic action of MP. Sodium pyruvate alone or in combination with 10 microM forskolin had no insulinotropic effect. In additional experiments islet phosphoinositide pools were labeled with myo-2-[3H]inositol, and the subsequent accumulation of labeled inositol phosphates was used to monitor the activation of phospholipase C. Methyl pyruvate stimulated a dose-dependent increase in inositol phosphate levels when measured after a 30-min incubation period with a maximal increase of about 300% at 20 mM methyl pyruvate. The increase in phosphoinositide hydrolysis caused by methyl pyruvate (20 mM) was, like insulin secretion, reduced by both diazoxide and nitrendipine but was immune to inhibition by mannoheptulose. Pyruvate (20 mM) had no effect on inositol phosphate accumulation. Prior short-term exposure to methyl pyruvate sensitized islets to subsequent stimulation with 15 mM glucose. Sodium pyruvate did not sensitize islets. These findings support the concept that the mitochondrial metabolism of nutrient molecules is an event sufficient to acutely augment insulin release from the beta cell, to increase phospholipase C-mediated phosphoinositide hydrolysis, and to induce time-dependent potentiation of insulin secretion.

Regulation of insulin secretion by phospholipase C

Biphasic insulin secretion in response to a sustained glucose stimulus occurs when rat or human islets are exposed to high levels of the hexose. A transient burst of hormone secretion is followed by a rising and sustained secretory response that, in the perfused rat pancreas, is 25- to 75-fold greater than prestimulatory insulin release rates. This insulin secretory response is paralleled by a significant five- to sixfold increase in the phospholipase C (PLC)-mediated hydrolysis of islet phosphoinositide (PI) pools by high glucose. In contrast, mouse islets, when stimulated under comparable conditions with high glucose, display a second-phase response that is flat and only slightly (two- to threefold) greater than prestimulatory release rates. The minimal second-phase insulin secretory response to high glucose is accompanied by the minimal activation of PLC in mouse islets as well. However, stimulation of mouse islets with the protein kinase C (PKC) activator tetradecanoyl phorbol acetate (TPA) or the muscarinic agonist carbachol, which significantly activates an isozyme of PLC distinct from that activated by high glucose, induces a rising and sustained second-phase insulin secretory response. When previously exposed to high glucose, both rat and human islets respond to subsequent restimulation with an amplified insulin secretory response. They display priming, sensitization, or time-dependent potentiation. In contrast, mouse islets primed under similar conditions with high glucose fail to display this amplified insulin secretory response on restimulation. Mouse islets can, however, be primed by brief exposure to either TPA or carbachol. Finally, whereas rat islets are desensitized by chronic exposure to high glucose, mouse islet insulin secretory responses are relatively immune to this adverse effect of the hexose. These and other findings are discussed in relationship to the role being played by agonist-induced increases in the PLC-mediated hydrolysis of islet phosphoinositide pools and the activation of PKC in these species-specific insulin secretory response patterns.

Effects of short-term culturing on islet phosphoinositide and insulin secretory responses to glucose and carbachol

The ability of glucose and carbachol, alone or in combination, to stimulate islet cell phosphoinositide (PI) hydrolysis and insulin secretory responses in freshly isolated or in 20-24 h cultured rat islets was assessed. In freshly isolated, 3H-inositol-prelabeled islets, 20 mM glucose alone or 1 mM carbachol alone stimulated significant increments in 3H-inositol efflux and inositol phosphate (IP) accumulation. When stimulated with both agonists, a dramatic and synergistic effect on IP accumulation was noted. Carbachol (1 mM) alone had no sustained stimulatory effect on insulin secretion. Glucose (20 mM) alone induced a biphasic insulin secretory response. When compared to prestimulatory secretory rates of 18 +/- 4 pg/islet/min, peak first and second phase responses now averaged 422 +/- 61 and 1016 +/- 156 pg/islet/min, respectively. In contrast to freshly studied islets, culturing islets for 20-24 h in CMRL-1066 medium attenuated all measured responses. The increases in 3H-inositol efflux rates in response to glucose, carbachol, or their combination were significantly less than those observed with fresh islets. The IP responses were also attenuated. Second phase insulin secretory responses to 20 mM glucose alone 68 +/- 9 pg/islet/min) or the combination of 20 mM glucose plus 1 mM carbachol (358 +/- 85 pg/islet/min) were also significantly decreased when compared with fresh islets. We conclude from these studies that the process of culturing islets for one day in CMRL-1066 significantly decreases islet cell PI hydrolysis and insulin secretory responsiveness. These observations may help to explain the discordant conclusions reached concerning the involvement of PI hydrolysis and protein kinase C activation in the regulation of insulin release from freshly isolated versus cultured islets.

Long-term action of lithium: a role for transcriptional and posttranscriptional factors regulated by protein kinase C

Lithium, a simple monovalent cation, represents one of psychiatry's most important treatments and is the most effective treatment for reducing both the frequency and severity of recurrent affective episodes. Despite extensive research, the underlying biologic basis for the therapeutic efficacy this drug remains unknown, and in recent years, research has focused on signal transduction pathways to explain lithium's efficacy in treating both poles of manic-depressive illness. Critical to attributions of therapeutic relevance to any observed biochemical effect, however, is the observation that the characteristic prophylactic action of lithium in stabilizing the profound mood cycling of bipolar disorder requires a lag period for onset and is not immediately reversed upon discontinuation of treatment. Biochemical changes requiring such prolonged administration of a drug suggest alterations at the genomic level but, until recently, little has been known about the transcriptional and posttranscriptional factors regulated by chronic drug treatment, although long-term changes in neuronal synaptic function are known to be dependent upon the selective regulation of gene expression. In this paper, we will present evidence to show that chronic lithium exerts significant transcriptional and posttranscriptional effects, and that these actions of lithium may be mediated via protein kinase C (PKC)-induced alterations in nuclear transcription regulatory factors responsible for modulating the expression of proteins involved in long-term neural plasticity and cellular response. Such target sites for chronic lithium may help unravel the processes by which a simple monovalent cation can produce a long-term stabilization of mood in individuals vulnerable to bipolar illness.

Ziskind-Somerfeld Research Award 1993. Biochemical, behavioral, and clinical studies of the role of inositol in lithium treatment and depression

Lithium (Li) reduces brain inositol levels by inhibiting the enzyme inositol monophosphatase. The enzyme inositol-1-phosphatase was measured in human red blood cells of controls, Li-free bipolar patients, and Li-treated bipolar patients and was found to be reduced by 80% in Li-treated bipolars, thus supporting the concept that chronic Li at therapeutic concentrations inhibits this enzyme. Two behaviors in rats caused by Li, reduction of rearing, and Li-pilocarpine seizures, are reversed by intracerebroventricular replenishment of inositol. The reversal is stereospecific to the naturally occurring myo-inositol; whereas the stereoisomer L-chiro-inositol is ineffective. The reversal is dose-dependent, requiring a dose consistent with known quantities of brain inositol depletion; and is time-dependent, as inositol must be given 1-8 h before stimulation. High-dose peripheral inositol also reverses the limbic seizures induced by Li-pilocarpine, and using gas chromatography was shown to increase brain inositol levels that had been reduced by Li treatment. Low-dose inositol could be shown to reverse a peripheral Li-induced side effect, polyuria/polydipsia, in rats and in patients treated with Li. A higher dose of inositol markedly reduced Hamilton Depression Ratings in 9 of 11 unipolar major depressive disorder patients previously unresponsive to tricyclics, in an open design, but had no effect on chronic schizophrenics in a controlled double-blind randomized crossover trial. A new inositol monophosphatase inhibitor, a fungal product originally discovered as a complement inhibitor, was found to act like Li and lower the seizure threshold for subconvulsant doses of pilocarpine. These data suggest that inositol monophosphatase inhibition is a key mechanism of Li's therapeutic action and that design of new inositol monophosphatase inhibitors may be a practical strategy to create new compounds with Li-like therapeutic effects.

Evidence against a direct role for inositol phosphate metabolism in the circadian oscillator and the blue-light signal transduction pathway in Neurospora crassa

The inositol-depletion hypothesis proposes that the effects of Li+ on cellular functions are the result of inhibition by Li+ of the inositol monophosphate phosphatase and subsequent depletion of inositol lipids. This mechanism has been proposed to account for the effects of Li+ on the period of the circadian oscillator. Inositol phosphate metabolism has also been proposed as part of the blue-light signal-transduction pathway through which the phase of the circadian oscillator can be reset by light pulses. Four predictions of these two hypotheses have been tested in the fungus Neurospora crassa and all have been found to fail: (1) inositol supplementation does not reverse the effects of Li+ on the period of the circadian rhythm; (2) inositol depletion of an inositol-requiring mutant does not mimic the effects of Li+; (3) depletion of inositol lipids does not inhibit the response to light; and (4) a phase-resetting pulse of light does not increase the levels of inositol phosphates, including Ins(1,4,5)P3.

Role of polyamines in the regulation of proliferation and hormone production by insulin-secreting cells

This paper focuses on the mechanisms regulating proliferation and insulin production by normal and tumoral pancreatic beta-cells. In particular, the evidence for involvement of polyamines is reviewed. Pancreatic islet cells contain high levels of polyamines, and based on findings obtained using enzyme-directed inhibitors, it appears that putrescine and spermidine are necessary for proinsulin biosynthesis, whereas spermine may exert a stimulatory or permissive role in RNA transcription-stabilization and long-term insulin release. Islet polyamine content is not altered by short-term secretory stimulation, nor is the acute secretory response impeded by polyamine synthesis inhibitors, making it unlikely that these amines play any major role in short-term insulin release. Various mitogens increase islet polyamine contents and DNA synthesis, but increases in cytosolic polyamines do not seem to mediate their mitogenicity. Nuclear polyamine content is not altered by the inhibitors, suggesting that maintenance of polyamines within this organelle may be sufficient to sustain elevated DNA synthesis. In tumoral RINm5F cells, polyamine depletion results in decreased proliferation and increased cellular content of insulin and insulin secretory granules without affecting insulin mRNA levels or translation. Moreover, polyamine-depleted RINm5F cells display improved substrate metabolism and sensitivity of the stimulus-secretion coupling. Possible levels of polyamine interaction with Ca2+ metabolism are discussed.

Restoration of brain myo-inositol levels in rats increases latency to lithium-pilocarpine seizures

Lithium pretreatment in rats potentiates the epileptogenic effects of pilocarpine and other cholinergic agonists. In order to determine if this effect of lithium could be reversed by myo-inositol, rats were pretreated with intracerebroventricular (ICV) injections of myoinositol, artificial CSF or L-chiro-inositol. Lithium chloride, 3 meq/kg was administered intraperitoneally 20-24 h prior to the subcutaneous injection of pilocarpine, 20 or 30 mg/kg. In both experiments, myo-inositol significantly prolonged the latency to the appearance of clonic seizures and lowered the pilocarpine seizure score. myo-Inositol prevented the development of clonic seizures in 50% of the rats receiving pilocarpine, 20 mg/kg. The levels of cortical myo-inositol in rats injected with myo-inositol were approximately double those of the CSF and L-chiro-inositol groups.

Spontaneous recovery of deficits in spatial memory and cholinergic potentiation of NMDA in CA1 neurons during chronic lithium treatment

The therapeutic action of lithium in affective disorders is still unclear. One effect of lithium is to deplete membrane inositol and consequently to exhaust the phosphoinositide (PI) pathway. Under chronic lithium treatment, rats showed persistent performance deficits in an active avoidance task and in a visually cued maze. The same treatment, however, resulted in only a transient deficit in the performance of rats in a spatial memory task. Lithium treatment caused a similarly transient deficit in the ability of acetylcholine to potentiate responses to N-methyl-D-aspartate (NMDA) in neurons of the hippocampal slice. The authors propose that the development of compensatory mechanisms may account for the lack of severe memory impairments during lithium treatment. It is suggested that the effects of lithium on the PI pathway are not sufficient to explain the behavioral consequences of chronic lithium treatment.

Role of protein kinase C and Ca2+ in glucose-induced sensitization/desensitization of insulin secretion

The role of protein kinase C and Ca2+ in glucose-induced sensitization/desensitization of insulin secretion was studied. A 22-24 h exposure of mouse pancreatic islets to glucose (16.7 mmol/l) in TCM 199 culture medium, with 0.26 mmol/l or 1.26 mmol/l Ca2+, reduced total islet protein kinase C activity to approx. 85% and 60% of control values, respectively. At 0.26 mmol/l Ca2+ in TCM 199 medium, exposure to glucose (16.7 mmol/l) led to a potentiation of both phase 1 and phase 2 of glucose-induced insulin secretion, and caused a shift in the dose-response curve with 10 mmol/l and 16.7 mmol/l glucose exhibiting equipotent effects in stimulation of insulin secretion. In glucose-sensitized islets, the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (0.16 mumol/l) did not further potentiate induction of secretion by 10 mmol/l or 16.7 mmol/l glucose. At 3.3 mmol/l glucose, however, phorbol ester-induced secretion was augmented, and was characterized by a faster onset of secretion in glucose-sensitized islets relative to control islets. In contrast, a partial reduction in arachidonic acid (100 mumol/l)-induced insulin release was observed in glucose-sensitized islets in the absence of extracellular Ca2+. Increasing the Ca2+ concentration to 1.26 mmol/l in TCM 199 during the 22-24 h exposure to glucose (16.7 mmol/l) led to inhibition of phase 1 and abolition of phase 2 of glucose (10 mmol/l, 16.7 mmol/l)-induced insulin secretion. In addition, this treatment abolished phorbol ester-induced and arachidonic acid-induced insulin secretion at 3.3 mmol/l glucose. Altogether, these data suggest that sensitization of insulin secretion is caused by a preferential down-regulation of the inhibitory effects of protein kinase C, leading to an increased first phase, and an increased coupling of glucose to the stimulatory effects of protein kinase C during the second phase of glucose-induced insulin secretion. Desensitization of insulin secretion appears to be a consequence of sustained Ca2+ influx, inducing extensive down-regulation of protein kinase C and also causing deleterious effects on islet cell function in protein kinase C-deprived islets.

Influence of staurosporine, nitrendipine and monooleoylglycerol on interleukin-1-induced insulin secretion and phosphoinositide hydrolysis

The monokine interleukin-1 alpha (IL-1) induces a glucose-dependent increase in insulin secretion, an effect tentatively attributed to its ability to increase beta cell phosphoinositide (PI) hydrolysis. In the present experiments, the effects of the protein kinase C inhibitor staurosporine (20 nM), the calcium channel antagonist nitrendipine (5 microM), and the diacylglycerol kinase inhibitor monooleoylglycerol (MOG, 25 microM) on 40 nM IL-1-induced increments in insulin release from perifused islets and inositol phosphate levels in [3H]inositol prelabeled islets were assessed. In perifused islets, insulin secretion in response to IL-1 in the presence of 7 mM glucose averaged 313 +/- 43 pg/islet/min 35-40 min after the onset of stimulation. Release from control islets perifused in the presence of 7 mM glucose alone averaged 56 +/- 6 pg/islet/min at this time point. The addition of staurosporine together with IL-1 reduced insulin secretion at this time point to 88 +/- 21 pg/islet/min. This level of IL-1 caused significant increases in inositol phosphate accumulation in the presence of 7 mM glucose but not 2.75 mM glucose. Staurosporine was without a significant effect on inositol phosphate accumulation in response to the monokine. In contrast, nitrendipine (5 microM) inhibited insulin release and inositol phosphate accumulation in a parallel fashion. Finally, MOG significantly amplified release to the monokine without significantly affecting its impact on inositol phosphate accumulation. Nitrendipine or staurosporine blocked this amplifying effect of MOG on secretion. These results emphasize the role of PI hydrolysis in IL-1-induced insulin secretion and suggest further that calcium influx is essential for IL-1 to fully activate both PI hydrolysis and insulin secretion.

Lithium and the phosphoinositide cycle: an example of uncompetitive inhibition and its pharmacological consequences

The ability of lithium to exert profound and selective psychopharmacological effects to ameliorate manic-depressive psychosis has been the focus of considerable research effort. There is increasing evidence that lithium exerts its therapeutic action by interfering with polyphosphoinositide metabolism in brain and prevention of inositol recycling by an uncompetitive inhibition of inositol monophosphatase. Stefan Nahorski, Ian Ragan and John Challiss discuss this unusual stimulus-dependent form of enzyme inhibition, emphasizing that the selectivity exhibited by lithium depends upon the degree of inositol lipid hydrolysis and polyphosphoinositide dephosphorylation.

Widespread synchronous [Ca2+]i oscillations due to bursting electrical activity in single pancreatic islets

Pancreatic beta cells, tightly organized in the islet of Langerhans, secrete insulin in response to glucose in a calcium-dependent manner. The calcium input required for this secretory activity is thought to be provided by an oscillatory electrical activity occurring in the form of "bursts" of calcium action potentials. The previous observation that islet intracellular free Ca2+ levels undergo spontaneous oscillations in the presence of glucose, together with the fact that islet cells are coupled through gap junctions, hinted at a highly effective co-ordination between individual islet cells. Through the use of simultaneous recordings of intracellular calcium and membrane potential it is now reported that the islet calcium waves are synchronized with the beta cell bursting electrical activity. This observation suggests that each calcium wave is due to Ca2+ entering the cells during a depolarized phase of electrical activity. Moreover, fura-2 fluorescence image analysis indicates that calcium oscillations occur synchronously across the whole islet tissue. The maximal phase shift between oscillations occurring in different islet cells is estimated as 2 s. This highly co-ordinated oscillatory calcium signalling system may underlie pulsatile insulin secretion and the islet behaviour as a secretory "syncytium". Since increasing glucose concentration lengthens calcium wave and burst duration without significantly affecting wave amplitude, we further propose that it is the fractional time at an enhanced Ca2+ level, rather than its amplitude, that encodes for the primary response of insulin-secreting cells to fuel secretagogues.

Intracerebroventricular myo-inositol antagonizes lithium-induced suppression of rearing behaviour in rats

Several biological effects of lithium have been reversed by in vitro myo-inositol. To determine if intracerebroventricular myo-inositol would reverse behavioural effects of lithium, rats were injected with 5 meq/kg lithium chloride or sodium chloride and injected intracranially with myo-inositol (10 mg) or artificial CSF 24 h and 15 min prior to measurement of activity in an automated activity monitor. Myo-inositol alone had no significant effect on behaviour, but significantly reversed suppression of rearing activity by lithium.

Influence of monooleoylglycerol on islet cell phosphoinositide hydrolysis and insulin secretion

The diacylglycerol kinase inhibitor monooleoylglycerol (MOG) produced a dose-dependent increase in both phosphoinositide (PI) hydrolysis and insulin secretion in the presence of a substimulatory glucose level (2.75 mM). This effect could not be reproduced by the combination of oleic acid plus glycerol, potential metabolic products derived from MOG catabolism. At a level (25 microM) which has no significant effect on beta cell insulin secretion or PI hydrolysis in the presence of 2.75 mM glucose, MOG significantly potentiated the insulin stimulatory effect of the sulfonylurea tolbutamide (200 microM) in the presence of 7 mM glucose. This heightened insulin secretory response and PI hydrolysis were effectively attenuated by the calcium channel blocker nitrendipine (0.5 microM). These findings indicate that MOG has complex effects on beta cell performance. It promises, however, to be a useful probe in assessing how events associated with increases in PI hydrolysis influence insulin secretion.