Ontogenesis of physiological responsiveness and guanine nucleotide sensitivity of cardiac muscarinic receptors during chick embryonic development

Classical and novel pharmacological insights offered by the simple chick cardiomyocyte cell culture model: a valuable teaching aid and a primer for "real" research

The chick embryo cardiomyocyte model of cell culture is a staple technique in many physiology and pharmacology laboratories. Despite the relative simplicity, robustness, and reproducibility inherent in this model, it can be used in a variety of ways to yield important new insights that help facilitate student understanding of underlying physiological and pharmacological concepts as well as, more generally, the scientific method. Using this model, this paper will show real data obtained by undergraduate students in the authors' laboratories. It will first demonstrate classical pharmacological concepts such as full and partial agonism, inverse agonism, and competitive reversible antagonism and then move on to more complex pharmacology involving the characterization of novel receptors in these cells.

Adenylyl cyclase signaling in the developing chick heart: the deranging effect of antiarrhythmic drugs

The adenylyl cyclase (AC) signaling system plays a crucial role in the regulation of cardiac contractility. Here we analyzed the key components of myocardial AC signaling in the developing chick embryo and assessed the impact of selected β-blocking agents on this system. Application of metoprolol and carvedilol, two commonly used β-blockers, at embryonic day (ED) 8 significantly downregulated (by about 40%) expression levels of AC5, the dominant cardiac AC isoform, and the amount of Gsα protein at ED9. Activity of AC stimulated by forskolin was also significantly reduced under these conditions. Interestingly, when administered at ED4, these drugs did not produce such profound changes in the myocardial AC signaling system, except for markedly increased expression of Giα protein. These data indicate that β-blocking agents can strongly derange AC signaling during the first half of embryonic heart development.

Effects of gestational age and cortisol treatment on ovine fetal heart function in a novel biventricular Langendorff preparation

Structural and functional maturation of a number of fetal organs and physiological systems occurs in the immediate period prior to term, in association with the prepartum increase in fetal plasma cortisol concentration. At present, little is known about how myocardial sensitivity to adrenergic and muscarinic cholinergic stimulation changes as the fetus approaches term, nor the role of the prepartum increase in plasma cortisol concentration in mediating these changes. This study used a novel Langendorff, biventricular, ovine fetal heart preparation to investigate the effects of advancing gestation and cortisol treatment on myocardial sensitivity to adrenergic (isoprenaline) and muscarinic cholinergic (carbachol) stimulation. It was hypothesized that cortisol infusion would fully mimic developmental changes in myocardial responsiveness to adrenergic and cholinergic stimulation. Sixteen Welsh Mountain sheep fetuses were surgically prepared under general anaesthesia with vascular catheters. At 125 +/- 1 days gestational age (dGA; term, 145 dGA) fetuses were infused with saline vehicle (n= 7; Premature Control) or with cortisol (n= 4; 2-3 mg kg(-1) d(-1)i.v.; Premature Cortisol) for 5 days. The Term Control group (n= 5) comprised fetuses that were surgically prepared at 130 dGA and infused with vehicle for 5 days prior to delivery (n= 2), or that received no surgery (n= 3). Under terminal anaesthesia, Premature Control and Premature Cortisol fetuses were delivered at 130 dGA and Term Control fetuses between 135 and 143 dGA. Following exsanguination under anaesthesia, fetal hearts were mounted in the Langendorff preparation, allowing measurement of left ventricular (LV) developed pressure and right ventricular (RV) developed pressure, heart rate (HR), coronary perfusion pressure and perfusate distribution to the myocardium. Cortisol infusion elevated fetal plasma cortisol concentrations to values similar to those measured close to term (45.0 +/- 7.1 ng ml(-1)). Advancing gestational age, but not cortisol treatment, enhanced fetal LV developed pressure, RV developed pressure and HR responses to carbachol (P < 0.05). Advancing gestational age, but not cortisol treatment, suppressed fetal LV developed pressure, RV developed pressure and HR responses to isoprenaline (P < 0.05). Maximum doses of either carbachol or isoprenaline had no effect on coronary perfusate distribution. Changes in myocardial responsiveness to adrenergic and muscarinic cholinergic stimulation with advancing gestation provide mechanisms that contribute to the maturation of the cardiovascular system as the ovine fetus approaches term. These changes in myocardial responsiveness are not solely dependent on preparturient elevations in fetal plasma cortisol concentration.

Development of peripheral autonomic synapses: neurotransmitter receptors, neuroeffector associations and neural influences

1. The functional innervation of autonomic target tissues occurs early during development, at a time when both the nerves and post-synaptic target tissues are still differentiating. 2. Physiological responses appear soon after the arrival of the first fibres when uptake and release mechanisms within the nerves are already functional. Initial responses differ from those in the mature animal, both in the form and, frequently, in the subtypes of receptors involved. 3. Results of a number of studies suggest that the initial expression of neurotransmitter receptors during development is largely independent of neural influences. Changes recorded in neurotransmitter receptor expression during development appear to be similarly independent of neural influences. 4. While signal transduction pathways coupling adrenergic neurotransmitter receptors to effector responses appear to develop independently of the nerves, the efficient coupling of muscarinic receptors often requires the action of the neurotransmitter, acetylcholine. 5. During the period of synapse formation, the neural plexus continues to expand. While developing varicosities can release the neurotransmitter, the capacity for neurotransmitter retention appears to be restricted. Developmental changes in the neurotransmitters that produce functional responses, while well known in the sweat glands, may also be seen in more subtle forms in other target tissues. 6. Ultrastructural studies suggest that close physical associations between the membranes of the release sites of the developing nerves and the target cells may form early during development when physiological responses are still immature. These close associations could enable more specific reciprocal interactions between nerves and target cells involving known and novel growth factors, neuropeptides and cytokines important in shaping the mature synaptic characteristics.

GATA factor-dependent regulation of cardiac m2 muscarinic acetylcholine gene transcription

The m2 subtype is the predominant muscarinic acetylcholine receptor subtype expressed in heart and regulates the rate and force of cardiac contraction. We have previously reported the isolation of the promoter region for the chick m2 receptor gene and defined a region of the chick m2 promoter sufficient for high level expression in cardiac primary cultures. In this manuscript we demonstrate transactivation of cm2 promoter by the GATA family of transcription factors. In addition, we define the GATA-responsive element in the chick m2 promoter and demonstrate that this element is required for expression in cardiac primary cultures. Finally, we demonstrate specific binding of both a chick heart nuclear protein and of cloned chick GATA-4, -5, and -6 to the GATA-responsive element.

G-protein expression in melanotropes changes coincident with innervation of the developing rat pituitary intermediate lobe

The two isoforms of the dopamine D2 receptor, the D2short and the D2long differ in a 29 amino acid insert in the third cytoplasmic loop with which G proteins interact. We have previously reported that in rat melanotropes, expression of D2short increases markedly at the end of the first postnatal week which is concurrent with innervation of the intermediate lobe. Using immunohistochemistry, this study examined expression of G alpha i1/2, G alpha i3, G alpha o and G alpha s proteins before and after dopaminergic innervation. G alpha i3 increased through gestational day 20, and then remained level to postnatal day 6. At this time, coinciding with the induction of D2short expression, G alpha i3 immunoreactive intensity increased markedly, possibly indicating co-regulation of these proteins. On postnatal day 6, G alpha s immunoreactive intensity increased in some, but not all, melanotropes. The resulting heterogeneity in Gs expression persisted in the adult. G alpha i1/2 immunoreactivity did not change and G alpha o was detected only subsequent to the event of innervation. Thus, dopamine released from axons and acting through D2 receptor stimulation could increase G alpha i3 immunoreactivity and decrease G alpha s immunoreactive intensity in some melanotropes.

Maintenance of cellular levels of G-proteins: different efficiencies of alpha s and alpha o synthesis in GH3 cells

G-proteins couple membrane-bound receptors to intracellular effectors. Each cell has a characteristic complement of G-protein alpha, beta and gamma subunits that partly determines the cell's response to external signals. Very little is known about the mechanisms that set and maintain cellular levels of G-proteins or about potential points of regulation. We have assayed the steady-state levels of mRNA and protein for two types of G-protein subunits, alpha s and alpha o, in rat brain, heart and GH3 cells, and found that in all these cases, it takes 9- to 20-fold more mRNA to produce a given amount of alpha s protein than to produce the same amount of alpha o protein. Such a situation could arise from a relatively rapid rate of alpha s protein degradation, requiring rapid protein synthesis to compensate, or from relatively inefficient translation of alpha s mRNA compared with alpha o mRNA. The latter appears to be the case in GH3 cells. These cells contain 94 times more mRNA for alpha s than for alpha o, yet the rate of alpha s protein synthesis is only 9 times greater than alpha o protein synthesis. The degradation rates of the two proteins are similar (13 h for alpha s and 18 h for alpha o). To begin to define the mechanism that accounts for the fact that it takes more mRNA to synthesize a given amount of alpha s than alpha o, we asked whether there is a pool of alpha s mRNA that does not participate in protein synthesis. We found that virtually all alpha s and alpha o mRNA is associated with ribosomes. Therefore, all the mRNA is likely to be capable of directing protein synthesis. Since the rate-limiting step in protein synthesis is usually binding of the ribosome to mRNA at initiation, our results suggest that the relatively slow rate of alpha s protein synthesis is regulated by a mechanism that acts beyond initiation at peptide elongation and/or termination.

Effects of adenosine on cAMP production during early development in the optic tectum of chicks

Accumulation of cyclic adenosine monophosphate (cAMP) elicited by adenosine was studied in slices and membrane preparations of optic tectum from chicks aged 1-13 days post-hatch. Accumulation of cAMP promoted by adenosine declined with age, the highest value being observed in three-day-old chicks and the lowest in 11-day-old chicks. However, when the slices were incubated with adenosine and the phosphodiesterase inhibitor-Ro 20-1724 the differences between the two ages were abolished, suggesting a higher phosphodiesterase activity in 11-day-old chicks. In membrane preparations, although basal adenylate cyclase activity was lower in three-day-old chicks, the guanylyl-imidodiphosphate (Gpp(NH)p) concentration curves for stimulation of adenylate cyclase activity indicated a higher sensitivity of G protein to Gpp(NH)p at this age. This hypothesis was reinforced by the observation that the binding of [3H]Gpp(NH)p to the membrane preparation was greater in three-day-old animals. In spite of these differences, the percentage of adenylate cyclase activity stimulation by 2-chloroadenosine (2CADO)+Gpp(NH)p was the same at both ages. These findings suggest that the decreased response evoked by adenosine during development is probably due to increased phosphodiesterase activity and a lower sensitivity of adenylate cyclase activity to Gpp(NH)p.

Regulation of glibenclamide receptors in cultured chick cardiomyocytes

We found that the expression of sulphonylurea receptors which control ATP-dependent potassium channels was up-regulated both by a receptor agonist and by potassium channel activators. Sulphonylurea receptors detected on cell membranes of cultured chick cardiomyocytes by [3H]glibenclamide were increased 2.8-fold following growth of the cells for 1-2 days in the presence of 30 nM glibenclamide. [3H]glibenclamide binding was also increased approximately 2-fold following growth of the cells in the presence of the ATP-dependent potassium channel activators, pinacidil, minoxidil and diazoxide, but was diminished to 75% of control following treatment with depolarizing concentrations of KCl.

Tissue-specific regulation of muscarinic acetylcholine receptor expression during embryonic development

We used solution hybridization, immunoprecipitation, and immunoblot analyses to examine the developmental expression of chicken m2 (cm2), cm3, and cm4 muscarinic acetylcholine receptor (mAChR) mRNA and protein in embryonic and post-hatched chick heart and retina in order to correlate developmental expression patterns with known physiological events. cm2 is the predominant mAChR subtype expressed in chick heart. cm3 and cm4 protein and mRNA expression is very low in chick heart, and cm3 expression is highest early in development. The decrease in cm3 expression correlates well with the developmental decrease in mAChR-mediated activation of phospholipase C. cm4 is the predominant mAChR subtype expressed in chick retina. The expression of both cm4 protein and mRNA is highest early in development and decreases as development progresses. cm2 and cm3 mAChR are expressed at approximately equivalent levels and have similar patterns of expression. The cm2 and cm3 protein levels increase throughout development, while cm2 and cm3 mRNA levels peak at embryonic day 15 and then decrease after hatching. Our data indicate that the three mAChR subtypes are differentially regulated in chick heart and retina and that the patterns of expression of mAChR may be important in the development and physiology of these tissues.

Characterization of a rat type 2 angiotensin II receptor stably expressed in 293 cells

A cDNA clone for the rat type 2 (AT2) angiotensin II receptor was stably transfected into human embryonic kidney 293 cells. Binding characteristics of CGP42112A (Kd = 0.18 nM, Bmax = 10.8 pmol/mg protein) and ligand specificity were indistinguishable from those obtained with the whole rat fetus and with transiently transfected COS-7 cells. Non-hydrolyzable guanine nucleotide analogs did not affect the ligand binding curve; interestingly, the guanine nucleotide analog's effect was observed in the presence of sulfhydryl reducing agent, suggesting that a certain redox condition may affect G protein coupling to this receptor. Using the established cell line, several second messenger systems were assessed. None of cAMP levels, cGMP levels, arachidonic acid release, or phosphotyrosine phosphatase activity was affected by angiotensin II stimulation of this receptor. Furthermore, the AT2 receptor did not undergo agonist-stimulated internalization. These results using the cloned receptor suggest that the transfected AT2 receptor fails to effectively couple to the major G protein-mediated signaling mechanisms and ligand-activated internalization in transfected 293 cells.

Characterization of muscarinic acetylcholine receptors expressed by an atrial cell line derived from a transgenic mouse tumor

The properties of muscarinic acetylcholine receptors in the cell line MCM1, derived from an SV40 T-antigen-induced atrial tumor in a transgenic mouse, were determined. Binding studies using the nonselective muscarinic antagonist [3H]quinuclidinyl benzilate, the M1-selective antagonist pirenzepine, and the M2-selective antagonist AFDX-116 indicate that the receptors have the pharmacological properties of the cardiac (M2) receptor subtype. The receptors could be immunoprecipitated with a monoclonal antibody specific for the cardiac receptor, thus confirming the identity of the receptors expressed in these cells. The types of G proteins expressed in the cells were determined by Northern blot analyses: mRNA encoding the alpha subunits of Gs, G(o), and Gi-2, but not Gi-1 or Gi-3, were detected, consistent with previous observations of neonatal mammalian atria. The muscarinic receptors were functionally active, as demonstrated by the ability of the agonist to stimulate phosphoinositide turnover and to inhibit adenylyl cyclase activity. The availability of a mammalian atrial cell line that continues to express the appropriate functionally coupled subtype of muscarinic receptor may provide a useful system for the investigation of the regulation of expression and function of cardiac muscarinic receptors.

Diminished functional activity of newly synthesized muscarinic acetylcholine receptors in stably transfected Y1 adrenal cells

We have examined the functional responsiveness of newly synthesized m2 muscarinic acetylcholine receptors in stably transfected Y1 adrenal cells. After inactivation of preexisting receptors with the covalent alkylating antagonist propylbenzilylcholine mustard, the number of cell surface receptors returned to control values over a 3-h period. After a 3-h recovery, the cells exhibited diminished sensitivity for muscarinic receptor-mediated inhibition of adenylyl cyclase activity, with much higher concentrations of agonist being required to elicit a response. The functional sensitivity returned to control values over a 12-18-h period. The decreased functional activity was not due to a decreased affinity of the newly synthesized receptors for agonist or to a decrease in the levels of inhibitory G proteins in the cells. The results suggest that muscarinic receptors may be synthesized in a form with diminished functional activity. The ability to study the maturation of receptor function in a transfected cell system should allow a combination of biochemical and molecular genetic approaches to analyze the synthesis and functional responsiveness of muscarinic receptors.

Uptake of 5-hydroxydopamine into non-sympathetic nerves of guinea-pig uterine artery in late pregnancy

Perivascular nerve fibres of the uterine artery of virgin and late pregnant guinea-pigs were examined under the electron microscope following loading with 5-hydroxydopamine, a marker for catecholamine uptake, and immunohistochemistry for dopamine beta hydroxylase, neuropeptide Y, vasoactive intestinal polypeptide, substance P and calcitonin gene-related peptide. Varicosities, loaded with 5-hydroxydopamine labelled vesicles, and immunoreactive axons were counted in whole transverse sections of uterine arteries. Localization of the immunoreactivities in 5-hydroxydopamine-labelled vesicles was also studied. Colocalization of substance P and dopamine beta hydroxylase immunoreactivities was investigated at the light microscopic level. Both total and relative number of varicosities with 5-hydroxydopamine-labelled vesicles in a whole section of the artery increased in late pregnancy (61.2 +/- 10.2 versus 24.5 +/- 3.2 in virgin, representing 35% and 27% respectively, of all varicosities). Also the number of neuropeptide Y, vasoactive intestinal polypeptide, substance P and calcitonin gene-related peptide-immunoreactive axons increased, but their relative proportion remained unchanged. In virgin guinea-pigs only calcitonin gene-related peptide and neuropeptide Y immunoreactivities were associated with varicosities loaded with small dense-cored vesicles, while in late pregnancy 5-hydroxydopamine-labelled vesicles were also seen in a number of vasoactive intestinal polypeptide, substance P and calcitonin gene-related peptide-immunoreactive axons. Double immunolabelling for dopamine beta hydroxylase and substance P immunoreactivity showed that substance P immunoreactivity was not present in dopamine beta hydroxylase-immunoreactive axons of the uterine artery, of neither virgin nor late pregnant guinea-pigs. It is concluded that vascular hypertrophy of the uterine artery in late pregnancy is associated with an increase in the number of perivascular nerve fibres, that involves many, if not all of the subpopulations of neurons supplying the uterine artery. Also 5-hydroxydopamine-labelled varicosities were increased, but the results of the present study indicate that some of the nerve fibres that are able to take up 5-hydroxydopamine in late pregnancy are not sympathetic (i.e. are sensory and/or parasympathetic in origin). The relevance of these findings in pregnancy is discussed.

Regulation of acetylcholinesterase in avian heart. Studies on ontogeny and the influence of vagotomy

This article examines the role of innervation in regulating expression of acetylcholinesterase (AchE), butyrylcholinesterase (BuchE), and the muscarinic acetylcholine receptor (mAchR) in avian heart. Two distinct approaches are taken. The first approach examines the relation between the onsets of parasympathetic and sympathetic innervation and the appearance of AchE and BuchE. All molecular forms of AchE and BuchE are present in early embryonic chick heart well before the onset of parasympathetic and sympathetic innervation. These molecular forms are characterized by sedimentation coefficients of 4.5S, 11S, 15S, and 19S. With further development, the amounts of AchE fall; the reductions in AchE parallel the onset of functional parasympathetic innervation. The amounts of BuchE increase progressively throughout embryonic development, independent of autonomic innervation, and in mature chick heart predominate over the much less abundant amounts of AchE. The 15S and 19S forms of AchE in heart are lost during early embryogenesis but reappear in skeletal muscle during later embryogenesis. The second approach examines the influence of vagotomy and sympathetic denervation of 8-day-old chick myocardium on expression of the molecular forms of AchE, BuchE, mAchR, and beta-adrenergic receptors. The amounts of AchE and BuchE molecular forms in avian heart are not measurably influenced by bilateral vagotomy for a duration of 4 days, unilateral vagotomy for a duration of 25 days, or sympathetic denervation. A measurable upregulation is observed in muscarinic receptors (35-46%) after vagotomy but not sympathectomy and in beta-adrenergic receptors (29%) after sympathectomy but not vagotomy. In all cases, results in atria and ventricles are nearly identical. The present results indicate that expression of AchE in the myocardium is unique and different from that in skeletal muscle and not directly linked with autonomic innervation.

Control of K+ channels by G proteins

Heterotrimeric G3 proteins are though to couple receptors to ionic channels via cytoplasmic mediators such as cGMP in the case of retinal rods, cAMP in the case of olfactory cells, and the cAMP cascade in the case of cardiac myocytes. G protein-mediated second messenger effects on K+ channels are dealt with elsewhere in this series. Recently, membrane-delimited pathways have been uncovered and an hypothesis proposed in which the alpha subunits of G proteins directly couple receptors to ionic channels, particularly K+ channels. While direct coupling has not been proven, the membrane-delimited nature has been established for specific G proteins and their specific K+ channel effectors.

Sulfhydryl reducing agents distinguish two subtypes of angiotensin II receptors in the rat brain

Two angiotensin II receptor subtypes were distinguished in the rat brain using in vitro receptor autoradiography based on the differential effects of sulfhydryl reducing agents on 125I-sarcosine1,isoleucine8 angiotensin II binding in various brain nuclei. At several nuclei, e.g. the hypothalamus, circumventricular organs and the dorsal medulla, 125I-sarcosine1,isoleucine8 angiotensin II binding was strongly inhibited by 30 mM beta-mercaptoethanol or 5 mM dithiothreitol, whereas at other nuclei, e.g. the lateral septum, colliculi, locus coeruleus and medial amygdala, sulfhydryl reducing agents had either little effect on radioligand binding or enhanced the binding. The distribution of the sulfhydryl reducing agent inactivated subtype corresponds exactly with the distribution of DuP 753 sensitive (designated as AII alpha) 125I-sarcosine1,isoleucine8 angiotensin II binding sites25. The subtype not inhibited by sulfhydryl reducing agents corresponds with the DuP 753 insensitive (designated as AII beta) sites in the brain25. The sulfhydryl reducing agent effect on brain angiotensin II receptor subtypes is similar to that seen in angiotensin II receptor subtypes in peripheral tissues. These observations indicate that many previous studies of brain angiotensin II receptor binding that included 5 mM dithiothreitol in the assay medium overlooked the sulfhydryl reducing agent inactivated (AII alpha) receptor subtype.

Molecular heterogeneity of the subclasses of islet-activating protein (pertussis toxin)-sensitive GTP-binding proteins in porcine thyroid tissue

From porcine thyroid cell membranes, we purified five GTP-binding proteins (G-proteins); Nos. 1 to 3 have 41-kDa alpha-subunits, and Nos. 4 and 5 have 40-kDa alpha-subunits. They were chromatographically (Mono Q) separable and served as specific substrates for islet-activating protein (pertussis toxin). G-proteins 1 and 2 were indistinguishable from porcine brain Gi1 with respect to three criteria, i.e., mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), pI of the ADP-ribosylated alpha-subunit, and immunoreactivity. G-protein 3 was identified as Gi3 by immunoreactivity. The SDS-PAGE and isoelectric focusing (IEF) analyses identified G-proteins 4 and 5 as being chromatographically heterogeneous subtypes of Gi2 in comparison with a pure porcine brain preparation. The IEF analysis also disclosed that each of the Gi1, Gi2, and Gi3 subspecies isolated in the present study has a minor component characterized by a slightly lower pI of its alpha-subunit. We conclude that porcine thyroid tissue contains at least Gi1, Gi2, and Gi3, and that each is made up of heterogeneous populations.

Aging of rat heart myocytes disrupts muscarinic receptor coupling that leads to inhibition of cAMP accumulation and alters the pathway of muscarinic-stimulated phosphoinositide hydrolysis

The biochemical responses to muscarinic stimulation (inhibition of isoproterenol-stimulated cAMP accumulation and stimulation of phosphoinositide turnover) were investigated in intact myocyte cultures prepared from the hearts of newborn rats. The studies employed young (5 days after plating) and aged (14 days old) myocyte cultures. Aging of the myocyte cultures was accompanied by marked alterations in both the inhibition of cAMP accumulation and the stimulation of the phosphoinositide metabolism via the muscarinic receptors. However, the effects on the two muscarinic responses were different. The first response was disrupted at the level of the coupling of the muscarinic receptors with adenylate cyclase through Gi. On the other hand, muscarinic stimulation of phosphoinositide hydrolysis still occurred in the aged myocyte cultures; however, the inositol trisphosphate generated was not converted to inositol 1-phosphate as in young cultures or as in aged cultures stimulated by norepinephrine. This raises the possibility that muscarinic activation of aged myocyte cultures shifts the metabolic state of the cells and alters the pathway of phosphoinositide hydrolysis. Treatment of aging cultures with phosphatidylcholine liposomes under conditions that yielded aged myocyte cultures with a lipid composition resembling that of young ones restored the muscarinic effect on cAMP accumulation, where the impairment in aged cultures was at the coupling stage (which takes place in the plasma membrane). This treatment had no effect on the response of the phosphoinositide metabolism to muscarinic stimulation.

Beta-adrenergic inhibition of cardiac sodium channels by dual G-protein pathways

The signaling pathways by which beta-adrenergic agonists modulate voltage-dependent cardiac sodium currents are unknown, although it is likely that adenosine 3'5'-monophosphate (cAMP) is involved. Single-channel and whole-cell sodium currents were measured in cardiac myocytes and the signal transducing G protein Gs was found to couple beta-adrenergic receptors to sodium channels by both cytoplasmic (indirect) and membrane-delimited (direct) pathways. Hence, Gs can act on at least three effectors in the heart: sodium channels, calcium channels, and adenylyl cyclase. The effect on sodium currents was inhibitory and was enhanced by membrane depolarization. During myocardial ischemia the sodium currents of depolarized cells may be further inhibited by the accompanying increase in catecholamine levels.

Hepatocyte beta-adrenergic responsiveness and guanine nucleotide-binding regulatory proteins

Hepatocytes isolated from hypothyroid, adrenalectomized, or partially hepatectomized rats display an enhanced beta-adrenergic responsiveness as compared with cells from control animals. The enhanced beta-adrenergic responsiveness is evidenced by both increased ureagenesis and adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in response to isoproterenol. The role of stimulatory guanine nucleotide-binding protein (Gs) and inhibitory guanine nucleotide-binding protein (Gi) in the enhanced responsiveness was studied. It was observed, contrary to what would have been anticipated, that the level of Gs [as reflected by cholera toxin-catalyzed ADP ribosylation, 5'-guanosine gamma-thiotriphosphate (GTP gamma S)-stimulated adenylate cyclase activity, and a functional reconstitution assay] was decreased in liver membranes from adrenalectomized and partially hepatectomized rats as compared with the controls. Furthermore, the level of Gi was increased in these conditions as reflected by pertussis toxin-catalyzed ADP ribosylation. The data suggest that changes in beta-adrenergic receptor levels rather than the levels of guanine nucleotide-binding (G) regulatory proteins predominate in regulation of hepatic beta-adrenergic responses by hypothyroidism, adrenalectomy, or partial hepatectomy.

Ontogeny of the GTP-binding protein Go in rat brain and heart

We determined the ontogeny of the GTP-binding protein Go in rat brain and heart by employing highly sensitive enzyme immunoassay methods. In the brain, the alpha subunit of Go (Go alpha) gradually increased and reached adult levels approximately 20 and 30 days after birth in cerebral cortex and cerebellum, respectively. Concentrations of beta subunits, which were also quantified by the immunoassay, were almost equal to those of Go alpha in the brain of rats younger than 10 days, but were higher than those of Go alpha after 10 days. These results suggest that late development of GTP-binding proteins other than Go. Go alpha was immunohistochemically positive in neuropils and negative in cell bodies at any age tested. In the heart, the concentrations of Go alpha increased up to several times of the adult level just after birth, and then gradually decreased after the 20th postnatal day. The level of Go alpha in the liver, however, was very low and constant throughout ontogenic development. An immunohistochemical study indicated that Go alpha was positive in the cardiac muscle of young rat, but negative in that of adult rat. These results indicate that Go alpha exists in cells other than those of nervous tissues and neuroendocrine cells in some periods of ontogenic development.

Developmental changes in the levels of substrates for cholera toxin-catalyzed and pertussis toxin-catalyzed ADP-ribosylation in rat cardiac cell membranes

Developmental changes in the substrates for cholera toxin (CTX)- and pertussis toxin (PTX)-catalyzed ADP-ribosylation in cardiac (ventricular) cell membranes were studied in fetal (16- to 20-day), neonatal (0- to 20-day) and adult (2- to 3-month) rats. The CTX and PTX substrates were determined by the method of CTX-catalyzed and PTX-catalyzed ADP-ribosylation of the alpha-subunit of GTP-binding (G) proteins, respectively. As early as fetal day 16, three substrates (45-, 47- and 52-kDa proteins) were identified for CTX-catalyzed ADP-ribosylation and one substrate (41-kDa protein) for PTX-catalyzed ADP-ribosylation. The levels of the three CTX substrates (fmol/mg tissue) increased with development between fetal day 16 and neonatal day 16, and then they decreased to their adult levels. The level of the one PTX substrate (fmol/mg tissue) changed as follows: the substrate decreased between fetal day 16 and the day of birth, increased abruptly for 4 days neonatal and increased slowly thereafter until neonatal day 16, and then decreased to the final adult level. The PTX substrate seems to reach a nearly maximum level earlier than the CTX substrates. This information is essential for understanding the developmental changes in the transmembrane signaling system between membrane receptors and their effectors which are coupled with the stimulatory and inhibitory G proteins.

Modification of neuronal muscarinic receptor-mediated responses by islet-activating protein

Islet-activating protein (IAP) was used to investigate the role of the guanosine triphosphate binding proteins Gi and/or Go in muscarinic acetylcholine receptor-mediated responses in neuroblastoma cells (clone N1E-115). Incubation of intact cells for 24 h with 20 ng/ml IAP resulted in inhibition of subsequent IAP catalyzed incorporation of [32P]ADP-ribose into a membrane protein doublet of molecular weight 40,000 (Gi alpha and Go alpha). IAP treatment fully blocked muscarinic receptor-mediated inhibition of cAMP accumulation. Incubation of intact cells with carbachol for 8 h resulted in the concentration dependent loss of membrane muscarinic receptor. Pretreatment of cells with IAP prior to carbachol exposure partially blocked the subsequent decrease in receptor number. Pretreatment of cells with IAP had no effect on the ability of carbachol to stimulate phosphoinositide hydrolysis in neuroblastoma cells. Thus, while the guanosine triphosphate binding proteins Gi and/or Go are involved in coupling the muscarinic receptor to some of the physiological responses in these cells, it is clear that activation of phospholipase C by the muscarinic receptor is a Gi/Go independent response.

Chronic membrane depolarization regulates the level of the guanine nucleotide binding protein Go alpha in cultured neuronal cells

Chronic membrane depolarization results in an increase in muscarinic acetylcholine receptor (mAChR) number in N1E-115 neuroblastoma cells. Because the mAChR interacts with the guanine nucleotide binding regulatory (G) proteins, Gi and Go, the effect of chronic membrane depolarization on the levels of subunits of these G proteins was examined. Quantitation of G protein subunit levels was performed using affinity-purified, monospecific antibodies in a quantitative immunoblot assay. Incubation with 50 microM veratridine (VTN), an activator of voltage-sensitive Na+ channels, induced a 48 +/- 15% increase in the level of the alpha subunit of Go. The effect of VTN was blocked by tetrodotoxin. On removal of VTN, the level of Go alpha decreased to control levels within 24 h. The levels of the alpha subunit of Gi and the common beta subunit were not affected by VTN treatment. These results show that in N1E-115 cells, the level of the alpha subunit of Go is regulated in a manner similar to the level of mAChR in response to chronic membrane depolarization.

Multiple species and isoforms of Bordetella pertussis toxin substrates

In purified G proteins from bovine brain cortex the ADP-ribosylated substrates of Bordetella pertussis toxin (PT) can be resolved in three polypeptides by polyacrylamide gel electrophoresis: a 39 kDa major substrate, corresponding to Go alpha and two others (40 and 41 kDa) assigned to alpha subunits of Gi-like proteins. These three polypeptides were also detected in membranes of normal cells or tissues from neuronal and endocrine origins. In contrast, in membranes from other origins, only two PT substrates at 41 and 40 kDa were resolved; the latter being the most abundant ADP-ribosylated substrate in human platelets and C6 glioma cells. In these cells, electrophoretic patterns of PT-radiolabeled proteolytic fragments derived from the 40 kDa peptide were different to those from the 39 and 41 kDa polypeptides of purified G proteins. However, isoelectrofocusing and two dimensional analyses showed that the 40 kDa and 39 kDa (but not the 41 kDa) PT substrate of purified G proteins exhibited similar isoforms.

Muscarinic acetylcholine receptors from avian retina and heart undergo different patterns of molecular maturation

Muscarinic acetylcholine receptors (mAChRs) from the avian CNS exist in two molecular weight forms whose concentrations change during development. Here, we have compared the development of mAChRs from embryonic hearts with those of the CNS. Analysis of [3H]-propylbenzilylcholine mustard (PrBCM)-labeled retina and heart mAChRs by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed two atropine-sensitive peaks for each tissue. Apparent molecular masses of retina mAChRs, 86 +/- 0.7 kilodaltons (kDa) and 72 +/- 0.7 kDa, were different from those of heart mAChRs, 77 +/- 1.0 kDa and 52 +/- 0.9 kDa. During retina development, the major receptor type changed from 86 kDa to 72 kDa. No such change occurred during heart development. Furthermore, the 52-kDa species appeared to be generated by endogenous proteolysis, as prolonged incubation of heart membranes at 37 degrees C increased the amount of 52-kDa peptide with a decrease of 77-kDa peptide. Protease inhibitors blocked this conversion. Incubation of retina membranes at 37 degrees C did not result in a conversion of the 86-kDa peptide into the 72-kDa peptide, but it did cause the appearance of a minor amount of 52-kDa peptide. The proteolysis of retina mAChRs was not enhanced by cohomogenizing them with heart tissue, arguing against the presence of releasable proteases in heart. Membrane-bound retina and heart mAChRs displayed similar sensitivity to exogenous (Staphylococcus aureus V8) protease, indicating that heart receptors were not unusually susceptible to proteolytic attack; analysis of the labeled polypeptides with the V8 protease showed different patterns of digestion for the retina and heart receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Subcellular distribution and isoelectric heterogeneity of the substrate for ADP-ribosyl transferase from Clostridium botulinum

When the homogenate of bovine adrenal gland was subjected to subcellular fractionation, an Mr 21,000 substrate for botulinum ADP-ribosyl transferase was found not only in the membrane fractions but also in the cytosol; the amounts in the 10,000 x g precipitates and the 100,000 x g supernatant were about 21 and 56% of the total amount, respectively. Each fraction gave a single ADP-ribosylated protein band on SDS-polyacrylamide gel electrophoresis, but yielded on isoelectric focusing at least three bands between pH 5.5 and 6.0, suggesting the presence of multiple forms of the substrate of a similar molecular weight but different isoelectric points. ADP-ribosylated protein bands from the membrane and cytosol overlapped each other on both electrophoreses. After ammonium sulfate fractionation, the substrate from the cytosol showed requirement of divalent cations or guanine nucleotides for the reaction. Among cations tested, calcium, magnesium and manganese stimulated, whereas cadmium and lanthanum inhibited the reaction. Guanine nucleotides such as GTP, GDP and GTP-gamma-S also stimulated the substrate activity in the cytosol as that in the membrane fraction. However, no additive effects were observed when the nucleotides and cations were added together.

Structural and functional relationships of guanosine triphosphate binding proteins

Information available at present documents the existence of three well-defined classes of guanine nucleotide binding proteins functioning as signal transducers: Gs and Gi which stimulate and inhibit adenylate cyclase, respectively, and transducin which transmits and amplifies the signal from light-activated rhodopsin to cGMP-dependent phosphodiesterase in ROS membranes. Go is a fourth member of this family. Its function is the least known among GTP binding signal transducing proteins. The family of G proteins has a number of properties in common. All are heterotrimers consisting of three subunits, alpha, beta, and gamma. Each of the subunits may be heterogeneous depending on species and tissue of origin and may be posttranslationally modified covalently. The alpha subunits vary in size from 39 to 52 kDa. The sequences for Gs alpha and transducin alpha have 42% overall homology and those of Gi alpha and Gs alpha 43%, whereas those of Gi alpha and transducin alpha have a higher degree (68%) of homology. All alpha subunits bind guanine nucleotides and are ADP-ribosylated by either pertussis toxin (Gi, transducin, Go) or cholera toxin (Gs, Gi, transducin). Thus, transducin and Gi, which have the highest degree of sequence homology, are also ADP-ribosylated by both toxins. The beta subunits have molecular weights of 36 and 35 kDa, respectively. While Gs, Gi, and Go contain a mixture of both, transducin contains only the larger (36-kDa) beta-polypeptide. The relationship of the 36- and the 35-kDa beta subunits is not defined. Although the complete sequence of the 36-kDa beta subunit of transducin has been deduced from the cDNA sequence, complete sequences of other beta subunits are not yet available so that detailed comparisons cannot be made at present. However, the proteolytic profiles of each class of the beta subunits of different G proteins are indistinguishable. The gamma subunit of bovine transducin has been completely sequenced. It has a Mr of 8400. Again complete sequences of other gamma subunits are not yet available. While the gamma subunits of Gs, Gi, and Go have identical electrophoretic mobility in SDS gels, they differ significantly in this respect from the gamma subunit of transducin. Moreover, crossover experiments point to functional differences between gamma subunits from G protein and transducin complexes. In addition, a role for beta, gamma in anchoring guanine nucleotide binding proteins to membranes has been postulated.(ABSTRACT TRUNCATED AT 400 WORDS)

G-protein distribution in canine cardiac sarcoplasmic reticulum and sarcolemma: comparison to rabbit skeletal muscle membranes and to brain and erythrocyte G-proteins

Herein we describe the distribution of G-proteins in canine cardiac sarcolemma (SL) and sarcoplasmic reticulum (SR) and in rabbit skeletal muscle SL, T-tubules, and junctional and longitudinal SR in comparison to G-proteins of human erythrocyte and bovine brain. G-proteins were unequivocally present in cardiac SL and SR and in skeletal T-tubules. Both cardiac fractions had two substrates specifically ADP-ribosylated by cholera toxin migrating on a sodium dodecyl sulfate-polyacrylamide gel at about 42 and 45 kDa. In skeletal muscle membranes, cholera toxi-labeled substrates migrated at about 42 and 62 kDa. Three substrates for pertussis toxin were resolved by sodium dodecyl sulfate/urea-polyacrylamide gel electrophoresis in cardiac SL at about 38, 40, and 43 kDa. Only the two higher molecular weight substrates were detected in cardiac SR and in any of several skeletal muscle membrane fractions. Comparison of G-proteins in muscle membrane fractions with G-proteins isolated from bovine brain and human erythrocyte as well as their reaction with antisera to either a common sequence of alpha subunits of G-proteins (G alpha common antibody) or to a unique sequence of the alpha subunit of Go (G alpha o antibody) indicated that the two lower molecular weight bands in cardiac SL are Go or Go-like, and therefore the upper band is probably Gi. These data demonstrate that pertussis toxin substrates are more heterogeneous than previously described and have implications for studies attempting to attribute physiological functions to G-protein isolates.

Development and properties of the secretory response in rat sweat glands: relationship to the induction of cholinergic function in sweat gland innervation

Previous studies suggest that the sympathetic innervation of the sweat glands in the rat is initially noradrenergic and during development undergoes a transition in neurotransmitter phenotype to become cholinergic. To characterize this system and its development further, we have examined the adrenergic and cholinergic components of the secretory response in adult and immature rats and have studied the onset of sweating in the plantar sweat glands of developing rats. Stimulation of the sciatic nerve in adult rats elicited a secretory response which was completely blocked by the cholinergic antagonist, atropine, and was unaffected by adrenergic antagonists, indicating that nerve-evoked secretion was cholinergic. In adult rats, the sweat glands were quite sensitive to cholinergic agonists. In addition to acetylcholine, the mature sweat gland innervation contains vasoactive intestinal peptide (VIP). In some rats, the injection of VIP alone elicited a secretory response which was blocked by atropine, suggesting that the response to VIP was mediated cholinergically. In contrast to cholinergic agonists, the glands responded relatively infrequently and with reduced volumes of sweat to the alpha- and beta-adrenergic agonists 6-fluoronorepinephrine and isoproterenol. However, when VIP, which is a potent vasodilator, was simultaneously injected with adrenergic agonists, glands in many of the injected footpads exhibited a secretory response. The response to adrenergic agonists in combination with VIP was reduced by atropine and by phentolamine plus propranolol, but was blocked completely only by a combination of the three antagonists, indicating that both adrenergic and cholinergic mechanisms were involved. In immature rats, sweating evoked by nerve stimulation first appeared at 14 days of age in 25% of the rats tested. Both the percentage of rats sweating and the number of active glands increased rapidly. At 16 days, 50% of the rats tested exhibited some active glands, and by 21 days all rats tested exhibited a secretory response. In 16-day-old rats, nerve-evoked sweating was almost completely inhibited by local injection of 1 microM atropine, but was unaffected by phentolamine and propranolol in concentrations up to 10 microM. Similarly, the glands were sensitive to 10 microM muscarine, but they exhibited no secretory response to the alpha-adrenergic agonists, clonidine and 6-fluoronorepinephrine, nor to the beta-adrenergic agonist, isoproterenol, at concentrations up to 50 microM. The simultaneous injection of VIP with adrenergic agonists did not reveal an adrenergically mediated secretory response in 16-day-old animals.(ABSTRACT TRUNCATED AT 400 WORDS)

Pertussis toxin-insensitive phosphoinositide hydrolysis, membrane depolarization, and positive inotropic effect of carbachol in chick atria

Muscarinic agonists can stimulate rather than inhibit cardiac muscle in some preparations. In left atria from hatched chicks, treatment with pertussis toxin reversed the membrane action of carbachol from hyperpolarization to depolarization and reversed the inotropic effect of carbachol from negative to positive. Acetylcholine also depolarized the membrane and increased the force of contraction in atria from pertussis-toxin-treated chicks although oxotremorine did not. These cholinergic responses were blocked by atropine but not by adrenoceptor antagonists, suggesting that they are mediated via muscarinic receptors and are not due to actions of endogenously released catecholamines. Muscarinic receptor stimulation leads to two distinct biochemical responses in chick atria: inhibition of adenylate cyclase and activation of phosphoinositide (PI) hydrolysis. The former is lost in atria from pertussis-toxin-treated chicks, whereas the PI response persists. The pharmacologic characteristics of the PI response resemble those of the depolarization and positive inotropic response. Both are insensitive to blockade by pertussis toxin, require high concentrations of carbachol, and are elicited by acetylcholine but not by oxotremorine. The present study suggests that muscarinic agonist-induced PI turnover may be responsible for the membrane depolarization and positive inotropic effects of carbachol and acetylcholine; that an increase in Na+ conductance underlies these responses; and that it is stimulated either by an increase of intracellular calcium mobilized by inositol triphosphate and/or by activation by protein kinase C.

Stimulation by GTP-binding proteins (Gi, Go) of partially purified phospholipase C activity from human platelet membranes

A phospholipase C exhibiting preferential hydrolytic activity for polyphosphoinositides was partially purified from the deoxycholate extract of human platelet membranes by Q-Sepharose and Heparin-Sepharose column chromatographies. The activity of this purified phospholipase C free of the GTP gamma S-binding activity was stimulated at a similar level by addition of purified rat brain Gi or Go. These results suggest that GTP-binding proteins may interact directly with a solubilized membrane phospholipase C to stimulate its activity.

Regulatory GTP-binding proteins: emerging concepts on their role in cell function

The last few years have evidenced a tremendous expansion in our appreciation of the role of regulatory GTP-binding proteins in cellular activation. The availability of cholera and pertussis toxins to detect G proteins as well as methodological advances in the study of cellular function has afforded the opportunity to examine G protein participation in many cellular events. Regulation of adenylyl cyclase and cyclic GMP phosphodiesterase by G proteins has been demonstrated. Phosphatidylinositol-4,5-biphosphate specific phospholipase C activity appears to be subject to G protein control. G proteins regulate inward K+ and Ca2+ channels through a mechanism which may be independent of effects on the above mentioned enzymes. Certainly, the number of G proteins which have been identified from sequencing of complementary DNA affords the potential for G protein involvement in many cellular events. Only three G proteins have however been isolated and functionally characterized, Gs, Gi and transducin. Whether all the functions of these proteins have been identified remains to be seen.

Development of physiological responsiveness to glucagon during embryogenesis of avian heart

Glucagon increases contractility of the heart muscle by stimulation of adenylate cyclase activity and elevation of cAMP. We have investigated the specific time of onset of glucagon sensitivity of heart muscle during development of the chick embryo. Using both isolated heart preparation and cultured cardiac cells, we have found that the contractile response to glucagon cannot be detected prior to Day 4 of development. Binding studies, carried out with heart cells prepared from 3-, 5-, 7-, and 11-day chick embryos, showed a significant increase in the number of glucagon binding sites between Days 3 and 5. Scatchard analysis showed that for Day 5 cells maximum binding capacity was 0.56 pmole/mg of protein with Kd of 16.0 nM, while for Day 3, maximal binding was only 0.16 pmole/mg with Kd of 15.1 nM. Therefore in this 2-day interval there was a marked increase in the receptor number, without changes in the receptor affinity. Since hormonal stimulation of adenylate cyclase depends on the presence of the regulatory component (Ns), we have used cholera toxin-induced chronotropic effect as an assay for functional Ns. No response to cholera toxin could be detected prior to Day 4 of chick heart development. Therefore, emergence of the cholera toxin sensitivity correlates well with the onset of responsiveness to glucagon. We conclude that as the heart develops it acquires a physiological responsiveness to glucagon. The acquisition of the hormonal sensitivity correlates with the increase in the receptor number and the functional levels of regulatory component.

Protein kinase C activation enhances cAMP accumulation in Swiss 3T3 cells: inhibition by pertussis toxin

Addition of phorbol 12,13-dibutyrate (PBt2) in the presence of forskolin or cholera toxin caused marked (6- to 8-fold) and rapid accumulation of cAMP in Swiss 3T3 cells. The effect of PBt2 is mediated by protein kinase C because the synthetic diacylglycerol 1-oleoyl-2 acetylglycerol substitutes for PBt2 in enhancing cAMP accumulation and because the enhancing effect of either PBt2 or 1-oleoyl-2-acetylglycerol was prevented by down-regulation of protein kinase C. Vasopressin, which activates protein kinase C but does not directly affect adenylate cyclase in 3T3 cells, also enhanced cAMP accumulation in cells treated with cholera toxin or forskolin. This effect was abolished by down-regulation of protein kinase C. Treatment with pertussis toxin blocked the enhancing effect of PBt2 in a concentration- and time-dependent manner. Pertussis toxin neither prevented protein kinase C activation by PBt2 nor other biologic responses elicited by PBt2. The results presented here suggest an unusual function for a pertussis toxin substrate--namely, coupling protein kinase C activation to cAMP production.

Pertussis toxin inhibits negative inotropic and negative chronotropic muscarinic cholinergic effects on the heart

We injected rats with pertussis toxin, known to cause ADP ribosylation of the Gi regulatory protein of the adenylate cyclase complex and of another closely related GTP binding protein in the heart, and after 7 days we examined several effects of muscarinic activation on the heart. The negative chronotropic effect of carbamoylcholine on spontaneously beating perfused hearts was conspicuously diminished. While 10(-5) mol/l carbamoylcholine invariably produced heart arrest in control rats, the heart rate did not decrease by more than 20% in the toxin-treated rats even when the concentration of carbamoylcholine was raised to 10(-2) mol/l. The negative inotropic effect of carbamoylcholine examined on electrically paced ventricles perfused with isoproterenol was reduced, while the maximum positive inotropic effect of isoproterenol was substantially increased after toxin treatment. The inhibitory action of carbamoylcholine on the isoproterenol-stimulated accumulation of cyclic AMP in the heart auricles was attenuated. The weakening by pertussis toxin of the negative inotropic effect of carbamoylcholine is probably mainly due to the ADP ribosylation of the Gi regulatory protein and the subsequent loss of influence of muscarinic receptors on adenylate cyclase. The blockade of the negative chronotropic action of carbamoylcholine by pertussis toxin strongly indicates, together with other recently published evidence, that the Gi or another closely related GTP binding protein in the cardiac pacemaker cells is involved in the coupling of muscarinic receptors to the K+ channels.