Formation of second messengers in response to activation of ion channels in excitable cells

Transmembrane H+ fluxes and the regulation of neural induction in Xenopus laevis

It has previously been reported that in ex vivo planar explants prepared from Xenopus laevis embryos, the intracellular pH (pHi) increases in cells of the dorsal ectoderm from stage 10.5 to 11.5 (i.e. 11-12.5 hpf). It was proposed that such increases (potentially due to H+ being extruded, sequestered, or buffered in some manner), play a role in regulating neural induction. Here, we used an extracellular ion-selective electrode to non-invasively measure H+ fluxes at eight locations around the equatorial circumference of intact X. laevis embryos between stages 9-12 (˜7-13.25 hpf). We showed that at stages 9-11, there was a small H+ efflux recorded from all the measuring positions. At stage 12 there was a small, but significant, increase in the efflux of H+ from most locations, but the efflux from the dorsal side of the embryo was significantly greater than from the other positions. Embryos were also treated from stages 9-12 with bafilomycin A1, to block the activity of the ATP-driven H+ pump. By stage 22 (24 hpf), these embryos displayed retarded development, arresting before the end of gastrulation and therefore did not display the usual anterior and neural structures, which were observed in the solvent-control embryos. In addition, expression of the early neural gene, Zic3, was absent in treated embryos compared with the solvent controls. Together, our new in vivo data corroborated and extended the earlier explant-derived report describing changes in pHi that were suggested to play a role during neural induction in X. laevis embryos.

A genome-wide linkage scan of familial benign recurrent vertigo: linkage to 22q12 with evidence of heterogeneity

Benign recurrent vertigo (BRV) is a common disorder affecting up to 2% of the adult population and may be etiologically related to migraine because of similarities in the clinical spectrum of the phenotypes and a high co-morbidity within families. Many families have multiple-affected genetically related individuals suggesting familial transmission of the disorder with moderate to high penetrance. While clinically similar to episodic ataxias, there are currently no genes identified that contribute to BRV and no systematic linkage studies performed. In an initial effort to genetically define BRV, we have selected from our Neurology Clinic population a subset of 20 multigenerational families with apparent autosomal dominant transmission, and performed genetic linkage mapping using both parametric and non-parametric linkage (NPL) approaches. The Affymetrix 10K SNP Mapping Assay was used for the genotyping. Heterogeneity LOD (HLOD) analysis reveals the evidence of genetic heterogeneity for BRV and evidence of linkage in a subset of the families to 22q12 (HLOD = 4.02). An additional region was identified by NPL analysis at 5p15 (LOD = 2.63). As migraine is observed substantially more commonly both within the BRV-affected individuals and the related family members, it is possible that a form of migraine is allelic to the BRV locus at 22q12. However, testing linkage or the chromosome 22q12 region to a broader migraine/vertigo phenotype by defining affectation status as either migrainous headaches or BRV greatly weakened the linkage signal, and no significant other peaks were detected. Thus, BRV and migraine does not appear to be allelic disorders within these families. We conclude that BRV is a heterogeneous genetic disorder, appears genetically distinct from migraine with aura and is linked to 22q12. Additional family and population-based linkage and association studies will be needed to determine the causative alleles.

Mechanisms of pyrethroid neurotoxicity: implications for cumulative risk assessment

The Food Quality Protection Act (FQPA) of 1996 requires the United States Environmental Protection Agency to consider the cumulative effects of exposure to pesticides having a 'common mechanism of toxicity.' This paper reviews the information available on the acute neurotoxicity and mechanisms of toxic action of pyrethroid insecticides in mammals from the perspective of the 'common mechanism' statute of the FQPA. The principal effects of pyrethroids as a class are various signs of excitatory neurotoxicity. Historically, pyrethroids were grouped into two subclasses (Types I and II) based on chemical structure and the production of either the T (tremor) or CS (choreoathetosis with salivation) intoxication syndrome following intravenous or intracerebral administration to rodents. Although this classification system is widely employed, it has several shortcomings for the identification of common toxic effects. In particular, it does not reflect the diversity of intoxication signs found following oral administration of various pyrethroids. Pyrethroids act in vitro on a variety of putative biochemical and physiological target sites, four of which merit consideration as sites of toxic action. Voltage-sensitive sodium channels, the sites of insecticidal action, are also important target sites in mammals. Unlike insects, mammals have multiple sodium channel isoforms that vary in their biophysical and pharmacological properties, including their differential sensitivity to pyrethroids. Pyrethroids also act on some isoforms of voltage-sensitive calcium and chloride channels, and these effects may contribute to the toxicity of some compounds. Effects on peripheral-type benzodiazepine receptors are unlikely to be a principal cause of pyrethroid intoxication but may contribute to or enhance convulsions caused by actions at other target sites. In contrast, other putative target sites that have been identified in vitro do not appear to play a major role in pyrethroid intoxication. The diverse toxic actions and pharmacological effects of pyrethroids suggest that simple additivity models based on combined actions at a single target are not appropriate to assess the risks of cumulative exposure to multiple pyrethroids.

Glucocorticoid-induced plasma membrane depolarization during thymocyte apoptosis: association with cell shrinkage and degradation of the Na(+)/K(+)-adenosine triphosphatase

Multiple signaling pathways are known to induce apoptosis in thymocytes through mechanisms that include the loss of mitochondrial membrane potential, cell shrinkage, caspase activation, and DNA degradation but little is known about the consequences of apoptosis on the properties of the plasma membrane. We have previously shown that apoptotic signals, including survival factor withdrawal and glucocorticoids, induce plasma membrane depolarization during rat thymocyte apoptosis, but the mechanisms involved in this process are unknown. We report here that inhibition of the Na(+)/K(+)-adenosine triphosphatase (Na(+)/K(+)-ATPase) with ouabain similarly depolarized control thymocytes and enhanced glucocorticoid-induced membrane depolarization, suggesting a link between Na(+)/K(+)-ATPase and plasma membrane depolarization of thymocytes. To determine whether repression of Na(+)/K(+)-ATPase levels within cells can account for the loss of plasma membrane potential, we assessed protein levels of the Na(+)/K(+)-ATPase in apoptotic thymocytes. Spontaneously dying thymocytes had decreased levels of both catalytic and regulatory subunits of Na(+)/K(+)-ATPase, and glucocorticoid treatment enhanced the loss of Na(+)/K(+)-ATPase protein. The pan caspase inhibitor (z-VAD) blocked both cellular depolarization and repression of Na(+)/K(+)-ATPase in both spontaneously dying and glucocorticoid-treated thymocytes; however, specific inhibitors of caspase 8, 9, and caspase 3 did not. Interestingly, glucocorticoid treatment simultaneously induced cell shrinkage and depolarization. Furthermore, depolarization and the loss of Na(+)/K(+)-ATPase protein were limited to the shrunken population of cells. The data indicate an important role for Na(+)/K(+)-ATPase in both spontaneous and glucocorticoid-induced apoptosis of rat thymocytes.

Regulation of Na+,K+-ATPase by persistent sodium accumulation in adult rat thalamic neurones

The present study investigated the regulatory mechanism of the Na+, K+-ATPase and the level of internal Na+ and Ca2+ in response to persistent Na+ influx in acutely dissociated rat thalamic neurones. Whole-cell patch-clamp recordings and Na+ imaging revealed a stable [Na+]i and low background pump activity. Exposure to veratridine (50 microM) for 1 h resulted in a progressive rise in [Na+]i (DeltaFNa = 64 +/-22%) and [Ca2+]i (DeltaFCa = 44 +/- 14%) over 3 h. Increases in [Na+]i and [Ca2+]i were also observed during neuronal exposure to the Na+ ionophore monensin (50 microM). Subcellular confocal immunofluorescence quantification of alpha3 catalytic Na+-K+ pump subunits showed that a veratridine-induced rise in [Na+]i was accompanied by a significant increase in pump density in both membrane and cytoplasmic compartments, by 39 and 54%, respectively. Similar results were also obtained in experiments when neurones were treated with monensin. A fluorescent 9-anthroylouabain binding assay detected a 60 and 110% increase in phosphorylated (active) pumps after veratridine and monensin exposure, respectively. During the entire experiment, application of ouabain or veratridine alone induced little cell swelling and death, but pump inhibition in cells pre-loaded with Na+ led to rapid cell swelling and necrosis. The above results indicate that a persistent influx of Na+ may trigger rapid enhancement of pump synthesis, membrane redistribution and functional activity. However, these compensatory mechanisms failed to prevent persistent Na+ accumulation.

Sodium overload through voltage-dependent Na(+) channels induces necrosis and apoptosis of rat superior cervical ganglion cells in vitro

Using the failure to exclude trypan blue as a criterion for cell death, we found that veratridine, the voltage-dependent Na(+) channel activator, exerted its toxicity to cultured sympathetic neurons in a dose-dependent manner (half-maximal toxicity occurred at 2 microM). The co-presence of tetrodotoxin completely reversed the toxicity only at concentrations of veratridine < 20 microM. Veratridine neurotoxicity was due to the influx of Na(+); a medium low in Na(+) (36 mM) completely abolished its neurotoxicity, whereas a Ca(2+)-free medium did not attenuate its neurotoxicity. Furthermore, the buffering action of 1, 2-Bis-(2-aminophenoxy)ethane-N,N,N',N',-tetraacetate (BAPTA) on veratridine-induced increase in intracellular Ca(2+) levels neither blocked veratridine neurotoxicity in normal medium, nor attenuated the low Na(+) effect. Elevated K(+) effectively blocked veratridine neurotoxicity in a Ca(2+)-dependent manner. Cytoplasmic pH measurements using a fluorescent pH indicator demonstrated that cellular acidification (from pH 7.0 to pH 6.5) occurred upon treatment with veratridine. Both veratridine-induced acidification and cell death were ameliorated by 5-(N-ethyl-N-isopropyl)amiloride, the specific inhibitor of the Na(+)/H(+) exchanger (IC(50) = 0.5 microM). Finally, necrosis occurred predominantly in veratridine neurotoxicity, but both staining with bis-benzimide and TUNEL analysis showed nuclear features of apoptosis in sympathetic neurons undergoing cell death.

Comparison of the effects of a series of kappa-opioid receptor agonists upon sodium channel function in rat brain miniprisms

The blockade of veratrine-stimulated phosphoinositide breakdown in rat cerebral cortical miniprisms has been used as a model of drug action on voltage-dependent sodium channels. The kappa-opioid agonists bremazocine, (+/-)- and (+)-trans-U-50488, U-62066 (spiradoline) and U-69593 inhibited the response to veratrine with IC50 values of 35, 13, 15, 9, and > 100 microM, respectively. Bremazocine, at concentrations inhibiting the response to veratrine, did not inhibit the phosphoinositide breakdown response to the sodium ionophore monensin, indicating the specificity of the assay for sodium channels. The inhibitory actions of bremazocine upon veratrine-stimulated phosphoinositide breakdown were not antagonised by naloxone. This study thus confirms previous data suggesting that the kappa-opioid receptor agonists can affect Na(+)-channel function in a manner unrelated to their actions at kappa-opioid receptors. However, for the compounds tested, such effects are only found at rather high concentrations of the compounds.

Veratridine delays apoptotic neuronal death induced by NGF deprivation through a Na(+)-dependent mechanism in cultured rat sympathetic neurons

Superior-cervical ganglion (SCG) cells dissociated from newborn rats depend on nerve growth factor (NGF) for survival. Membrane depolarization with elevated K+ is known to prevent neuronal death following NGF deprivation and/or to promote survival via a Ca(2+)-dependent mechanism. Here we have exploited the possibility of whether or not a Na(+)-dependent pathway for neuronal survival is present in these cells. Veratridine (EC50 = 40 nM), a voltage-dependent Na+ channel activator, significantly delayed the onset of apoptotic cell death in NGF-deprived SCG neurons that had been cultured for 7 days in the presence of NGF. This effect was blocked completely by Na+ channel blockers including tetrodotoxin (TTX, 1 microM), benzamil (25 microM) and flunarizine (1 microM), but was not attenuated by nimodipine (1 microM), an L-type Ca2+ channel blocker. The saving effect of veratridine on cultured neurons was observed even in low Ca2+ media (0-1.0 mM), but was completely abolished in a low Na+ medium (38 mM). Sodium-binding benzofuran is isophthalate was employed as a fluorescent probe for monitoring the level of cytoplasmic free Na+, which revealed a sustained increase in its level (12.9 mM, 307% of that of control) in response to veratridine (0.75 microM). The TTX or flunarizine completely blocked veratridine-induced Na+ influx in these cultured neurons. Moreover, no appreciable increase in intracellular Ca2+ was detected under these conditions. Though Na+ channels were effectual in SCG neurons which were freshly isolated from newborn rats, the Na(2+)-dependent saving effect of veratridine was not observed in these young neurons. These lines of evidence suggest that the death-suppressing effect of veratridine on cultured SCG neurons depends on the Na+ influx via voltage-dependent Na+ channels, and suggest the presence of Na(+)-dependent regulatory mechanism(s) in neuronal survival.

Effects of excitatory amino acids on phosphoinositide metabolism in frog retina

The effects of excitatory amino acid receptor agonists on the hydrolysis of phosphoinositides were examined using frog retinal membranes prelabeled in vitro with either 32PO4 or [3H]inositol. Glutamate stimulated release of [3H]inositol phosphates (IPs) from the retinas and altered the 32P-labeling pattern of phosphatidylinositol (PI) cycle intermediates. This indicates that glutamate affects not only the hydrolysis of phosphoinositides but possibly other steps involved in the PI cycle. Among glutamate analogs, kainate (KA), quisqualate (QA), and, to a lesser extent, N-methyl-D-aspartate (NMDA) mimicked the glutamate effect, whereas L-2-amino-4-phosphonobutyrate (L-AP4) was not effective in causing either the accumulation of [3H]IPs or the alteration of the 32P-labeling pattern of PI cycle intermediates. Among QA specific agonists, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), but not ibotenate (IBO) or trans-1-aminocyclopentane-1,3-dicarboxylate (ACPD) was active in stimulating IPs formation. KA effect on IPs formation may be due to indirect (polysynaptic) activation of receptor(s) other than L-AP4, IBO, or ACPD specific QA receptors. To avoid activating polysynaptic pathways, retinal synaptoneurosomes prelabeled with [3H]inositol were used to examine the hydrolysis of phosphoinositides. As in whole retinas, KA, carbachol (CARB), and NMDA stimulated the release of IPs while L-AP4 had minimal effect. Glycine (GLY) had no effect. Our results show CARB and KA to be the most effective in stimulating the production of IPs. Their effects were exerted directly through separate receptors and not through polysynaptic pathways. ACPD and IBO were the least effective in eliciting the release of IPs. Our studies provide evidence that ionotropic and not metabotropic glutamate receptors are involved in PI metabolism in the retina.

Potassium-Induced Depolarization of Rat Telencephalic Synaptoneurosomes Increases [3H]Amino-3-Hydroxy-5-Methylisoxazole-4-Propionate Receptor Binding

Potassium-induced depolarization of synaptoneurosomes prepared from rat telencephalon was found to increase [3H]amino-3-hydroxy-5-methylisoxazole-4-propionate ([3H]AMPA) binding to the AMPA receptor. The effect required the presence of calcium because it was blocked by the calcium chelator EGTA but was not blocked by an antagonist of the N-methyl-D-aspartate receptor, aminophosphonopentanoate. The depolarization-induced increase in [3H]AMPA binding was markedly reduced by a blocker of voltage-dependent calcium channels, verapamil. Saturation kinetic experiments revealed that the increase in [3H]AMPA binding produced by potassium depolarization was due to an increase in the affinity of the AMPA receptor. These results provide additional support for a critical role of calcium in the regulation of the AMPA receptors. The synaptoneurosome preparation might represent an interesting tool to determine the role of different calcium-dependent enzymes involved in the regulation of the AMPA receptor.

A specific transduction mechanism for the glutamate action on phosphoinositide metabolism via the quisqualate metabotropic receptor in rat brain synaptoneurosomes: II. Calcium dependency, cadmium inhibition

In this article, we demonstrate that an increase in intracellular Ca2+ concentration may represent a specific common step(s) in the mechanism(s) of action of glutamate (Glu) and depolarizing agents on formation of inositol phosphates (IPs) in 8-day-old rat forebrain synaptoneurosomes. In fact, A23187, a Ca2+ ionophore, induces a dose-dependent accumulation of IPs, which is not additive with that evoked by Glu and K+ but is slightly synergistic with that induced by carbachol. In addition, Glu and K+ augment the intracellular Ca2+ concentration in synaptoneurosome preparations as measured by the fura-2 assay. The absence of external Ca2+ decreases basal and Glu-, and K(+)-stimulated formation of IPs. Cd2+ (100 microM) fully inhibits both Glu- and K(+)-evoked formation of IPs without affecting the carbachol-elicited response of IPs. Zn2+ inhibits Glu- and K(+)-stimulated accumulation of IPs (IC50 approximately 0.4 mM) but with a lower affinity than Cd2+ (IC50 approximately 0.035 mM). The organic Ca2+ channel blockers verapamil (10 microM), nifedipine (10 microM), omega-conotoxin (2 microM), and amiloride (10 microM) as well as the inorganic blockers Co2+ (100 microM) and La3+ (100 microM) block neither Glu- nor K(+)-evoked formation of IPs, a result suggesting that the opening of the L-, T-, N-, or P-type Ca2+ channels does not participate in these responses. All these data suggest that an increase in intracellular Ca2+ concentration resulting from an influx of Ca2+, sensitive to Cd2+ but not to other classical Ca2+ antagonists, may play a key role in the transduction mechanism activated by Glu or depolarizing agents.

A specific transduction mechanism for the glutamate action on phosphoinositide metabolism via the quisqualate metabotropic receptor in rat brain synaptoneurosomes: I. External Na+ requirement

The characteristics of the transduction mechanism(s) activated by glutamate (Glu) via the quisqualate metabotropic receptor, as well as by depolarizing agents, to trigger formation of inositol phosphates (IPs) were investigated in 8-day-old rat forebrain synaptoneurosomes. The replacement of external Na+ by various compounds (Li+, Tris+, N-methyl-D-glucamine+, and sucrose) induces an increase in basal accumulation of IPs and depolarizes synaptoneurosome membranes. Under these conditions, Glu- and K(+)-induced accumulations of IPs are inhibited, whereas the carbachol (Carb)-elicited response of IPs parallels the basal one. Agents increasing Na+ influx, such as veratridine and monensin, depolarize synaptoneurosomes and stimulate formation of IPs. These stimulations are not additive with responses of IPs elicited by Glu or K+. These data suggest that (a) Glu activates phosphoinositide metabolism via a specific mechanism (distinct from that of cholinergic agonists), (b) depolarizing agents and Glu share at least one common intermediate step in their mechanisms of activation of the metabolism of IPs, and (c) the depolarization may correspond to this common step. In addition, Na+ seems to be required for Glu stimulation of metabolism of IPs. The depolarization associated with the action of Glu on formation of IPs results neither from an influx via tetrodotoxin-sensitive voltage-dependent Na+ channels nor from an entry via the classically characterized Na+/Ca2+ or Na+/H+ exchangers. In fact, tetrodotoxin (2 microM) has no effect on the Glu- or K(+)-elicited response of IPs. Amiloride (greater than 50 microM) and some of its derivatives similarly inhibit not only Glu- and K(+)- but also Carb-evoked formation of IPs.

Effects of different types of stimulation on cyclic AMP content in the rabbit carotid body: functional significance

Cyclic AMP levels in rabbit carotid bodies incubated under control conditions, 100% O2- or 95% O2/5% CO2- equilibrated medium, are close to 1 pmol/mg wet tissue (range 0.4-2.43 pmol/mg). Isobutylmethylxanthine (0.5 mM) increases cyclic AMP levels by a factor of 14 and 8 in HEPES- and CO2/CH3O(-)-buffered medium, respectively. Forskolin (0.5-10 microM) applied during 30 min increases cyclic AMP levels in a dose-dependent manner. Incubation of carotid bodies at low O2 tensions resulted in an elevation of cyclic AMP levels both in the absence and in the presence of isobutymethylxanthine. In the latter conditions cyclic AMP increase was maximum at an O2 tension of 46 mm Hg and tended to decrease at extremely low PO2. In isobutylmethylxanthine-containing Ca2(+)-free medium, cyclic AMP increased linearly with decreasing PO2 from 66 to 13 mm Hg; the absolute cyclic AMP levels attained in Ca2(+)-free medium were smaller than those observed in Ca2(+)-containing medium at any PO2. The differences between Ca2(+)-free and Ca2(+)-containing media appear to be due to the action of released neurotransmitters in the latter conditions, because dopamine and norepinephrine, which are known to be released by hypoxia in a Ca2(+)-dependent manner, increase cyclic AMP in the carotid body. Low pH/high PCO2 and high [K+]e increase cyclic AMP levels only in Ca2(+)-containing medium. Forskolin potentiates the release of catecholamines induced by low PO2. These results suggest that cyclic AMP plays an important role in the modulation of the chemoreception process.

Neurotoxicological effects and the mode of action of pyrethroid insecticides

Neuroexcitatory symptoms of acute poisoning of vertebrates by pyrethroids are related to the ability of these insecticides to modify electrical activity in various parts of the nervous system. Repetitive nerve activity, particularly in the sensory nervous system, membrane depolarization, and enhanced neurotransmitter release, eventually followed by block of excitation, result from a prolongation of the sodium current during membrane excitation. This effect is caused by a stereoselective and structure-related interaction with voltage-dependent sodium channels, the primary target site of the pyrethroids. Near-lethal doses of pyrethroids cause sparse axonal damage that is reversed in surviving animals. After prolonged exposure to lower doses of pyrethroids axonal damage is not observed. Occupational exposure to pyrethroids frequently leads to paresthesia and respiratory irritation, which are probably due to repetitive firing of sensory nerve endings. Massive exposure may lead to severe human poisoning symptoms, which are generally treated well by symptomatic and supportive measures.

Modification of the Na-dependent action potential in myelinated fibers of rat sciatic nerve exposed to phorbol ester

Exposure of rat sciatic nerve to the active phorbol 1,2-beta-myristate-13-acetate (b-PMA), but not to the active analogue 4-alpha-phorbol-12,13-didecanoate (a-PDD), is followed by a decrease of the compound action potential amplitude, rate of rise, and conduction velocity, and an increase of the threshold, and of the duration of the refractory period. The effect is concentration-dependent, the Kd being 250 nM. The attenuated Na-dependent action potential is tetrodotoxin (TTX)-sensitive, but after exposure to b-PMA the sensitivity to TTX is decreased from Kd = 45 nM to 400 nM. Action potential depression is larger when Ca is replaced by Mg (but not by Ba), or when Na is replaced by Li. The replacement of K by Cs, or exposure to potassium channel blockers such as 4-aminopyridine (4AP) and tetra-ethyl ammonium (TEA) has no effect. The results indicate that in the myelinated axons of rat sciatic nerve, exposure to b-PMA induces modification of Na channels.

Calcium-mobilizing receptors, polyphosphoinositides, generation of second messengers and contraction in the mammalian iris smooth muscle: historical perspectives and current status

It is well established now that activation of Ca2+ -mobilizing receptors results in the phosphodiesteratic breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2), instead of phosphatidylinositol (PI), into myoinositol 1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DG). There is also accumulating experimental evidence which indicates that IP3 and DG may function as second messengers, the former to mobilize Ca2+ from intracellular sites and the latter to activate protein kinase C (PKC). In this review, I have recounted our early studies, which began in 1975 with the original observation that activation of muscarinic cholinergic and adrenergic receptors in the rabbit iris smooth muscle leads to the breakdown of PIP2, instead of PI, and culminated in 1979 in the discovery that the stimulated hydrolysis of PIP2 results in the release of IP3 and DG and that this PIP2 breakdown is involved in the mechanism of smooth muscle contraction. In addition, I have summarized more recent work on the effects of carbachol, norepinephrine, substance P, the platelet-activating factor, prostaglandins, and isoproterenol on PIP2 hydrolysis, IP3 accumulation, DG formation, myosin light chain (MLC) phosphorylation, cyclic AMP production, arachidonic acid release (AA) and muscle contraction in the iris sphincter muscle. These studies suggest: (a) that the IP3-Ca2+ signalling system, through the Ca2+ -dependent MLC phosphorylation pathway, is probably the primary determinant of the phasic component of the contractile response; (b) that the DG-PKC pathway may not be directly involved in the tonic component of muscle contraction, but may play a role in the regulation of IP3 generation; (c) that there are biochemical and functional interactions between the IP3-Ca2+ and the cAMP second messenger systems, cAMP may act as regulator of muscle responses to agonists that exert their action through the IP3-Ca2+ system; and (d) that enhanced PIP2 turnover is involved in desensitization and sensitization of alpha 1-adrenergic- and muscarinic cholinergic-mediated contractions of the dilator and sphincter muscles of the iris, respectively. The contractile response is a typical Ca2+ -dependent process, which makes smooth muscle an ideal tissue to investigate the second messenger functions of IP3 and DG and their interactions with the cAMP system.