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

Cardiomegaly induced by pressure overload in newborn rats is accompanied by altered expression of the long isoform of G(s)alpha protein and deranged signaling of adenylyl cyclase

G proteins-coupled signaling pathways appear to play a role in the development of cardiac hypertrophy and its progression to heart failure. The present study aimed to investigate trimeric G proteins and adenylyl cyclase signaling in immature as well as in adult rat myocardium during this process caused by pressure overload. Pressure overload was induced in newborn (2-day-old) rats by abdominal aortic banding and myocardial preparations from left ventricular myocardium of immature (10-day-old) and adult (90-day-old) animals were analyzed for the relative content of different G protein subunits and adenylyl cyclase (AC) by immunoblotting with specific antibodies. A functional status of the AC signaling system was also evaluated. Normal maturation of rat heart was accompanied by increased expression of AC type V/VI and VII and of the long isoform (G(s)alphaL) of G(s)alpha protein. In parallel, the amounts of myocardial G(i)alpha/G(o)alpha proteins tended to decrease, and G(q)alpha/G(11)alpha and Gbeta did not change. Interestingly, whereas fluoride-stimulated AC activity increased in the course of maturation, activity of AC measured under other experimental conditions (stimulation by Mn2+, forskolin or isoproterenol) was lower in adult than in young rat myocardium. Pressure overload did not influence distribution of G proteins in immature myocardium, but considerably decreased the content of G(s)alphaL and increased G(o)alpha proteins in hearts of 90-day-old rats. These hearts exhibited worsened functional reserve as compared to age-matched controls and activity of AC was also markedly lower. A considerable reduction in Mn(2+)-stimulated AC activity together with similar decrease in AC activity determined under other stimulation conditions suggests that it is a function of AC catalytic subunit that is primarily impaired in this model of pressure overload.

Membrane-Bound and cytosolic forms of heterotrimeric G proteins in young and adult rat myocardium: influence of neonatal hypo- and hyperthyroidism

Membrane and cytosolic fractions prepared from ventricular myocardium of young (21-day-old) hypo- or hyperthyroid rats and adult (84-day-old) previously hypo- or hyperthyroid rats were analyzed by immunoblotting with specific anti-G-protein antibodies for the relative content of Gs alpha, Gi alpha/Go alpha, Gq alpha/G11 alpha, and G beta. All tested G protein subunits were present not only in myocardial membranes but were at least partially distributed in the cytosol, except for Go alpha2, and G11 alpha. Cytosolic forms of the individual G proteins represented about 5-60% of total cellular amounts of these proteins. The long (Gs alpha-L) isoform of Gs alpha prevailed over the short (Gs alpha-S) isoform in both crude myocardial membranes and cytosol. The Gs alpha-L/Gs alpha-S ratio in membranes as well as in cytosol increased during maturation due to a substantial increase in Gs alpha-L. Interestingly, whereas the amount of membrane-bound Gi alpha/Go alpha and Gq alpha/G11 alpha proteins tend to lower during postnatal development, cytosolic forms of these G proteins mostly rise. Neonatal hypothyroidism reduced the amount of myocardial Gs alpha and increased that of Gi alpha/Go alpha proteins. By contrast, neonatal hyperthyroidism increased expression of Gs alpha and decreased that of Gi alpha and G11 alpha in young myocardium. Changes in G protein content induced by neonatal hypo- and hyperthyroidism in young rat myocardium were restored in adulthood. Alterations in the membrane-cytosol balance of G protein subunits associated with maturation or induced by altered thyroid status indicate physiological importance of cytosolic forms of these proteins in the rat myocardium.

Establishment of beta-adrenergic modulation of L-type Ca2+ current in the early stages of cardiomyocyte development

beta-Adrenergic modulation of the L-type Ca2+ current (ICaL) was characterized for different developmental stages in murine embryonic stem cell-derived cardiomyocytes using the whole-cell patch-clamp technique at 37 degreesC. Cardiomyocytes first appeared in embryonic stem cell-derived embryoid bodies grown for 7 days (7d). ICaL was insensitive to isoproterenol, forskolin, and 8-bromo-cAMP in very early developmental stage (VEDS) cardiomyocytes (from 7+1d to 7+2d) but highly stimulated by these substances in late developmental stage (LDS) cardiomyocytes (from 7+9d to 7+12d), indicating that all signaling cascade components became functionally coupled during development. In early developmental stage (EDS) cells (from 7+3d to 7+5d), the stimulatory response to forskolin and 8-bromo-cAMP was relatively weak. The forskolin effect was strongly augmented by ATP-gamma-S. At this stage, basal ICaL was stimulated by the nonselective phosphodiesterase (PDE) inhibitor isobutylmethylxanthine, by PDE inhibitors selective for the PDE II, III, and IV isoforms, as well as by the phosphatase inhibitor okadaic acid. Stimulation of ICaL by the catalytic subunit of the cAMP-dependent protein kinase A (PKA) was found to be similar (about 3 times) throughout development and in adult mouse ventricular cardiomyocytes, indicating that no structural changes of the Ca2+ channel related to phosphorylation occurred during development. ICaL was stimulated by isoproterenol in the presence of a PKA inhibitor and GTP-gamma-S in LDS but not VEDS cardiomyocytes, suggesting the development of a membrane-delimited stimulatory pathway mediated through the stimulatory GTP binding protein, Gs. We conclude that uncoupling and/or low expression of Gs protein accounted for the ICaL insensitivity to beta-adrenergic stimulation in VEDS cardiomyocytes. Furthermore, in EDS cells at the 7+4d stage, the reduced beta-adrenergic response is due, at least in part, to high intrinsic PDE and phosphatase activities.

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.

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.

Perioperative lymphocyte adenylyl cyclase function in the pediatric cardiac surgical patient

OBJECTIVE

To examine intraoperative and postoperative lymphocyte adenylyl cyclase activities in children undergoing repair of congenital cardiac defects with hypothermic cardiopulmonary bypass.

DESIGN

A prospective study.

SETTING

Tertiary university pediatric hospital.

PATIENTS

Twelve children were enrolled into the study to examine intraoperative lymphocyte adenylyl cyclase activities and 12 children were enrolled to examine postoperative lymphocyte adenylyl cyclase activities.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Basal (unstimulated), isoproterenol, and prostaglandin E-1 stimulated adenylyl cyclase activities, and plasma norepinephrine and epinephrine concentrations were measured. Intraoperative basal (unstimulated), beta-adrenergic receptor-stimulated (in response to isoproterenol), and prostaglandin E1 (PGE1)-stimulated lymphocyte adenylyl cyclase activities all increased during cardiopulmonary bypass, then decreased immediately after cardiopulmonary bypass. In the postoperative group, a significant decrease in basal (unstimulated), beta-adrenergic receptor- and PGE1-stimulated adenylyl cyclase activities were observed on postoperative day 1 as compared with precardiopulmonary bypass values.

CONCLUSIONS

In the pediatric cardiac surgical patient, there was an intraoperative enhancement of lymphocyte adenylyl cyclase activities. This increase in adenylyl cyclase activities was followed by reduced lymphocyte adenylyl cyclase activities, including beta-adrenergic receptor desensitization, postoperatively, as we have previously documented in adults.

Developmental changes in modulation of calcium currents of rabbit ventricular cells by phosphodiesterase inhibitors

BACKGROUND

We have previously shown major differences in beta-adrenergic and muscarinic modulation of L-type calcium currents (ICa) in newborn and adult rabbit heart. However, little is known about developmental changes in modulation of ICa by phosphodiesterases (PDEs), which also regulate intracellular cAMP concentration by its hydrolysis.

METHODS AND RESULTS

Enzymatically isolated adult and newborn (1- to 3-day-old) rabbit ventricular myocytes were used to study the effects of PDE inhibitors on ICa measured by the whole-cell patch-clamp method. 3-Isobutyl-1-methyl-xanthine (IBMX), a nonselective PDE inhibitor, increased ICa in a dose-dependent manner for both groups. The maximal effect of IBMX, expressed as percentage increase in ICa over control levels, was greater for newborn myocytes than for adult myocytes, but the effects of IBMX applied alone were observed only at concentrations > 10 mumol/L. The concomitant use of 0.1 mumol/L isoproterenol produced a significant potentiation of the IBMX effect on ICa, with a significant additive effect of IBMX in newborn myocytes even at 0.05 mumol/L IBMX. The concomitant use of a subthreshold concentration of IBMX (0.1 mumol/L) did not potentiate the dose dependence of adult ICa on isoproterenol but did markedly potentiate the dose dependence of newborn ICa on isoproterenol. The Emax and EC50 of isoproterenol in the presence of 0.1 mumol/L IBMX on newborn ICa were 235% and 8 nmol/L, respectively, whereas the Emax and EC50 of isoproterenol in the absence of IBMX on newborn ICa were 111% and 81 nmol/L, respectively. The addition of 50 mumol/L IBMX to 10 mumol/L isoproterenol markedly increased the newborn ICa density up to a level equivalent to that reached with 200 mumol/L cAMP in the pipette (14.9 +/- 1.2 versus 13.4 +/- 0.7 pA/pF). Our data suggest that the inhibition constant (Ki) of IBMX for inhibiting PDEs that participate in the regulation of ICa is much lower in newborn than in adult myocytes. Milrinone 1 mumol/L, a selective PDE III inhibitor, increased the 0.1 mumol/L isoproterenol-stimulated ICa of adult myocytes but had no significant additive effect for the 0.1 mumol/L isoproterenol-stimulated ICa of newborn myocytes. Rolipram 1 mumol/L, a selective PDE IV inhibitor, increased the 0.1 mumol/L isoproterenol-stimulated ICa for newborn myocytes but had no significant additive effect for the 0.1 mumol/L isoproterenol-stimulated ICa for adult myocytes.

CONCLUSIONS

These results suggest that the most important PDE isozyme for regulation of ICa of rabbit myocytes changes from PDE IV to PDE III during the postnatal period.

Effects of fetal dexamethasone exposure on postnatal control of cardiac adenylate cyclase: beta-adrenergic receptor coupling to Gs regulatory protein

In the adult, glucocorticoids have been shown to upregulate beta-adrenergic control of adenylate cyclase by a variety of mechanisms; glucocorticoids are also thought to play a role in development of cardiac adrenergic function. In the current study, pregnant rats were given 0.2 mg/kg of dexamethasone on gestational days 17, 18, and 19 and the effects on the development of cardiac beta-receptors and their linkage to the stimulatory G-protein, Gs, were examined at 4 days postpartum. beta-Receptor numbers and affinity were unaffected by dexamethasone exposure, nor was there any change in the ability of the GTP analog, Gpp(NH)p, to shift the affinity state of the receptor. Addition of Gpp(NH)p to cardiac membranes enhanced basal and isoproterenol-stimulated adenylate cyclase activity, but the total response to isoproterenol, with or without Gpp(NH)p, represented a very small fraction of total enzymatic activity. Quantitative analysis of Gs indicated no changes attributable to dexamethasone treatment. Although prenatal dexamethasone has been shown to increase adenylate cyclase reactivity to beta-adrenergic input, the effect appears to be at the level of the catalytic subunit of adenylate cyclase, rather than at receptor or G-protein stages.

The effect of isobutylmethylxanthine, forskolin, and cholera toxin on growth hormone release from pituitary cell cultures of perinatal and mature rats

The factors that regulate growth hormone (GH) release during the perinatal period are not well understood. Circulating GH levels are markedly elevated in mammalian fetuses and newborns compared with mature animals, and the immature pituitary is highly responsive to the GH-stimulatory effect of GH-releasing factor (GHRF). The etiology of these developmental changes in GH secretion is not known. In order to investigate the mechanisms underlying GH release from immature pituitaries, we tested the effects of agents that increase intracellular cyclic adenosine 3',5' monophosphate (cAMP) production independent of the GHRF receptor on GH release from pituitaries of developing and mature rats. Pituitary cell cultures from fetal (day 20 of gestation), newborn (postnatal day 2), juvenile (postnatal day 12-15), adult male (3-4 months), and adult female (3-4 months) rats were tested with isobutylmethylxanthine (IBMX; 0.001-1.0 mM), forskolin (0.01-10 microM), and cholera toxin (0.025-25 ng/ml). IBMX, forskolin, and cholera toxin stimulated GH release in a dose-dependent manner from pituitary cultures of all age groups. However, the magnitude of the GH responses to these agents was highly age-dependent. Perinatal pituitaries exhibited markedly greater GH responses to IBMX, forskolin, and cholera toxin than did those of mature animals (P < 0.001 for age effect with each agent). GH release in response to the highest dose of IBMX (1 mM) was 301 +/- 8, 389 +/- 37, 296 +/- 33, 198 +/- 14, and 187 +/- 19% of control values from pituitary cell cultures of fetal, newborn, juvenile, adult male, and adult female rats, respectively (P < 0.001). In response to the highest dose of forskolin (10 microM) GH release was 537 +/- 46, 601 +/- 75, 274 +/- 22, 270 +/- 37, and 248 +/- 35% of control values in the same respective age groups (P < 0.001). Similarly, the highest dose of cholera toxin (25 ng/ml) stimulated GH release to 407 +/- 55, 365 +/- 43, 249 +/- 26, 186 +/- 11, and 186 +/- 1% of controls in these respective age groups (P < 0.003). The marked stimulation of GH release from perinatal pituitaries by IBMX, forskolin, and cholera toxin is consistent with the concept that cAMP is a potent mediator of GH release from immature as well as mature somatotrophs. The developmental changes in the GH secretory response to these agents further suggest that signal transduction pathways mediating GH release may undergo maturation, at least in part, at intrasomatotroph loci distal to the GHRF receptor.

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)

Characterization of high affinity GTPase activity correlated to beta-adrenergic receptor stimulation of adenylyl cyclase in rat parotid membranes

beta-Adrenergic receptor stimulation of adenylyl cyclase involves the activation of a GTP-binding regulatory protein (G-protein, termed here Gs). Inactivation of this G-protein is associated with the hydrolysis of bound GTP by an intrinsic high affinity GTPase activity. In the present study, we have characterized the GTPase activity in a Gs-enriched rat parotid gland membrane fraction. Two GTPase activities were resolved; a high affinity GTPase activity displaying Michaelis-Menten kinetics with increasing concentrations of GTP, and a low affinity GTPase activity which increased linearly with GTP concentrations up to 10 mM. The beta-adrenergic agonist isoproterenol (10 microM) increased the Vmax of the high affinity GTPase component approx. 50% from 90 to 140 pmol/mg protein per min, but did not change its Km value (approximately 450 nM). Isoproterenol also stimulated adenylyl cyclase activity in parotid membranes both in the absence or presence of GTP. In the presence of a non-hydrolyzable GTP analogue, guanosine 5'-(3-O-thio)triphosphate (GTP gamma S), isoproterenol increased cAMP formation to the same extent as that observed with AlF-4. Cholera toxin treatment of parotid membranes led to the ADP-ribosylation of two proteins (approximately 45 and 51 kDa). Cholera toxin also specifically decreased the high affinity GTPase activity in membranes and increased cAMP formation induced by GTP in the absence or the presence of isoproterenol. These data demonstrate that the high affinity GTPase characterized here is the 'turn-off' step for the adenylyl cyclase activation seen following beta-adrenergic stimulation of rat parotid glands.

An update to silent myocardial ischemia: pathophysiological, diagnostic, and therapeutic approaches

Silent myocardial ischemia (SMI) is a common ischemia process which can be defined as objective evidence of myocardial ischemia without chest pain or other equivalent indications. SMI can occur in totally asymptomatic patients, as well as in patients who have documented coronary artery disease (CAD) and who, on exercise and/or during Holter monitoring, may show ischemic changes. The precise mechanism for pathogenesis and pathophysiology of SMI remains to be clarified. A great deal has been learned about the role of beta-adrenergic receptors, adenylyl cyclase, and guanine nucleotide binding proteins (G proteins) in the myocardial ischemic process of SMI. Moreover, standard exercise test and long-term ECG recordings have proved to be of great value, especially when performed jointly, however, in this field there is still room for expanded knowledge. Pharmacological interventions to date have demonstrated the beneficial effects of beta-adrenergic receptor antagonists and/or calcium antagonists as contributing substantially to reducing both frequency and duration of SMI episodes. However, therapeutic options to improve the prognosis of SMI appear to be limited.

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.