Muscarinic acetylcholine receptors in the heart of rats before and after birth

Evaluation of drug-muscarinic receptor affinities using cell membrane chromatography and radioligand binding assay in guinea pig jejunum membrane

AIM

To study if cell membrane chromatography (CMC) could reflect drug-receptor interaction and evaluate the affinity and competitive binding to muscarinic acetylcholine receptor (mAChR).

METHODS

The cell membrane stationary phase (CMSP) was prepared by immobilizing guinea pig jejunum cell membrane on the surface of a silica carrier, and was used for the rapid on-line chromatographic evaluation of ligand binding affinities to mAChR. The affinity to mAChR was also evaluated from radioligand binding assays (RBA) using the same jejunum membrane preparation.

RESULTS

The capacity factor (k') profiles in guinea pig jejunum CMSP were: (-)QNB (15.4)>(+)QNB (11.5)>atropine (5.35)>pirenzepine (5.26)>4-DAMP (4.45)>AF-DX116 (4.18)>pilocarpine (3.93)>acetylcholine (1.31). These results compared with the affinity rank orders obtained from radioligand binding assays indicated that there was a positive correlation (r2= 0.8525, P<0.0001) between both data sets.

CONCLUSION

The CMC method can be used to evaluate drug-receptor affinities for drug candidates.

Quantitation of mRNAs for M(1) to M(5) subtypes of muscarinic receptors in rat heart and brain cortex

It has been generally accepted that, of the five subtypes of muscarinic receptors (M(1)-M(5)), only the M(2) subtype is expressed in mammalian heart. This notion has recently been challenged by a series of reports indicating that mRNAs for some or all non-M(2) subtypes are also present in mammalian heart, in parallel with the M(2) mRNA. However, the quantities of relevant mRNAs reported to be present in the heart are not known, which makes it difficult to evaluate their likely significance. We measured the concentrations of the five muscarinic mRNAs by competitive reverse transcription-polymerase chain reaction and discovered that the M(2) mRNA represents more than 90% of total muscarinic mRNAs in rat atria and in either ventricle. The concentrations of total muscarinic mRNAs and of the M(2) mRNA were more than twice as high in the atria than in the ventricles. mRNAs for all non-M(2) muscarinic receptor subtypes were also detected but represented less than 1% (M(1) and M(4)), less than 3% (M(3)), and less than 5% (M(5)) of total muscarinic RNAs in the atria and ventricles. The findings support the concept of the prevalent role of the M(2) muscarinic receptors in the cholinergic control of the heart. When the same method of quantitation was applied to rat cerebral cortex, mRNAs for individual subtypes were found to represent 36% (M(1)), 21% (M(2)), 25% (M(3)), 11% (M(4)), and 7% (M(5)) of total muscarinic mRNAs.

In vitro effects of chlorpyrifos, parathion, methyl parathion and their oxons on cardiac muscarinic receptor binding in neonatal and adult rats

Organophosphorus insecticides elicit toxicity by inhibiting acetylcholinesterase. Young animals are generally more sensitive than adults to these toxicants. A number of studies reported that some organophosphorus agents also bind directly to muscarinic receptors, in particular the m(2) subtype, in tissues from adult rats. As both the density and agonist affinity states of cardiac muscarinic receptors (primarily m(2)) have been reported to change in an age-related manner, we evaluated the relative in vitro sensitivity of cardiac muscarinic receptors in tissues from neonatal (7-11 days of age) and adult (90 days of age) rats to selected organophosphorus compounds (chlorpyrifos, parathion, methyl parathion and their oxygen analogs or oxons). The effects of the cholinergic agonist carbachol (100 pM-5 microM) or an organophosphorus toxicant (50 pM-10 microM) on muscarinic receptor binding were determined using the nonselective muscarinic ligand [3H]quinuclidinyl benzilate or the m(2)-preferential ligand [3H]oxotremorine-M acetate. Carbachol displaced [3H]oxotremorine labeling in adult and neonatal membranes in a relatively similar manner (IC(50)=7-20 nM). The oxons all displaced [3H]oxotremorine binding in a concentration-dependent manner, with chlorpyrifos oxon being the most potent (IC(50): neonates, 15 nM; adults, 7 nM) and efficacious (maximum displacement: neonates, 42%; adults, 56%). Interestingly, methyl parathion was an extremely potent displacer of [3H]oxotremorine binding in adult tissues (IC(50)=0.5 nM, maximum displacement=37%) but had no effect in neonatal tissues. The displacement of [3H]oxotremorine binding by chlorpyrifos oxon (10 microM) was still apparent after washing the tissues, suggesting the oxon irreversibly blocked agonist binding to the receptor while interaction with MePS appeared reversible. As effective concentrations of the oxons were relatively similar to their anticholinesterase potencies, these findings suggest that direct interaction with cardiac muscarinic receptors by some organophosphorus agents may occur at relevant exposure levels and contribute to cardiac toxicity.

Ontogeny of humoral heart rate regulation in the embryonic mouse

Catecholamines, acetylcholine, and adenosine are known to influence cardiac function, yet the effects of these agents on mammalian embryonic myocardium are largely unknown. To address this issue, we compared the chronotrophic effects of adenosinergic, adrenergic, and muscarinic agents on cultured murine embryos from postcoital day (PC) 8.0, when the fusing heart tubes first begin to beat, to PC 14, when cardiogenesis is essentially complete. At PC 8.0 and older, A(1)-adenosine receptor (A(1)AR) activation significantly decreased heart rates. Adrenergic stimulation caused modest increases in heart rates (145-155% of baseline) beginning at PC 9.0. Muscarinic activation decreased heart rates only after PC 13. When receptor gene expression was examined, A(1)ARs and beta(1)ARs were expressed in isolated hearts as early as PC 9.0, and beta(2)ARs and m(2)-muscarinic receptor genes were expressed at PC 11.0. These results identify the adenosinergic system as the earliest and most potent regulator of embryonic cardiac function and show that prenatal responsiveness to catecholamines and acetylcholine develops at later embryonic stages.

Carbachol inhibition of Ca2+ currents in ventricular cells obtained from neonatal and adult rats

We investigated the postnatal developmental changes produced by the muscarinic receptor agonist, carbachol, on the L-type Ca2+ current (ICa(L)) in neonatal (aged 5 to 7 days) and adult (aged 2 to 5 months) rat ventricular cells by using the whole-cell voltage clamp technique. Carbachol inhibited the isoproterenol-stimulated ICa(L). The maximal inhibition was 89.3 +/- 4.8% (n = 5) in neonatal cells and 17.7 +/- 7.7% (n = 9) in adult cells. Carbachol inhibited the forskolin-stimulated ICa(L) to almost same extent as the isoproterenol-stimulated ICa(L). In the cells pretreated with pertussis toxin, carbachol failed to inhibit the isoproterenol-stimulated ICa(L), indicating that carbachol produced its effect via a pertussis toxin-sensitive G-protein pathway. The effects of carbachol in adult cells became more pronounced, increasing from 17.7% to 54.8% (n = 11), with the addition of the synthetic inhibitory G-protein alpha subunit (Gi alpha) (1 microM) to the reaction. Conversely, the alpha subunit of another pertussis toxin-sensitive synthetic G-protein (G(o) alpha, 1 microM) failed to mimic the effect of Gi alpha. These results suggest that, in rat ventricular cells, (1) the action of carbachol on ICa(L) showed a marked decrease during development; (2) the decrease in the effect of carbachol in adult cells is in part due to a decrease in the activity of pertussis toxin-sensitive G protein, especially Gi alpha.

Methylene blue is a muscarinic antagonist in cardiac myocytes

We studied the mechanism of action of methylene blue (Mblue), a putative guanylyl cyclase inhibitor, on the L-type calcium current (ICa) and the muscarinic activated K+ current (IK,ACh) in rat ventricular and atrial myocytes, respectively, and on the binding of [3H]quinuclidinyl benzylate in rat ventricular membranes. Superfusion, but not internal dialysis, with 30 microM Mblue antagonized the inhibitory effect of acetylcholine (ACh, 1 microM) on beta-adrenergic stimulation of ICa with isoprenaline (Iso, 10 nM or 1 microM). However, Mblue had no effect on the basal ICa or on the stimulation of ICa by Iso in the absence of ACh. The activation of IK,ACh by 3 microM ACh was also antagonized by Mblue in a dose-dependent manner. In contrast, Mblue had no effect on the activation of IK,ACh by either guanosine-5'-O-(3-thio)triphosphate or guanosine-5'-(beta,gamma-imido)triphosphate. Chlorpromazine (CPZ), a piperazine derivative like Mblue, also inhibited the muscarinic activation of IK,ACh in a dose-dependent manner. The specific binding of [3H]QNB, a muscarinic ligand, to rat ventricular membranes was displaced in a dose-dependent manner by Mblue and CPZ. The piperazine derivatives behaved like competitive antagonists of [3H]QNB binding, exhibiting equilibrium dissociation constant (Ki) values of 187 nM for Mblue and 366 nM for CPZ. In conclusion, Mblue exerts antimuscarinic effects on ICa and IK,ACh in rat cardiac myocytes that are best explained by the binding of Mblue to the M2 subtype of muscarinic receptors. This property probably contributes to the antimuscarinic effect of the putative guanylyl cyclase inhibitor reported in previous studies.

Differences in isoproterenol stimulation of Ca2+ current of rat ventricular myocytes in neonatal compared to adult

The developmental changes in the isoproterenol stimulation of the L-type calcium current (ICa(L)) were studied in freshly isolated neonatal (3-5-day-old) and adult (2-3-month-old) rat ventricular myocytes using whole-cell voltage clamp (at room temperature). ICa(L) was measured as the peak inward current at a test potential of +10 mV (or +20 mV) by applying a 300 ms pulse from a holding potential of -40 mV. The pipette solution was Cs(+)-rich and Ca(2+)-free. The external solution was Na(+)-free and K(+)-free. Isoproterenol stimulated ICa(L) in a dose-dependent manner. The concentrations of isoproterenol for half-maximal effect were 6.8 nM in neonatal and 13.3 nM in adult. The maximal stimulation of ICa(L) was 147 +/- 14% in neonatal and 97 +/- 7% in adult. The steady-state inactivation curves were not affected by isoproterenol, whereas the steady-state activation curve was shifted to the left in both neonatal and adult. Forskolin (10 microM) increased ICa(L) by 105 +/- 10% in neonatal and 90 +/- 12% in adult. After stimulating ICa(L) by forskolin, the addition of isoproterenol produced a further increase of ICa(L) by 99 +/- 27% in neonatal, but only by 19 +/- 3% in adult. The presence of an inhibitor of cAMP-dependent protein kinase in the pipette did not affect this marked difference between neonatal (87 +/- 23%) and adult (11 +/- 8%). We conclude that, in rat ventricular myocytes, (1) stimulation of ICa(L) by the beta-adrenoceptor agonist, isoproterenol, is already fully developed in the neonatal stage and actually decreases during development; (2) there is evidence for a cAMP-independent stimulation of Ca2+ channels by isoproterenol, and this is greater in neonatal than in adult. We believe that the cAMP-independent pathway is the direct pathway mediated by Gs alpha protein.

Post-natal decrease in chronotropic sensitivity to acetylcholine in rat heart

1. The negative chronotropic effects of acetylcholine and carbachol on isolated rat right atria were examined at 0, 4, 8, and 16 weeks after birth. 2. Acetylcholine produced negative chronotropic responses at all ages and completely abolished spontaneous beating at its maximum effective concentration. 3. The sensitivity to acetylcholine, expressed in terms of ED50 values, was higher at 0 and 4 weeks than at 8 and 16 weeks, ED50 values (microM) at 0, 4, 8 and 16 weeks being 9.5 +/- 1.8 (n = 12), 13.2 +/- 3.4 (n = 11), 59.3 +/- 10.9 (n = 14) and 51.5 +/- 17.5 (n = 5), respectively. 4. Neostigmine produced a leftward shift of the concentration-response curve for acetylcholine both at 4 and 8 weeks after birth. The shift was larger at 8 weeks and no difference in sensitivity to acetylcholine was observed between the two ages in the presence of neostigmine. 5. Further, no developmental changes were observed in the sensitivity to carbachol, which is not hydrolyzed by cholinesterase. 6. We concluded that the chronotropic sensitivity to acetylcholine of rat atria decreases post-natally during the period between 4 and 8 weeks after birth due to increase in cholinesterase activity.

Developmental and biochemical characteristics of the cardiac membrane-bound arginine-specific mono-ADP-ribosyltransferase

ADP-ribosylation of protein in heart membrane preparations has been shown to be present in adult tissue but absent from early neonate tissue [Piron and McMahon (1990) Biochem. J. 270, 591-597]. To further this observation, the cardiac membrane-bound form of arginine-specific mono-ADP-ribosyltransferase (EC 2.4.2.31) has been characterized. Apparent Km values of 330 and 470 microM were found in heart membrane preparations from rat and quail respectively. The Vmax. value depended greatly on the species of animal studied, and was 1.1 and 48 nmol/min per mg in rat and quail preparations respectively. The specific activity of the enzyme was lowest in pig, intermediate in rat, dog and rabbit, and highest in mouse and quail cardiac membranes. In the rat, the ADP-ribosylation of protein and enzyme activity were very low in heart preparations from 1-15-day-old animals. Thereafter the ADP-ribosylation and enzyme activity increased gradually to adulthood. Bacillus cereus phosphatidylinositol-specific phospholipase C, known to hydrolyse glycosylphosphatidylinositol anchors of proteins, released the mono-ADP-ribosyltransferase from membrane preparations of both rat and quail in a dose-dependent, Zn(2+)-inhibited manner. Thus it appears that a membrane-bound form of arginine-specific mono-ADP-ribosyltransferase is present in heart membranes from a variety of species and is not species-specific. The activity of this ADP-ribosyltransferase appears to be developmentally regulated and to be bound to the cardiac membranes by a glycosylphosphatidylinositol anchor.

Developmental changes in the beta-adrenergic modulation of calcium currents in rabbit ventricular cells

We studied the developmental changes in the beta-adrenergic modulation of L-type calcium current (ICa) in enzymatically isolated adult (AD) and newborn (NB, 1-4-day-old) rabbit ventricular cells using the whole-cell patch-clamp method. ICa was measured as the peak inward current at a test potential of +15 mV by applying a 180-450-msec pulse from a holding potential of -40 mV with Cs(+)-rich pipettes and a K(+)-free bath solution at room temperature. In control, ICa density (obtained by normalizing ICa to the cell capacitance) was significantly higher in AD cells (5.5 +/- 0.2 [mean +/- SEM] pA/pF, n = 65) than in NB cells (2.6 +/- 0.1 pA/pF, n = 60). Isoproterenol (ISO, 1 nM-30 microM) increased ICa in a dose-dependent manner for both groups. The maximal effect (Emax) of ISO, expressed as percent increase in ICa over control levels, and the concentration for one half of the maximal effect (EC50) were 203% and 51 nM, respectively, for AD cells and 111% and 81 nM, respectively, for NB cells. The effect of ISO (1 microM) on ICa was decreased as the test potential was increased from -10 to +40 mV. However, the ratio of the percent increase in ICa for AD versus NB cells was almost constant (2.09-2.45) at each test potential. Dose-response curves of forskolin (FOR, 0.3-50 microM) gave Emax and EC50 of 268% and 0.74 microM, respectively, for AD cells and 380% and 1.15 microM, respectively, for NB cells. After stimulating ICa by 10 microM ISO, the addition of 10 microM FOR produced a further increase in ICa of only 12 +/- 2% in AD cells (n = 4) but a further increase of 140 +/- 41% in NB cells (n = 6). FOR (10 microM) did not produce any increase in ICa for AD and NB cells after stimulating ICa by intracellular application of 200 microM cAMP. ICa density stimulated by 10 microM ISO (17.8 +/- 1.1 pA/pF, n = 7), 10 microM FOR (21.0 +/- 1.3 pA/pF, n = 8), or 200 microM cAMP (18.0 +/- 1.3 pA/pF, n = 5) was equivalent in AD cells, whereas ICa density stimulated by 10 microM ISO (5.8 +/- 0.6 pA/pF, n = 9) was significantly lower than that stimulated by either 10 microM FOR (13.8 +/- 1.5 pA/pF, n = 7) or 200 microM cAMP (13.4 +/- 0.7 pA/pF, n = 7) in NB cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Characteristics of binding of endothelin-1 and endothelin-3 to rat hearts. Developmental changes in mechanical responses and receptor subtypes

Endothelin-1 (ET-1) and endothelin-3 (ET-3) produced positive inotropic effects on electrically stimulated left atria and increased the frequency of spontaneously beating right atria of adult rats. The potency of the inotropic effect of ET-1 was greater than that of ET-3, but the potencies of the chronotropic effects of ET-1 and ET-3 were not significantly different. In the neonatal atria, ET-1 and ET-3 also induced positive inotropic and chronotropic responses. ET-1 and ET-3 showed weak or no cardiotonic effects on the adult ventricles, whereas they caused marked positive inotropy in the neonatal ventricles. The characteristics of binding sites for ET-1 and ET-3 were very similar between the atria and the ventricles of the rat neonate. Saturation and competition binding experiments have shown that neonatal cardiac membranes from both atria and ventricle have two distinct binding sites for endothelin, that is, a low-affinity and a high-affinity site. ET-1 was found to bind to the low-affinity sites with a significantly lower Kd than ET-3, whereas the estimated Kd values for ET-1 and ET-3 at the high affinity sites were similar. In contrast, the binding sites in adult atria were different from those of the ventricles: only a single binding site for both ET-1 and ET-3 was detected. Adult atrial membranes, on the other hand, had two distinct binding sites similar to those of neonatal membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in beta-adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in rat ventricular muscles

1. In an attempt to explain the previous electrophysiological data on the ontogeny of beta-adrenergic and muscarinic cholinergic interactions on cardiac Ca2+ current, biochemical studies were performed on the ontogeny of beta-adrenoceptors, muscarinic cholinoceptors and Ca2+ channels in cardiac muscle of developing rats: 16-20 days old foetuses, 0-20 days old neonates, and 2-3 months old adults. 2. Developmental changes in cardiac beta-adrenoceptors, muscarinic cholinoceptors, and Ca2+ channels were determined with the use of specific radioligands, [3H]-dihydroalprenolol (DNA), [3H]-quinuclidinyl benzilate (QNB), and [3H]-nitrendipine (NTD), respectively. 3. The Bmax value (fmol mg-1 tissue) for [3H]-DNA binding started to increase on post-gestation day 20, reached almost its maximum level on neonatal day 6, kept almost the same level until neonatal day 20, and then decreased slightly to its adult level. 4. The Bmax value (fmol mg-1 tissue) for [3H]-QNB binding started to increase on post-gestation day 16, reached almost its maximum level on neonatal day 0, remained almost constant until neonatal day 15, and then decreased to its adult level. 5. The Bmax value (fmol mg-1 tissue) for [3H]-NTD binding increased with age between post-gestation day 18 and neonatal day 15, stayed almost constant until neonatal day 20, and then decreased to its adult level. 6. The Kd values for [3H]-DHA, [3H]-QNB, and [3H]-NTD bindings remained almost constant during the developmental period examined. 7. Isoprenaline (Iso) increased the kx of slow action potentials (APs) from post-gestation day 18, and the adult level was reached at about 2 weeks after birth; this developmental time course is similar to that of Ca2+ channels. The number of beta-adrenoceptors also started to increase a few days before birth, but attained its peak about one week earlier than did the Pax of slow APs or the number of Ca2 + channels. 8. Acetylcholine (ACh) almost completely abolished the Iso-induced increase in m,,ax observed from postgestation day 18 to neonatal day 20; this developmental time course for the ACh effect is consistent with the finding that the number of muscarinic cholinoceptors started to increase on post-gestation day 16 and reached a peak on the day of birth. 9. Previous electrophysiological and the present biochemical findings strongly suggest that the functional coupling between muscarinic cholinoceptors and Ca2+ current is already established when the coupling between beta-adrenoceptors and Ca2 + current starts to operate in developing rat hearts.

Developmental changes in beta-adrenergic and cholinergic interactions on calcium-dependent slow action potentials in rat ventricular muscles

1. Developmental changes in the effect of isoprenaline (Iso) and acetylcholine (ACh) interactions on Ca2(+)-dependent slow action potentials (APs) were studied in the ventricular muscles of foetal (12-20 days post-gestation), neonatal (0-20 days old), and adult (2-3 months old) rats. The slow APs were recorded at 0.2 Hz in partially depolarized preparations (an extracellular K+ concentration of 25 mM). 2. Iso (1 nM to 10 microM) began to increase the Vmax of the slow APs (an approximate indicator of Ca2+ current) on foetal day 18; its potentiating effect became greater with age and reached the adult level about 2 weeks after birth. 3. ACh (10 microM) abolished the Iso (1 microM)-induced increased in the Vmax observed in the late foetal and neonatal periods. 4. The inhibitory effect of ACh on the Vmax was antagonized by atropine but not by pirenzepine, suggesting that ACh reduces Ca2+ current (in the presence of beta-adrenoceptor agonists) by stimulating muscarinic (M2) cholinoceptors. 5. These results suggest that developmental changes in the modulatory effects of beta-adrenoceptor and cholinoceptor agonists on Ca2+ channels occur from a few days before birth to 2 weeks after birth and that the functional coupling between muscarinic cholinoceptors and Ca2+ channels has already been established when the coupling between beta-adrenoceptors and Ca2+ channels starts to operate.

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.

The action of acetylcholine upon heart rate changes markedly with development in bullfrogs

Dose-response curves for the action of acetylcholine (ACh) and its antagonist, atropine, upon heart rate in situ were made for three larval stages and adults of the bullfrog, Rana catesbeiana. The absolute magnitude of heart rate inhibition at any ACh concentration progressively increased throughout larval development. Unexpectedly, metamorphosis to the adult resulted in a sharp decrease in cholinergic cardiac sensitivity down to levels similar to the earliest larvae. Thus, cholinergic sensitivity of heart rate, reflecting properties of the cardiac pacemaker, is greatest immediately before metamorphosis and lowest in early larvae and adults.

Autoradiographic localization of alpha-adrenoceptors, muscarinic acetylcholine receptors and opiate receptors in the heart

Using in vitro autoradiography the distribution of [3H]rauwolscine, [3H]prazosin and [3H]quinuclidinyl benzilate binding sites has been demonstrated in cardiac tissue taken from the cat and rat. A similar distribution of both alpha 1- and alpha 2-adrenoceptor sites was seen but the distribution of muscarinic acetylcholine sites was markedly different. alpha-Adrenoceptors were present predominantly in ventricular muscle whereas muscarinic acetylcholine receptors exhibited a greater density in atrial tissue compared to ventricular muscle. Opiate receptors were absent from cardiac tissue.