Evidence for the participation of calmodulin in stimulus-secretion coupling in the pancreatic beta-cell

A distinct difference in the metabolic stimulus-response coupling pathways for regulating proinsulin biosynthesis and insulin secretion that lies at the level of a requirement for fatty acyl moieties

The regulation of proinsulin biosynthesis in pancreatic beta-cells is vital for maintaining optimal insulin stores for glucose-induced insulin release. The majority of nutrient fuels that induce insulin release also stimulate proinsulin biosynthesis, but since insulin exocytosis and proinsulin synthesis involve different cellular mechanisms, a point of divergence in the respective metabolic stimulus-response coupling pathways must exist. A parallel examination of the metabolic regulation of proinsulin biosynthesis and insulin secretion was undertaken in the same beta-cells. In MIN6 cells, glucose-induced proinsulin biosynthesis and insulin release shared a requirement for glycolysis to generate stimulus-coupling signals. Pyruvate stimulated both proinsulin synthesis (threshold 0.13-0.2 mM) and insulin release (threshold 0.2-0.3 mM) in MIN6 cells, which was eliminated by an inhibitor of pyruvate transport (1 mM alpha-cyano-4-hydroxycinnamate). A combination of alpha-oxoisohexanoate and glutamine also stimulated proinsulin biosynthesis and insulin release in MIN6 cells, which, together with the effect of pyruvate, indicated that anaplerosis was necessary for instigating secondary metabolic stimulus-coupling signals in the beta-cell. A consequence of increased anaplerosis in beta-cells is a marked increase in malonyl-CoA, which in turn inhibits beta-oxidation and elevates cytosolic fatty acyl-CoA levels. In the beta-cell, long-chain fatty acyl moieties have been strongly implicated as metabolic stimulus-coupling signals for regulating insulin exocytosis. Indeed, it was found in MIN6 cells and isolated rat pancreatic islets that exogenous oleate, palmitate and 2-bromopalmitate all markedly potentiated glucose-induced insulin release. However, in the very same beta-cells, these fatty acids in contrast inhibited glucose-induced proinsulin biosynthesis. This implies that neither fatty acyl moieties nor beta-oxidation are required for the metabolic stimulus-response coupling pathway specific for proinsulin biosynthesis, and represent an early point of divergence of the two signalling pathways for metabolic regulation of proinsulin biosynthesis and insulin release. Therefore alternative metabolic stimulus-coupling factors for the specific control of proinsulin biosynthesis at the translational level were considered. One possibility examined was an increase in glycerophosphate shuttle activity and change in cytosolic redox state of the beta-cell, as reflected by changes in the ratio of alpha-glycerophosphate to dihydroxyacetone phosphate. Although 16.7 mM glucose produced a significant rise in the alpha-glycerophosphate/dihydroxyacetone phosphate ratio, 1 mM pyruvate did not. It follows that the cytosolic redox state and fatty acyl moieties are not necessarily involved as secondary metabolic stimulus-coupling factors for regulation of proinsulin biosynthesis. However, the results indicate that glycolysis and the subsequent increase in anaplerosis are indeed necessary for this signalling pathway, and therefore an extramitochondrial product of beta-cell pyruvate metabolism (that is upstream of the increased cytosolic fatty acyl-CoA) acts as a key intracellular secondary signal for specific control of proinsulin biosynthesis by glucose at the level of translation.

Human islets of Langerhans express multiple isoforms of calcium/calmodulin-dependent protein kinase II

Previous studies have provided evidence for the presence of calcium/calmodulin-dependent protein kinase II (CaM kinase II) in rodent islets of Langerhans, and beta-cell CaM kinase II activity has been correlated with insulin secretion. In this study we provide the first conclusive evidence for the expression of CaM kinase II in human islets of Langerhans and show that multiple isoforms are expressed. Screening of a human islet cDNA library resulted in the isolation of a 999bp partial cDNA clone encoding CaM kinase II. The nucleotide sequence of the islet clone showed a high degree of homology (94.8%) to the two gamma isoforms of CaM kinase II previously isolated from human T lymphocytes (gammaB and gammaC). In order to obtain full length sequence for the islet clone, rapid amplification of cDNA ends (RACE) was used to amplify the 3' end of the islet clone from human islet poly A+ RNA. Two distinct gamma isoforms of CaM kinase II were amplified from the islet RNA. They were identified as gammaB and gammaE; the latter is distinguished from gammaB by a 114bp insertion within the association domain of the cDNA. Using reverse transcriptase polymerase chain reaction (RT-PCR) we also detected in human islets of Langerhans the novel beta3 isoform of CaM kinase II previously reported to be expressed in neonatal rat islets.

Differential sensitivity of interleukin-1 alpha and -beta precursor proteins to cleavage by calpain, a calcium-dependent protease

In view of the observations that the calcium ionophores, A23187 and ionomycin, enhance the processing and secretion of interleukin-1 (IL-1 alpha) and IL-1 beta from macrophages, and IL-1 alpha processing is mediated by calpain, a calcium-dependent protease, we evaluated the possibility that calpain might also play a role in the processing of IL-1 beta. Whereas calpain-containing P388D1 macrophage lysates and purified calpain processed precursor IL-1 alpha to its mature 17-kDa form, precursor IL-1 beta was degraded by both sources of calpain. However, the activation of calpain in P388D1 cells that were transiently transfected with a cDNA expression vector encoding the precursor form of IL-1 beta did not result in the degradation of precursor IL-1 beta, but did result in the processing and secretion of IL-1 alpha, implying that precursor IL-1 beta is protected from calpain degradation in vivo. Furthermore, calpain did not enhance the processing of the IL-1 beta precursor by the IL-1 beta-converting enzyme. These results indicate that calpain is not involved in the processing of precursor IL-1 beta in vitro or in vivo. The IL-1 beta precursor may be protected from calpain degradation by a sequestering mechanism that involves a cytoplasmic factor(s) that reduces the sensitivity of IL-1 beta to attack by calpain or localizes IL-1 beta to a site that precludes any interaction with the protease. Although MDL 28,170, a calpain inhibitor, prevented the ionomycin-induced processing of precursor IL-1 alpha to the mature protein in P388D1 cells, it did not inhibit the ionomycin-induced secretion of the mature IL-1 alpha and -beta proteins expressed in these cells. These results indicate that a calcium-dependent factor other than calpain is involved in the secretion of the mature IL-1 proteins.

Protein phosphorylation and beta-cell function

The central role of reversible protein phosphorylation in regulation of beta-cell function is reviewed and the properties of the protein kinases so far defined in beta cells are summarised. The key effect of Ca2+ to initiate insulin secretion involves activation of a Ca2+/calmodulin-dependent protein kinase. Potentiation of secretion by agents activating protein kinase A or C appears to involve an increase in the sensitivity of the secretory system to intracellular Ca2+. The effects of MgATP on the binding of [3H]-glibenclamide to the beta-cell sulphonylurea receptor suggest that the properties of this receptor, which controls the activity of ATP-sensitive K-channels, are modulated by phosphorylation. The identity of the kinases and phosphatases responsible is not known but the presence in beta-cell membranes of various kinases not dependent on Ca2+ or cyclic AMP, and including tyrosine kinase, is documented, together with the presence of both Ca(2+)-dependent and Ca(2+)-independent protein phosphatases. Protein phosphorylation is also involved in regulation of beta-cell Ca2+ fluxes and evidence is presented that protein kinase C activation inhibits Ca2+ signalling by reducing influx of Ca2+ into the beta cell. The identity of the Ca2+/calmodulin-dependent protein kinase activity in beta cells is discussed. Comparison of its properties towards substrates and inhibitors with those of brain Ca2+/calmodulin-dependent protein kinase II suggests that the beta-cell enzyme may be similar or identical to the brain enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of calcium and calmodulin dependent protein kinase II during stimulation of insulin secretion

Pancreatic islets contain an alloxan sensitive, calcium and calmodulin dependent protein kinase (CaM-PK) which may play an important part in the cellular control of insulin secretion. We have studied this activity in islets and the insulin secreting tumor cell line RINm5f with particular interest in the changes in kinase activity that accompany stimulation of secretion. Initial experiments showed that the CaM-PK activity enriched in microsomal preparations from RIN cells was similar to the islet cell kinase in terms of apparent endogenous substrates, Ca2+ and calmodulin dependence, and inactivation by alloxan. For studies of protein substrate specificity, tumor cell CaM-PK was isolated from other kinase activities and substantially purified by affinity chromatography with calmodulin-agarose. The major protein substrates of CaM-PK (54 kD and 57 kD) co-purified with the kinase activity, representing autophosphorylation of subunits of the enzyme. Exogenous substrates phosphorylated by these preparations included microtubule-associated protein 2, synapsin, and glycogen synthase; this pattern of substrate utilization identified the kinase as the Type II multifunctional kinase which has been extensively characterized in brain. A polyclonal antibody to rat brain CaM-PK II was employed to immunoprecipitate the kinase from RINm5f cells incubated with secretagogues to measure the effect of stimulation of secretion on autophosphorylation of CaM-PK (which reflects kinase activation). D-Glyceraldehyde (22 mM) and depolarizing concentrations of potassium increased autophosphorylation and insulin secretion in a parallel fashion. Potassium stimulated autophosphorylation was dose dependent and saturable, and was increased to near maximal levels at times as short as 1 min. These studies demonstrate that pancreatic islets and RINm5f cells contain a Type II CaM-PK which is activated during the secretion process.

Insulin secretion from pancreatic B cells caused by L-arginine-derived nitrogen oxides

L-arginine causes insulin release from pancreatic B cells. Data from three model systems support the hypothesis that L-arginine-derived nitrogen oxides (NOs) mediate insulin release stimulated by L-arginine in the presence of D-glucose and by the hypoglycemic drug tolbutamide. The formation of NO in pancreatic B cells was detected both chemically and by the NO-induced accumulation of guanosine 3',5'-monophosphate. NG-substituted L-arginine analogs inhibited the release of both insulin and NO. Protein immunoblot and histochemical analysis with antiserum to type I NO synthase suggest that the formation of NO in pancreatic B cells is catalyzed by an NADPH- (reduced form of nicotinamide adenine dinucleotide phosphate), Ca2+/calmodulin-dependent type I NO synthase of about 150 kilodaltons.

Identification of the calmodulin-regulated protein phosphatase, calcineurin, in rat pancreatic islets

The aim of this work was the identification of the calmodulin-stimulated protein phosphatase, calcineurin, in rat pancreatic islets. For this purpose, a high-affinity calcineurin antibody and the Western blotting technique were used to detect the presence of calcineurin in freshly collagenase-isolated islets. The calcineurin content detected by this method was about 0.30 ng islet (approx. 0.07% of the total islet protein). The subunit composition and Mr of islet calcineurin were similar to those of bovine brain calcineurin. Incubation of nitrocellulose membranes of the Western blotting, containing the islet protein fractions, with 125I-labeled calmodulin and 45Ca2+ demonstrated that the A subunit bound calmodulin, while the B subunit bound Ca2+. The presence of calcineurin in the islets of Langerhans would suggest its possible participation, as a counterpart of the kinases effect, in the regulatory mechanism of insulin secretion.

Calmodulin and insulin secretion: use of naphthalenesulfonamide compounds

Studies have been performed on the involvement of calmodulin in the control of stimulated insulin secretion. The W-compounds, which are sulfonamide-derivative calmodulin inhibitors, were used in analogue pairs to control for hydrophobic and nonspecific effects. N-(4-aminobutyl)-5-chloro-2-naphthalenesulfonamide HCl (W-13) inhibited glucose-stimulated insulin secretion from rat pancreatic islets over the concentration range appropriate for an action involving calmodulin. The control compound N-(4-aminobutyl)-2-naphthalenesulfonamide HCl (W-12) inhibited slightly only at higher concentrations. Similar differential inhibition by the analogue pairs was found using W-13 and W-12 and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide HCl (W-7) and N-(6-aminohexyl)-1-naphthalenesulfonamide HCl (W-5) against depolarization-induced insulin secretion in the RINm5F clonal beta-cell line. A sufficient explanation for the inhibition of insulin release was found in the differential inhibition of depolarization-induced Ca2+ uptake, which located the action of the W-compounds at the voltage-dependent Ca2+ channels. These data suggest an action of the W-compounds at a point before elevation of intracellular Ca2+ concentration [( Ca2+]i) in stimulus-secretion coupling. In subsequent studies, with the voltage-dependent Ca2(+)-channel agonist BAY K 8644, which increases Ca2+ uptake and the rate of insulin secretion, it was demonstrated that the W-compounds had no effect on the stimulation of 45Ca2+ uptake or insulin secretion. Thus the W-compounds inhibit stimulus-secretion coupling before the elevation of [Ca2+]i but appear to have no action subsequently. These data cast doubt on the hypothesis that calmodulin is involved in stimulus-secretion coupling.

Modulation of pp60c-src tyrosine kinase activity during secretion in stimulated bovine adrenal chromaffin cells

High levels of the proto-oncogene product, pp60c-src, have been found in developing and adult neural tissues as well as in certain fully mature cells of the hematopoietic lineage, e.g., platelets and myelomonocytes. Adrenal medullary chromaffin cells exhibit characteristics of both types of cells, i.e., they are derived from the neural crest and carry out exocytosis in response to specific stimuli. Earlier studies have shown that pp60c-src localizes not only to the plasma membrane of chromaffin cells but also to the membranes of chromaffin granules, the secretory vesicles of these cells that store catecholamines and other secretory products. To investigate the possible involvement of pp60c-src in exocytosis, cultured bovine chromaffin cells were analyzed for changes in c-src tyrosine kinase activity in response to stimulation by several secretagogues. Results of in-vitro immune complex kinase assays showed that pp60c-src, derived from cells that had been stimulated for various lengths of time, exhibited decreased auto- and transphosphorylating activities as compared to pp60c-src immunoprecipitated from control cells. The greatest reduction in activity was observed 10 min post-stimulation, while normal levels were regained 2-6 hr after secretagogue treatment. Western immunoblot analysis of the immunoprecipitated pp60c-src revealed that approximately 50% less c-src protein was present in immune complexes prepared 10 min after stimulation as compared to those prepared from mock-stimulated controls, resulting in a specific autophosphorylating activity that was 42-47% of control and little or no reduction in the transphosphorylating specific activity. In experiments in which the rate of secretion of [3H]-norepinephrine from cells preloaded with this compound was compared to the rate of modulation of pp60c-src activity, 50% of the maximal reduction in pp60c-src activity occurred within 2-4 min while 50% maximal release of [3H]-norepinephrine occurred within 1-3 min. Taken together, these results suggest that pp60c-src may play some role (direct or indirect) in the exocytotic process.

CGS 9343B and W7 (calmodulin antagonists) inhibit KCl-induced increase in cytosolic free calcium and insulin secretion of RINm5F cells

CGS 9343B:1,3-Dihydro-1-[1-[4-methyl-4H,6H-pyrrolo[1,2-a]-[4,1] benzoxazepin-4-yl)methyl)-4-piperidinyl]-2H-benzimidazol-2-o ne maleate and W7:N-6(aminohexyl)-5-chloro-1-naphthalenesulfonamide) are calmodulin antagonists with different specificities. The effects of CGS 9343B and W7 on cytosolic free calcium concentration ([ Ca2+]i) and insulin release were investigated in rat insulinoma cells (RINm5F). As measured with the Quin-2 technique, preincubation with CGS 9343B (0.3-10 microM) and W7 (5-50 microM) concentration dependently decreased KCl (25 mM)-mediated accumulation of cytosolic calcium. Both, CGS 9343B (10 microM) and W7 (50-100 microM) almost abolished the alanine- and KCl-induced increase in [Ca2+]i and significantly inhibited KCl (25 mM)- and alanine (10 mM)-mediated insulin release. W5 (100 microM), the chlorine-deficient analogue of W7 with decreased affinity for calmodulin, did not inhibit the KCl-induced increase in [Ca2+]i and enhanced basal and KCl-mediated insulin release by 56% and 189%, respectively. Our data suggest that CGS 9343B and W7 inhibit the depolarization-induced calcium uptake and subsequent increase in [Ca2+]i.

The use of fluorescein diacetate and ethidium bromide as a viability stain for isolated islets of Langerhans

There is a need for a simple, rapid, sensitive method for assessing the viability of isolated islets of Langerhans. In this study the fluorescent dyes fluorescein diacetate (FDA) and ethidium bromide (EB) have been used to provide a viability assay for isolated rat islets. Discrimination of living from dead islets is efficient; in a blind sorting experiment using freshly isolated islets and islets killed by either heat or alcohol, viability determined by insulin secretion in response to glucose stimulation correlated well with viability as determined by FDA/EB staining. Furthermore, it is possible to discriminate degrees of viability, and a scoring system is described for this purpose which is shown to correlate with another index of viability, the ATP content. A reliable viability stain should not itself be toxic; FDA/EB stained islets remain viable after staining, showing normal response to glucose stimulation and normal function after transplantation.

Energy metabolism in rat mast cells in relation to histamine secretion

1. The relation between the energy metabolism and the secretory activity of rat peritoneal mast cells has been studied by determination of the cellular content of ATP and the rate of lactate production reflecting the rate of ATP synthesis under various experimental conditions. Secretion of histamine was induced by the antigen-antibody reaction, the polymeric amine compound 48/80, and the divalent ionophore A23187. 2. In presence of low concentrations of metabolic inhibitors (oligomycin or antimycin A) a linear relation between the secretion of histamine induced by all three liberators and the cellular ATP content at the time of cell activation was demonstrated. This may indicate a direct link between ATP and the secretory mechanism. 3. The possibility of an increased utilization of ATP during histamine secretion was explored in mast cells exposed to metabolic inhibitors. Incubation of mast cells with 2-deoxyglucose (2-DG) decreased the ATP content of the cells, and a long-lasting and stable level of mast cell ATP was observed. This is explained by a small decrease in the rate of ATP-synthesis by 2-DG. In 2-DG-treated cells secretion of histamine in response to compound 48/80 or the antigen-antibody reaction could still occur, and the secretion is shown to be associated with a decrease of the cellular ATP level. This ATP decrease indicates that secretion occurs by an ATP-requiring mechanism, that causes an increased cellular utilization of ATP in association with the secretory response. 4. The possibility has been considered that increased cytosol concentration of calcium in association with secretion may decrease the rate of ATP-synthesis due to accumulation of calcium by the mitochondria. This possibility can be excluded by the observation that in presence of respiratory inhibitors an identical ATP decrease was found during the secretion as in the case of the glycolytic inhibitor alone. This indicates that increased utilization of ATP by the cells was related directly to the secretory response. This observation was made with all three secretagogues. 5. A quantitative evaluation of the increased amount of ATP utilized by the cells in relation to the secretory response was performed by use of metabolic inhibitors. The ATP requirement of compound 48/80-induced secretion was 0.15 pmol/10(3) cells. This was observed when the ATP-synthesis was decreased to a negligible value.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+-dependent phosphorylation by endogenous kinases of Mr 95 K and 50 K-55 K proteins in PC12 pheochromocytoma cells

Endogenous protein phosphorylation of PC12 cells was investigated with the homogenate as well as intact cells. In the case of the homogenate, the major proteins that were phosphorylated in the presence of Ca2+ were found to be of Mr 95 K and Mr 50 K-55 K. Ca2+/calmodulin-dependent protein kinase appeared to be responsible for phosphorylation of Mr 50 K-55 K proteins and partly of Mr 95 K protein. The apparent Km's for Ca2+ of Mr 95 K and 50 K-55 K protein phosphorylation were 2.2 x 10(-7) M and around 1.5 x 10(-6) M, respectively. Since several cell lines of neuroblastoma exhibited Mr 95 K protein phosphorylation of similar type, the protein phosphorylation may be a common process shared by neuronal cells. Depolarization of intact PC12 cells by high K+ concentrations induced Mr 95 K protein phosphorylation. The results suggest that a physiological increase by excitation in the intracellular Ca2+ concentration triggers phosphorylation of Mr 95 K protein in neuronal cells and this phosphorylation may play a role in the regulation of transmitter release.

Glucose-induced phospholipid-dependent protein phosphorylation in neonatal rat islets

The participation of calcium-activated, phospholipid-dependent protein kinase in the phosphorylation of endogenous islet proteins following the exposure of cultured, neonatal pancreatic islets to stimulatory glucose concentrations was investigated by two techniques. In the first technique, islets were prelabeled with 32Pi. The major endogenous substrates for glucose-induced phosphorylation had apparent molecular masses of 130,100 +/- 1010, 100,000 +/- 700, 80,400 +/- 890, 58,100 +/- 1200, 39,800 +/- 700, and 29,400 +/- 700 Da. In the presence of 12-O-tetradecanoylphorbol 13-acetate (2 microM), an activator of calcium-activated phospholipid-dependent kinase, there was enhanced phosphorylation of proteins of 80,000, 40,000, and 29,000 Da. In the second technique, exogenous phosphorylation by [gamma-32P]ATP of proteins in a postnuclear particulate fraction was studied in the presence and absence of cofactors for Ca2+-activated, phospholipid-dependent protein kinase (Ca2+, phosphatidylserine, and unsaturated diolein). These studies were performed in islets preexposed to low (1.7 mM) or high (16.7 mM) glucose concentration prior to preparation of the postnuclear particulate fraction. Following exposure of islets to low glucose concentration, three substrates (apparent molecular masses 40,500 +/- 600, 57,100 +/- 700, and 79,400 +/- 600 Da) in the postnuclear particulate fraction exhibited enhanced phosphorylation in the presence of calcium ions, phosphatidylserine, and unsaturated diolein. In preparations of islets preexposed to 16.7 mM glucose, the phosphorylation of the protein of molecular mass about 40,000 Da was significantly reduced, indicating prior phosphorylation of the acceptor sites on this substrate in response to glucose exposure. It is concluded that stimulation of neonatal cultured islets by glucose induces the acute changes in calcium ion, phospholipid, and diacylglycerol concentration required to activate the calcium-activated phospholipid-dependent protein kinase and that the islet postnuclear particulate fraction contains at least one specific substrate for this kinase.

An inhibitory role for polyamines in protein kinase C activation and insulin secretion in mouse pancreatic islets

The occurrence and function of polyamines in protein kinase C activation and insulin secretion in mouse pancreatic islets were studied. Determination of polyamines in mouse islets revealed 0.9 +/- 0.3 (mean +/- S.E.M., n = 6) pmol of putrescine, 11.7 +/- 3.2 (8) pmol of spermidine and 3.7 +/- 0.6 (8) pmol of spermine per islet, corresponding to intracellular concentrations of 0.3-0.5 mM-putrescine, 3.9-5.9 mM-spermidine and 1.2-1.9 mM-spermine in mouse islets. Stimulation of insulin secretion by glucose, the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) or the sulphonylurea glibenclamide did not affect these polyamine contents. In accordance with a role for protein kinase C in insulin secretion, TPA stimulated both protein kinase C activity and insulin secretion. Stimulation of insulin secretion by TPA was dependent on a non-stimulatory concentration of glucose and was further potentiated by stimulatory concentrations of glucose, glibenclamide or 3-isobutyl-1-methylxanthine, suggesting that protein kinase C activation, Ca2+ mobilization and cyclic AMP accumulation are all needed for full secretory response of mouse islets. Spermidine (5 mM) and spermine (1.5 mM) at concentrations found in islets inhibited protein kinase C stimulated by TPA + phosphatidylserine by 55% and 45% respectively. Putrescine (0.5 mM) was without effect, but inhibited the enzyme at higher concentrations (2-10 mM). Inhibition of protein kinase C by polyamines showed competition with Ca2+, and Ca2+ influx in response to glucose or glibenclamide prevented inhibition of insulin secretion by exogenous polyamines at concentrations where they did not affect glucose oxidation. It is suggested that inhibition of protein kinase C by polyamines may be of significance for regulation of insulin secretion in vivo and that Ca2+ influx may function by displacing inhibitory polyamines bound to phosphatidylserine in membranes.

A role for transglutaminase in glucose-stimulated insulin release from the pancreatic beta-cell

Preincubation of rat islets of Langerhans with the potent inhibitors of islet transglutaminase activity, monodansylcadaverine (30-100 microM) and N-(5-aminopentyl)-2-naphthalenesulphonamide (100-200 microM), led to significant inhibition of glucose-stimulated insulin release from islets. In contrast, the respective N'-dimethylated derivatives of these two compounds, which did not inhibit islet transglutaminase activity, were much less effective as inhibitors of glucose-stimulated insulin release. None of the compounds inhibited rat spleen protein kinase C activity at concentrations which gave rise to inhibition of glucose-stimulated insulin release. When tested for their effects on calmodulin-stimulated bovine heart phosphodiesterase activity, of the compounds that inhibited insulin release, only monodansylcadaverine did not act as an effective antagonist of calmodulin at concentrations (up to 50 microM) that gave rise to significant inhibition of glucose-stimulated insulin release. Furthermore, at 50 microM, monodansylcadaverine did not inhibit methylation of islet lipids. The inhibition of glucose-stimulated insulin release by monodansylcadaverine is therefore likely to be attributable to its interference with islet transglutaminase activity. The sensitivity of islet transglutaminase to activation by Ca2+ was investigated by using a modified assay incorporating dephosphorylated NN'-dimethylcasein as a substrate protein. The Km for Ca2+ obtained (approx. 3 microM) was an order of magnitude lower than previously reported for the islet enzyme [Bungay, Potter & Griffin (1984) Biochem. J. 219, 819-827]. Mg2+ (2 mM) was found to have little effect on the sensitivity of the enzyme to Ca2+. Investigation of the endogenous substrate proteins of islet transglutaminase by using the Ca2+-dependent incorporation of [14C]methylamine into proteins of islet homogenates demonstrated that most of the incorporated radiolabel was present in cross-linked polymeric aggregates which did not traverse 3% (w/v) acrylamide gels. The radiolabelled polymeric aggregates were present in 71 000 g-sedimented material of homogenates, and their formation was transglutaminase-mediated. These findings provide new evidence for the involvement of islet transglutaminase in the membrane-mediated events necessary for glucose-stimulated insulin release.

Calmodulin in various insulin secreting cell types

The involvement of the Ca2+ binding protein, calmodulin, in the regulation of insulin release was studied. Calmodulin was measured in isolated rat islets, rat insulinoma cells, the insulin secreting cell line (RINm5F) and in islets isolated from normal and diabetic Chinese hamsters. Total content of calmodulin was determined by a radioimmunoassay using a rabbit anti-calmodulin serum and was found to lie in the range of 4 to 7 micrograms/ml protein. When rat islets were maintained in tissue culture for 6 days at 2.8 or 8.3 mM glucose, the content of calmodulin of the two groups was similar. Likewise there was no difference in calmodulin content between islets from normal and diabetic hamsters. This study suggests that a variation of the total cellular calmodulin does not play a role in the process of insulin secretion.

Calcium calmodulin and hormone secretion

As long ago as 1970, it was proposed that Ca2+ can act as a 'second messenger' like cAMP (Rasmussen & Nagata, 1979). The recognition that calmodulin is a major Ca2+ binding protein in non-muscle cells has prompted the suggestion that calmodulin may serve an analogous role for Ca2+ to that served by protein kinase for cAMP (Wang & Waisman, 1979), or at least to the regulatory subunit of the cyclic nucleotide-dependent kinases. It is becoming clear that calmodulin probably does play a role in stimulus secretion coupling in endocrine cells. Nevertheless, some of the experimental approaches which have led to this rather tentative conclusion do induce some doubts, as we have attempted to indicate. Many of the pharmacological agents used in the studies cited in this review are not specific in their interaction with calmodulin. For example, the phenothiazines also inhibit phospholipid-sensitive protein kinase. The introduction of more specific drugs, such as the naphthalene sulphonamides, may lead to a clearer picture of the role of calmodulin in hormone secretion. Relationships probably exist between cyclic nucleotides, calcium, calmodulin, phosphatidylinositol (PI) turnover and phospholipids in the overall control of the secretory process (see Fig. 1). There is considerable evidence that calcium is the primary internal signal initiating exocytosis of hormone from many glands. However, it appears that cyclic nucleotides can modulate the calcium signal either positively or negatively and it is possible that cAMP and calcium can separately activate secretion. The presence of both calmodulin-activated adenylate cyclase and cyclic nucleotide phosphodiesterase in the same tissue would appear to suggest either spatial or temporal control mechanisms or that (diagram; see text) the calcium requirement for calmodulin activation differs between the two enzymes. The true explanation is probably far more complex and involves perhaps as yet unknown factors that can differentially influence the activity of calmodulin itself in membranes and in cytosol. Berridge (1982) and Rasmussen (1980) give detailed accounts and review current hypotheses regarding relationships between the cyclic nucleotide and calcium second messenger systems. The various possible interrelationships of the putative messengers have been encompassed by the term 'Synarchic regulation' (Rasmussen, 1980). These concepts and the elucidation of the mechanisms by which cyclic AMP and calcium are involved in the control of secretion from particular cell types will make fascinating reading over the next few years.(ABSTRACT TRUNCATED AT 400 WORDS)

Newly synthesized and preformed acetylcholine are released from Torpedo synaptosomes by different pathways

In this study, we investigated the mechanisms underlying the release of preformed and of newly synthesized acetylcholine (ACh) from isolated Torpedo nerve terminals (synaptosomes). This was pursued by examining and comparing the effects of anticytoskeletal and anticalmodulin drugs and of activating the presynaptic muscarinic ACh receptors on the release of preformed endogenous ACh and of newly synthesized radiolabeled ACh. The anticytoskeletal drugs vinblastine, cytochalasin B, and colchicine inhibit the Ca2+-dependent K+-mediated release of newly synthesized radiolabeled ACh, but have no effect on the release of preformed ACh. By contrast, the muscarinic agonist oxotremorine markedly inhibits the release of preformed ACh, but has little effect on the release of newly formed ACh. Treatment of the synaptosomes with the calmodulin antagonist trifluoperazine inhibits the release of both ACh pools concomitantly. These findings show that preformed and newly synthesized ACh are released by different routes and suggest that their secretion is mediated by converging pathways. The significance of these results in view of the previously demonstrated preferential release of newly synthesized ACh is discussed.

Role of phospholipase and calmodulin inhibitors on insulin, arachidonic acid and prostaglandin E2 release

Using several experimental approaches, we have studied simultaneously the effect of glucose upon insulin, arachidonic acid and prostaglandin E2 release by rat pancreatic islets. A 16.6 mmol/l glucose concentration stimulated the release of insulin, arachidonic acid and prostaglandins. All these effects were significantly reduced either by calmodulin and phospholipase A2 inhibitors, or by the omission of calcium in the incubation medium. Phospholipase A2 inhibitors do not modify the glucose-induced net 45Ca2+ uptake by isolated islets. Our results would suggest that activation of phospholipases, particularly A2, is involved in the mechanism by which glucose stimulates insulin release. This activation increases the intracellular concentration of arachidonic acid, prostaglandins and probably phospholipid degradation products, that could act as messengers for the stimulus-secretion coupling of insulin. The calcium-calmodulin complex would take part in this effect. Conversely, the glucose-induced net calcium uptake by the islets might either be preceded by phospholipase activation or not significantly affected by the blockade of its activity.

Properties of isolated human islets of Langerhans: insulin secretion, glucose oxidation and protein phosphorylation

In the present study, human islets were isolated by collagenase digestion from the pancreases of three kidney donors. Maintainance of the islets in tissue culture enabled insulin release, glucose oxidation and Ca2+ -calmodulin-dependent protein phosphorylation to be determined using the same islets. Increasing glucose over a range 0-20 mmol/l resulted in a sigmoidal stimulation of insulin release (28.8 +/- 5.2 to 118.4 +/- 25.8 microU . islet-1 . h-1, n = 10; threshold less than 4 mmol/l). There was a marked correlation between the insulin secretory response of the islets to glucose and their rate of glucose oxidation (5.9 +/- 0.3 at glucose 2 mmol/l up to 25.8 +/- 1.8 pmol . islet-1 . h-1 at 20 mmol/l, r = 0.98). N-acetylglucosamine (20 mmol/l) failed to elicit a secretory response from the islets. Stimulation of insulin secretion by glucose was dependent upon the presence of extracellular Ca2+. Extracts of the islets contained a Ca2+ -calmodulin-dependent protein kinase which phosphorylated a 48-kdalton endogenous polypeptide. Myosin light-chain kinase activity was demonstrated in the presence of exogenous myosin light chains. This report demonstrates for the first time the sigmoidal nature of glucose-stimulated insulin release from isolated human islets, and its correlation with enhanced glucose oxidation. Furthermore, this is the first report of the presence of Ca2+ -dependent protein kinases in human islets.

Depolarisation-dependent protein phosphorylation in rat cortical synaptosomes is inhibited by fluphenazine at a step after calcium entry

The sequence of molecular events linking depolarisation-dependent calcium influx to calcium-stimulated protein phosphorylation is unknown. In this study the effect of the neuroleptic drug fluphenazine on depolarisation-dependent protein phosphorylation was investigated using an intact postmitochondrial pellet isolated from rat cerebral cortex. Fluphenazine, in a dose-dependent manner, completely inhibited the increases in protein phosphorylation observed previously. The concentration of fluphenazine required for 50% inhibition varied for different phosphoproteins but for synapsin I was 123 microM. Other neuroleptics produced effects similar to fluphenazine with their order of potency being thioridazine greater than haloperidol greater than trifluoperazine greater than fluphenazine greater than chlorpromazine. Fluphenazine also increased the phosphorylation of proteins in nondepolarised controls at concentrations of 20 and 60 microM. The inhibition of depolarisation-dependent phosphorylation was apparently not due to a loss of synaptosomal integrity or viability, a decrease in calcium uptake, a change in substrate availability, or to a change in protein phosphatase activity. The data are most consistent with an inhibition of protein kinase activity by blockade of calmodulin or phospholipid activation.

Activation of protein kinase C by a tumor-promoting phorbol ester in pancreatic islets

Rat pancreatic islet homogenates display protein kinase C activity. This phospholipid-dependent and calcium-sensitive enzyme is activated by diacylglycerol or the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). In the presence of TPA, the Ka for Ca2+ is close to 5 microM. TPA does not affect phosphoinositide turnover but stimulates [32P]- and [3H]choline-labelling of phosphatidylcholine in intact islets. Exogenous phospholipase C stimulates insulin release, in a sustained and glucose-independent fashion. The secretory response to phospholipase C persists in media deprived of CaCl2. It is proposed that protein kinase C participates in the coupling of stimulus recognition to insulin release evoked by TPA, phospholipase C and, possibly, those secretatogues causing phosphoinositide breakdown in pancreatic islets.

Studies on rat parotid-cell actomyosin

Actomyosin was partially purified from rat parotid cells dispersed by collagenase digestion and found to possess different solubility characteristics from that from (undispersed) rat parotid tissue. This is attributed to the decrease in vascular contamination effected by the isolation of parotid cells, yielding a non-muscle actomyosin [Adelstein, Conti, Johnson, Pastan & Pollard (1972) Proc. Natl. Acad. Sci. U.S.A. 69, 3693-3697]. Myosin light-chain kinase was partially purified from dispersed rat parotid cells by calmodulin affinity chromatography and shown to be activated by Ca2+-calmodulin. The calmodulin content of dispersed rat parotid cells was shown to be 6.50 +/- 0.59 ng of calmodulin/micrograms of rat parotid-cell protein (mean +/- S.E.M.), as determined by the activation of purified bovine brain phosphodiesterase by heat-treated extracts of dispersed rat parotid cells.

Identification and characterization of Ca2+-phospholipid-dependent protein kinase in rat islets and hamster beta-cells

We show that extracts of rat islets of Langerhans and of cloned hamster beta-cells (HIT-T15 cells) contain Ca2+-phospholipid-dependent protein kinase and endogenous protein substrates for the kinase. We purified Ca2+-phospholipid-dependent protein kinase from HIT-T15 beta-cells and report here its physical and kinetic properties.

Ca2+-sensitive phosphatidylinositol 4-phosphate metabolism in a rat beta-cell tumour

Subcellular fractions were isolated from a rat beta-cell tumour by centrifugation of homogenates on Percoll and Urografin density gradients. Fractions were incubated with [gamma-32P]ATP, and labelling of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was used to measure phosphatidylinositol kinase and phosphatidylinositol 4-phosphate kinase activities, respectively. The distribution of enzyme markers in density gradients indicated that phosphatidylinositol kinase was located in both the plasma membrane and the secretory-granule membrane. Phosphatidylinositol 4-phosphate kinase activity was low in all fractions. Phosphatidylinositol kinase activity of secretory granules and plasma membranes was decreased to 10-20% of its initial value by raising the free [Ca2+] from 1 microM to 5 microM. The enzyme had a Km (apparent) for ATP of 110 microM (secretory granule) or 120 microM (plasma membrane) and a Ka for Mg2+ of 7 mM (secretory granule) or 6 mM (plasma membrane). Ca2+-sensitivity of phosphatidylinositol kinase in calmodulin-depleted secretory granules and plasma membranes was not affected by addition of exogenous calmodulin, although activity was stimulated by trifluoperazine in the presence of 0.1 microM or 40 microM-Ca2+. Trifluoperazine oxide had no effect on the enzyme activity of secretory granules. Plasma membranes had a phosphatidylinositol 4-phosphate phosphatase activity which was stimulated by raising the free [Ca2+] from 0.1 to 40 microM. The secretory granule showed no phosphatidylinositol 4-phosphate-degrading activity. These results suggest the presence in the tumour beta-cell of Ca2+-sensitive mechanisms responsible for the metabolism of polyphosphoinositides in the secretory granule and plasma membrane.

Calcium-dependent protein phosphorylation in bovine anterior pituitary membranes and intact cells

A calmodulin-activated protein kinase has been identified in bovine anterior pituitary membranes. This enzyme phosphorylated one endogenous substrate of subunit molecular weight 53,000 in the membranes. Phosphorylation of this protein was rapid, was half-maximal at 2.5 microM calcium in the presence of saturating concentrations of calmodulin (CaM), and was inhibited by trifluoperazine and thioridazine. A second protein was phosphorylated by an endogenous protein kinase in anterior pituitary membranes. Phosphorylation of this 42,000 Mr protein was reduced by calcium, was independent of exogenously added CaM, and was increased by trifluoperazine or thioridazine. The 42,000 Mr protein may be the alpha-subunit of pyruvate dehydrogenase. Calcium-dependent protein phosphorylation was also observed in intact cells; the largest increases were seen in proteins of Mr 42,000, 21,000 and 17,000.

Cyclic AMP-dependent protein phosphorylation and insulin secretion in intact islets of Langerhans

Effects on insulin release, cyclic AMP content and protein phosphorylation of agents modifying cyclic AMP levels have been tested in intact rat islets of Langerhans. Insulin release induced by glucose was potentiated by dibutyryl cyclic AMP, glucagon, cholera toxin and 3-isobutyl-1-methylxanthine (IBMX); the calmodulin antagonist trifluoperazine reversed these potentiatory effects. Inhibition by trifluoperazine of IBMX-potentiated release was, however, confined to concentrations of IBMX below 50 microM; higher concentrations, up to 1 mM, were resistant to inhibition by trifluoperazine. IBMX-potentiated insulin release was also inhibited by 2-deoxyadenosine, an inhibitor of adenylate cyclase. In the absence of glucose, IBMX at concentrations up to 1 mM did not stimulate insulin release and in the presence of 3.3 mM-glucose IBMX was effective only at a concentration of 1 mM; under the latter conditions trifluoperazine again did not inhibit insulin secretion. The maximum effect on insulin release was achieved with 25 microM-IBMX. Islet [cyclic AMP] was increased by IBMX, with the maximum rise occurring with 100 microM-IBMX. The increase in [cyclic AMP] elicited by IBMX was more rapid than that induced by cholera toxin. Trifluoperazine did not significantly affect islet cyclic AMP levels under any of the conditions tested. When islets were incubated with [32P]Pi, radioactivity was incorporated into islet ATP predominantly in the gamma-position. The rate of equilibration of label was dependent on medium Pi and glucose concentration and at optimal concentrations of these 100% equilibration of internal [32P]ATP with external [32P]Pi required a period of 3h. Radioactivity was incorporated into islet protein and, in response to an increase in islet [cyclic AMP], the major effect was on a protein of Mr 15 000 on sodium dodecyl sulphate/polyacrylamide gels. The extent of phosphorylation of the Mr-15 000 protein was correlated with the level of cyclic AMP: phosphorylation in response to IBMX was inhibited by 2-deoxyadenosine but not by trifluoperazine. Fractionation of islets suggested that the Mr-15 000 protein was of nuclear origin: the protein co-migrated with histone H3 on acetic acid/urea/Triton gels. In the islet cytosol a number of proteins were phosphorylated in response to elevation of islet [cyclic AMP]: the major species had Mr values of 18 000, 25 000, 34 000, 38 000 and 48 000. Culture of islets with IBMX increased the rate of [3H]-thymidine incorporation.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential control of insulin secretion and somatostatin-receptor recruitment in isolated pancreatic islets

Somatostatin receptors appear to be localized to secretory granules in pancreatic islet homogenates. Recruitment of these receptors to the islet-cell surfaces may mark the contact event between secretory granules and plasma membranes before release of insulin by fission. Isethionate, an impermeant anionic replacement for chloride, blocks the release step but does not affect receptor recruitment. By contrast, low concentrations of phenothiazine drugs, such as trifluoperazine and promethazine, inhibit both receptor recruitment and secretion. Scatchard analysis of phenothiazine effects on somatostatin receptors reveals that these drugs reduce the number of receptors but do not affect the affinity of the receptor for somatostatin. These data indicate that membrane contact and fission steps during exocytosis can be biochemically separated.

Correlation of Ca2+-and calmodulin-dependent protein kinase activity with secretion of insulin from islets of Langerhans

A Ca2+-activated and calmodulin-dependent protein kinase activity which phosphorylates predominantly two endogenous proteins of 57kDa and 54kDa was found in a microsomal fraction from islet cells. Half-maximal activation of the protein kinase occurs at approx. 1.9 microM-Ca2+ and 4 micrograms of calmodulin/ml (250 nM) for phosphorylation of both protein substrates. Similar phosphoprotein bands (57kDa and 54kDa) were identified in intact islets that had been labelled with [32P]Pi. Islets prelabelled with [32P]Pi and incubated with 28 mM-glucose secreted significantly more insulin and had greater incorporation of radioactivity into the 54 kDa protein than did islets incubated under basal conditions in the presence of 5 mM-glucose. Thus the potential importance of the phosphorylation of these proteins in the regulation of insulin secretion is indicated both by activation of the protein kinase activity by physiological concentrations of free Ca2+ and by correlation of the phosphorylation of the substrates with insulin secretion in intact islets. Experiments undertaken to identify the endogenous substrates indicated that this calmodulin-dependent protein kinase may phosphorylate the alpha- and beta-subunits of tubulin. These findings suggest that Ca2+-stimulated phosphorylation of islet-cell tubulin via a membrane-bound calmodulin-dependent protein kinase may represent a critical step in the initiation of insulin secretion from the islets of Langerhans.

Secretagogue effect of barium on output of melanocyte-stimulating hormone from pars intermedia of the mouse pituitary

Ba ions caused an intense and prolonged discharge of melanocyte-stimulating hormone (MSH) from perifused neurointermediate lobes of mouse pituitaries and dispersed pars intermedia cells. The effect persisted in chronically cultured lobes or cells. It did not require Ca, but, like the Ca-dependent response to excess K, was blocked by cyanide combined with glucose lack. The secretagogue effect of Ba was blocked or prevented by Co or by excess Ca, both of which can reduce inward Ba currents through Ca channels. Prior exposure to excess K partially reduced the secretagogue effect of Ba, suggesting that depolarization caused some inactivation of Ba current. In contrast to Ba, excess K elicited secretion that was transient, and prior exposure of preparations to excess K (in the absence of Ca) profoundly suppressed the secretagogue effect of Ca. The evidence is consistent with the view that inward Ca current rapidly inactivates in these cells. It is concluded that Ba ions have a potent and persistent direct secretagogue effect on the melanotrophs that may reflect, in part, their ability to penetrate Ca channels more easily than Ca ions. The strong secretagogue effects of Ba on melanotrophs may be of considerable utility in studies on MSH secretion since a physiological secretagogue has yet to be discovered. Moreover, since the responses of melanotrophs (and other endocrine cells) to Ba can be distinguished from those of various other secretory cells and neurones, it is suggested that Ba may provide a tool for characterizing the distinctive membrane properties of the Ba-responsive endocrine cells.

Modulation of active Ca2+ uptake by the islet-cell endoplasmic reticulum

The possible effects of calmodulin and cyclic AMP on active Ca2+ uptake by the islet-cell endoplasmic reticulum were investigated. Neither calmodulin nor cyclic AMP affected the rate of active Ca2+ uptake, or the steady-state filling capacity of the endoplasmic reticulum when measured in the absence of oxalate. Consistent with these results, calmodulin did not activate the Ca2+-stimulated ATPase activity associated with this cell fraction. During the course of these experiments., it was unexpectedly discovered that the rate of Ca2+ uptake, as well as the steady-state Ca2+ filling capacity of the endoplasmic reticulum, were markedly increased by unidentified factor(s) in the cytosol. This effect could be demonstrated by reconstitution of the membranes in cytosol, or by direct addition of fresh or dialysed cytosol to the Ca2+ uptake assays. The degree of activation by the cytosol indicates that the endoplasmic reticulum may play a prominent role in controlling beta-cell Ca2+ concentrations and that the unidentified activator(s) present in the cytosol may be involved in regulation of this function.

Calcium/calmodulin-dependent phosphorylation of vimentin in rat sertoli cells

Ca2+-dependent protein phosphorylation and the role of calmodulin in this process was investigated in subcellular fractions of primary cultures of rat Sertoli cells. Significant Ca2+/calmodulin-dependent protein phosphorylation in Sertoli cells was restricted to the cytosol fraction. The calmodulin dependence of these effects was confirmed by using the calmodulin inhibitor trifluoperazine. One of the Ca2+/calmodulin-dependent phosphoproteins was identified as the intermediate filament protein vimentin, based on the following criteria: (i) migration pattern in two-dimensional polyacrylamide gels, (ii) Ca2+/calmodulin-dependent phosphorylation of a 58-kilodalton protein present in detergent-insoluble intermediate filament protein extract of Sertoli cells, and (iii) peptide mapping of the phosphoprotein. These data support a role for Ca2+/calmodulin-dependent protein phosphorylation in the modulation of Sertoli cell cytoskeletal components.

Alterations of the cytoplasmic organization of WIRL cells induced by trifluoperazine

The antipsychotic drug trifluoperazine (TFP) causes a reversible rounding of cells of the rat liver epithelial cell line, WIRL. We have investigated the cytoplasmic organization of these cells after TFP treatment using SEM, TEM and immunofluorescence and have observed significant differences between the control and treated cells. Mitochondria are converted to the condensed configuration with distended cristae and the endoplasmic reticulum becomes tubular with distended cisternae. Intermediate filaments, visualized with a monoclonal antibody, are aggregated to a cap on the nucleus in an arrangement different from that induced by colcemid.

Dissection of stages in exocytosis in the adrenal chromaffin cell with use of trifluoperazine

Stimulation of isolated chromaffin cells with carbamylcholine led to a number of morphological changes, indicative of exocytosis, apparently resulting from translocation of secretory granules to the plasma membrane and their subsequent fusion with the plasma membrane to release their contents. However, stimulation in the presence of trifluoperazine resulted only in the accumulation of secretory granules close to the plasma membrane. Thus exocytosis could be divided into two stages: a trifluoperazine-insensitive stage involving translocation of secretory granules to the plasma membrane and a second trifluoperazine-sensitive stage resulting in granule-plasma membrane fusion.

Differential effects of Ca2+-calmodulin on adenylate cyclase activity cyclase activity in mouse and rat pancreatic islets

The effects of Ca2+-calmodulin on adenylate cyclase activity in EGTA-washed, 27000 g particulate fractions of mouse and rat pancreatic islets were studied. Ca2+ (10 microM)-calmodulin (1 microM) stimulated adenylate cyclase activity 53.1 +/- 5.2 (N = 6)% in the particulate fraction of rat islets. Trifluoperazine (50 microM), a specific inhibitor of calmodulin, inhibited the Ca2+-calmodulin activation of the adenylate cyclase activity of this fraction of rat islets. These results confirm previous reports dealing with Ca2+-Calmodulin and rat islet adenylate cyclase [Valverde, Vandermeers. Anjaneyulu & Malaisse (1979) Science 206, 225-227; Sharp, Wiedenkeller, Kaelin, Siegel & Wollheim (1980) Diabetes 29, 74-77]. In contrast, however, Ca2+ (1-100 microM)-calmodulin (1-10 microM) did not stimulate the adenylate cyclase activity in the EGTA-washed particulate fraction of mouse islets, and trifluoperazine (50 microM) did not inhibit the adenylate cyclase activity of this fraction of mouse islets, although some remaining calmodulin [0.18 +/- 0.05 (n = 3) microgram/mg of protein] could be demonstrated. GTP (10 microM) enhanced islet adenylate cyclase activity considerably, but did not confer any sensitivity towards Ca2+-calmodulin on mouse islet adenylate cyclase. The results question the role of calmodulin in the Ca2+-dependent rise in cyclic AMP evoked by glucose in pancreatic islets.

Interaction of drugs with calmodulin. Biochemical, pharmacological and clinical implications

Calmodulin is a widely distributed, highly active, calcium-binding protein that influences a number of important biological events. Accordingly, agents that inhibit the activity of calmodulin should have profound pharmacological effects. Within the past few years, a number of compounds have been identified that inhibit calmodulin. The most potent of these described so far include certain antipsychotic drugs, smooth muscle relaxants, alpha-adrenergic blocking agents and neuropeptides. Studies of the physicochemical and structural properties of a variety of calmodulin inhibitors have shown that there are ionic and hydrophobic interactions between the drug and calmodulin. From the limited studies conducted so far, we conclude that, for a compound to inhibit calmodulin, it should carry a positive charge at physiological pH, presumably to interact with negative charges on the highly acidic calmodulin, and have hydrophobic groups, presumably to interact with lipophilic regions on calmodulin. But these two factors are not the only ones that are involved in inhibiting calmodulin, for many highly charged and highly hydrophobic agents have relatively little effect on calmodulin activity. The structural relationships between these ionic and hydrophobic regions and other, as yet identified, factors are also important. Many of the biochemical actions of the phenothiazine antipsychotic agents can be explained by the common mechanism of their binding to, and inhibiting, calmodulin. The question of whether these biochemical actions can explain their pharmacological and clinical effects is still unclear. The fundamental role calmodulin plays in biology suggests that this calcium binding protein may provide a new site for the pharmacological manipulation of biological activity. The calmodulin inhibitors described thus far hardly scratch the surface of this fertile area of research.