Novel isoform of Ca2+ channel in rat fetal cardiomyocytes

NFAT5-mediated CACNA1C expression is critical for cardiac electrophysiological development and maturation

Entry of calcium into cardiomyocyte via L-type calcium channel (LTCC) is fundamental to cardiac contraction. CACNA1C, a type of LTCC and a hallmark of a matured ventricular myocyte, is developmentally regulated. Here, we identified 138 potential transcription factors by a comparative genomic study on 5-kb promoter regions of CACNA1C gene across eight vertebrate species, and showed that six factors were developmentally regulated with the expression of Cacna1c in mouse P19cl6 in vitro cardiomyocyte differentiation model. We further demonstrated that the nuclear factor of activated T cells 5 (Nfat5) bound to a consensus sequence TGGAAGCGTTC and activated the transcription of Cacna1c. The siRNA-mediated knockdown of Nfat5 suppressed the expression of Cacna1c and decreased L-type calcium current in mouse neonatal cardiomyocytes. Furthermore, morpholino-mediated knockdown of nfat5 in zebrafish prohibited the expression of cacna1c and resulted in a non-contractile ventricle, while over-expression of either cacna1c or nfat5 rescued this impaired phenotype. Thus, NFAT5-mediated expression of CACNA1C is evolutionarily conserved and critical for cardiac electrophysiological development and maturation of cardiomyocyte.

KEY MESSAGE

Nfat5 binds to a consensus sequence TGGAAGCGTTC in the promoter of Cacna1c. Nfat5 activates the transcription of Cacna1c. Nfat5 knockdown suppresses Cacna1c expression, decreases L-type calcium current, and results in non-beating ventricle. NFAT5-mediated expression of CACNA1C is evolutionarily conserved. NFAT5-mediated CACNA1C expression is critical for cardiac electrophysiological development and maturation.

Developmental changes of intracellular Ca2+ transients in beating rat hearts

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

Functional and molecular characterization of a T-type Ca(2+) channel during fetal and postnatal rat heart development

T-type calcium current (I(CaT)) is distributed among a large variety of species and tissues. The main functions of I(CaT) are thought to be related to pacemaker activity and to the cell cycle. Using the whole-cell patch-clamp configuration, we showed that fetal rat ventricular cells exhibit an I(CaT) with electrophysiological and pharmacological characteristics similar to those already described for this current. We investigated I(CaT) density and found that this current was mainly expressed in fetal cells and remained stable until birth (3.1+/-0.3 pA/pF for 18-day-old fetus, n=9). I(CaT) density decreased soon after birth (2.0+/-0.3 pA/pF, n=6, 1.1+/-0.2 pA/pF, n=5, for 1- and 5-day-old rats, respectively) and was no longer detected in 21-day-old rats. The rat ventricular cells express an alpha 1H isoform in addition to a homologous alpha 1G variant. Interestingly, the Ni(2+) sensitivity of I(CaT) indicates that in newborn myocytes, I(CaT) is only generated by alpha 1G subunits, whereas both alpha 1G and alpha 1H subunits participate in the fetal I(CaT). Moreover, the relative contribution of each subunit varies during fetal developmental stages, with a major contribution of alpha 1H in 16-day-old fetuses. Through quantitative RT-PCR we showed that the amount of both alpha 1G and alpha 1H transcripts are developmentally regulated. In fetuses of less than 18 days and in newborn rats after 1 day old, the transcriptional levels of alpha 1G and alpha 1H subunits clearly mismatch the functional contribution of these subunits to I(CaT). However, in perinatal period, the amount of alpha 1G mRNA seems to be in accordance to alpha 1G-related I(CaT) density. In conclusion, we showed that I(CaT) is mainly expressed during fetal stages, that alpha 1G and alpha 1H differentially participate to I(CaT) and that alpha 1G and alpha 1H isoforms are regulated by both transcriptional and post-transcriptional mechanisms.

Identification of the t-type calcium channel (Ca(v)3.1d) in developing mouse heart

During cardiac development, there is a reciprocal relationship between cardiac morphogenesis and force production (contractility). In the early embryonic myocardium, the sarcoplasmic reticulum is poorly developed, and plasma membrane calcium (Ca(2+)) channels are critical for maintaining both contractility and excitability. In the present study, we identified the Ca(V)3.1d mRNA expressed in embryonic day 14 (E14) mouse heart. Ca(V)3.1d is a splice variant of the alpha1G, T-type Ca(2+) channel. Immunohistochemical localization showed expression of alpha1G Ca(2+) channels in E14 myocardium, and staining of isolated ventricular myocytes revealed membrane localization of the alpha1G channels. Dihydropyridine-resistant inward Ba(2+) or Ca(2+) currents were present in all fetal ventricular myocytes tested. Regardless of charge carrier, inward current inactivated with sustained depolarization and mirrored steady-state inactivation voltage dependence of the alpha1G channel expressed in human embryonic kidney-293 cells. Ni(2+) blockade discriminates among T-type Ca(2+) channel isoforms and is a relatively selective blocker of T-type channels over other cardiac plasma membrane Ca(2+) handling proteins. We demonstrate that 100 micromol/L Ni(2+) partially blocked alpha1G currents under physiological external Ca(2+). We conclude that alpha1G T-type Ca(2+) channels are functional in midgestational fetal myocardium.

Characterization of nifedipine-resistant calcium current in neonatal rat ventricular cardiomyocytes

Calcium current was recorded from ventricular cardiomyocytes of rats at various stages of postnatal development using the whole cell patch-clamp technique. In cultured 3-day-old neonatal cells, the current carried by Ca(2+) or Ba(2+) (5 mM) was not completely inhibited by 2 microM nifedipine. A residual current was activated in the same voltage range as the L-type, nifedipine-sensitive Ca(2+) current, but its steady-state inactivation was negatively shifted by 16 mV. This nifedipine-resistant calcium current was not further inhibited by other organic calcium current antagonists such as PN200-110, verapamil, and diltiazem nor by nickel, omega-conotoxin, or tetrodotoxin. It was completely blocked by cadmium and increased by isoproterenol and forskolin. This current was >20% of total calcium current in ventricular myocytes freshly isolated from neonatal rats, and it decreased during postnatal maturation, disappearing at the adult stage. This suggests that this current could be caused by an isoform of the L-type calcium channel expressed in a way that reflects the developmental stage of the rat heart.

Ca(2+) channel modulation by recombinant auxiliary beta subunits expressed in young adult heart cells

L-type Ca(2+) channels contribute importantly to the normal excitation-contraction coupling of physiological hearts, and to the functional derangement seen in heart failure. Although Ca(2+) channel auxiliary beta(1-4) subunits are among the strongest modulators of channel properties, little is known about their role in regulating channel behavior in actual heart cells. Current understanding draws almost exclusively from heterologous expression of recombinant subunits in model systems, which may differ from cardiocytes. To study beta-subunit effects in the cardiac setting, we here used an adenoviral-component gene-delivery strategy to express recombinant beta subunits in young adult ventricular myocytes cultured from 4- to 6-week-old rats. The main results were the following. (1) A component system of replication-deficient adenovirus, poly-L-lysine, and expression plasmids encoding beta subunits could be optimized to transfect young adult myocytes with 1% to 10% efficiency. (2) A reporter gene strategy based on green fluorescent protein (GFP) could be used to identify successfully transfected cells. Because fusion of GFP to beta subunits altered intrinsic beta-subunit properties, we favored the use of a bicistronic expression plasmid encoding both GFP and a beta subunit. (3) Despite the heteromultimeric composition of L-type channels (composed of alpha(1C), beta, and alpha(2)delta), expression of recombinant beta subunits alone enhanced Ca(2+) channel current density up to 3- to 4-fold, which argues that beta subunits are "rate limiting" for expression of current in heart. (4) Overexpression of the putative "cardiac" beta(2a) subunit more than halved the rate of voltage-dependent inactivation at +10 mV. This result demonstrates that beta subunits can tune inactivation in the myocardium and suggests that other beta subunits may be functionally dominant in the heart. Overall, this study points to the possible therapeutic potential of beta subunits to ameliorate contractile dysfunction and excitability in heart failure.

Developmental regulation of the L-type calcium channel alpha1C subunit expression in heart

We used Northern analyses, RNase protection assays and immunoblot analyses to examine the relationship among developmental age of the heart, abundance of mRNA and L-type calcium channel alpha1C subunit protein, and to establish the size of the native protein in heart. Northern analysis, RNase protection assays, and immunoblots were used to study RNA and protein from rat heart of various ages. In fetal and adult ventricles there was a predominant 8.3-kb transcript for the alpha1C subunit with no change in transcript size during development. RNase protection assays demonstrated a 2-fold increase in abundance of the DHP receptor message during postnatal development. Immunoblots identified a 240 kD protein, corresponding to the predicted molecular mass of the full length alpha1C subunit. No change in size of protein for the alpha1C subunit was observed at any developmental stage and there was no evidence for a truncated isoform. There was an approximate 2-fold increase in alpha1C subunit protein in ventricular homogenates during postnatal development. Thus, in the developing rat heart, alterations in calcium channel properties during development appear to result neither from alternative splicing that produces a smaller transcript for the alpha1C subunit nor from expression of a truncated protein, but at least in part from transcriptionally-regulated expression of the 240 kDa polypepde.

Expression of Ca(2+) channel subunits during cardiac ontogeny in mice and rats: identification of fetal alpha(1C) and beta subunit isoforms

Functional cardiac L-type calcium channels are composed of the pore-forming alpha(1C) subunit and the regulatory beta(2) and alpha(2)/delta subunits. To investigate possible developmental changes in calcium channel composition, we examined the temporal expression pattern of alpha(1C) and beta(2) subunits during cardiac ontogeny in mice and rats, using sequence-specific antibodies. Fetal and neonatal hearts showed two size forms of alpha(1C) with 250 and 220 kDa. Quantitative immunoblotting revealed that the rat cardiac 250-kDa alpha(1C) subunit increased about 10-fold from fetal days 12-20 and declined during postnatal maturation, while the 220-kDa alpha(1C) decreased to undetectable levels. The expression profile of the 85-kDa beta(2) subunit was completely different: beta(2) was not detected at fetal day 12, rose in the neonatal stage, and persisted during maturation. Additional beta(2)-stained bands of 100 and 90 kDa were detected in fetal and newborn hearts, suggesting the transient expression of beta(2) subunit variants. Furthermore, two fetal proteins with beta(4) immunoreactivity were identified in rat hearts that declined during prenatal development. In the fetal rat heart, beta(4) gene expression was confirmed by RT-PCR. Cardiac and brain beta(4) mRNA shared the 3 prime region, predicting identical primary sequences between amino acid residues 62-519, diverging however, at the 5 prime portion. The data indicate differential developmental changes in the expression of Ca(2+) channel subunits and suggest a role of fetal alpha(1C) and beta isoforms in the assembly of Ca(2+) channels in immature cardiomyocytes.

Predominant distribution of nifedipine-insensitive, high voltage-activated Ca2+ channels in the terminal mesenteric artery of guinea pig

We have found nifedipine-insensitive (NI), rapidly inactivating, voltage-dependent Ca2+ channels (current, NI-I(Ca)) with unique biophysical and pharmacological properties in the terminal branches of guinea pig mesenteric artery, by using a whole-cell mode of the patch-clamp technique. The fraction of NI-I(Ca) appeared to increase dramatically along the lower branches of mesenteric artery, amounting to almost 100% of global I(Ca) in its periphery. With 5 mmol/L Ba2+ as the charge carrier, NI-I(Ca) was activated with a threshold of -50 mV, peaked at -10 mV, and was half-activated and inactivated at -11 and -52 mV, respectively, generating a potential range of constant activation near the resting membrane potential. The NI-I(Ca) was rundown resistant, was not subject to Ca(2+)-dependent inactivation, and exhibited the pore properties typical for high voltage-activated Ca2+ channels; Ba2+ is approximately 2-fold more permeable than Ca2+, and Cd2+ is a better blocker than Ni2+ (IC(50), 6 and 68 micromol/L, respectively). Relatively specific blockers for N- and P/Q-type Ca2+ channels such as omega-conotoxins GVIA and MVIIC (each 1 micromol/L) and omega-agatoxin IVA (1 micromol/L) were ineffective at inhibiting NI-I(Ca), whereas nimodipine partially (10 micromol/L; approximately 40%) and amiloride potently ( approximately 75% with 1 mmol/L; IC(50); 107 micromol/L) blocked the current. Although these properties are reminiscent of R-type Ca2+ channels, expression of the alpha(1E) mRNA was not detected using reverse transcriptase-polymerase chain reaction. These results strongly suggest the predominant presence of NI, high voltage-activated Ca2+ channels with novel properties, which may be abundantly expressed in peripheral small arterioles and contribute to their tone regulation.

Unitary current analysis of L-type Ca2+ channels in human fetal ventricular myocytes

INTRODUCTION

L-type calcium channels were studied in cell-attached patches from ventricular cell membranes of human fetal heart.

METHODS AND RESULTS

Experiments were performed in the presence of 70 mM Ba2+ as the charge carrier at 22 degrees C to 24 degrees C. Unitary current sweeps were evoked by 300-msec depolarizing pulses to 0 mV from a holding potential of -50 mV at 0.5 Hz. Recorded currents were blocked by nisoldipine (1 microM) and stimulated by (-)Bay K 8644 (1 microM). During control, channel activity was seen in 13.9%+/-4.2% of the total 200 sweeps. Ensemble average current amplitude was 0.03+/-0.01 pA (n = 6) and average conductance was 20.4+/-0.2 pS (n = 5). Analysis of single channel kinetics showed open time and closed time histograms were best fit by one and two exponentials, respectively. Mean open time was tau(o) = 0.99+/-0.05 msec (n = 6). Mean closed time fast (tau(cf)) and slow (tau(cs)) component values were tau(cf) = 0.85+/-0.09 msec and tau(cs) = 8.0+/-0.94 msec (n = 6), respectively. With intrapipette (-)Bay K 8644 (1 microM), mean open time was best fit by two exponentials, tau(of) = 0.9+/-0.2 msec (n = 10) and tau(os) = 13.4+/-2.6 msec (n = 10); mean close time values were tau(cf) = 0.6+/-0.1 msec (n = 10) and tau(cs) = 9.8+/-1.9 msec (n = 10), respectively. With (-)Bay K 8644, channel activity was 66.5%+/-7.4%, the ensemble average current was 0.52+/-0.04 pA (n = 10) and the conductance 20.7+/-0.5 pS (n = 5).

CONCLUSION

(1) the data establishes the characteristics of L-type Ca channels of human fetal hearts and their modulation by dihydropyridines; (2) the open time kinetics differ from those of avian embryonic and rat fetal hearts; and (3) the findings provide new and relevant information for understanding the physiologic behavior of unitary Ca2+ channels in the developing human heart and the baseline comparison for diseases that implicate Ca2+ channels in their etiology, such as autoimmune-associated congenital heart block.

Functional characteristics of ES cell-derived cardiac precursor cells identified by tissue-specific expression of the green fluorescent protein

In contrast to terminally differentiated cardiomyocytes, relatively little is known about the characteristics of mammalian cardiac cells before the initiation of spontaneous contractions (precursor cells). Functional studies on these cells have so far been impossible because murine embryos of the corresponding stage are very small, and cardiac precursor cells cannot be identified because of the lack of cross striation and spontaneous contractions. In the present study, we have used the murine embryonic stem (ES, D3 cell line) cell system for the in vitro differentiation of cardiomyocytes. To identify the cardiac precursor cells, we have generated stably transfected ES cells with a vector containing the gene of the green fluorescent protein (GFP) under control of the cardiac alpha-actin promoter. First, fluorescent areas in ES cell-derived cell aggregates (embryoid bodies [EBs]) were detected 2 d before the initiation of contractions. Since Ca2+ homeostasis plays a key role in cardiac function, we investigated how Ca2+ channels and Ca2+ release sites were built up in these GFP-labeled cardiac precursor cells and early stage cardiomyocytes. Patch clamp and Ca2+ imaging experiments proved the functional expression of the L-type Ca2+ current (ICa) starting from day 7 of EB development. On day 7, using 10 mM Ca2+ as charge carrier, ICa was expressed at very low densities 4 pA/pF. The biophysical and pharmacological properties of ICa proved similar to terminally differentiated cardiomyocytes. In cardiac precursor cells, ICa was found to be already under control of cAMP-dependent phosphorylation since intracellular infusion of the catalytic subunit of protein kinase A resulted in a 1.7-fold stimulation. The adenylyl cyclase activator forskolin was without effect. IP3-sensitive intracellular Ca2+ stores and Ca2+-ATPases are present during all stages of differentiation in both GFP-positive and GFP-negative cells. Functional ryanodine-sensitive Ca2+ stores, detected by caffeine-induced Ca2+ release, appeared in most GFP-positive cells 1-2 d after ICa. Coexpression of both ICa and ryanodine-sensitive Ca2+ stores at day 10 of development coincided with the beginning of spontaneous contractions in most EBs. Thus, the functional expression of voltage-dependent L-type Ca2+ channel (VDCC) is a hallmark of early cardiomyogenesis, whereas IP3 receptors and sarcoplasmic Ca2+-ATPases are expressed before the initiation of cardiomyogenesis. Interestingly, the functional expression of ryanodine receptors/sensitive stores is delayed as compared with VDCC.

Regulation of the slow Ca++ channels of myocardial cells

Contraction of the heart is regulated by a number of mechanisms, such as neurotransmitters, hormones, autacoids, pH, intracellular ATP, and Ca++ ions. These actions are mediated, at least in part, by actions on the sarcolemmal slow (L-type) Ca++ channels, exerted directly or indirectly. The major mechanisms for the regulation of the slow Ca++ channels of myocardial cells includes the following. cAMP/PK-A phosphorylation stimulates the slow Ca++ channel activity, whereas cGMP/PK-G phosphorylation inhibits. DAG/PK-C phosphorylation and tyrosine kinase phosphorylation are suggested to stimulate the slow Ca++ channel activity. Intracellular application of Gs alpha protein increases the slow Ca++ currents (ICa(L)). Lowering of intracellular ATP inhibits ICa(L). Acidosis and increase in [Ca]i inhibits ICa(L). A number of changes in the Ca++ channels also occur during development and aging. Thus, it appears that the slow Ca++ channel is a complex structure, including perhaps several associated regulatory proteins, which can be regulated by a number of extrinsic and intrinsic factors, and thereby control can be exercised over the force of contraction of the heart.

Intracellular free Ca2+ movements in cultured cardiac myocytes as shown by rapid scanning confocal microscopy

Two-dimensional images of intracellular free Ca2+ movements in cultured cardiac myocytes were obtained at 33-ms intervals with a Ca(2+)-sensitive fluorescence probe, fluo-3, and a rapid scanning confocal laser microscope, a prototype of Nikon RCM8000. The cells used were isolated from the ventricular myocardium of neonatal mice, cultured for approximately 72 h and loaded with fluo-3. One type of cytoplasmic Ca2+ movement observed was a simultaneous increase in [Ca2+] throughout the cytoplasm, termed a "spike"; another type was a local increase in [Ca2+] propagating in the cytoplasm, termed a "wave." Cells with either spike or wave or both types of movements were observed. Tetrodotoxin (TTX) 10(-5) M, nicardipine 10(-6) M, and increased extracellular potassium concentration (40 mM) selectively inhibited spike, and ryanodine 10(-6) M and cyclopiazonic acid (CPA) 3 x 10(-6) M selectively inhibited wave. These results indicate that spike was triggered by depolarization-induced Ca2+ influx across the sarcolemma, whereas wave was a propagating local increase in Ca2+ due to Ca2+ release from the sarcoplasmic reticulum (SR). On spike, nuclear [Ca2+] was shown to increase and decrease synchronously with cytoplasmic [Ca2+], with a delay and slower time course.

Regulation of calcium channel expression in neonatal myocytes by catecholamines

Expression of the dihydropyridine (DHP) receptor (alpha 1 subunit of L-type calcium channel) in heart is regulated by differentiation and innervation and is altered in congestive heart failure. We examined the transmembrane signaling pathways by which norepinephrine regulates DHP receptor expression in cultured neonatal rat ventricular myocytes. Using a 1.3-kb rat cardiac DHP receptor probe, and Northern analysis quantified by laser densitometry, we found that norepinephrine exposure produced a 2.2-fold increase in DHP receptor mRNA levels at 2 h followed by a decline to 50% of control at 4-48 h (P < 0.02). The alpha-adrenergic agonist phenylephrine and a phorbol ester produced a decline in mRNA levels (8-48 h). The beta-adrenergic agonist isoproterenol and 8-bromo-cAMP produced a transient increase in mRNA levels. After 24 h of exposure to isoproterenol, 3H-(+)PN200-110 binding sites increased from 410 +/- 8 to 539 +/- 39 fmol/mg (P < 0.05). The number of functional calcium channels, estimated by whole-cell voltage clamp experiments, was also increased after 24 h of exposure to isoproterenol. Peak current density (recordings performed in absence of isoproterenol) increased from -10.8 +/- 0.8 (n = 23) to -13.9 +/- 1.0 pA/pF (n = 27) (P < 0.01). Other characteristics of the calcium current (voltage for peak current, activation, and inactivation) were unchanged. Exposure for 48 h to phenylephrine produced a significant decline in peak current density (P < 0.01). We conclude that beta -adrenergic transmembrane signaling increases DHP receptor mRNA and number of functional calcium channels and that alpha - adrenergic transmembrane signaling produces a reciprocal effect. Regulation of cardiac calcium channel expression by adrenergic pathways may have physiological and pathophysiological importance.

Regulation of slow calcium channels of myocardial cells and vascular smooth muscle cells by cyclic nucleotides and phosphorylation

The slow Ca2+ channels (L-type) of the heart are stimulated by cAMP. Elevation of cAMP produces a very rapid increase in number of slow channels available for voltage activation during excitation. The probability of a Ca2+ channel opening and the mean open time of the channel are increased. Therefore, any agent that increases the cAMP level of the myocardial cell will tend to potentiate ICa, Ca2+ influx, and contraction. The action of cAMP is mediated by PK-A and phosphorylation of the slow Ca2+ channel protein or an associated regulatory protein (stimulatory type). The myocardial slow Ca2+ channels are also regulated by cGMP, in a manner that is opposite or antagonistic to that of cAMP. We have demonstrated this at both the macroscopic level (whole-cell voltage clamp) and the single-channel level. The effect of cGMP is mediated by PK-G and phosphorylation of a protein, as for example, a regulatory protein (inhibitory-type) associated with the Ca2+ channel. Introduction of PK-G intracellularly causes a relatively rapid inhibition of ICa(L) in both chick and rat heart cells. Such inhibition occurs for both the basal and stimulated ICa(L). In addition, the cGMP/PK-G system was reported to stimulate a phosphatase that dephosphorylates the Ca2+ channel. In addition to the slower indirect pathway--exerted via cAMP/PK-A--there is a faster more-direct pathway for ICa(L) stimulation by the beta-adrenergic receptor. This latter pathway involves direct modulation of the channel activity by the alpha subunit (alpha s*) of the Gs-protein. In vascular smooth muscle cells the two pathways (direct and indirect) also appear to be present, although the indirect pathway produces inhibition of ICa(L). PK-C and calmodulin-PK also may play roles in regulation of the myocardial slow Ca2+ channels. Both of these protein kinases stimulate the activity of these channels. Thus, it appears that the slow Ca2+ channel is a complex structure, including perhaps several associated regulatory proteins, which can be regulated by a number of factors intrinsic and extrinsic to the cell, and thereby control can be exercised over the force of contraction of the heart.

Developmental changes in Ca2+ currents from newborn rat cardiomyocytes in primary culture

Electrophysiological characteristics of neonatal rat ventricular cardiomyocytes in primary culture were studied using the whole-cell patch-clamp recording technique. Cell size, estimated by measurement of membrane capacitance, was significantly increased throughout the culture from 22.4 +/- 5.4 pF at day 2 to 55.0 +/- 16.1 pF at day 7, reflecting the hypertrophic process which characterises postnatal cell development. The Ca2+ current was investigated at day 2 and 7 of the culture which constituted the early postnatal and maximally developed stages, respectively, of isolated cells in our experimental conditions. At 2 days of culture, two types of Ca2+ current could be distinguished, as also observed in freshly dissociated newborn ventricular cells. From their potential dependence and pharmacological characteristics, they could be attributed to the T- (ICa-T) and L-type (ICa-L) Ca2+ current components. After 7 days of culture, only the latter ICa-L was present and its density was significantly increased when compared to the density in 2-day-old cells, but lower than that obtained in freshly dissociated adult cells. As the age of the culture progressed, the steady-state inactivation curve was shifted toward negative potentials, in the direction of the inactivation curve obtained for adult cells. Compared to the serum-free control conditions, the density of ICa-L was significantly increased in the presence of fetal calf serum throughout the culture. Consequently, the density of ICa-L obtained in 7-day-old cells was similar to the density of ICa-L obtained in freshly dissociated adult cardiac cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological properties of neonatal mouse cardiac myocytes in primary culture

1. The increasing utility of transgenic mice in molecular studies of the cardiovascular system has motivated us to characterize the ionic currents in neonatal mouse ventricular myocytes. 2. Cell capacitance measurements (30 +/- 1 pF, n = 73) confirmed visual impressions that neonatal mouse ventricular myocytes in primary culture are considerably smaller than freshly isolated adult ventricular myocytes. With the use of electron microscopy, mitochondria and sarcoplasmic reticulum were found in close association with myofibrils, but transverse tubules were not observed. 3. Action potential durations, measured at 50 and 90% repolarization, were 23 +/- 1 and 42 +/- 2 ms respectively (n = 46). Application of 4-aminopyridine (4-AP; 5 mM) prolonged action potential duration at 50% repolarization by 26 +/- 5% (n = 3). The brevity of the action potential is explained by the rapid activation of a transient outward K+ current upon voltage-clamp depolarization to plateau potentials. 4. Potassium currents identified include an inward rectifier, a large 4-AP-sensitive transient outward, a slowly inactivating 4-AP-insensitive outward, a slowly activating delayed rectifier and a small rapidly activating E-4031 (10 microM)-sensitive delayed rectifier K+ current. 5. Sodium currents (-305 +/- 50 pA pF-1, n = 21) were recorded in 40 mM Na+ with Ni2+ (1 mM) to block Ca2+ currents and with K+ replaced by Cs+. The relative insensitivity of the Na+ current to block by tetrodotoxin (IC50 = 2.2 +/- 0.3 microM, n = 4) is distinctive of the cardiac Na+ channel isoform. 6. Nitrendipine-insensitive (10 microM) Ba2+ currents elicited during steps from -90 to -30 mV measured -25 +/- 5 pA pF-1 (n = 7, 30 mM Ba2+). Decay of these currents was complete during 180 ms depolarizations, even with Ba2+ as the charge carrier. These currents were not present when the holding potential was set at -50 mV. These data support the presence of a low threshold, T-type Ca2+ current. 7. The maximal nitrendipine-sensitive L-type Ca2+ current density was -10 +/- 2 pA pF-1 (n = 8) in 2 mM Ca2+ and -38 +/- 5 pA pF-1 (n = 9) in 30 mM Ba2+. Exposure to isoprenaline (1 microM) resulted in an 82% increase (n = 3) in the amplitude of the Ba2+ currents elicited at 0 mV. 8. Neonatal mouse cardiac myocytes in primary culture possess surprisingly large inward currents given the brevity of their action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantification of two splicing events in the L-type calcium channel alpha-1 subunit of intestinal smooth muscle and other tissues

cDNA fragments encoding a representative region of the L-type calcium channel alpha-1 subunit of rabbit intestine smooth muscle were amplified by polymerase chain reaction (PCR). The nucleotide sequences of these intestine clones shared a high similarity with aorta, lung and heart calcium channels. However, in the extracellular loop between the third and fourth segments of domain IV and in the transmembrane IVS3 segment itself, we observed primary sequence variations corresponding to alternative splicing phenomenons. Since structural differences of L-type calcium channel alpha-1 subunits could result in functional variations, the respective expression frequency of these isoforms was determined in various tissues and species, and in the embryonic A7r5 cell line. The ontogeny of these splicing events was also examined from tissues of different ages. From this quantitative study, carried out by PCR of reverse-transcribed mRNA, it clearly appears that the observed splicing processes in the IVS3-IVS4 region are not only tissue-dependent but also regulated during development.

Cyclic GMP regulation of calcium slow channels in cardiac muscle and vascular smooth muscle cells

The effects of cAMP and cGMP on the slow Ca2+ channels in cardiac muscle, VSM, and skeletal muscle fibers are summarized in Table V. As shown, in cardiac muscle, cAMP stimulates and cGMP inhibits. In VSM, both cAMP and cGMP inhibit. In skeletal muscle, both cAMP and cGMP stimulate.

Voltage-dependent modulation of L-type Ca2+ current by manidipine in guinea-pig heart cells

Effects of manidipine, a dihydropyridine derivative, on L-type Ca2+ currents were examined in guinea-pig ventricular cells, using the whole-cell patch-clamp method. At a holding potential of -37 mV, manidipine decreased the Ca2+ current at concentrations above 0.1 nM, and abolished it at 100 nM (IC50 = 2.6 nM). At a holding potential of -78 mV, manidipine did not suppress the Ca2+ current at concentrations less than 100 nM, but increased the Ca2+ current slightly at a concentration of 10 nM. The antagonistic effect of manidipine was significant at concentrations above 100 nM (IC50 = 400 nM). The voltage-dependent effect of manidipine on the Ca2+ current may explain the weak negative inotropism of manidipine in ventricular muscles, and provide an electrophysiological basis for its vascular selectivity.