Involvement of novel protein kinase C isoforms in carbachol-stimulated insulin secretion from rat pancreatic islets

The roles of protein kinase C (PKC) isoforms in cholinergic potentiation of glucose-induced insulin secretion were investigated in rat pancreatic islets. Western-blot analysis showed the presence of PKC-alpha, betaII, delta, epsilon, eta, and zeta, but not PKC-betaI, gamma, or iota, in the islets. Carbachol (CCh) caused translocations of PKC-alpha, betaII, delta, and epsilon from the cytosol to the plasma membrane. CCh facilitated 7-mM glucose-induced insulin secretion from isolated rat islets. The CCh-stimulated insulin secretion was significantly suppressed by the generic PKC inhibitor chelerythrine. In contrast, Go 6976, an inhibitor of conventional PKC isoforms, had no effect on the insulin secretion stimulated by CCh, although it significantly inhibited that induced by phorbol 12-myristate 13-acetate. These results suggest that the novel PKC isoforms activated by CCh, i.e., PKC-delta and/or epsilon, participate in the stimulatory effect of CCh on insulin secretion.     

Activation of protein kinase C is essential for sustained insulin secretion in response to cholinergic stimulation

Insulin secretion from isolated rat islets of Langerhans is enhanced by cholinergic agonists, such as carbachol (CCh), in the presence of a stimulatory concentration of glucose. Depletion of islet protein kinase C activity by prolonged exposure to a tumour-promoting phorbol ester did not prevent the initial secretory response to CCh, but markedly reduced the duration of CCh-induced elevated secretory rates. These results suggest that the major action of PKC is in maintaining rather than initiating the insulin secretory response to cholinergic agonists.     

Glucose-stimulated insulin secretion is not dependent on activation of protein kinase A

The involvement of cyclic AMP-dependent protein kinase A (PKA) in the exocytotic release of insulin from rat pancreatic islets was investigated using the Rp isomer of adenosine 3',5'-cyclic phosphorothioate (Rp-cAMPS). Preincubation of electrically permeabilised islets with Rp-cAMPS (1 mM, 1 h, 4 degrees C) inhibited cAMP-induced phosphorylation of islet proteins of apparent molecular weights in the range 20-90 kDa, but did not affect basal (50 nM Ca2+) nor Ca2(+)-stimulated (10 microM) protein phosphorylation. Similarly, Rp-cAMPS (500 microM) inhibited both cAMP- (100 microM) and 8BrcAMP-induced (100 microM) insulin secretion from electrically permeabilised islets without affecting Ca2(+)-stimulated (10 microM) insulin release. In intact islets, Rp-cAMPS (500 microM) inhibited forskolin (1 microM, 10 microM) potentiation of insulin secretion, but did not significantly impair the insulin secretory response to a range of glucose concentrations (2-20 mM). These results suggest that cAMP-induced activation of PKA is not essential for either basal or glucose-stimulated insulin secretion from rat islets.     

Parallel effects of arachidonic acid on insulin secretion, calmodulin-dependent protein kinase activity and protein kinase C activity in pancreatic islets.

A potential role of arachidonic acid in the modulation of insulin secretion was investigated by measuring its effects on calmodulin-dependent protein kinase and protein kinase C in islet subcellular fractions. The results were interpreted in the light of arachidonic acid effects on insulin secretion from intact islets. Arachidonic acid could replace phosphatidylserine in activation of cytosolic protein kinase C (K0.5 of 10 microM) and maximum activation was observed at 50 microM arachidonate. Arachidonic acid did not affect the Ca2+ requirement of the phosphatidylserine-stimulated activity. Arachidonic acid (200 microM) inhibited (greater than 90%) calmodulin-dependent protein kinase activity (K0.5 = 50-100 microM) but modestly increased basal phosphorylation activity (no added calcium or calmodulin). Arachidonic acid inhibited glucose-sensitive insulin secretion from islets (K0.5 = 24 microM) measured in static secretion assays. Maximum inhibition (approximately 70%) was achieved at 50-100 microM arachidonic acid. Basal insulin secretion (3 mM glucose) was modestly stimulated by 100 microM arachidonic acid but in a non-saturable manner. In perifusion secretion studies, arachidonic acid (20 microM) had no effect on the first phase of glucose-induced secretion but nearly completely suppressed second phase secretion. At basal glucose (4 mM), arachidonic acid induced a modest but reproducible biphasic insulin secretion response which mimicked glucose-sensitive secretion. However, phosphorylation of an 80 kD protein substrate of protein kinase C was not increased when intact islets were incubated with arachidonic acid, suggesting that the small increases in insulin secretion seen with arachidonic acid were not mediated by protein kinase C. These data suggest that arachidonic acid generated by exposure of islets to glucose may influence insulin secretion by inhibiting the activity of calmodulin-dependent protein kinase but probably has little effect on protein kinase C activity.     

Protein kinase C isoform specificity of cholinergic potentiation of glucose-induced pulsatile 5-HT/ insulin release from mouse pancreatic islets

Thymeleatoxin (TMX), an activator of Ca2+-sensitive protein kinase C (cPKC) isoforms, was used to assess the PKC isoform specificity of cholinergic potentiation of glucose (11 mM)-induced pulsatile 5-HT/insulin release (PIR) from single mouse pancreatic islets. TMX (100 nM) and carbachol (Cch, 50 microM) enhanced PIR approximately 3-fold while reducing the underlying [Ca2+]i oscillations (duration and amplitude) by approximately 40-50%. Both effects were ablated by the specific PKC inhibitor bisindolylmaleimide and chronic TMX pretreatment. Cch also evoked an initial transient [Ca2+]i rise and surge of 5-HT release, which remained unaffected by chronic TMX pretreatment. It is concluded that the immediate cholinergic responses are insensitive to cPKC. In contrast, specific activation of a cPKC isoform mediates sustained cholinergic potentiation of glucose-induced insulin secretion.     

Effects of combined secretagogues and extracellular calcium on neonatal insulin release

The insulin response of 3-day old neonatal rat islets was evaluated following a 1 h incubation with glucose alone and in the presence of 30 nM sulfated cholecystokinin octapeptide (CCK) and/or 20 microM carbachol (CCh). Insulin secretion was found to be incrementally increased from the lowest glucose concentration and enhanced several fold in the presence of CCK and/or CCh. In combination, CCK and CCh increased glucose-stimulated insulin secretion by an amount equivalent to the sum of their individual increases. The presence of either CCK alone or CCK plus CCh increased phosphoinositide hydrolysis by the same relative amount that they increased insulin secretion when compared to 8.3 mM glucose. Glucose-stimulated insulin secretion was totally inhibited when calcium was omitted from the incubation buffer; this effect was partially negated by CCK alone and more so by CCK combined with CCh. Insulin secretion in response to 8.3 mM glucose alone was unchanged when calcium in the incubation buffer was increased from 1 to 5 mM; however, the insulin response to 16.7 mM glucose alone and 8.3 mM glucose in the presence of CCK and/or CCh was increased under this condition. Thus, we have shown that, even at 3 days postpartum, insulin secretion from isolated islets is a complex response capable of being molded by several secretagogues at once and ultimately determined by interplay of different signaling systems activated.     

Difference in mechanism between glyceraldehyde- and glucose-induced insulin secretion from isolated rat pancreatic islets

The effects of D-glyceraldehyde and glucose on islet function were compared in order to investigate the difference between them in the mechanism by which they induce insulin secretion. The stimulation of insulin secretion from isolated rat islets by 10 mM glyceraldehyde was not completely inhibited by either 150 microM diazoxide (an opener of ATP-sensitive K(+) channels) or 5 microM nitrendipine (an L-type Ca(2+)-channel blocker), whereas the stimulation of insulin secretion by 20 mM glucose was completely inhibited by either drug. The insulin secretion induced by glyceraldehyde was less augmented by 100 microM carbachol (a cholinergic agonist) than that induced by glucose. The stimulation of myo-inositol phosphate production by 100 microM carbachol was more marked in islets incubated with the hexose than with the triose. The content of glyceraldehyde 3-phosphate, a glycolytic intermediate, in islets incubated with glyceraldehyde was far higher than that in islets incubated with glucose, whereas the ATP content in islets incubated with the triose was significantly lower than that in islets incubated with the hexose. These results suggest that glyceraldehyde not only mimics the effect of glucose on insulin secretion but also has the ability to cause the secretion of insulin without the influx of Ca(2+ )through voltage-dependent Ca(2+) channels. The reason for the lower potency of the triose than the hexose in stimulating insulin secretion is also discussed.     

Insulin secretion and protein phosphorylation in PKC-depleted islets of Langerhans.

Protein kinase C (PKC)-dependent phosphorylation of endogenous substrates was measured in electrically permeabilised rat islets of Langerhans. The PKC-activating phorbol ester, 4 beta-phorbol myristate acetate (PMA), caused a slow but prolonged increase in insulin secretion from permeabilised islets, which was accompanied by increased 32P incorporation into several islet proteins of apparent M.W. 30-50 kDa. Depletion of islet PKC by prolonged exposure to PMA abolished subsequent secretory and phosphorylating responses to the phorbol ester. However, PKC-depleted islets did not show diminished responses to glucose, suggesting that PKC-mediated phosphorylation of these proteins is not essential for nutrient-induced insulin secretion.     

Activation of protein kinase C is not required for glyceraldehyde-stimulated insulin secretion from rat islets

Glyceraldehyde-induced insulin release from rat islets of Langerhans was not affected following down-regulation of protein kinase C (PKC) by prolonged exposure to the tumour-promoting phorbol ester, 4 beta-phorbol myristate acetate (PMA). Glyceraldehyde did not cause translocation of islet PKC under conditions in which PMA stimulated redistribution of enzyme activity. These results indicate that activation of PKC is not required for glyceraldehyde stimulation of insulin secretion from normal rat islets.     

Chronic exposure to high leucine impairs glucose-induced insulin release by lowering the ATP-to-ADP ratio

Exposure of rat pancreatic islets to 20 mM leucine for 24 h reduced insulin release in response to glucose (16.7 and 22.2 mM). Insulin release was normal when the same islets were stimulated with leucine (40 mM) or glyburide (1 microM). To investigate the mechanisms responsible for the different effect of these secretagogues, we studied several steps of glucose-induced insulin secretion. Glucose utilization and oxidation rates in leucine-precultured islets were similar to those of control islets. Also, the ATP-sensitive K(+) channel-independent pathway of glucose-stimulated insulin release, studied in the presence of 30 mM K(+) and 250 microM diazoxide, was normal. In contrast, the ATP-to-ADP ratio after stimulation with 22.2 mM glucose was reduced in leucine-exposed islets with respect to control islets. The decrease of the ATP-to-ADP ratio was due to an increase of ADP levels. In conclusion, prolonged exposure of pancreatic islets to high leucine levels selectively impairs glucose-induced insulin release. This secretory abnormality is associated with (and might be due to) a reduced ATP-to-ADP ratio. The abnormal plasma amino acid levels often present in obesity and diabetes may, therefore, affect pancreatic islet insulin secretion in these patients.     

Effects of galnon, a non-peptide galanin-receptor agonist, on insulin release from rat pancreatic islets

Galanin is a neurotransmitter peptide that suppresses insulin secretion. The present study aimed at investigating how a non-peptide galanin receptor agonist, galnon, affects insulin secretion from isolated pancreatic islets of healthy Wistar and diabetic Goto-Kakizaki (GK) rats. Galnon stimulated insulin release potently in isolated Wistar rat islets; 100 microM of the compound increased the release 8.5 times (p<0.001) at 3.3 mM and 3.7 times (p<0.001) at 16.7 mM glucose. Also in islet perifusions, galnon augmented several-fold both acute and late phases of insulin response to glucose. Furthermore, galnon stimulated insulin release in GK rat islets. These effects were not inhibited by the presence of galanin or the galanin receptor antagonist M35. The stimulatory effects of galnon were partly inhibited by the PKA and PKC inhibitors, H-89 and calphostin C, respectively, at 16.7 but not 3.3 mM glucose. In both Wistar and GK rat islets, insulin release was stimulated by depolarization of 30 mM KCl, and 100 microM galnon further enhanced insulin release 1.5-2 times (p<0.05). Cytosolic calcium levels, determined by fura-2, were increased in parallel with insulin release, and the L-type Ca2+-channel blocker nimodipine suppressed insulin response to glucose and galnon. In conclusion, galnon stimulates insulin release in islets of healthy rats and diabetic GK rats. The mechanism of this stimulatory effect does not involve galanin receptors. Galnon-induced insulin release is not glucose-dependent and appears to involve opening of L-type Ca2+-channels, but the main effect of galnon seems to be exerted at a step distal to these channels, i.e., at B-cell exocytosis.     

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 effect of calcium on somatostatin inhibition of insulin release and cyclic AMP production in mouse pancreatic islets.

The effect of somatostatin on glucose-induced insulin secretion and cyclic AMP accumulation in isolated islets from obese, hyperglycemic ob/ob mice was studied in a microperifusion system. The normal biphasic pattern of insulin release as well as the inhibitory pattern of insulin release produced by somatostatin (0.5--1 microgram/ml) was matched by similar changes in the intracellular concentration of cyclic AMP. When islets were stimulated by glucose (3 mg/ml) plus 3-isobutyl-1-methylxanthine (0.1 mM), somatostatin (0.5 microgram/ml) failed to inhibit insulin secretion or cyclic AMP formation in the second phase whereas in the first phase both parameters were significantly reduced by somatostatin (0.5 microgram/ml). In batch-type incubations it was shown that addition of excess calcium (to 6 mM) reversed this inhibition. In the second phase calcium potentiated the (glucose + 3-isobutyl-1-methylxanthine)-stimulated insulin secretion without affecting the cyclic AMP production. This potentiation was inhibited by somatostatin (0.1 microgram/ml). Somatostatin (1 microgram/ml) inhibited adenylate cyclase activity in islet homogenates. No effect of somatostatin on islet glucose utilization could be demonstrated. The results indicate a dual action of somatostatin in the inhibition of insulin release, one involving the islet adenylate cyclase and one affecting the islet uptake of calcium.     

Effects of insulin secretagogues on protein kinase C-catalyzed phosphorylation of an endogenous substrate in isolated pancreatic islets

The influence of the insulin secretagogues, carbachol and glucose, on protein kinase C activation in isolated pancreatic islets has been examined by determination of the phosphorylation state of an endogenous 80-kDa protein substrate of protein kinase C. The islet 80-kDa protein was identified as the myristoylated alanine-rich C kinase substrate previously described (Stumpo D. J., Graff, J. M., Albert, K. A., Greengard, P., and Blackshear, P. J. (1989) Proc. Natl. Acad. Sci. U. S. A. 86, 4012-4016) by immunoprecipitation studies. The muscarinic agonist, carbachol (500 microM), induced insulin secretion and a time-dependent increase in the phosphorylation state of this protein in islets. This phosphorylation was maximal (220 +/- 24% of control) at 5 min and was suppressed by the protein kinase C inhibitor, staurosporine. Concentrations of glucose (28 mM) which induce maximal insulin secretion did not induce a statistically significant increase in 80-kDa phosphorylation. The combination of carbachol and a submaximally stimulatory concentration of glucose (10 mM), when added simultaneously, exerted a marked synergistic effect on insulin secretion and a synergistic effect on the phosphorylation of the 80-kDa protein kinase C substrate. These data suggest that the activation of protein kinase C may play an important role in carbachol-induced insulin secretion and in the potentiation by carbachol of insulin secretion induced by glucose. However, the activation of protein kinase C does not appear to be a primary determinant of insulin secretion induced by glucose alone.     

Phorbol-ester-induced down-regulation of protein kinase C in mouse pancreatic islets. Potentiation of phase 1 and inhibition of phase 2 of glucose-induced insulin secretion.

The influence of down-regulation of protein kinase C on glucose-induced insulin secretion was studied. A 22-24 h exposure of mouse pancreatic islets to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA; 0.16 microM) in RPMI 1640 culture medium (8.3 mM-glucose, 0.43 mM-Ca2+) abolished TPA (0.16 microM)-induced insulin secretion and led to a potentiation of phase 1 and a decrease in phase 2 of glucose-induced insulin secretion. Thus, although the total insulin release during 40 min of perfusion with glucose (16.7 mM) (45-85 min) was unaffected, the percentage released during phase 1 (45-55 min) was increased from 12.9 +/- 1.5 (4)% in controls to 35.8 +/- 3.9 (4)% in TPA-treated islets (P less than 0.01), and the percentage released during phase 2 (65-85 min) was decreased from 63.2 +/- 3.9 (4)% to 35.3 +/- 1.4 (4)% (P less than 0.005). In contrast, TPA exposure in TCM 199 medium (5.5 mM-glucose, 1.26 mM-Ca2+) caused a total abolition of both phases 1 and 2 of glucose-induced secretion. However, inclusion of the alpha 2-adrenergic agonists adrenaline (10 microM) or clonidine (10 microM), or lowering of the Ca2+ concentration in TCM 199 during down-regulation, preserved and potentiated phase 1 of glucose-induced secretion. Furthermore, perifusion of islets in the presence of staurosporine (1 microM), an inhibitor of protein kinase C, potentiated phase 1 and inhibited phase 2 of glucose-induced secretion. In addition, down-regulation of protein kinase C potentiated phase 1 and inhibited phase 2 of carbamoylcholine (100 microM)-induced insulin secretion at 3.3 mM-glucose, and abolished the potentiating effect of carbamoylcholine (100 microM) at 16.7 mM-glucose. These results substantiate a role for protein kinase C in insulin secretion, and suggest that protein kinase C inhibits phase 1 and stimulates phase 2 of both glucose-induced and carbamoylcholine-induced insulin secretion.     

Tacrolimus suppresses glucose-induced insulin release from pancreatic islets by reducing glucokinase activity

Tacrolimus is widely used for immunosuppressant therapy, including various organ transplantations. One of its main side effects is hyperglycemia due to reduced insulin secretion, but the mechanism remains unknown. We have investigated the metabolic effects of tacrolimus on insulin secretion at a concentration that does not influence insulin content. Twenty-four-hour exposure to 3 nM tacrolimus reduced high glucose (16.7 mM)-induced insulin secretion (control 2.14 +/- 0.08 vs. tacrolimus 1.75 +/- 0.02 ng.islet(-1).30 min(-1), P < 0.01) without affecting insulin content. In dynamic experiments, insulin secretion and NAD(P)H fluorescence during a 20-min period after 10 min of high-glucose exposure were reduced in tacrolimus-treated islets. ATP content and glucose utilization of tacrolimus-treated islets in the presence of 16.7 mM glucose were less than in control (ATP content: control 9.69 +/- 0.99 vs. tacrolimus 6.52 +/- 0.40 pmol/islet, P < 0.01; glucose utilization: control 103.8 +/- 6.9 vs. tacrolimus 74.4 +/- 5.1 pmol.islet(-1).90 min(-1), P < 0.01). However, insulin release from tacrolimus-treated islets was similar to that from control islets in the presence of 16.7 mM alpha-ketoisocaproate, a mitochondrial fuel. Glucokinase activity, which determines glycolytic velocity, was reduced by tacrolimus treatment (control 65.3 +/- 3.4 vs. tacrolimus 49.9 +/- 2.8 pmol.islet(-1).60 min(-1), P < 0.01), whereas hexokinase activity was not affected. These results indicate that glucose-stimulated insulin release is decreased by chronic exposure to tacrolimus due to reduced ATP production and glycolysis derived from reduced glucokinase activity.     

Effects of muscarinic receptor type 3 knockout on mouse islet secretory responses

The impact of muscarinic type 3 receptor knockout (M3KO) on the cholinergic regulation of insulin secretion and phospholipase C (PLC) activation was determined. Islets isolated from control, wild-type mice or heterozygotes responded with comparable insulin secretory responses to 15 mM glucose. This response was markedly amplified by the inclusion of 10 microM carbachol. While 15 mM glucose-induced release remained similar to wild-type and heterozygote responses in M3KO mice, the stimulatory impact of carbachol was abolished. Stimulation with 15 mM glucose plus 50 microM carbachol increased fractional efflux rates of myo-[2-3H]inositol from control wild-type and heterozygote islets but not from M3KO islets. Fed plasma insulin levels of M3KO mice were reduced 68% when compared to values obtained from combined wild-type and heterozygote animals. These studies support the conclusion that the M3 receptor in islets is coupled to PLC activation and insulin secretion and that cholinergic stimulation of the islets may play an important role in the regulation of plasma insulin levels.     

The effect of calcium on somatostatin inhibition of insulin release and cyclic AMP production in mouse pancreatic islets

The effect of somatostatin on glucose-induced insulin secretion and cyclic AMP accumation in isolated islets from obese, hyperglycemic ob/ob mice was studied in a microperifusion system. The normal biphasic pattern of insulin release as well as the inhibitory pattern of insulin release produced by somatostatin (0.5–1 μg/ml) was matched by similar changes in the intracellular concentration of cyclic AMP. When islets were stimulated by glucose (3 mg/ml) plus 3-isobutyl-1-methylxanthine (0.1 mM), somatostatin (0.5 μg/ml) failed to inhibit insulin secretion or cyclic AMP formation in the second phase whereas in the first phase both parameters were significantly reduced by somatostatin (0.5 μg/ml). In batch-type incubations it was shown that addition of excess calcium (to 6 mM) reversed this inhibition. In the second phase calcium potentiated the (glucose + 3-isobutyl-1-methylxanthine)-stimulated insulin secretion without affecting the cyclic AMP production. This potentiation was inhibited by somatostatin (0.1 μg/ml). Somatostatin (1 μg/ml) inhibited adenylate cyclase activity in islet homogenates. No effect of somatostatin on islet glucose utilization could be demonstrated.The results indicate a dual action of somatostatin in the inhibition of insulin release, one involving the islet adenylate cyclase and one affecting the islet uptake of calcium.     

Acute and chronic effects of glucose and carbachol on insulin secretion and phospholipase C activation: studies with diazoxide and atropine

The acute and chronic effects of 20 mM glucose and 10 microM carbachol on beta-cell responses were investigated. Acute exposure of rat islets to 20 mM glucose increased glucose usage rates and resulted in a large insulin-secretory response during a dynamic perifusion. The secretory, but not the metabolic, effect of 20 mM glucose was abolished by simultaneous exposure to 100 microM diazoxide. Glucose (20 mM) significantly increased inositol phosphate (IP) accumulation, an index of phospholipase C (PLC) activation, from [(3)H]inositol-prelabeled islets. Diazoxide, but not atropine, abolished this effect as well. Unlike 20 mM glucose, 10 microM carbachol (in the presence of 5 mM glucose) increased IP accumulation but had no effect on insulin secretion or glucose (5 mM) metabolism. The IP effect was abolished by 50 microM atropine but not by diazoxide. Chronic 3-h exposure of islets to 20 mM glucose or 10 microM carbachol profoundly reduced both the insulin-secretory and PLC responses to a subsequent 20 mM glucose stimulus. The adverse effects of chronic glucose exposure were abolished by diazoxide but not by atropine. In contrast, the adverse effects of carbachol were abolished by atropine but not by diazoxide. Prior 3 h of exposure to 20 mM glucose or carbachol had no inhibitory effect on glucose metabolism. Significant secretory responses could be evoked from 20 mM glucose- or carbachol-pretreated islets by the inclusion of forskolin. These findings support the concept that an early event in the evolution of beta-cell desensitization is the impaired activation of islet PLC.     

Muscarinic-agonist and guanine nucleotide stimulation of myo-inositol trisphosphate formation in membranes isolated from bovine iris sphincter smooth muscle: effects of short-term cholinergic desensitization

The effect of short-term cholinergic desensitization on muscarinic acetylcholine receptor (mAChR)-mediated activation of phospholipase C was investigated in membranes isolated from the bovine iris sphincter smooth muscle. Membranes prepared from normal or desensitized muscles, prelabeled with either [3H]myo-inositol or 32P from [gamma-32P]ATP, were incubated with a hydrolysis-resistant analogue of GTP, GTP gamma S, or GTP gamma S plus carbachol (CCh), and the production of [3H]myo-inositol 1,4,5-trisphosphate (IP3) and the breakdown of polyphosphoinositides were assessed. In normal membranes, GTP (greater than or equal to 1 mM), GTP gamma S (greater than 10 microM) and GTP gamma S (1 microM) plus CCh (10 microM), but not GDP or GDP beta S, increased phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and IP3 production. GTP gamma S increased IP3 accumulation in a time- and dose-dependent manner, and CCh, which had no effect on phospholipase C activity in the absence of GTP gamma S, potentiated the effects of GTP gamma S. The effect of CCh plus GTP gamma S on IP3 production was inhibited by atropine, had an absolute requirement for nM amounts of Ca2+ and was not affected by pertussis toxin. At higher concentrations (greater than 1 microM), Ca2+ alone induced PIP2 hydrolysis. Short-term exposure (less than 60 min) of the muscle to CCh (100 microM) did not affect the total number (Bmax) of mAChRs nor their affinity (KD) for [3H]-N-methylscopolamine. Desensitization did, however, result in: (1) a loss of the CCh-high affinity binding state of the sphincter mAChRs in a manner analogous to that produced by GTP gamma S; (2) a loss of the ability of GTP gamma S to affect CCh binding to the receptors; and (3) an attenuation of the GTP gamma S plus CCh-stimulated PIP2 hydrolysis. In conclusion, the data presented suggest that, in the iris smooth muscle, G-proteins are involved in the coupling of mAChRs to phospholipase C and that short-term cholinergic desensitization results in (1) the uncoupling of the receptor-G-protein complex and (2) the attenuation of mAChR-activation of phospholipase C.