The role of the sarcolemmal Ca(2+)-ATPase in the pH transients associated with contraction in rat smooth muscle

1. We have investigated the origin of the intracellular acid pH transients that accompany myometrial contraction. Intra- and extracellular pH were measured with SNARF and intracellular Ca2+ concentration ([Ca2+]i) with indo-1. 2. An intracellular acidification accompanied spontaneous contractions and those elicited by KCl depolarization or the addition of the agonists carbachol or prostaglandin F2 alpha. The size of the acidification increased with the magnitude of the contraction. 3. The intracellular acidification was accompanied by an extracellular alkalinization, showing that it results from proton movement across the surface membrane. Furthermore, it was decreased either by addition of Cd2+ (20 nM, an inhibitor of the sarcolemmal Ca(2+)-ATPase) or by elevating [Ca2+]o. 4. Extracellular alkalinization increased the magnitude of the rise of [Ca2+]i and force produced by KCl. 5. An intracellular acidification was also associated with contraction in the portal vein and ureter. 6. We conclude that the sarcolemmal Ca(2+)-ATPase produces a significant intracellular acidification while removing Ca2+. Both the acidification and decrease of [Ca2+]i will promote relaxation. Since Ca2+ and protons have opposite effects on many cellular processes, this dual regulation by these two ions may be of general importance.

Simultaneous measurements of electrical activity, intracellular [Ca2+] and force in intact smooth muscle

Although both electrical activity and changes in intracellular [Ca2+] are known to be important determinants of smooth muscle force, little is known about the relationship between the three in intact muscle. This is due to a lack of simultaneous measurements of these parameters. In this paper we describe how we have combined the sucrose gap technique with microspectrophotometry and force recording in rat ureteric smooth muscle. We have investigated the timecourses of the changes in these parameters following physiological stimulation i.e. the action potential. The results of this paper show that it is possible to simultaneously measure electrical activity, [Ca] and force in intact smooth muscle and that (i) about a third of the change in Ca2+ occurs on the upstroke of the action potential and the remainder during the plateau. (ii) 50% of the decline in [Ca2+] occurs after the repolarization of the action potential and (iii) the kinetics of force development and relaxation are significantly slower than those of Ca2+, suggesting that [Ca2+] is not rate-limiting. These are the first such simultaneous measurements in any smooth muscle.

Agonist mobilization of sarcoplasmic reticular calcium in smooth muscle: functional coupling to the plasmalemmal Na+/Ca2+ exchanger?

There is a close association of peripheral sarcoplasmic reticulum (SR), containing IP3 receptors, and regions of the plasma membrane enriched in the Na+/Ca2+ exchanger in smooth muscle. We have tested the possibility in rat uterine smooth muscle that Ca2+ released from the SR is preferentially removed from the cytosol by the Na+/Ca2+ exchanger. In Ca(2+)-free solution, carbachol stimulation of myometria of non-pregnant rats resulted in transient increases in [Ca2+]i and force due entirely to the release of SR Ca2+. Inhibition of Na+/Ca2+ exchange by removal of extracellular Na+ did not alter the agonist-induced transients suggesting that Na+/Ca2+ exchange was not involved in the removal of SR released Ca2+. However, in myometria of pregnant rats, Na+/Ca2+ exchange inhibition resulted in changes in the agonist-induced [Ca2+]i transient profiles. The peak amplitude, duration and integral of carbachol-induced [Ca+]i transients were enhanced in Ca(2+)-free/Na(+)-free solution without significantly affecting force transients. The lower rate of decay of [Ca2+]i transients in Na(+)-free solution leads us to suggest that up to 35% of the SR released Ca2+ may be extruded by the Na+/Ca2+ exchanger in myometria of pregnant rats. Thus, in uterine smooth muscle, there is a gestational-dependent coupling of SR releasable Ca2+ and plasmalemmal Na+/Ca2+ exchange activity.

External alkalinization decreases intracellular Ca++ and spontaneous contractions in pregnant rat myometrium

OBJECTIVES

As plasma pH rises during pregnancy, the effect of raising external pH on spontaneous contractions in pregnant rat myometrium was investigated to test the hypothesis that elevated external pH depresses contraction.

STUDY DESIGN

Strips of longitudinal myometrium were loaded with SNARF (seminaphthorhodafluor) or Indo-1 for simultaneous intracellular pH or Ca++ and force measurements. Results were obtained from a minimum of five animals in each group, and significant differences were tested for by paired Student t tests.

RESULTS

Raising the external pH significantly reduced spontaneous force and calcium transient in the pregnant uterus. Raising the external pH led to a slow rise in intracellular pH, but this could not account for the functional effect. K+ rubidium 86-labeled efflux rates were unaffected by external pH, suggesting no hyperpolarization. The Ca++ channel agonist Bay K8644 (5 mumol/L) restored contractions abolished by raised external pH.

CONCLUSIONS

Raised external pH reduces spontaneous contractions in the pregnant rat uterus, probably by an external effect on Ca++ entry. This effect may contribute to uterine quiescence before term.

Neuronal dopamine subpopulations maintained in hypothalamic slice explant cultures exhibit distinct tyrosine hydroxylase mRNA turnover rates

Changes in mRNA stability have been shown to regulate critical intracellular processes. In this investigation, we studied tyrosine hydroxylase (TH) mRNA turnover in functionally and anatomically distinct dopaminergic (DA) populations of the rat hypothalamus. To this end, long-term slice explant cultures from postnatal, preoptic area/hypothalami, containing three anatomically discrete DA populations, were generated and maintained under defined conditions. The organotypic cultures were treated with the transcription inhibitors 5,6-dichloro-1-D-ribofuranosylbenzimidazole or actinomycin D and processed for in situ hybridization histochemistry. Relative TH mRNA content per cell was quantitated. Single-cell analysis showed marked differences in basal TH mRNA turnover rates between DA neuronal populations. Anterior and midhypothalamic DA neurons exhibited half-time turnovers of 9-12 and 11-23 hr, respectively. In contrast, in the caudal hypothalamus, DA neurons of the arcuate nucleus had a significantly lower baseline level and more rapid turnover (6-7 hr) of TH mRNA. This investigation shows that basal turnover of a phenotypic mRNA, TH mRNA in DA neurons, is not an intrinsic property of the phenotypic marker. Furthermore, we found that destabilization of TH mRNA in the caudal hypothalamus corresponds to the known rhythmic output displayed by arcuate DA cells and, as such, may be critical for normal function of this population. We propose that intrinsic differences in the post-transcriptional regulation of TH permits neuronal subpopulations, which subserve different physiological functions, an additional mechanism to control DA biosynthesis in response to their unique needs.

An alternative gonadotropin-releasing hormone (GnRH) RNA splicing product found in cultured GnRH neurons and mouse hypothalamus

Gonadotropin-releasing hormone (GnRH) is encoded by the proGnRH gene which contains four exons and three introns. In this study, two immortalized GnRH-expressing cell lines (Gn11 and NLT) were characterized. The NLT and Gn11 cells, derived from a same brain tumor in a transgenic mouse, display neuronal morphology and neuron-specific markers. However, NLT cells secrete much higher levels of GnRH than Gn11 cells. To delineate the mechanism underlying this difference, reverse transcriptase-polymerase chain reaction and RNase protection assays were performed to examine proGnRH gene expression. While the mature proGnRH mRNA was predominately expressed in NLT cells, Gn11 cells express an abundant short transcript. Sequence analysis revealed that this short transcript contains exons 1, 3, and 4, but not exon 2, which encodes the GnRH decapeptide. RNase protection assays demonstrated that NLT cells express much higher levels of mature proGnRH mRNA than Gn11 cells. The lower level of GnRH secreting capacity in Gn11 cells is due, in part, to decreased expression of mature proGnRH mRNA. When proGnRH gene expression in the mouse brain was examined, the same short splicing variant was observed in the olfactory area and preoptic area-anterior hypothalamus. But the prevalent transcript in these regions was the mature proGnRH mRNA. In contrast, only the mature proGnRH mRNA was found in the caudal hypothalamus. These results suggest that alternative splicing may be one of the mechanisms regulating proGnRH gene expression in the animal brain.

Effect of metabolic inhibition on intracellular Ca2+, phosphorylation of myosin regulatory light chain and force in rat smooth muscle

1. The effect of the inhibition of oxidative phosphorylation on intracellular calcium concentration ([Ca2+]i), phosphorylation of the 20 kDa regulatory light chain of myosin (MLC20) and contractility was investigated in isolated longitudinal smooth muscle from rat uteri. 2. Cyanide (2 mM) application to normally polarized preparations resulted in an elevation of basal [Ca2+]i but an inhibition of [Ca2+]i transients and the accompanying contractions. 3. Depolarization with high-K+ solution (40 mM KCI) resulted in elevation of [Ca2+]i and maintained force production. Phosphorylation of MLC20 was transiently increased followed by a steady-state augmentation above resting levels. 4. Carbachol (100 microM) produced a transient elevation of [Ca2+]i and force of depolarized tissues followed by a steady-state augmentation of both parameters. PGF2 alpha (1 microM) did not significantly potentiate [Ca2+]i or force in depolarized preparations. Both carbachol and PGF2 alpha potentiated phosphorylation of MLC20 in depolarized tissues. 5. Addition of cyanide to depolarized preparations, in the presence or absence of carbachol or PGF2 alpha, resulted in significant attenuation of force under each condition. The magnitude and normalized rates of force inhibition by cyanide were not significantly different for each stimulus condition. MLC20 phosphorylation levels were unaltered by cyanide treatment. However, cyanide increased the maintained level of [Ca2+]i under each experimental protocol. 6. It is concluded that the inhibition of oxidative phosphorylation with cyanide results in dissociation of both the [Ca2+]i-force and MLC20 phosphorylation-force relationships in rat uterine smooth muscle.

GABAergic neurons in the embryonic olfactory pit/vomeronasal organ: maintenance of functional GABAergic synapses in olfactory explants

In previous work, we showed a robust gamma-aminobutyric acid (GABAergic) synaptic input onto embryonic luteinizing hormone-releasing hormone (LHRH) neurons maintained in olfactory explants. In this study, we identify GABAergic neurons in olfactory pit (OP) of embryonic mice in vivo and study, using patch-pipet whole-cell current and voltage clamp techniques, synaptic interactions of these neurons in explant cultures. In vivo, glutamate decarboxylase (GAD, the enzyme which synthesizes GABA) mRNA was first detected in nasal regions on Embryonic Day (E) 11.5. From E12.5 to E13.5, robust GAD expression was localized to cells primarily in the ventral aspect of the OP. GAD mRNA was not detected over dorsally located cells in olfactory sensory or respiratory epithelium. In addition, GAD mRNA was not observed in cells along olfactory axons. GAD mRNA was dramatically reduced in the OP/vomeronasal organ by E16.5. Using antibodies against both GABA and GAD, immunopositive axonal-like tracts were detected in the nasal septum on E12.5. GABAergic staining decreased by E13.5. To examine synaptic interactions of these GABAergic cells, embryonic olfactory explants were generated and maintained in serum-free media. As explants spread, neuron-like cells migrated into the periphery, sometimes forming ganglion-like clusters. Cells were recorded, marked intracellularly with Lucifer Yellow and post-fixation, immunocytochemically examined. Forty-six cells, typically multipolar, were GABAergic, had resting potentials around -50 mV, and exhibited spontaneous action potentials which were generated by spontaneous depolarizing GABAergic (GABAA) synaptic activity. OP neurons depolarized in response to GABA by increasing Cl- conductance. The biophysical properties of OP-derived GABAergic neurons were distinct from those reported for olfactory receptor neurons but similar to embryonic LHRH neurons. However, unlike LHRH neurons, GABAergic neurons did not migrate large distances in olfactory explants or appear to leave the olfactory pit in vivo.

Stimulus-dependent modulation of smooth muscle intracellular calcium and force by altered intracellular pH

Measurements of simultaneous force and intracellular Ca2+ concentration ([Ca2+]i) in rat uterine smooth muscle have been made to elucidate the mechanisms involved when force produced spontaneously, by high-K+ depolarization or carbachol is altered by a change of intracellular pH (pHi). Rises in force and [Ca2+]i were closely correlated for all forms of contraction, with the Ca2+ transient peaking before force. In spontaneously active preparations, alkalinization significantly increased, and acidification decreased, force and [Ca2+]i. Inhibition of the sarcoplasmic reticulum ATPase (cyclopiazonic acid) did not affect these changes, whereas removal of external Ca2+ abolished both responses, suggesting that the effect of pHi is on Ca2+ entry. Alkalinization caused a prolongation of the action potential complex, associated with a potentiation of contractile activity. Acidification produced hyperpolarization and abolition of action potentials and spontaneous activity, but did not prevent brief applications of carbachol or high-K+ from producing depolarization and increasing force, suggesting no impairment of the mechanism of generation of the action potential. For depolarized preparations, acidification increased tonic force and [Ca2+]i; the increase in the calcium signal persisted in zero-external calcium. In the presence of carbachol, acidification transiently increased force and [Ca2+]i, followed by a reduction in both. It is concluded that changes in pHi act at more than one step in excitation-contraction coupling and that changes in [Ca2+]i can account for most of the changes in uterine force.

Cell-specific expression of the human gonadotropin-releasing hormone gene in transgenic animals

We have previously demonstrated that 1131 base pairs (bp) of the human gonadotropin-releasing hormone (hGnRH) gene promoter can target simian virus 40 T antigen expression to GnRH neurons in transgenic mice. In these animals, GnRH neurons were transformed before they migrated to their final location in the rostral hypothalamus, complicating an analysis of cell-specific expression. To localize regions of the hGnRH promoter that are important for cell-specific expression, we created transgenic mice with various 5'-flanking regions of the hGnRH gene fused to the luciferase reporter gene. When 3828 or 1131 bp of the hGnRH promoter 5'-flanking DNA were used (-3828/+5LUC and -1131/+5LUC, respectively), luciferase expression in adult transgenic mice was observed in the rostral hypothalamus and olfactory tissues, regions which have been shown to be loci of GnRH-expressing neurons. Luciferase expression was not observed in other brain or peripheral tissues. Double-labeled in situ hybridization further demonstrated that luciferase expression was invariably colocalized with GnRH expression. When transgenic animals were created with a construct consisting of 484 bp of the hGnRH 5'-flanking DNA fused to the luciferase gene (-484/+5LUC), luciferase expression was not observed in the hypothalamus or in olfactory tissues. This is the first report localizing DNA sequences responsible for cell-specific expression of the GnRH gene in vivo.

An investigation into the mechanism whereby pH affects tension in guinea-pig ureteric smooth muscle

1. We have altered intracellular (pHi) and extracellular pH (pHo) in the smooth muscle of guinea-pig ureter and determined the effects on evoked phasic contractions. In order to investigate the mechanisms underlying the effects of pH alteration, intracellular Ca2+ ([Ca2+]i), pHi, electrical activity and force were measured. 2. Intracellular acidification, produced by the weak acid butyrate, application of CO2 at constant pHo or removal of weak bases, greatly increased phasic contractions. Alkalinization with weak bases or by removal of CO2 inhibited contractions. The results were similar whether Hepes or CO2-HCO3-buffered the solutions. 3. Phasic contractions were preceded by intracellular Ca2+ transients in the ureter. Acidification of the cytoplasm led to an increase in the amplitude of the Ca2+ transient, and alkalinization decreased its magnitude. 4. In the ureter the action potential leads to Ca2+ influx, therefore electrophysiological recordings of its configuration were made during alteration of pHi. Acidification led to the action potential duration and amplitude being increased, whereas alkalinization shortened the action potential and reduced its amplitude. 5. As the effects of acidification on the action potential resembled the effects of blocking of K+ channels, we investigated whether pHi alteration was able to alter tension when K+ channels were blocked by tetraethylammonium. Acidification was unable to potentiate force under these conditions nor did alkalinization decrease force. 6. External pH over the range 6.8-8.0 had little or no effect on pHi, phasic contractions and [Ca2+]i. Tonic contractions were enhanced, however, when pHo was increased. 7. These data suggest that pHi alteration in the guinea-pig ureter modulates the action potential, probably by alteration of K+ currents. Subsequent changes in [Ca2+]i and contraction then occur. A potentiating effect of acidic pH on force is not common in muscle, but may be a characteristic of the smooth muscle of the urinary tract. Changes of pHo had little effect on phasic force or pHi, but modulated tonic contractions. The possible physiological significance of these results is discussed.

Simultaneous measurement of intracellular pH, calcium, and tension in rat mesenteric vessels: effects of extracellular pH

In rat mesenteric vessels changes in external pH (pHo) alter tension. Changes in intracellular pH (pHi) and calcium ([Ca2+]i) have both been suggested to underlie the tension changes. As no simultaneous measurements of pHi and [Ca2+]i had been made, however, the relative importance and temporal relationship between pHi [Ca2+]i and tension were unknown. In order therefore to gain a clearer understanding of the mechanisms involved, we have made simultaneous measurements of these parameters using carboxy-SNARF and INDO-1. Raising pHo caused rises in pHi, Ca2+ and tension. The increases in pHi preceded increases in [Ca2+]i, which preceded the increases in tension. Similar but opposite effects were observed when pHo was decreased. We conclude that the change in [Ca2+]i upon alteration of pHo is subsequent to that of pHi.

Identification of a novel protein containing two C2 domains selectively expressed in the rat brain and kidney

We have isolated and characterized a rat brain cDNA clone which encodes a new protein of 474 amino acids in length which contains two C2 domains structurally homologous to those present in synaptotagmins. The overall amino acid identity in C2 domains between this protein and the synaptotagmins is 36-44%. This protein also contains 3 putative consensus sequences for phosphorylation by cAMP-dependent protein kinase. RNA blot hybridization revealed a 3.0 kb transcript abundantly expressed only in the rat brain and the kidney. Thus, we called this brain/kidney protein (B/K). In situ hybridization and Northern blot analyses showed that the B/K transcript was found in forebrain including the olfactory bulb, cerebral cortex, hippocampus, and hypothalamus. In the kidney, high levels of B/K transcript were expressed in the papillary region of the inner medulla, the inner stripe of the outer medulla and the cortex. The selective expression in forebrain and kidney suggests that B/K may be involved in similar cAMP-dependent processes at these very different sites.

Major difference between rat and guinea-pig ureter in the ability of agonists and caffeine to release Ca2+ and influence force

1. We have investigated the internal Ca2+ store and its ability to affect contraction by simultaneously measuring force and Ca2+ in the ureter from guinea-pig and rat. Both species responded in a similar manner to electrical stimulation and depolarization with high-K+, generating plateau-type action potentials and increasing intracellular calcium ([Ca2+]i) and force. 2. In the guinea-pig, carbachol had no effect on [Ca2+]i and force in the resting ureter. In contrast, resting rat ureter always responded with a large [Ca2+]i rise and maintained force to carbachol in Ca(2+)-containing solution, and in Ca(2+)-free solution it showed a transient increase in [Ca2+]i and force. This Ca2+ release and force development was also present in both polarized and high-K(+)-depolarized preparations and was insensitive to nifedipine, suggesting the presence of a receptor-coupled pathway of Ca2+ release in rat ureter. 3. Caffeine was able to produce a release of Ca2+ from the internal store of guinea-pig ureter and elicit contraction. However, rat ureter failed to respond to caffeine. In the presence of La3+, the caffeine response in the guinea-pig ureter and carbachol response in the rat ureter, elicited in Ca(2+)-free solutions, were always increased and prolonged and could be repeatedly evoked, suggesting similarity in Ca2+ uptake behaviour of the store in both species. 4. Ryanodine blocked the caffeine responses of the guinea-pig ureter elicited both in Ca(2+)-containing and Ca(2+)-free solutions, both in the absence and presence of La3+. However, ryanodine failed to prevent the rat ureter responding to carbachol, suggesting that carbachol was releasing Ca2+ from a ryanodine-insensitive channel in the sarcoplasmic reticulum (SR). 5. Cyclopiazonic acid, which inhibits the SR Ca(2+)-ATPase, abolished the effects of both caffeine and carbachol in Ca(2+)-free solutions in guinea-pig and rat, respectively. 6. We conclude that there is a major difference in the mechanisms of Ca2+ release in the internal Ca2+ store of smooth muscle from guinea-pig and rat ureter. The data suggest that the guinea-pig store is purely a calcium-induced calcium release (CICR)-type store and that the rat store is a pure receptor-operated Ca2+ store.

Changes of pH affect calcium currents but not outward potassium currents in rat myometrial cells

Spontaneous contraction of uterine smooth muscle is enhanced by alkalinization and depressed by acidification. We have investigated the ionic currents responsible for this in single myometrial cells. Intracellular acidification (20 mM butyrate) at constant external pH depressed the magnitude of the calcium current to 58+/-6% of control, but had little effect on outward currents. Similar but slower effects were also observed when the extracellular pH was lowered to 6.9 (56+/-9% of control). Correspondingly, when the intracellular or extracellular pH was elevated (20 mM NH4Cl or pH 7.9 respectively) the calcium current magnitude increased (165+/-15% in NH4Cl; 136+/-2% at pH 7.9) and there was, again, no effect on the outward currents. These observations are consistent with the effects of pH on spontaneous contractile activity being due to an effect on the membrane calcium current.

The effects of extracellular pH and calcium change on force and intracellular calcium in rat vascular smooth muscle

1. In order to investigate the mechanism whereby changes in external pH (pHo) alter tone in rat mesenteric resistance vessels, we have made simultaneous measurements of tension and intracellular Ca2+ [Ca2+]i. Strips of mesenteric artery were loaded with the Ca(2+)-sensitive indicator indo-1 and superfused with physiological salt solution at pH 7.4 and 37 degrees C. 2. An increase of pHo from 7.4 to 7.9 produced an increase in tension. This was accompanied by an increase in [Ca2+]i in resting and high-K(+)-depolarized vessels. Acidification to 6.9 reduced tension and was associated with a fall in [Ca2+]i. Over the pHi range examined, 6.6-7.9, parallel changes in [Ca2+]i and tension were found in K(+)-activated vessels. 3. In contrast to the relatively slow change in [Ca2+]i, pHi and tension with change of pHo, depolarization produced rapid changes in [Ca2+]i and tension, consistent with a more direct action on Ca2+ mobilization. 4. Reducing the external [Ca2+] below 1 mM produced a pronounced fall in [Ca2+]i and force. Changes in [Ca2+]i, produced by alteration of external [Ca2+] (Cao2+) were used to examine the relation between [Ca2+]i and tension. A linear relation was found. Alteration of pHo to 6.9 or 7.9 did not significantly change this relation. When the tension data were normalized to their own maxima, no shift in the tension-Ca2+ relation occurred, suggesting little or no effect of pH on the Ca2+ sensitivity of force production by the contractile proteins. 5. To determine further whether the changes in [Ca2+]i produced by alteration of pHo could account for all the changes observed in tension, [Ca2+]i was restored to control levels while maintaining an altered pHo. When this was done, restoration of [Ca2+]i led to restoration of force. Thus, in this preparation, the changes in [Ca2+]i produced by altering pHo in depolarized vessels can account for the changes in vascular tone.

Changes in intracellular pH close to term and their possible significance to labour

The contractile pattern of uterine smooth muscle is markedly altered by even modest manipulations of intracellular pH (pHi); an acidification can abolish contractions, while alkalinization increases contractility. As at the end of gestation the uterus changes from being relatively quiescent to highly contractile, we investigated whether pHi changes with pregnancy in women. Isolated tissue was loaded with the pH-sensitive fluorophore, carboxy-SNARF. We found a significant alkalinization over the last few weeks of pregnancy. Such an increase in pHi will increase contractility of the uterus, and may therefore contribute to the mechanisms ensuring strong and efficient contractions occur during labour.

Functional effects of intracellular pH alteration in the human uterus: simultaneous measurements of pH and force

Changes in intracellular pH have differing effects on contraction in different types of muscle, and between species. Because of the large number of women requiring a caesarean section due to failure to progress in labour, it is important to know how the human myometrium responds to the changes in intracellular pH that may occur during labour. The pH-sensitive dye, carboxy-SNARF, was used to measure intracellular pH in small strips of human myometrium. Intracellular pH and tension were simultaneously recorded in pregnant and nonpregnant tissue. Intracellular pH was altered by the addition of weak acids and bases. Intracellular alkalinization caused an increase in the frequency and amplitude of contractions. Intracellular acidification led to an initial increase in the frequency and/or the amplitude of the contractions, followed by abolition of contractions. Alterations in intracellular pH had profound effects on contraction in human uterine smooth muscle. Possible mechanisms are discussed whereby pH could influence force production, and changes in contraction are related to the speed and extent of the change in intracellular pH.

Effects of insulin-like growth factors I and II and insulin on the immortalized hypothalamic GTI-7 cell line

Insulin and insulin-like growth factor I (IGF-I) participate in energy metabolism, regulate cellular growth and differentiation, and are thought to act locally in a paracrine manner through specific receptors. Systemic levels of these peptides in humans and primates are directly associated with levels of activity of the reproductive axis. To date, it is unclear whether these peptides participate in reproductive function by acting at the level of the GnRH neuron. In this study we examined the effects of IGF-I, IGF-II and insulin on immortalized GnRH-secreting neurons, the GTI-7 cell line. The GTI-7 cells expressed all three members of the insulin receptor family as determined by analysis of 125I-IGF-I, 125I-IGF-II and 125I-insulin binding sites. Insulin receptors bound insulin, IGF-II and IGF-I with a ratio of potency of 1:5:20. IGF-I and IGF-II receptors bound both IGF-I and IGF-II. The ratio of potency of IGF-I/IGF-II was 1:5 for the IGF-I receptor and 100:1 for the IGF-II receptor. The binding characteristics of the growth factors at 22 degrees C suggested the possibility that these cells may secrete IGF binding proteins. To ensure that changes in GnRH levels in the media were due to secretion and not to changes in cell number, the mitogenic effect of these peptides on GTI cells was evaluated. Both insulin and IGF-I were strong mitogens (48-hour incubation), restoring cell number to that of serum-replete cultures at a dose of 0.1 ng/ml. A 100-fold higher dose of IGF-II was required to produce a similar level of mitogenicity, implicating an action through the IGF-I and/or insulin receptor. Due to these mitogenic effects, the effect of insulin, IGF-I and IGF-II on GnRH secretion was studied after short-term exposure. Insulin and IGF-I did not affect GnRH secretion, but IGF-II had a biphasic effect on GnRH release after 2 h of incubation (a maximum stimulatory effect occurred with a 0.1 ng/ml dose). In order to examine the signal transduction mechanism, the role of cytoplasmic calcium mobilization in IGF-II-induced GnRH secretion was examined in single cells using calcium imaging. The effect of IGF-II on GnRH secretion appeared to operate via a calcium-independent mechanism. The studies document an insulin/IGF system in the GTI-7 neuronal cell line and show that insulin and IGFs can exert direct effects on the immortalized GnRH neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of metabolic inhibition on rat uterine intracellular pH and its role in contractile failure

Although intracellular pH (pHi) is expected to change during inhibition of oxidative phosphorylation and to affect force, there have been no simultaneous measurements of its effect on pHi and force in smooth muscle. Therefore, we have investigated the relationship between force and pHi in strips of longitudinal rat myometrium, loaded with the pH-sensitive indicator carboxy-SNARF and simultaneously measured tension. The application of cyanide produced an abolition of spontaneous contractions and a rapid initial fall in pHi. In 9/19 preparations pHi then started to return to resting values but there was no corresponding restoration of force. Cyanide and weak base were simultaneously added to uterine preparations to prevent any acidification; force still fell. Addition of cyanide to depolarized preparations also produced an acidification and a fall in force. Depolarization of preparations in which spontaneous force had been abolished by cyanide often produced a transient rise in force, despite further acidification of the cytoplasm. Cyanide produced an acidification in zero Ca(2+)-containing solution, similar to that in the presence of Ca2+ indicating little role for changes in [Ca2+] in producing the acidification. It is concluded that cyanide decreases pHi and force in the uterus, but that there is not a simple relationship between the two.

Molecular characterization of a neuronal-specific protein that stimulates the activity of Cdk5

Cyclin-dependent kinase, Cdk5, has been identified in neural tissue in connection with neurofilament and tau protein phosphorylation. This report describes the characterization of a 62-kDa protein that copurifies with Cdk5 from rat spinal cord homogenates. Dissociation of the protein from neural Cdk5 is concomitant with a reversible loss in kinase activity. Amino acid sequence information from tryptic peptide fragments was used to clone the complementary DNA from rat brain. A single full-length cDNA was characterized coding for a 67.5-kDa protein (p67). Exogenously expressed p67 stimulated Cdk5 kinase activity in vitro in a dose-dependent manner and when presented as an affinity matrix, selectively adsorbed Cdk5 from a cleared rat brain homogenate. In situ hybridization analysis of E18 rat embryos and adult rat brain demonstrated that p67 transcript expression is restricted to neural tissue. Immunohistochemical staining with an amino-terminal peptide-specific antibody further indicated that p67 is exclusively expressed in neurons. Localization in vivo and in cultured rat hippocampal neurons showed that p67 is highly enriched in axons. We propose that p67, by virtue of its regulation of Cdk5, participates in the dynamics of axonal architecture through the modulation of phosphorylation of cytoskeletal components.

Electrical and synaptic properties of embryonic luteinizing hormone-releasing hormone neurons in explant cultures

Voltage- and ligand-activated channels in embryonic neurons containing luteinizing hormone-releasing hormone (LHRH) were studied by patch-pipette, whole-cell current and voltage clamp techniques. LHRH neurons were maintained in explant cultures derived from olfactory pit regions of embryonic mice. Cells were marked intracellularly with Lucifer yellow following recording. Sixty-two cells were unequivocally identified as LHRH neurons by Lucifer yellow and LHRH immunocytochemistry. The cultured LHRH neurons had resting potentials around -50 mV, exhibited spontaneous discharges generated by intrinsic and/or synaptic activities and contained a time-dependent inward rectifier (Iir). Voltage clamp analysis of ionic currents in the LHRH neuron soma revealed a tetrodotoxin-sensitive Na+ current (INa) and two major types of K+ currents, a transient current (IA), a delayed rectifier current (IK) and low- and high-voltage-activated Ca2+ currents. Spontaneous depolarizing synaptic potentials and depolarizations induced by direct application of gamma-aminobutyrate were both inhibited by picrotoxin or bicuculline, demonstrating the presence of functional gamma-aminobutyrate type A synapses on these neurons. Responses to glutamate were found in LHRH neurons in older cultures. Thus, embryonic LHRH neurons not yet positioned in their postnatal environment in the forebrain contained a highly differentiated repertoire of voltage- and ligand-gated channels.

The relationship between plasma urea levels and some muscle trimethylamine levels in Xenopus laevis: a 31P and 14N nuclear magnetic resonance study

Urea is known to be disruptive to proteins, yet high levels occur in a variety of tissues. It has been suggested that trimethylamines counteract the effects of urea on protein. We have, therefore, directly investigated whether elevating tissue urea levels produces an increase in trimethylamine concentrations by using 31P and 14N nuclear magnetic resonance (NMR) spectroscopy to detect two trimethylamines (glycine betaine and glycerylphosphorylcholine) in intact, living gastrocnemius muscle from Xenopus laevis. Xenopus laevis naturally elevates its tissue urea levels under dry conditions. This ability was used to obtain plasma urea levels ranging from 1 to 110 mmol l-1. This procedure did not alter pH or levels of ATP or phosphocreatine in the muscles, but did raise trimethylamine levels. We find that there is a significant relationship between plasma urea concentrations and the trimethylamine levels studied. This relationship was, however, limited to the lower range of urea levels. We propose that other trimethylamines or mechanisms come into play at urea concentrations above 20 mmol l-1.