Phosphorylation of the spinach chloroplast 24 kDa RNA-binding protein (24RNP) increases its binding to petD and psbA 3' untranslated regions.

The chloroplast 24 kDa RNA binding protein (24RNP) from Spinacea oleracea is a nuclear encoded protein that binds the 3' untranslated region (3'UTR) of some chloroplast mRNAs and seems to be involved in some processes of mRNA metabolism, such as 3'UTR processing, maturation and stabilization. The 24RNP is similar to the 28RNP which is involved in the correct maturation of petD and psbA 3'UTRs, and when phosphorylated, decreases its binding affinity for RNA. In the present work, we determined that the recombinant 24RNP was phosphorylated in vitro either by an animal protein kinase C, a plant Ca(2+)-dependent protein kinase, or a chloroplastic kinase activity present in a protein extract with 3'-end processing activity in which the 24RNP is also present. Phosphorylation of 24RNP increased the binding capacity (B(max)) 0.25 time for petD 3'UTR, and three times for psbA 3'UTR; the affinity for P-24RNP only increased when the interaction with petD was tested. Competition experiments suggested that B(max), not K(d), might be a more important factor in the P-24RNP-3'UTR interaction. The data suggested that the 24RNP role in chloroplast mRNA metabolism may be regulated in vivo by changes in its phosphorylation status carried out by a chloroplastic kinase.

Oxidative phosphorylation supported by an alternative respiratory pathway in mitochondria from Euglena.

The effect of antimycin, myxothiazol, 2-heptyl-4-hydroxyquinoline-N-oxide, stigmatellin and cyanide on respiration, ATP synthesis, cytochrome c reductase, and membrane potential in mitochondria isolated from dark-grown Euglena cells was determined. With L-lactate as substrate, ATP synthesis was partially inhibited by antimycin, but the other four inhibitors completely abolished the process. Cyanide also inhibited the antimycin-resistant ATP synthesis. Membrane potential was collapsed (<60 mV) by cyanide and stigmatellin. However, in the presence of antimycin, a H(+)60 mV) that sufficed to drive ATP synthesis remained. Cytochrome c reductase, with L-lactate as donor, was diminished by antimycin and myxothiazol. Cytochrome bc(1) complex activity was fully inhibited by antimycin, but it was resistant to myxothiazol. Stigmatellin inhibited both L-lactate-dependent cytochrome c reductase and cytochrome bc(1) complex activities. Respiration was partially inhibited by the five inhibitors. The cyanide-resistant respiration was strongly inhibited by diphenylamine, n-propyl-gallate, salicylhydroxamic acid and disulfiram. Based on these results, a model of the respiratory chain of Euglena mitochondria is proposed, in which a quinol-cytochrome c oxidoreductase resistant to antimycin, and a quinol oxidase resistant to antimycin and cyanide are included.

Modulation by protein kinase C of the hormonal responsiveness of hepatocytes from lean (Fa/fa?) and obese (fa/fa) Zucker rats.

The effect of phorbol myristate acetate (PMA) on the hormonal responsiveness of hepatocytes from lean and obese Zucker rats was studied. Phenylephrine-stimulated phosphatydylinositol labeling and phosphorylase activation were antagonized by PMA in cells from obese and lean animals; bigger residual effects were observed in cells from obese animals even at high PMA concentrations. Cyclic AMP accumulation induced by isoproterenol, glucagon, forskolin and cholera toxin was higher in cells from lean animals than in those from obese rats. PMA diminished glucagon- and cholera toxin-induced cyclic AMP accumulation; cells from lean animals were more sensitive to PMA. Two groups of isoforms of protein kinase C (PKC) were observed in hepatocytes from Zucker rats using DEAE-cellulose column chromatography: PKC 1 and PKC 2. The PKC 1 isozymes were separated into four peaks using hydroxylapatite: aa, 1a (PKC-beta), 1b (PKC-alpha) and 1c. Short treatment with PMA decreased the activity of PKC 1 (peaks 1b (PKC-alpha) and 1c) and to a lesser extent of PKC 2; cells from lean animals were more sensitive to PMA than those obtained from obese rats. Our results indicate that cells from genetically obese Zucker rats are in general less sensitive to this activator of protein kinase C than those from their lean littermates. The possibility that alterations in the phosphorylation/dephosphorylation cycles, that control metabolism and hormonal responsiveness, may contribute to this obese state is suggested.

Modulation of glucagon actions by phorbol myristate acetate in isolated hepatocytes. Effect of hypothyroidism.

Phorbol myristate acetate (PMA) inhibits glucagon-stimulated cyclic AMP accumulation and shifts to the right the dose-response curve to glucagon for ureagenesis. In cells from hypothyroid rats the effect of PMA on glucagon-stimulated ureagenesis was much more pronounced, but its effect on cyclic AMP accumulation was similar to that observed in the control cells. The stimulations of ureagenesis by the glucagon analogue THG and dibutyryl cyclic AMP (But2-cAMP) were also diminished by PMA, to a greater extent in cells from hypothyroid rats than in those from euthyroid rats. PMA inhibited the increases in cytoplasmic [Ca2+] induced by glucagon. THG or But2-cAMP; the effect of PMA was much more marked in cells from hypothyroid rats than in the controls. Treatment of the cells with glucagon or THG increased the production of citrulline by subsequently isolated mitochondria, whereas PMA diminished their effects. The results suggest that PMA alters glucagon actions at least at two levels; (i) cyclic AMP production and (ii) elevation of cytosol calcium. The increased sensitivity to PMA of some glucagon effects in hypothyroid rats seems to be related to the latter action.

Influence of protein kinases on the osmosensitive release of taurine from cerebellar granule neurons.

The role of phosphorylation events on the activation and modulation of the osmosensitive (3)H-taurine release (OTR) was examined in cultured cerebellar granule neurons (CGN) stimulated with 30% hyposmotic solutions. OTR was not decreased when [Ca(2+)](i) rise evoked by hyposmolarity was prevented by EGTA-AM (50 microM) or depleted by treatment with 1 microM ionomycin in Ca(2+)-free medium. Accordingly, OTR was not inhibited by Ca(2+)-dependent signaling events. The calmodulin (CAM) blocker W-7 (50 microM) potentiated OTR while the Ca(2+)/CAM kinase blocker KN-93 (10 microM) was without effect. Blockade of PKC by H-7, H-8 (50 microM) and Gö6976 (1 microM), as well as activation by phorbol myristate acetate (PMA) (100 nM) did not influence OTR, but chronic treatment to down regulate PKC decreased it by 30%. Forskolin (20 microM) and 8-BrcAMP (10 microM) did not change OTR. Protein tyrosine phosphorylation seems to be of crucial importance in the activation and modulation of OTR, as it was markedly inhibited (90%) by tyrphostine A23 (50 microM) and potentiated by the tyrosine phosphatase inhibitor ortho-vanadate (100 microM). The PI3 kinase blocker wortmannin 100 nM essentially abolished OTR but LY294002 (10-100 microM) was without effect. This difference may be accounted for PI3K isoforms in neurons with different sensitivity to the blockers. Alternatively, the effect of wortmannin may be exerted not in PI3 kinase but instead on phospholipases, which are also sensitive to this blocker. The hyposmotic stimulus induced activation of Erk1/Erk2, but blockade of this effect by PD 98059 (50 microM) only marginally decreased OTR suggesting that the Erk1/Erk2 is an epiphenomenon, not directly involved in OTR activation.

Alpha 1-adrenoceptor subtypes in aorta (alpha 1A) and liver (alpha 1B).

The effect of several alpha 1 adrenoceptor antagonists on the alpha 1-adrenoceptor-mediated stimulation of phosphatidylinositol labeling was studied comparatively in rat hepatocytes and rabbit aorta. It was observed that 5-methyl urapidil and WB 4101 were much more potent in rabbit aorta than in hepatocytes. The orders of potency were prazosin much greater than 5-methyl urapidil greater than or equal to WB 4101 in liver cells and WB 4101 greater than or equal to 5 methyl urapidil = prazosin in aorta. Treatment with chlorethylclonidine inhibited 70-80% of the stimulation of labeling induced by epinephrine in rat liver, but only 30-40% of that in aorta. Our data suggest the existence of two pharmacologically distinct receptors in these tissues i.e.m alpha 1A-adrenoceptors in aorta and alpha 1B in liver cells.

Characterization of the human liver alpha 1-adrenoceptors: predominance of the alpha 1A subtype.

The alpha 1-adrenoceptor subtype present in human liver membranes was studied using radioligand binding techniques. [3H]Prazosin binding was rapid, saturable and reversible. A kinetically derived Kd of 0.22 nM was obtained. Rosenthal analysis of saturation isotherms indicated a single class of binding sites with a Kd of 0.47 nM and a Bmax of 70 fmol/mg of protein. Membrane preincubation with chloroethylclonidine markedly decreased total binding (62% decrease) without altering the Kd for the radioligand. Binding competition experiments were performed and the order of potency for agonists was: oxymetazoline > epinephrine > or = norepinephrine > methoxamine. The binding affinity for epinephrine was modulated by the GTP analogue guanosine-5'-(beta,gamma-imido)triphosphate. For antagonists the potency order was: WB4101 > or = prazosin > or = (+)-niguldipine = 5-methylurapidil > or = benoxathian > or = phentolamine. The pharmacological profile of the [3H]prazosin binding sites of human liver membranes suggests that alpha 1A-adrenoceptors predominate (75%-85% of the alpha 1-adrenoceptors) in this tissue.

Identification of a functional Gs protein in Euglena gracilis.

We found a Gs protein coupled to adenylyl cyclase in a free-living protist, Euglena gracilis. This Gs protein of approximately 42 kDa is substrate for cholera toxin and is recognized by an antibody against the C-terminal decapeptide of Gs. Furthermore, this protein is coupled to adenylyl cyclase, as shown by: (a) the activation of the enzyme by GTP-analogues and (b) the effect of cholera toxin on cAMP accumulation in intact cells and the continuous activation of adenylyl cyclase activity in membranes. These data indicate that the Gs-adenylyl cyclase-coupled system is already apparent in the protist kingdom.

Evidence for two mechanisms of amino acid osmolyte release from hippocampal slices.

A 30% decrease in osmolarity stimulated 3H-taurine, 3H-GABA and glutamate (followed as 3H-D-aspartate) efflux from rat hippocampal slices. 3H-taurine efflux was activated rapidly but inactivated slowly. It was decreased markedly by 100 microM 5-nitro-(3-phenylpropylamino)benzoic acid (NPPB) and 600 microM niflumic acid and inhibited strongly by tyrphostins AG18, AG879 and AG112 (25-100 microM), suggesting a tyrosine kinase-mediated mechanism. Hyposmolarity activated the mitogen-activated protein kinases (MAPK) extracellular-signal-related kinase-1/2 (ERK1/ERK2) and p38, but blockade of this reaction did not affect 3H-taurine efflux. Hyposmosis also activated phosphatidylinositide 3-kinase (PI3K) and its prevention by wortmannin (100 nM) essentially abolished 3H-taurine efflux. 3H-taurine efflux was insensitive to the protein kinase C (PKC) blocker chelerythrine (2.5 microM) or to cytochalasin E (3 microM). The release of 3H-GABA and 3H-D-aspartate occurred by a different mechanism, characterized by rapid activation and inactivation, insensitivity to NPPB, niflumic acid, tyrphostins or wortmannin. 3H-GABA and 3H-D-aspartate efflux was not due to external [NaCl] decrease, cytosolic Ca2+ increase or depolarization, or to reverse operation of the carrier. This novel mechanism of amino acid release may be mediated by Ca2+-independent exocytosis and modulated by PKC and actin cytoskeleton disruption, as suggested by its inhibition by chelerythrine and potentiation by 100 nM phorbol-12-myristate-13 acetate (PMA) and cytochalasin E. GABA and glutamate osmosensitive efflux may explain the hyposmolarity-elicited increase in amplitude of inhibitory and excitatory postsynaptic potentials in hippocampal slices as well as the hyperexcitability associated with hyponatraemia.

Modulation of the ATP induced [Ca2+]c increase in AS-30D hepatoma cells.

1. The regulation of the increase in the cytosolic calcium concentration ([Ca2+]c) induced by extracellular ATP in AS-30D hepatoma cells was studied. 2. Homologous desensitization involving the refilling of intracellular calcium pools and the participation of protein kinase C was found. 3. Isoproterenol, forskolin and dibutyryl-cyclic AMP also induced an increase in [Ca2+]c. 4. Interestingly, synergism was found for isoproterenol or forskolin and ATP. 5. The results suggest that there are two pathways for mobilizing [Ca2+]c in AS-30D hepatoma cells; one is activated by ATP receptors and the other by cyclic AMP.

Substrate oxidation and ATP supply in AS-30D hepatoma cells.

The oxidation of several metabolites in AS-30D tumor cells was determined. Glucose and glycogen consumption and lactic acid production showed high rates, indicating a high glycolytic activity. The utilization of ketone bodies, oxidation of endogenous glutamate, and oxidative phosphorylation were also very active: tumor cells showed a high respiration rate (100 ng atoms oxygen (min x 10(7) cells)(-1)), which was 90% oligomycin-sensitive. AS-30D tumor cells underwent significant intracellular volume changes, which preserved high concentrations of several metabolites. A high O(2) concentration, but a low glucose concentration were found in the cell-free ascites liquid. Glutamine was the oxidizable substrate found at the highest concentration in the ascites liquid. We estimated that cellular ATP was mainly provided by oxidative phosphorylation. These data indicated that AS-30D hepatoma cells had a predominantly oxidative and not a glycolytic type of metabolism. The NADH-ubiquinol oxido reductase and the enzyme block for ATP utilization were the sites that exerted most of the control of oxidative phosphorylation (flux control coefficient = 0.3-0.42).

The Membrane-bound L- and D-lactate dehydrogenase activities in mitochondria from Euglena gracilis.

The activity of the pyridine nucleotide-independent lactate dehydrogenase (iLDH) was characterized in mitochondria isolated from the protist Euglena gracilis. The dissociation constants for L- and D-lactate were similar, but the V(max) was higher with the d isomer. A ping-pong kinetic mechanism was displayed with 2,4-dichlorophenol-indolphenol (DCPIP), or coenzyme Q(1), reacting as the second substrate with the modified, reduced enzyme. Oxamate was a competitive inhibitor against both L- and D-lactate. Oxalate exerted a mixed-type inhibition regarding L- or D-lactate and also against DCPIP. The rate of L-lactate uptake was partially inhibited by mersalyl and lower than the rate of dehydrogenation, which was mersalyl-insensitive. These data suggested that the active site of L-iLDH was orientated toward the intermembrane space. The following observations indicated the existence of two stereo-specific iLDH enzymes in the inner membrane of Euglena mitochondria: a greater affinity of the D-iLDH for both inhibitors, D-iLDH thermo-stability at 70 degrees C and denaturation of L-iLDH, opposite signs in the enthalpy change for the association reaction of the isomers to the enzyme, differential solubilization of both activities with detergents, and different molecular mass.

Amino acid osmolytes in regulatory volume decrease and isovolumetric regulation in brain cells: contribution and mechanisms.

Brain adaptation to hyposmolarity is accomplished by loss of both electrolytes and organic osmolytes, including amino acids, polyalcohols and methylamines. In brain in vivo, the organic osmolytes account for about 35% of the total solute loss. This review focus on the role of amino acids in cell volume regulation, in conditions of sudden hyposmosis, when cells respond by active regulatory volume decrease (RVD) or after gradual exposure to hyposmotic solutions, a condition where cell volume remains unchanged, named isovolumetric regulation (IVR). The amino acid efflux pathway during RVD is passive and is similar in many respects to the volume-activated anion pathway. The molecular identity of this pathway is still unknown, but the anion exchanger and the phospholemman are good candidates in certain cells. The activation trigger of the osmosensitive amino acid pathway is unclear, but intracellular ionic strength seems to be critically involved. Tyrosine protein kinases markedly influence amino acid efflux during RVD and may play an important role in the transduction signaling cascades for osmosensitive amino acid fluxes. During IVR, amino acids, particularly taurine are promptly released with an efflux threshold markedly lower than that of K(+), emphasizing their contribution (possibly as well as of other organic osmolytes) vs inorganic ions, in the osmolarity range corresponding to physiopathological conditions. Amino acid efflux also occurs in response to isosmotic swelling as that associated with ischemia or trauma. Characterization of the pathway involved in this type of swelling is hampered by the fact that most osmolyte amino acids are also neuroactive amino acids and may be released in response to stimuli concurrent with swelling, such as depolarization or intracellular Ca(++) elevation.

Control of oxidative phosphorylation in AS-30D hepatoma mitochondria.

1. The distribution of control of the rate of state 3 respiration of AS-30D hepatoma mitochondria was determined. 2. The ATP/ADP carrier (flux control coefficient, Ci = 0.70) and the ATP synthase (Ci = 0.19-0.32) were the only steps that exerted significant control on the phosphorylating flux supported by either glutamate+malate, pyruvate+malate, or succinate+rotenone. This is in contrast to liver mitochondria where the control is distributed between several steps. 3. It is suggested that this pattern of control of phosphorylation in hepatoma mitochondria is a consequence of a lower content of adenine nucleotides or a higher content of Mg2+.

Dietary fatty acids effects on sucrose-induced cardiovascular syndrome in rats.

Cardiovascular disease is one of the leading causes of morbidity and mortality in Mexico. We investigated the effects of omega-3 (n-3) and omega-6 (n-6) polyunsaturated fatty acids on the metabolic syndrome associated with cardiovascular disease in a high-sucrose-fed rat model. The metabolic syndrome-induced rats showed a significant increase in systolic blood pressure, serum insulin, nonfasting serum triglyceride and serum cholesterol levels. Experimental high-sucrose-fed animals received either a n-3 or n-6 enriched diet or a control diet during 6 weeks. Animals fed the n-3 enriched diet had a significant reduction in blood pressure and serum insulin and triglyceride levels. Serum triglyceride levels were also significantly reduced in the n-6-rich diet animals.

Entamoeba histolytica: identification of functional Gs and Gi proteins as possible signal transduction elements in the interaction of trophozoites with fibronectin.

Trophozoites of Entamoeba histolytica adhere to several components of the extracellular matrix. Binding is mediated by specific receptors identified in the parasite surface. Interaction of trophozoites with FN induces the formation of special adhesion structures that are dynamic cytoskeleton membrane complexes and facilitate both adhesion and substrate degradation. The process requires activation of signaling pathways in which PLC, IP3, Ca2-, and PKC participate. These observations, and recent experiments showing increments in cAMP in the trophozoites during the interaction with FN, suggest that FN receptors in the amebic surface could be coupled to G-proteins. We report here that trophozoite plasma membrane peptides of 92, 49, 42, 37, and 21 kDa are ADP-ribosylated by Vibrio cholerae and Bordetella pertussis toxins. Three of them are also recognized by antibodies prepared against the alpha-subunit of Gs-and Gi-proteins. Adenylyl cyclase activity detected in isolated membranes was strongly stimulated by treatment with the toxins. Forskolin (an agonist of the enzyme) and FN also induced increments in the enzymatic activity. Live amebas incubated with the toxins showed enhanced adhesion to FN substrates and a striking reorganization of polymerized actin. The actin rearrangement is reminiscent of the one induced by either forskolin or dibutyril cyclic AMP treatment. Our present data show the presence and the functionality of Gs- and Gi-like proteins and their apparent activation during in vitro interaction of amebas with FN and complement previous observations indicating the operation of signal transduction mechanisms in E. histolytica.

The role of cytosolic Ca2+, protein kinase C, and protein kinase A in hormonal stimulation of phospholipase D in rat hepatocytes.

Ca(2+)-dependent and protein kinase C-dependent mechanisms of phospholipase D (PLD) activation were studied in rat hepatocytes by measuring phosphatidylethanol (Peth) formation in the presence of ethanol. Stimulation of Peth formation by 12-O-tetradecanoyl-phorbol 13-acetate (TPA), vasopressin, or A23187 was inhibited by multiple protein kinase C inhibitors or by protein kinase C down-regulation, indicating that this enzyme is involved in the action of all these agents. A controlled elevation of the cytosolic Ca2+ concentration ([Ca2+]cyt) over the range of 0.1-2.0 microM activated Peth formation in the absence of other agonists. Staurosporin potentiated Ca(2+)-induced Peth formation by shifting the [Ca2+]cyt dose-response curve to the left. Other protein kinase C inhibitors (calphostin C, bisindolylmaleimide) inhibited Ca(2+)-mediated Peth formation, but this inhibition was reduced in staurosporin-treated cells. Okadaic acid potentiated PLD activation by TPA, but suppressed PLD activation by elevated [Ca2+]cyt. Desensitization of TPA-induced PLD activity did not affect PLD activation by Ca2+. These data indicate that [Ca2+]cyt and protein kinase C control distinct pathways of PLD activation, but the Ca(2+)-mediated pathway is suppressed by a staurosporin-sensitive protein kinase. Both mechanisms contribute to vasopressin-induced Peth formation in intact hepatocytes. Activation of protein kinase A enhanced vasopressin-induced Peth formation, but not TPA-stimulated or Ca(2+)-stimulated stimulated Peth formation. Protein kinase A acted by enhancing hormonal Ca2+ mobilization, rather than by directly activating PLD, and thereby shifted the balance of Ca(2+)-dependent and protein kinase C-dependent activation mechanisms of PLD in intact cells.

Contrasting effects of phorbol dibutyrate and phorbol myristate acetate in rabbit aorta.

In rat hepatocytes, active phorbol esters inhibited the alpha 1-adrenergic stimulation of phosphatidylinositol labeling with the expected potency order: phorbol myristate acetate (PMA) greater than phorbol dibutyrate (PDB). In contrast, in rabbit aorta the alpha 1-adrenergic action was inhibited dose-dependently by PDB but not by PMA. Similarly PDB (but not PMA) induced a strong contraction in rabbit aorta. The phorbol ester-induced contraction developed slowly, was dose-dependent and independent of extracellular calcium. These effects of PDB in rabbit aorta were neither inhibited by the protein kinase inhibitor H-7 nor mimicked by the synthetic diacylglycerol, OAG. Our results raise some doubts on the mechanism(s) through which the actions of PDB take place in rabbit aorta.

Characterization of rat white fat cell alpha 1B-adrenoceptors.

In isolated rat white adipocytes, epinephrine (in the presence of 10 microM propranolol) increased the uptake of [32P]Pi into phosphatidylinositol in a dose-dependent fashion. When the cells were pretreated with the irreversible antagonist chlorethylclonidine, this alpha 1-adrenergic effect was markedly diminished. The effect of epinephrine was dose-dependently antagonized by selective alpha 1-adrenergic antagonists, with the potency order prazosin greater than 5-methylurapidil greater than or equal to WB4101. Binding studies using crude membrane preparations were performed with the ligands [3H]bunazosin and 125I-HEAT. Both ligands bound to membrane sites with high affinity (Kd values of 0.75 +/- 0.20 nM for [3H]bunazosin and 125 +/- 20 pM for 125I-HEAT), in a rapid, reversible, and saturable (Bmax, 9-12 fmol/mg of protein) fashion, and with the expected pharmacological characteristics for alpha 1-adrenoceptors. Binding displacement studies with these ligands indicated a potency order of prazosin greater than 5-methylurapidil greater than or equal to WB4101. Northern blot analysis using receptor subtype-specific gene probes showed that adipocyte mRNA hybridized with the alpha 1B-adrenergic probe. All these data suggest that the alpha 1-adrenoceptors of rat white adipocytes belong to the alpha 1B subtype.

Assembly and sealing of tight junctions: possible participation of G-proteins, phospholipase C, protein kinase C and calmodulin.

The making and sealing of a tight junction (TJ) requires cell-cell contacts and Ca2+, and can be gauged through the development of transepithelial electrical resistance (TER) and the accumulation of ZO-1 peptide at the cell borders. We observe that pertussis toxin increases TER, while AIF3 and carbamil choline (carbachol) inhibit it, and 5-guanylylimidodiphosphate (GTPTs) blocks the development of a cell border pattern of ZO-1, suggesting that G-proteins are involved. Phospholipase C (PLC) and protein kinase C (PKC) probably participate in these processes since (i) activation of PLC by thyrotropin-1 releasing hormone increases TER, and its inhibition by neomycin blocks the development of this resistance; (ii) 1,2-dioctanoylglycerol, an activator of PKC, stimulates TER development, while polymyxin B and 1-(5-isoquinoline sulfonyl)-2-methyl-piperazine dihydrochloride (H7), which inhibit this enzyme, abolish TER. Addition of 3-isobutyl-1-methyl-xanthine, dB-cAMP or forskolin do not enhance the value of TER, but have just the opposite effect. Trifluoperazine and calmidazoline inhibit TER development, suggesting that calmodulin (CaM) also plays a role in junction formation. These results indicate that junction formation may be controlled by a network of reactions where G-proteins, phospholipase C, adenylate cyclase, protein kinase C and CaM are involved.