Lowering of the extracellular Na+ concentration enhances high-K+-induced formation of inositol phosphates in the guinea-pig ileum

Effects of membrane depolarization and changes in intra- and extracellular calcium concentration on phosphoinositide hydrolysis in bovine tracheal smooth muscle

Agonist-stimulated phosphoinositide metabolism plays a central role in pharmacomechanical coupling in airways smooth muscle (ASM). In many other tissues and cells, most noteably excitable cells, membrane depolarization or an increase in intracellular Ca2+ ([Ca2+]i) generated by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-induced Ca2+ release or agonist-mediated Ca2+ influx is able to trigger or augment phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis and/or initiate PtdIns4P/PtdIns hydrolysis by direct stimulation of PIC. To assess the importance of these mechanisms in ASM the effects of KCl-induced membrane depolarization, extracellular Ca2+ ([Ca2+]e) chelation, and addition of ionomycin to elevate [Ca2+]i on basal and agonist-stimulated Ins(1,4,5)P3 concentration and [3H]-InsPx accumulation have been examined. Reducing [Ca2+]e from 1.8 mM to 6 or 0.8 microM caused a progressive inhibition of agonist-stimulated [3H]inositol polyphosphate accumulation over 30 min with the histamine-stimulated response being significantly more sensitive to [Ca2+]e chelation than the response to carbachol. In contrast, the initial accumulation of Ins(1,4,5)P3 was completely unaffected by such reductions in [Ca2+]e. Incubation of [3H]inositol-prelabelled BTSM slices with buffer containing 80 mM KCl failed to stimulate [3H]InsPx accumulation, causing instead a small inhibition of carbachol-stimulated [3H]InsPx accumulation with a similar effect seen with respect to Ins(1,4,5)P3 accumulation. Addition of 5 microM ionomycin to BTSM slices similarly did not stimulate Ins(1,4,5)P3 generation and only increased [3H]InsPx accumulation after prolonged stimulation in the presence of high (mM) [Ca2+]e. These data indicated that in ASM, membrane depolarization or physiological increases in [Ca2+]i did not result in either independent activation of PIC or augmentation of initial agonist-stimulated PtdIns(4,5)P2 hydrolysis. However, while the initial agonist-stimulated generation of Ins(1,4,5)P3 was not dependent on [Ca2+]e, a normal plasmalemmal Ca2+ gradient was required to sustain maximal rates of agonist-stimulated PtdIns(4,5)P2 hydrolysis.

Effects of muscarinic agonists and depolarizing agents on inositol monophosphate accumulation in the rabbit vagus nerve

The effects of muscarinic agonists and depolarizing agents on inositol phospholipid hydrolysis in the rabbit vagus nerve were assessed by the measurement of [3H]inositol monophosphate production in nerves that had been preincubated with [3H]inositol. After 1 h of drug action, carbachol, oxotremorine, and arecoline increased the inositol monophosphate accumulation, though the maximal increase induced by these agonists differed. Addition of the muscarinic antagonists atropine or pirenzepine shifted the carbachol dose-response curves to the right, without decreasing the carbachol maximal stimulatory effects. The KB for pirenzepine was 35 nM, which is characteristic of muscarinic high-affinity binding sites coupled to phosphoinositide turnover and often associated with the M1 receptor subtype. On the other hand, agents known to depolarize or to increase the intracellular Ca2+ concentration, e.g., elevated extracellular K+, ouabain, Ca2+, and the Ca2+ ionophore A23187, also increased inositol monophosphate accumulation. These effects were not mediated by the release of acetylcholine, as suggested by the fact that they could not be potentiated by the addition of physostigmine nor inhibited by the addition of atropine. The Ca(2+)-channel antagonist Cd2+, also known to inhibit the Na+/Ca2+ exchanger, was able to block the effects of K+ and ouabain, but did not alter those of carbachol. These results suggest that depolarizing agents increase inositol monophosphate accumulation in part through elevation of the intracellular Ca2+ concentration and that muscarinic receptors coupled to phosphoinositide turnover are present along the trunk of the rabbit vagus nerve.

Bradykinin stimulation of phosphoinositide hydrolysis in guinea-pig ileum longitudinal muscle

1. Bradykinin (BK)-induced contraction of ileal smooth muscle is assumed to be due to phosphoinositide hydrolysis but this has never been reported. We have investigated whether BK receptors are linked to this transduction mechanism in guinea-pig ileum longitudinal muscle and determined whether these receptors are equivalent to those labelled in [3H]-BK binding assays. 2. In membranes prepared from longitudinal muscle, [3H]-BK bound to a single class of sites with high affinity. Characterization of the binding with BK analogues indicated that the radioligand selectivity labelled a B2 type receptor. 3. BK significantly elevated tissue levels of [3H]-inositol phosphates in longitudinal muscle slices preincubated with [3H]-myo-inositol. The agonists potencies of BK, Lys-BK, Met-Lys-BK, Tyr5-BK and Tyr8-BK were in agreement with their relative potencies in the binding assay. The B1 receptor agonist des-Arg9-BK, did not stimulate inositol phosphate production. The response to BK was blocked by known B2 receptor antagonists but not by the B1 antagonist des-Arg9, Leu8-BK. 4. BK-induced phosphoinositide hydrolysis was unaffected by exposure of muscle slices to either atropine or indomethacin. 5. The results indicate that the B2 receptors linked to phosphoinositide turnover in ileal longitudinal muscle exhibit properties similar to those involved in contractile responses. Also, the receptor mediating the phosphoinositide response is likely to be that labelled in the [3H]-BK binding studies.

The inhibition of agonist- or depolarisation-evoked formation of inositol phosphate by excitatory amino acids in rat cerebral cortex is due to the neurotoxic action of this class of neurotransmitter and is mediated by sodium influx

Previous work has shown that excitatory amino acids inhibit agonist or depolarisation evoked formation of inositol phosphate in brain. In this paper, possible mechanisms by which this may be occurring have been investigated. The inhibition of carbachol-stimulated formation of inositol phosphate by kainic acid (KA) was abolished if the tissue was incubated in a sodium-free medium. The sodium channel activator, veratridine (10 microM) and the sodium ionophore, monensin (3 microM), also inhibited the response of inositol phosphate to carbachol; tetrodotoxin (300 nM) reversed the effect of veratridine but not monensin or KA. Incubation with cadmium (0.3 mM) or removal of extracellular calcium did not alter the effects of KA, monensin or veratridine. The effects of KA were significantly reduced with the Na+/K(+)-ATPase inhibitor, ouabain (10-100 microM). Inhibition by KA was still observed in tissue that had been prestimulated with KA and then washed to remove the agonist. Incorporation of [3H]inositol into inositol lipids was significantly reduced by KA, in the absence or presence of carbachol. It is suggested that the inhibition of the turnover of stimulated phosphoinositide, by excitatory amino acids, is related to the neurotoxic actions of these transmitters and is mediated by Na+ influx, with a consequent activation of Na+/K(+)-ATPase, depletion of cellular ATP and reduction in synthesis of inositol lipid.

Reduction of Na+ enhances phosphoinositide hydrolysis and differentiates the stimulatory and inhibitory responses to quisqualate in rat brain slices

The concentration of Na+ in the incubation medium significantly influenced phosphoinositide hydrolysis induced by some, but not all, agonists in rat cerebral cortical slices. Reductions of the Na+ concentration below 120 mM resulted in incremental increases in basal and norepinephrine-stimulated accumulation of [3H]inositol monophosphate in cortical slices that had been prelabelled with [3H]inositol, and maximal responses were obtained with 0 and 5 mM Na+. In contrast, the responses to carbachol and ibotenate were similar in medium containing 120 or 5 mM Na+. In medium with 120 mM Na+, quisqualate has two effects on phosphoinositide hydrolysis in cortical slices, including a relatively weak stimulatory effect and an inhibitory modulation of the stimulation induced by norepinephrine. These two responses to quisqualate were differentially modulated by Na+; in 5 mM compared with 120 mM Na+ the stimulatory response was greatly increased and the inhibitory effect was mostly eliminated. That these were two separate events was confirmed by the use of L-BOAA (beta-N-oxalyl-L-alpha, beta-diaminopropionic acid), which reproduces the inhibitory, but not the stimulatory effect of quisqualate on phosphoinositide hydrolysis. In 5 mM Na+, inhibition by L-BOAA of norepinephrine-stimulated phosphoinositide hydrolysis was completely eliminated. These results demonstrate that a physiological concentration of Na+ maintains phosphoinositide hydrolysis at a submaximal level of sensitivity to some, but not all, agonists. The differential effects of Na+ on the stimulatory and inhibitory effects of quisqualate further substantiate the suggestion that these are two separate processes and indicate that alterations of the Na+ concentration may influence the effects of quisqualate, and other agonists, on phosphoinositide hydrolysis.

Characterization of ouabain-induced phosphoinositide hydrolysis in brain slices of the neonatal rat

The effect of the Na/K-ATPase inhibitor ouabain on phosphoinositide (Ptdlns) hydrolysis was studied in rat brain cortical slices. Ouabain induced a dose-dependent accumulation of inositol phosphates (InsPs) which was much higher in neonatal rats (1570 +/- 40% of basal) than in adult animals (287 +/- 18% of basal). For this reason, all experiments were conducted with 7 day-old rats. Strophantidin caused a similar stimulation of Ptdlns hydrolysis, although it was less potent than ouabain. The order of potency for ouabain-stimulated InsPs accumulation in brain areas was hippocampus greater than cortex greater than brainstem greater than cerebellum. The effect of ouabain was not blocked by antagonists for the muscarinic, alpha1 -adrenergic and glutamate receptors. Also ineffective were the K+ channel blockers 4-aminopyridine and tetraethylammonium, the sodium channel blocker tetrodotoxin, and the calcium channel blocker verapamil, whereas the Na/Ca exchanger blocker amiloride partially antagonized the effect of ouabain. The accumulation of InsPs induced by ouabain was additive to that of carbachol and norepinephrine, as well as to that induced by high K+ and veratrine, but not to that of glutamate. Removal of Na+ ions from the incubation buffer completely prevented the accumulation of InsPs induced by ouabain. The effect of ouabain was also dependent upon extracellular calcium and was under negative feedback control of protein kinase C. Despite the higher effect of ouabain on Ptdlns hydrolysis of immature rats, the density of [3H]ouabain binding sites, as well as the activity of Na/K-ATPase were higher in adult animals. Furthermore, a poor correlation was found between ouabain-stimulated Ptdlns hydrolysis and [3H]ouabain binding in brain regions.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences between muscarinic-receptor- and Ca2(+)-induced inositol polyphosphate isomer accumulation in rat cerebral-cortex slices

Muscarinic-receptor stimulation or depolarization by elevated K+ leads to increased accumulation of [3H]Ins(1,4,5)P3, [3H]Ins(1,3,4,5)P4 and several degradation products of these polyphosphates separated by h.p.l.c. On the other hand, agents such as ionomycin and maitotoxin, which increase intracellular Ca2+ directly, produce a small accumulation of [3H]Ins(1,4,5)P3 and markedly increase [3H]Ins(1,4)P2, but [3H]Ins(1,3,4,5)P4, [3H]Ins(1,3,4)P3 and [3H]Ins(1,3)P2 are virtually unaffected. Ca2(+)-dependent [3H]inositol polyphosphate metabolism may involve different pools of lipids and/or phosphoinositidases.

Increased intracellular calcium stimulates 3H-inositol polyphosphate accumulation in rat cerebral cortical slices

Agents that increase the intracellular Ca2+ concentration have been examined for their ability to stimulate 3H-inositol polyphosphate accumulation in rat cerebral cortex slices. Elevated extracellular K+ levels, the alkaloid sodium channel activator veratrine, the calcium ionophore ionomycin, and the marine toxin maitotoxin were all able to stimulate phosphoinositide metabolism. Certain features appear common to the agents studied. Thus, although [3H]inositol monophosphate, [3H]inositol bisphosphate ([3H]InsP2), and [3H]inositol trisphosphate were all stimulated, a proportionally greater effect was observed on [3H]InsP2 in comparison to stimulation by the muscarinic receptor agonist carbachol. However, only an elevated K+ level stimulated [3H]inositol tetrakisphosphate ([3H]InsP4) accumulation alone or produced marked synergy with carbachol on the formation of this polyphosphate. The results suggest that agents that elevate the cytoplasmic Ca2+ concentration in cerebral cells can increase the hydrolysis of membrane polyphosphoinositides. The pattern of the response differs from that produced by muscarinic receptor agonists and indicate that Ca2(+)-dependent hydrolysis may involve different pools of lipids, phosphoinositidase C enzymes, or both. However, clear differences in the ability of these agents to stimulate InsP4, alone or in the presence of muscarinic agonist, suggest that factors other than a simple elevated intracellular Ca2+ concentration are implicated.

Phorbol esters inhibit smooth muscle contractions through activation of Na(+)-K(+)-ATPase

1. The role of protein kinase C (PKC) in agonist-induced contractions of guinea-pig ileum longitudinal smooth muscle has been investigated. 2. The phorbol esters, phorbol 12,13-dibutyrate (PDBu), phorbol 12,13-diacetate (PDA) and phorbol 12-myristate 13-acetate (PMA), relaxed tissues precontracted by submaximal concentrations of carbachol, histamine or substance P. 3. This inhibitory action of the phorbol esters was reversed following the application of ouabain, a specific inhibitor of Na(+)-K(+)-ATPase. Similarly, pretreatment with ouabain inhibited the ability of phorbol esters to relax tissues precontracted by the above agonists. 4. The slow relaxation of the tonic component of contraction induced by submaximal concentrations of carbachol and histamine, and all concentrations of substance P, was abolished in the presence of ouabain. 5. In Na(+)-loaded tissues, PDBu and carbachol caused a concentration-dependent increase of Na(+)-K(+)-ATPase activity, assessed by ouabain-sensitive 86Rb(+)-uptake. Extrusion of Na+, assessed by the cellular content of the ion, was also stimulated by PDBu (the effect of carbachol was not investigated). 6. We conclude that phorbol esters inhibit the tonic component of contractions induced by submaximal concentrations of these agonists through activation of Na(+)-K(+)-ATPase. We suggest that PKC may exert feedback control over the tonic component of agonist contractions through stimulation of the pump.

K(+)-stimulation of the phosphoinositide pathway in guinea-pig ileum longitudinal smooth muscle is predominantly neuronal in origin and mediated by the entry of extracellular Ca2+

1. K+ and scorpion toxin stimulate formation of inositol phosphates in guinea-pig ileum longitudinal smooth muscle slices. The response to these two agents is not additive. 2. The response to K+ is inhibited partially by nifedipine and partially by omega-conotoxin. When given together the effect of these two Ca2+ channel blockers is additive and the response to K+ is reduced by more than 80%. 3. The response to scorpion toxin is inhibited completely by tetrodotoxin, partially by omega-conotoxin but not by atropine or nifedipine. Scorpion toxin induces a similar formation of inositol phosphates in collagenase-dispersed cells to that seen in cross-chopped slices. 4. The responses to scorpion toxin and K+ are inhibited completely when the extracellular Ca2+ concentration is reduced to below cytosolic levels (less than 100 nM). 5. Neither nifedipine nor omega-conotoxin, either alone or in combination, inhibited formation of inositol phosphates by substance P or carbachol. Both of these agonists induced a significant formation of inositol phosphates even when the extracellular Ca2+ concentration was reduced to 10 nM. 6. These results indicate that K+ and scorpion toxin induce formation of inositol phosphates through the mobilisation of extracellular Ca2+. The response to K+ appears to occur predominantly in neuronal cells.

Protein kinase C regulates the tonic but not the phasic component of contraction in guinea-pig ileum

1. We have investigated the effect of phorbol esters and the down-regulation of protein kinase C on contraction of guinea-pig ileum longitudinal smooth muscle to carbachol and high K+. 2. Phorbol 12,13-dibutyrate (PDBu) enhanced the phasic component and inhibited or enhanced, respectively, the tonic component of contraction to carbachol and high K+. In contrast, 4 alpha-phorbol, which does not activate protein kinase C, had no effect on these responses. 3. Exposure to phorbol 12-myristate 13-acetate (PMA; 1 microM) for up to 8 h induced a time-dependent loss of [3H]-PDBu binding sites, consistent with the down-regulation of protein kinase C by this treatment. 4. The phasic component of contraction to carbachol or high K+ was unaffected following the down-regulation of protein kinase C. The tonic component of contraction to carbachol was markedly enhanced by this treatment while that to high K+ was partially suppressed. 5. These data suggest that although the activation of protein kinase C can lead to potentiation of the phasic component of contraction to carbachol or high K+, this appears to have little physiological significance since the response is not altered in tissues in which protein kinase C has been down-regulated. On the other hand, protein kinase C may limit the tonic contraction to carbachol but potentiate that to high K+.

Dual effects of K+ depolarisation on inositol polyphosphate production in rat cerebral cortex

Depolarisation of [3H]inositol-prelabelled slices of rat cerebral cortex with elevated extracellular K+ induced a rapid and marked increase in inositol polyphosphate accumulation. Addition of the muscarinic antagonist atropine (10 microM) markedly inhibited the K+-induced accumulation of inositol tetrakisphosphate (InsP4), with only a slight reduction in stimulated inositol bis- and trisphosphate levels. Inhibitory effects on InsP4 were noted at the earliest time period measured (30 s) and suggested the involvement of released endogenous acetylcholine in part of the response. The atropine-insensitive component of depolarisation did not appear to be secondary to release of noradrenaline, histamine, or 5-hydroxytryptamine, because addition of prazosin, mepyramine, or ketanserin was without effect on the K+ response. Furthermore, secretion of a neuropeptide that could stimulate phosphoinositide hydrolysis was unlikely, because the peptidase inhibitor bacitracin was also without effect. The results suggest that endogenous acetylcholine can stimulate phosphoinositide metabolism by interacting with muscarinic receptors and that this is particularly evident on InsP4 accumulation. Atropine-insensitive responses may be secondary to Ca2+ entry via voltage-sensitive channels.

Phosphoinositide hydrolysis induced by depolarization and sodium channel activation in mouse cerebrocortical slices

Carbachol, a muscarinic receptor agonist and the sodium channel-activating agents, scorpion venom, veratridine, batrachotoxin and aconitine, were shown to stimulate the formation of [3H]inositol phosphates in [3H]inositol-labelled miniprisms, obtained from the cerebral cortex of the mouse. The inositol response to the Na+ channel-activating agents was inhibited by the sodium channel blocker tetrodotoxin (TTX), while the response induced by carbachol was partially resistant to TTX. The response to scorpion venom and the TTX-insensitive portion of the response to carbachol was additive, indicating different mechanisms. The presence of high potassium (K+) induced hydrolysis of inositide in a TTX-insensitive manner and was not additive with that resulting from sodium channel activators, thus indicating a common mechanism. The addition of large concentrations of magnesium to block the release of acetylcholine, did not inhibit the inositol response to high K+ or to veratridine. Calcium channel blockers such as nickel or cobalt, or the dihydropyridine calcium (Ca2+) channel activator BAY K 8644 and the calcium channel blocker nifedipine, nimodipine or PN-200 110 had little effect. Monensin, a sodium ionophore, stimulated the turnover of phosphatidylinositol at non-depolarizing concentrations and the omission of Na+ ions inhibited the response to sodium channel agents and to high K+. Thus, membrane potential and gradients of K+, Na+ and Ca2+ are all important factors determining the final effect on the turnover of phosphatidylinositol. The data are consistent with a model in which all these factors impinge on the Na+/Ca2+ exchanger regulating internal Ca2+ that, in turn, activates phospholipase C.

Ketamine-induced relaxation in intact and skinned smooth muscles of the rabbit ear artery

1. The effects of ketamine, an intravenous anaesthetic, on the rabbit ear artery were investigated by measuring the tension in intact and saponin-treated skinned smooth-muscle fibres. 2. Ketamine dose-dependently inhibited contractions of intact smooth-muscle fibres induced by high K+ solution and by noradrenaline (NA) or histamine in Krebs solution. This drug similarly attenuated both phasic and tonic contractions induced by high K+ solution. 3. Ketamine also inhibited NA- or histamine-induced contractions in Ca2+-free solution containing 2mM EGTA, but it did not affect the caffeine-induced contraction in this solution. 4. Because the pCa-tension relationship of saponin-treated skinned smooth-muscle fibres was not affected, it can be proposed that ketamine does not have an effect on the contractile proteins. 5. In the presence of 5mM NaN3, 20 microM inositol 1,4,5-trisphosphate (InsP3) or 25mM caffeine produced a contraction in skinned smooth-muscle fibres after accumulation of Ca2+ by intracellular stores. Analysis of the InsP3- or caffeine-induced contractions indicates that ketamine does not have an effect on the Ca2+ accumulation into and Ca2+ release from the intracellular stores. 6. These results indicate that the relaxant effects produced by ketamine in the rabbit ear artery are not likely to be due to an intracellular action. The inhibitory effects of ketamine could be caused by a decrease of the Ca2+ influx through the plasma membrane or interference with the process of signal transduction between receptors on the plasma membrane and intracellular stores.

Enhancement by potassium of carbachol-stimulated inositol phospholipid breakdown in rat cerebral cortical miniprisms: comparison with other depolarising agents

Increasing the [K+] in the assay medium from 5.7 to 17.8 mM produces a large enhancement of the inositol phospholipid breakdown response to the muscarinic agonist carbachol in rat cerebral cortical miniprisms, with minor effects on basal inositol phospholipid breakdown. This effect is also found with Rb+. The enhancement by a raised [K+] is not accompanied by a change in the composition of the labelled polyphosphoinositides. The carbachol-stimulated inositol phospholipid breakdown at 17.8 and 42.7 mM K+ was antagonised by veratrine (5-80 microM), 4-aminopyridine (5 mM), and tetraethylammonium (20 mM). These compounds, however, also inhibited the binding of [3H]quinuclidinyl benzilate to cortical membranes. BRL 34915 (0.2-20 microM) was without significant effect on carbachol-stimulated inositol phospholipid breakdown at either 5.7 or 17.8 mM K+.Mg2+ (10 mM) considerably reduced the carbachol-stimulated inositol phospholipid breakdown at 17.8, but not 42.7, mM K+. Inositol phospholipid breakdown was also stimulated, albeit to a small extent, by L-glutamate (100-3,000 microM) and quisqualate (1-100 microM), with the stimulation being additive to that produced by carbachol at both 5.7 and 17.8 mM K+. N-Methyl-D-aspartate (10-1,000 microM in Mg2+-free medium) had no significant effect on basal inositol phospholipid breakdown and had little or no effect on carbachol-stimulated inositol phospholipid breakdown at either 5.7 or 17.8 mM K+. It is concluded that it may not be correct to ascribe wholly the enhancement by K+ of carbachol-stimulated inositol phospholipid breakdown to the tissue-depolarising actions of this ion and that other actions of K+ may be involved.

Evidence for the involvement of Na+/Ca2+ exchange in the stimulation of inositol phospholipid hydrolysis by sodium channel activation and depolarization

Amiloride, an inhibitor of the Na+/Ca2+ exchanger, blocked the hydrolysis of inositol phospholipids in mouse cerebrocortical slices induced by the sodium channel activator veratridine, by KCl, or by the sodium ionophore monensin; there was no inhibition by A 23187, a Ca2+ ionophore, or by serotonin. It is concluded that agents that increase intracellular Na2+ stimulate inositide hydrolysis by an indirect effect via Na+/Ca2+ exchange.

Fluoride inhibits agonist-induced formation of inositol phosphates in rat cortex

Sodium fluoride inhibited carbachol, 5-hydroxytryptamine and noradrenaline stimulated formation of inositol phosphates in rat cerebral cortex. For example, carbachol (1 mM) induced a 337% increase of inositol phosphates above basal in 30 min which was reduced to 69% in the presence of NaF (10 mM). The IC50 for NaF was approximately 1.5 mM and inhibition was mediated by a decrease in maxima of the carbachol dose response curve rather than a shift to the right. This inhibitory action was not mimicked by NaBr or NaI, or by agents which increase cAMP. Inhibition did not appear to result from a toxic action of NaF since it had no effect on the formation of inositol phosphates by high K+; moreover, in higher concentrations NaF stimulated phospholipase C activity. Since fluoride ions are known to activate G-proteins in the concentrations used in this study, these results may indicate the existence of a novel G-protein linked to receptor inhibition of phospholipase C.