Participation of H+ in the Ca2(+)-induced conformational transition of 4-nitro-2,1,3-benzoxadiazole-labeled sarcoplasmic reticulum ATPase

The binding of Ca2+ to 4-nitro-2,1,3-benzoxadiazole (NBD)-labeled sarcoplasmic reticulum Ca2(+)-ATPase was accelerated markedly when the pH was changed at 11 degrees C from 6.5 to 8.0 at the time of Ca2+ addition. We examined the effect of pH on the enzyme conformational transition by measuring the kinetics of NBD fluorescence rises induced by a pH jump under various ligand conditions. The fast fluorescence rise following a pH jump from 6.0 or 6.5 to various test pHs in the presence and absence of Ca2+ proceeded monoexponentially. The amplitude of this fluorescence rise in the presence of Ca2+ was independent of the test pH, whereas the observed rate constant (kobs) increased markedly as the test pH increased. In contrast, the amplitude of the fast fluorescence rise in the absence of Ca2+ increased with increasing test pH, whereas kobs decreased. MgATP or Mg2+ influenced the pH dependences of these parameters in a complex way except for the amplitudes measured in the presence of Ca2+. These data could be simulated by using a reaction model in which Ca2+ binding is preceded by a rate-limiting enzyme conformational transition from a low to a high NBD fluorescence state and 1 mol each of H+ is liberated before and after this conformational transition. MgATP or Mg2+ appeared to promote this conformational transition by enhancing deprotonation of the enzyme. These results suggest that deprotonation may be the primary event in the activation of the unphosphorylated enzyme by Ca2+.

Increasing effect of sodium 3-ethyl-7-isopropyl-1-azulenesulfonate 1/3 hydrate (KT1-32), a novel antiulcer agent, on gastric mucosal blood flow in anesthetized

Effect of a novel azulene derivative KT1-32 on gastric mucosal blood flow (GMBF) was studied to elucidate its mucosal defensive mechanisms underlying the antiulcer action. The GMBF was measured by two methods which were based on heat clearance and laser Doppler velocimetry in the rat. Intravenous administration of KT1-32 produced a dose-dependent increase in GMBF at doses of 0.3-10 mg/kg; significant increases in GMBF were observed at doses not less than 1 or 3 mg/kg by the laser Doppler or heat clearance method, respectively. The duration of action was about 10 min at 10 mg/kg of KT1-32. A good parallelism in these GMBF changes was noted between both methods. Cetraxate, an antiulcer agent, elicited an increase in GMBF at 30 mg/kg, i.v., when GMBF was measured by the heat clearance method. Sodium guaiazulene 3-sulfonate, another azulene antiulcer agent, showed no increase in GMBF. It is suggested that the increasing effect of KT1-32 on GMBF contributes to its antiulcer activity through strengthening mucosal defensive mechanisms.

Effects of topical application of KT1-32 on transmucosal potential difference and acid secretion in the rat stomach

Effects of intragastric application of azuletil sodium (KT1-32), a novel antiulcer drug, on transmucosal potential difference (PD) and acid secretion were investigated in the rat stomach. The stomach was mounted on a Lucite chamber and perfused with saline before and after exposure to KT1-32 for 10 min. KT1-32 (3-30 mg/kg) produced an elevation of PD in a dose-dependent manner with a rise of the luminal pH. The increased PD response caused by KT1-32 (10 mg/kg) persisted after removal of the agent from the stomach, but this PD generating effect was significantly mitigated by pretreatment with omeprazole (60 mg/kg, i.p.). KT1-32 raised PD under basal conditions, but did not significantly affect the reduced PD response caused by 30% ethanol. In addition, topical application of KT1-32 significantly reduced acid secretion caused by histamine (4 mg/kg/hr, i.v.) and carbachol (20 micrograms/kg/hr, i.v.). In the in vitro study, KT1-32 at 3.9 x 10(-4) M showed 50% inhibition of the H/K ATPase activity prepared from the hog gastric mucosa. These results suggest that KT1-32 exerts locally antisecretory and PD generating effects. The latter may be accounted for by the antisecretory action, which is probably related to the H/K ATPase inhibition.

[General pharmacological properties of an anti-ulcer drug, azuletil sodium (KT1-32)]

Azuletil sodium (AZE, 100 mg/kg, p.o.) did not affect the general behaviors, spontaneous motor activity, pentobarbital-induced hypnosis and body temperature. Furthermore, it did not elicit anticonvulsant and muscle relaxant actions. However, AZE (300 mg/kg, p.o.) elicited a stiff gate and slightly inhibited the spontaneous motor activity and electroshock-induced convulsions. It had no influence on spontaneous EEG activities, even at 30 mg/kg, i.v. AZE inhibited acetic acid-induced writhing moderately at doses above 100 mg/kg. AZE at concentrations up to 10(-5) g/ml did not affect agonist-induced contractions of the isolated ileum, trachea, vas deference and uterus, but inhibited serotonin and oxytocin-induced contraction at concentrations above 3 x 10(-4) and 10(-5) g/ml, respectively; and it also depressed spontaneous movements of the ileum and uterus at concentrations above 3 x 10(-4) g/ml. AZE caused no changes in blood pressure (BP), heart rate (HR), left ventricular pressure, ECG, tracheal pressure (TP), femoral blood flow (FBF) and coronary blood flow (CBF) at doses up to 10 mg/kg, i.v. in anesthetized dogs, but it caused an increase or a decrease in BP, an increase in TP and an increase in CBF at 30 mg/kg, i.v. However, even at 300 mg/kg, p.o., it caused no changes in BP and HR in conscious rats. AZE moderately promoted the charcoal transport. AZE at doses up to 300 mg/kg, p.o. did not affect urine volume, urinary electrolyte excretion, blood glucose and prothrombin time. These results suggest that AZE at anti-ulcer doses of 10-100 mg/kg, p.o. does not have noticeable effects on general pharmacological properties, and there is no marked differences as compared with those of GAS.

Tertiary structure of oxidized flavodoxin from an eukaryotic red alga Chondrus crispus at 2.35-A resolution. Localization of charged residues and implication for interaction with electron transfer partners

The crystal structure of the oxidized form of a flavodoxin from an eukaryotic red alga, Chondrus crispus, has been determined by multiple isomorphous replacement and anomalous scattering methods. A model of the 173 residues and flavin mononucleotide (FMN) has been refined by a restrained least squares method to a crystallographic R-factor of 22.6% using 6236 reflections between 6.0 and 2.35 A with F greater than 3 sigma F. This molecule has a sheet consisting of five parallel beta-strands with two alpha-helices on one side of the sheet and three on the other side, and has a (beta alpha)5 structure. The molecule incorporates a substantial insertion in beta 5, as in Anacystis nidulans flavodoxin, which distinguishes these flavodoxins from the short-chain type. The isoalloxazine ring of FMN is sandwiched between the side chains of Trp-56 and Tyr-98, with its C-7 and C-8 methyl groups being exposed to solvent. The phosphate group of FMN is located at the N-terminal end of alpha 1, and forms extensive hydrogen bonds with the loop (T8-T13) between beta 1 and alpha 1 of the protein. Six of the total 11 lysine residues are clustered at the opposing face to the FMN-binding site, while about two-thirds of the total 35 acidic residues are located in the half of the molecule which includes the FMN-binding site. Such localization of charged residues produces a dipole within the molecule, which may be important in its recognition of the other proteins participating in electron transfer reactions.

Effects of KT-362, a new calcium release blocker, on vascular selectivity and hemodynamic actions in anesthetized dogs

Pharmacological properties of 5-(3-((2-(3,4-dimethoxyphenyl)ethyl)-amino)-1- oxopropyl)-2,3,4,5-tetrahydro-1,5-benzothiazepine fumarate (KT-362), a newly synthesized calcium release blocker, were studied by comparing its vascular selectivity and cardiovascular actions with those of verapamil, a calcium entry blocker. The relaxing effect of KT-362 in rabbit femoral and basilar artery strips contracted with norepinephrine was greater than that in aortic and coronary artery strips. In anesthetized mongrel dogs, KT-362 (0.1-3.0 mg/kg, i.v.) decreased the mean blood pressure, heart rate and total peripheral resistance in a dose-dependent manner, while cardiac output increased slightly despite a decrease in left ventricular pressure. This is consistent with the data on verapamil. Both i.a. and i.v. injections of KT-362 produced a marked dose-dependent increase in vertebral and femoral blood flow. Pretreatment of atropine, propranolol or diphenhydramine exerted no significant effect on the KT-362-induced vasodilation. Verapamil caused a marked increase in the vertebral and coronary blood flows after the injections, but only a slight increase in femoral blood flow. KT-362 at the dose of 10 mg/kg, i.v., had no significant effect on the PQ interval on the electrocardiogram in anesthetized dogs, but 0.1 mg/kg of verapamil increased this interval significantly. These results suggest that KT-362 has properties similar to calcium entry blockers such as verapamil on systemic hemodynamic actions except for the reactivity of vasculatures.

Mechanism for activation of the 4-nitrobenzo-2-oxa-1,3-diazole-labeled sarcoplasmic reticulum ATPase by Ca2+ and its modulation by nucleotides

The mechanism for activation of sarcoplasmic reticulum ATPase by Ca2+ was investigated in 2 mM MgCl2 and 0.1 M KCl at pH 6.5 and 11 degrees C by using enzyme preparations in which a specific amino acid residue (Cys-344) was labeled with 4-nitrobenzo-2-oxa-1,3-diazole (NBD) [Wakabayashi, S., Imagawa, T., & Shigekawa, M. (1990) J. Biochem. (Tokyo) 107, 563-571]. We compared the kinetics of binding and dissociation of Ca2+ from the enzyme with those of the accompanying NBD fluorescence changes. The fluorescence rise following addition of Ca2+ proceeded monoexponentially. At 2-100 microM Ca2+ and in the absence of nucleotides, the Ca2(+)-induced fluorescence rise and Ca2+ binding to the enzyme proceeded at similar rates, which were almost independent of the Ca2+ concentration. In contrast, the fluorescence decrease induced by Ca2+ removal was slower than the Ca2+ dissociation, and both of these processes were inhibited markedly by increasing medium Ca2+. ATP by binding at 1 mol/mol of the phosphorylation site markedly accelerated both the Ca2(+)-induced fluorescence rise and Ca2+ binding, ADP and AMPPNP but not GTP also being effective. In contrast, ADP minimally affected the NBD fluorescence decrease and the Ca2+ dissociation. These data are consistent with a reaction model in which binding of Ca2+ occurs after the conformational transition of the free enzyme from a state (E2) having low affinity for Ca2+ to one (E1) having high affinity for Ca2+ and in which ATP bound at the catalytic site of E2, whose affinity for ATP is about 30-fold less than that of E1, accelerates this conformational transition.

Protein kinase-dependent phosphorylation of cardiac sarcolemmal Ca2(+)-ATPase, as studied with a specific monoclonal antibody

Phosphorylation of the Ca2(+)-pump ATPase of cardiac sarcolemmal vesicles by exogenously added protein kinases was examined to elucidate the molecular basis for its regulation. The Ca2(+)-pump ATPase was isolated from protein kinase-treated sarcolemmal vesicles using a monoclonal antibody raised against the erythrocyte Ca2(+)-ATPase. Protein kinase C (C-kinase) was found to phosphorylate the Ca2(+)-ATPase. The stoichiometry of this phosphorylation was about 1 mol per mol of the ATPase molecule. The C-kinase activation resulted in up to twofold acceleration of Ca2+ uptake by sarcolemmal vesicles due to its effect on the affinity of the Ca2+ pump for Ca2+ in both the presence and absence of calmodulin. Both the phosphorylation and stimulation of ATPase activity by C kinase were also observed with a highly-purified Ca2(+)-ATPase preparation isolated from cardiac sarcolemma with calmodulin-Sepharose and a high salt-washing procedure. Thus, C-kinase appears to stimulate the activity of the sarcolemmal Ca2(+)-pump through its direct phosphorylation. In contrast to these results, neither cAMP-dependent protein kinase, cGMP-dependent protein kinase nor Ca2+/calmodulin-dependent protein kinase II phosphorylated the Ca2(+)-ATPase in the sarcolemmal membrane or the purified enzyme preparation, and also they exerted virtually no effect on Ca2+ uptake by sarcolemmal vesicles.

Transient global amnesia and Raynaud's phenomenon in scleroderma

Transient global amnesia (TGA) is a well-recognized clinical entity, but its pathophysiology and prognosis have remained arguable. We reported that a 63-year-old woman with scleroderma developed two TGA episodes. The patient sometimes suffered from headaches when Raynaud's phenomenon appeared in her fingers, but she did not experience further cerebrovascular events. This case suggests that the unique clinical presentation of this syndrome may result from an ischemic event, possibly triggered by a vasospastic mechanism like Raynaud's phenomenon.

Electroconvulsive treatment: effects on phospholipase C activity and GTP binding activity in rat brain

The effect of electroconvulsive treatment (ECT) on activities of phospholipase C hydrolyzing phosphatidylinositol (PI-PLC) and phosphatidylinositol 4,5-bisphosphate (PIP2-PLC) and guanosine-5'-(3-O-thio)triphosphate (GTP gamma S) binding activity were examined in membrane and cytosol fractions from four discrete areas (prefrontal cortex, hippocampus, striatum, and amygdala) of the rat brain. A single ECT resulted in an increase in cytosolic activities of PI-PLC in the prefrontal cortex and of PIP2PLC in all 4 brain regions examined. There were no significant changes in either PI-PLC or PIP2-PLC activity in membrane fractions after a single ECT. Repeated ECT caused regionally specific changes in PLC activities as follows: in the prefrontal cortex, both cytosolic PI-PLC and PIP2-PLC and membranous PI-PLC activities were decreased; in the hippocampus, no changes in any PLC activities were seen; in the striatum, only membranous PI-PLC activities were increased; and, in the amygdala, cytosolic and membranous PI-PLC and cytosolic PIP2-PLC activities were increased. The pattern of changes in GTP gamma S binding activity following repeated ECT resembled those found in PLC activity as follows: in the prefrontal cortex, GTP gamma S binding activities were significantly reduced in both membrane and cytosol; in the hippocampus, the activity was decreased in membrane; in the striatum, no changes in GTP gamma S binding activity were seen in any fraction; and, in the amygdala, the activity was increased in cytosol. These findings suggest that ECT has complex effects on the G protein-phospholipase C system, possibly affecting neuronal signal transduction.

Mitochondrial energy-linked nicotinamide nucleotide transhydrogenase: effect of substrates on the sensitivity of the enzyme to trypsin and identification of tryptic cleavage sites

The mitochondrial nicotinamide nucleotide transhydrogenase catalyzes hydride ion transfer between NAD(H) and NADP(H) in a reaction that is coupled to proton translocation across the inner mitochondrial membrane. The enzyme (1043 residues) is composed of an N-terminal hydrophilic segment (approximately 400 residues long) which binds NAD(H), a C-terminal hydrophilic segment (approximately 200 residues long) which binds NADP(H), and a central hydrophobic segment (approximately 400 residues long) which appears to form about 14 membrane-intercalating clusters of approximately 20 residues each. Substrate modulation of transhydrogenase conformation appears to be intimately associated with its mechanism of proton translocation. Using trypsin as a probe of enzyme conformation change, we have shown that NADPH (and to a much lesser extent NADP) binding alters transhydrogenase conformation, resulting in increased susceptibility of several bonds to tryptic hydrolysis. NADH and NAD had little or no effect, and the NADPH concentration for half-maximal enhancement of trypsin sensitivity of transhydrogenase activity (35 microM) was close to the Km of the enzyme for NADPH. The NADPH-promoted trypsin cleavage sites were located 200-400 residues distant from the NADP(H) binding domain near the C-terminus. For example, NADPH binding greatly increased the trypsin sensitivity of the K410-T411 bond, which is separated from the NADP(H) binding domain by the 400-residue-long membrane-intercalating segment. It also enhanced the tryptic cleavage of the R602-L603 bond, which is located within the central hydrophobic segment. These results, which suggest a protein conformation change as a result of NADPH binding, have been discussed in relation to the mechanism of proton translocation by the transhydrogenase.

Does fluorescence of 4-nitrobenzo-2-oxa-1,3-diazole incorporated into sarcoplasmic reticulum ATPase monitor putative E1-E2 conformational transition?

When a purified preparation of sarcoplasmic reticulum Ca2(+)-ATPase was labeled with 0.3 mM 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) in 1 mM AMPPNP and 1 mM CaCl2 at 25 degrees C and pH 7.0 for 60 min and then was treated with 10 mM dithiothreitol for 7 min, about 1 mol of NBD was incorporated per mol of the enzyme, and this inhibited the enzyme activity by 90 to 95%. The modified residue was identified as Cys-344 that is located near the phosphorylation site of the ATPase, Asp-351. The NBD-inhibition of enzyme activity could be reversed by treatment with membrane-acting agents such as C12E8, suggesting that Cys-344 is not directly involved in enzyme catalysis. A detailed study of partial reactions of ATP hydrolysis by the modified enzyme and associated changes in the fluorescence intensity of the incorporated NBD label revealed that a predominant effect of the NBD-modification was the inhibition of Ca2+ release from the ADP-sensitive phosphoenzyme intermediate and that two major fluorescent states of the enzyme alternated during ATP hydrolysis. The latter fluorescent data are consistent with the E1-E2 model of Ca2(+)-ATPase reaction.

The amino acid sequence of a flavodoxin from the eukaryotic red alga Chondrus crispus

The amino acid sequence of the constitutive flavodoxin from the red alga Chondrus crispus was determined from the analyses of peptide fragments derived by enzymic digestions of the carboxymethylated protein. This is the first sequence reported for a flavodoxin from a eukaryote. The protein is composed of 173 amino acid residues and is a member of the longer-chain group of flavodoxins. The extent of sequence homology to the three other flavodoxins in the group for which sequences are available is in the range 36-39%, with the most strongly conserved regions being those implicated in binding of the FMN, the redox-active prosthetic group. Nevertheless, Chondrus crispus flavodoxin stands apart in a number of respects, in particular the possession of an unusually high content of proline, with these residues distributed more or less regularly along the peptide chain.

Amino acid sequence and function of rubredoxin from Desulfovibrio vulgaris Miyazaki

Rubredoxin was purified from Desulfovibrio vulgaris Miyazaki. It was sequenced and some of its properties determined. Rubredoxin is composed of 52 amino acids. It is highly homologous to that from D. vulgaris Hildenborough. Its N-terminal methionyl residue is partially formylated. The millimolar absorption coefficients of the rubredoxin at 489 nm and 280 nm are 8.1 and 18.5, respectively, and the standard redox potential is +5 mV, which is slightly higher than those of other rubredoxins. Rubredoxin, as well as cytochrome c-553, was reduced with lactate by the action of lactate dehydrogenase of this organism, and the reaction was stimulated with 2-methyl-1,4-naphthoquinone. It is suggested that rubredoxin, in collaboration with membranous quinone, functions as a natural electron carrier for cytoplasmic lactate dehydrogenase of this organism, whereas cytochrome c-553 plays the same role for periplasmic lactate dehydrogenase.

Guanosine triphosphate utilization by canine cardiac muscle sarcoplasmic reticulum

We investigated the reaction mechanism for GTP-dependent Ca2+ uptake by canine cardiac microsomes enriched in fragmented sarcoplasmic reticulum (SR), because previous studies reported that GTP utilization in cardiac SR occurs via a pathway very different from that for ATP utilization (for a review, see "Entman, M.L., Bick, R., Chu, A., Van Winkle, W.B., & Tate, C.A. (1986) J. Mol. Cell. Cardiol. 18, 781-792"). In cardiac microsomes, we detected slow but distinct oxalate-dependent Ca2+ accumulation, which reached 550 nmol/mg protein in 10 min, and similarly slow Ca2+-dependent GTP hydrolysis. In 50 microM [gamma-32P]-GTP at 0 degrees C, we detected Ca2+-dependent formation of phosphoprotein whose level in the steady state was about a half of the maximum obtained with [gamma-32P]ATP. Kinetic properties of the phosphoprotein, its molecular weight and its chemical stability after the acid treatment are consistent with the conclusion that the phosphoprotein is an acylphosphate intermediate for Ca2+-dependent GTP hydrolysis catalyzed by the Ca2+-pump ATPase. Analysis of the kinetics of the turnover of phosphoprotein revealed that slow GTP hydrolysis is due to slow phosphoprotein formation; at 25 degrees C, the latter arises mainly from slow binding of Ca2+ to the dephosphorylated enzyme. These results indicate that, contrary to the previous data, the reaction pathway for GTP-dependent Ca2+ transport in cardiac SR is basically the same as that for ATP-dependent transport.

Ferredoxin and rubredoxin from Butyribacterium methylotrophicum: complete primary structures and construction of phylogenetic trees

Complete amino acid sequences of ferredoxin and rubredoxin from Butyribacterium methylotrophicum, a methylotrophic hetero-acetogen, were determined by combination of protease digestion, Edman degradation, carboxypeptidase digestion, and/or partial acid hydrolysis. The ferredoxin was composed of 55 amino acids with a molecular weight of 5,732 excluding iron and sulfur atoms and showed a typical 2[4Fe-4S]-type ferredoxin sequence with an internal repeat at the 14-23 and 42-51 positions. The rubredoxin was composed of 53 amino acids with a molecular weight of 5,672 excluding iron atom and showed a sequence similar to those of other anaerobic rubredoxins. The sequences were compared to those of corresponding proteins from six different bacteria to construct phylogenetic trees, which showed essentially the same topology. The relationships between the ferredoxin sequences from this bacterium and those of Clostridium thermoaceticum and Methanosarcina barkeri, both of which possess a carbonyl-dependent acetyl-CoA metabolic system, are also discussed.

Molecular cloning and nucleotide sequence of a cDNA encoding Euglena gracilis cytochrome c1

The amino acid sequence of the mature protein of Euglena gracilis cytochrome c1 was determined by sequencing of its cDNA. A cDNA expression library was constructed from Euglena poly(A)+ RNA in phage lambda gt11 and screened with an antiserum raised against cytochrome c1 polypeptide isolated from purified E. gracilis complex III. An isolated cDNA clone consisted of 872 base pairs and encoded the mature protein with 243 amino acids. The deduced amino acid sequence contained the unusual heme binding sequence-Phe-Ala-Pro-Cys-His- (Mukai, K. et al. (1989) Eur. J. Biochem. 178, 649-656) instead of the typical sequence,-Cys-X-Y-Cys-His-, commonly found in C-type cytochromes. Comparison of the sequence with those of several other cytochromes c1 revealed that Euglena cytochrome c1 conserved the residues probably ligating heme-iron, those supposed to interact with cytochrome c and regions anchoring the mitochondrial inner membrane.

A single amino acid substitution in B subunit of Escherichia coli enterotoxin affects its oligomer formation

We isolated a mutant strain of enterotoxigenic Escherichia coli by nitrosoguanidine mutagenesis, which produces an immunologically altered B subunit of heat-labile enterotoxin. This mutant B subunit was detected as a monomer on sodium dodecyl sulfate-polyacrylamide gel electrophoresis even without prior heating, suggesting a problem in oligomer formation. Furthermore, this mutant B subunit could not form holotoxin with the native A subunit, and the affinity to GM1-ganglioside receptor was 10-fold lower than that of the native B subunit. The amino acid sequence analysis of this mutant B subunit revealed only one amino acid substitution compared with the native B subunit, at the 64th position from the N terminus (valine instead of alanine). These data suggest that the alanine at position 64 from the N terminus is important for the native B subunit to form an oligomer structure and express its functions.

Synthesis and biological activity of novel carbacyclins having bicyclic substituents on the omega-chain

A number of carbacyclins having bicyclic substituents on the omega-chain have been synthesized and tested for antiplatelet aggregation activity in vitro (against collagen-induced aggregation of rat platelet), for reduction of systemic blood pressure in vivo (ability to reduce the blood pressure in anesthetized rat by iv injection), and for cytoprotective activity (protection against ethanol-induced rat gastric lesion). The most effective compound for each activity was [3aS-[2E,3a alpha,4 alpha (3R),5 beta,6a alpha]]-5-[hexahydro-5- hydroxy-4-[3-hydroxy-3-(2-indanyl)-1-propynyl]-2(1H)-pentalenylidene+ ++] pentanoic acid (compound 11a), while some 1,4-benzodioxan analogues had selectivity for organ-protective activity, and indan analogues showed selectivity in their antiaggregation activity.

[Adrenergic receptors regulation of insulin secretion due to 12-O-tetradecanoyl phorbol-13-acetate--a perifusion study]

A possible role for the activation of protein kinase C and adenylate cyclase in the regulation of insulin secretion by monolayer islet B cells of neonatal rats was examined in a perifusion system. Stimulation of 60 min of 16.7 mM glucose secreted a first component of insulin for the first 10-min period. In the presence of 200 nM 12-O-tetradecanoyl phorbol-13-acetate(TPA), a protein kinase C activator, however, the same dose of glucose significantly increased cAMP accumulation in medium and dramatically enhanced a second component of insulin secretion in which the average secretion rate was 1.81 +/- 0.25 of insulin for the first 10-min period. In the presence of 200 nM 12-O-tetradecanoyl phorbol-13-acetate(TPA), a protein kinase C activator, however, the same dose of glucose significantly increased cAMP accumulation in medium and dramatically enhanced a second component of insulin secretion in which the average secretion rate was 1.81 +/- 0.25 of insulin for the first 10-min period. In the presence of 200 nM 12-O-tetradecanoyl phorbol-13-acetate(TPA), a protein kinase C activator, however, the same dose of glucose significantly increased cAMP accumulation in medium and dramatically enhanced a second component of insulin secretion in which the average secretion rate was 1.81 +/- 0.25 ng/ml/min for the 10-30 min (second phase IIa) and 3.39 +/- 0.18 ng/ml/min for the 30-60 min (second phase IIb) respectively. Addition of 10 microM epinephrine significantly decreased the medium cAMP content and abolished the sustained second-phase secretion, resulting in a monophasic response of approximately the same magnitude as evoked by 16.7 mM glucose alone. In contrast, joint addition of 10 microM epinephrine and 10 microM phentolamine, which activates adenylate cyclase, exerted a more pronounced increase in cAMP accumulation and the second component of insulin secretion than in the presence of TPA; the average secretion rates were 3.62 +/- 0.38 and 6.44 +/- 0.35 ng/ml/min respectively for the second phase IIa and IIb. However, no antagonistic effect of propranolol (10 microM) was found. Likewise, either phenylephrine (10 microM) or clonidine (1 microM) inhibited enhancement of insulin secretion that a combination of glucose with TPA caused, and alpha 1-adrenergic blockade of 10 microM prazosin was weak as compared to the blockade of yohimbine at the same concentration.(ABSTRACT TRUNCATED AT 400 WORDS)