Phosphoinositide synthesis and Ca2+ gating in blowfly salivary glands exposed to 5-hydroxytryptamine

Partial characterization of protein kinase C from an insect cell line

The characteristics of protein kinase C (EC 2.7.1.37) from an insect cell line (Choristoneura fumiferana) have been described. DEAE-cellulose chromatography produced a major peak of activity which eluted at 0.04-0.055 M NaCl. The enzyme was sensitive to phosphatidylserine in the presence of calcium. Phorbol 12-myristate 13-acetate (PMA) in nanomolar concentrations stimulated protein kinase C activity 8-fold over basal levels and reduced the enzymes requirement for Ca2+. The enzyme had a Ka of 10 nM for PMA. Diacylglycerols tested included diolein, dilinolein, diarachidonin, oleoyl-acetyl-glycerol, dioctonoyl-sn-glycerol, dipalmitin and distearin. A 2.5- to 3-fold activation was obtained in the presence of 26 microM diolein, 40 microM oleoyl-acetyl-glycerol and 46 microM dioctonoyl-sn-glycerol. The enzyme activity was sensitive to the inhibitor H-7 and 50% inhibition was achieved at a concentration of 52 microM H-7. Phosphatidylinositol enhanced enzyme activity in the absence of phosphatidylserine but phosphatidylethanolamine had no effect.

Determination of mass changes in phosphatidylinositol 4,5-bisphosphate and evidence for agonist-stimulated metabolism of inositol 1,4,5-trisphosphate in airway smooth muscle

Stimulation of muscarinic receptors in bovine tracheal smooth muscle (BTSM) causes a sustained increase in muscle tone, but a transient increase in the second messenger Ins(1,4,5)P3. To examine whether this brief increase in Ins(1,4,5)P3 mass results from transient formation or is due to agonist-stimulation of Ins(1,4,5)P3 metabolism, we have studied the relationship between mass changes in PtdIns(4,5)P2 and Ins(1,4,5)P3 accumulation, and changes in [3H]InsP3, [3H]PtdIns, [3H]PtdInsP1 and [3H]PtdInsP2 in carbachol-stimulated myo-[3H]inositol-prelabelled BTSM slices. Carbachol (0.1 mM) caused a rapid transient increase in Ins(1,4,5)P3 concentration (basal, 12.9 +/- 0.8 pmol/mg of protein; 5 s carbachol treatment, 27.1 +/- 1.5 pmol/mg of protein), with values returning to basal levels by 30 s, but a sustained accumulation of total [3H]InsP3s, with [3H]Ins(1,3,4)P3 being the predominant isomer present at later time points. In contrast, PtdIns(4,5)P2 mass, determined by radioreceptor assay of Ins(1,4,5)P3 in desalted alkaline hydrolysates of acidified chloroform/methanol tissue extracts, declined rapidly (basal, 941 +/- 22 pmol/mg of protein; 120 s carbachol, 365 +/- 22 pmol/mg of protein; t1/2 14 s) and remained at this new steady-state level for at least 20 min in the continued presence of carbachol. Addition of 10 microM-atropine 2 min after carbachol caused a prompt return of PtdIns(4,5)P2 concentration to prestimulated values (t1/2 210 s). Ongoing resynthesis of PtdIns(4,5)P2 after carbachol stimulation was demonstrated in [3H]inositol-labelled tissue by observing a persistent increase in the specific radioactivity of [3H]PtdInsP2, shown to be exclusively [3H]PtdIns(4,5)P2, over a 10 min period. These findings strongly suggest the occurrence of persistent receptor-mediated increases in PtdIns(4,5)P2 hydrolysis and Ins(1,4,5)P3 formation which, in conjunction with the transient accumulation of Ins(1,4,5)P3 observed, provide evidence that regulation of the metabolism of Ins(1,4,5)P3 is a major determinant of Ins(1,4,5)P3 concentration in this tissue under agonist-stimulated conditions.

5-Hydroxytryptamine modulation of rat parotid salivary gland secretion

5-Hydroxytryptamine (5-HT) has been reported to produce significant responses in blowfly salivary glands, but little information is available concerning its action on mammalian salivary glands. When 5-HT (0.1 mumol/L to 10 mumol/L) is infused i.a. into anesthetized rats, no salivary secretion is obtained from either parotid or submandibular glands. However, when 5-HT is infused along with a threshold concentration of acetylcholine (0.1 mmol/L), potentiation of parotid secretory response is seen with 5-HT (1 mumol/L, 260% increase; 10 mumol/L, 146% increase). Substance P (0.3 mumol/L) combined with 5-HT (1 mumol/L) also resulted in a potentiation of parotid secretion (160% increase). Protein and calcium concentrations were not altered during such treatments. No potentiation of submandibular secretion was noted. Experiments in vitro with parotid cell aggregates exhibited no potentiation associated with the combined use of 5-HT and carbachol, as measured by amylase secretion and inositol trisphosphate accumulation. The experiments indicate that 5-HT substantially modulates parotid salivary secretion in vivo; however, the in vitro findings suggest that 5-HT does not act directly on surface glandular receptors. The magnitude of the in vivo potentiation could very well implicate circulating or released 5-HT as a physiological modulator of endogenous neurotransmitter action.

Calcium mobilizing hormones activate the plasma membrane Ca2+ pump of pancreatic acinar cells

45Ca fluxes and free-cytosolic Ca2+([Ca2+]i) measurements were used to study the effect of Ca2+-mobilizing hormones on plasma membrane Ca2+ permeability and the plasma membrane Ca2+ pump of pancreatic acinar cells. We showed before (Pandol, S.J., et al., 1987. J. Biol. Chem. 262:16963-16968) that hormone stimulation of pancreatic acinar cells activated a plasma membrane Ca2+ entry pathway, which remains activated for as long as the intracellular stores are not loaded with Ca2+. In the present study, we show that activation of this pathway increases the plasma membrane Ca2+ permeability by approximately sevenfold. Despite that, the cells reduce [Ca2+]i back to near resting levels. To compensate for the increased plasma membrane Ca2+ permeability, a plasma membrane Ca2+ efflux mechanism is also activated by the hormones. This mechanism is likely to be the plasma membrane Ca2+ pump. Activation of the plasma membrane Ca2+ pump by the hormones is time dependent and 1.5-2 min of cell stimulation are required for maximal Ca2+ pump activation. From the effect of protein kinase inhibitors on hormone-mediated activation of the pump and the effect of the phorbol ester 12-0-tetradecanoyl phorbol, 13-acetate (TPA) on plasma membrane Ca2+ efflux, it is suggested that stimulation of protein kinase C is required for the hormone-dependent activation of the plasma membrane Ca2+ pump.

Modulation of octopamine-mediated production of cyclic AMP by phorbol-ester-sensitive protein kinase C in an insect cell line

The presence of protein kinase C (EC 2.7.1.37) in an insect cell line has been demonstrated. Phorbol 12-myristate 13-acetate (PMA), in micromolar concentrations, activated protein kinase C with a translocation of the enzyme from the cytosol to the particulate fraction. Cyclic AMP production in the presence of PMA, octopamine and a combination of both increased in a dose-dependent and time-dependent fashion. The biologically inactive 4 alpha-phorbol 12,13-didecanoate had no effect on protein kinase C activity or on octopamine-mediated cyclic AMP production. Pretreatment of the cells with pertussis toxin had no effect on the response of cells to octopamine or PMA. However, pretreatment with cholera toxin resulted in increased cyclic AMP production which was further enhanced when both cholera toxin and PMA were used in combination. Our data indicate that the octopamine-mediated cyclic AMP production is modulated by protein kinase C.

The end organ defect in cystic fibrosis; a hypothesis: disinhibited inositol cycle activation?

Activation of the inositol cycle by a factor capable of by-passing the normal controls on exocrine secretion by an interaction with a coupling protein could produce effects similar to a calcium ionophore or the ciliary dyskinesia factor. The chloride permeability defect may represent a secondary adaptive change, able to limit the consequences of this via an acid shift in intracellular pH. The model predicts that lithium treatment would limit the effects of the disease.

Activation of muscarinic receptors in PC12 cells. Stimulation of Ca2+ influx and redistribution

Ca2+ homoeostasis was investigated in pheochromocytoma neurosecretory (PC12) cells both before and after treatment with nerve growth factor, which induces a neuronal-like differentiation accompanied by a large increase in the number of muscarinic receptors. The resting concentration of free cytosolic Ca2+, [Ca2+]i, measured by the quin2 technique, was found to be higher and more variable in differentiated cells. Moreover, the [Ca2+]i rises induced by the Ca2+ ionophore ionomycin and by depolarizing concentrations of KC1 were greater and more transient. Exposure to carbachol induced modest, but long-lasting, [Ca2+]i rises, which were faster and greater in differentiated than in non-differentiated cells. These effects were due to the activation of the muscarinic receptor, because they were unaffected by nicotinic blockers (hexamethonium and D-tubocurarine) and completely eliminated by low concentrations of the muscarinic antagonists atropine and pirenzepine [IC50 (concn. causing 50% inhibition) = 2 and 60 nM respectively]. The muscarinic-receptor-dependent [Ca2+]i rises were the result of two concomitant processes: (1) redistribution of Ca2+ from cytoplasmic stores to the cytosol, possibly mediated by generation of inositol 1,4,5-trisphosphate as a consequence of the muscarinic-receptor-coupled hydrolysis of polyphosphoinositides, and (2) increased Ca2+ influx through a pathway of the plasmalemma insensitive to verapamil and thus different from the voltage-dependent Ca2+ channel. The existence of this second process was documented: (a) by the difference of the [Ca2+]i responses brought about by carbachol in Ca2+-containing and Ca2+-free media; (b) by the occurrence of [Ca2+]i rise and increased 45Ca accumulation in cells exposed to 1 mM-CaCl2 after having been treated for 2 min with carbachol in Ca2+-free medium; (c) by typical differences in the quin2 signal kinetics observed in parallel samples of PC12 cells loaded with different concentrations of the dye.