Perforated-patch recording does not enhance effect of 3-isobutyl-1-methylxanthine on cardiac calcium current

Long-term patch recordings from adult spinal neurons herald new era of opportunity

Recently, Andreas Husch, Nathan Cramer, and Ronald M. Harris-Warrick achieved a remarkable breakthrough in patch-clamp recordings of ventral horn neurons in the adult spinal cord slice preparation. This landmark study that breaks the "age barrier" is titled "Long-duration perforated patch recordings from spinal interneurons of adult mice" (Husch et al., in press). In it, the authors demonstrate the unprecedented ability to undertake day-long (up to 12 h), and utterly stable perforated patch recordings. A description of the methodology is detailed in their paper. Here, I give a brief overview before providing context to this extraordinary achievement.

Inhibition of nitric oxide synthase enhances contractile response of ventricular myocytes from streptozotocin-diabetic rats

The contractile hyporesponsiveness of the streptozotocin diabetic rat heart in vitro to beta-adrenergic agonists is eliminated when the heart is perfused with N(G)-nitro-L-arginine methyl ester (L-NAME), a non-selective inhibitor of nitric oxide synthase (NOS). The following study evaluated the hypothesis that an increased production of NO/cGMP within the diabetic myocyte inhibits the beta-adrenergic-stimulated increase in calcium current and contractile response. Male Sprague-Dawley rats were given an intravenous injection of streptozotocin (60 mg/kg). After 8 weeks, L-type calcium currents were recorded in ventricular myocytes using the whole cell voltage-clamp method. Shortening of isolated myocytes was determined using a video edge detection system. cAMP and cGMP were measured using radioimmunoassay. Nitric oxide production was determined using the Griess assay kit. Basal cGMP levels and nitric oxide production were elevated in diabetic myocytes. Shortening of the diabetic myocytes in response to isoproterenol (1 microM) was markedly diminished. However, there was no detectable difference in the isoproterenol-stimulated L-type calcium current or cAMP levels between control and diabetic myocytes. Acute superfusion of the diabetic myocyte with L-NAME (1 mM) decreased basal cGMP and markedly enhanced the shortening response to isoproterenol but did not alter isoproterenol-stimulated calcium current. These data suggest that increased production of NO/cGMP within the diabetic myocyte suppressed beta-adrenergic stimulated shortening of the myocyte. However, NO/cGMP apparently does not suppress shortening of the myocyte by inhibition of the beta-stimulated calcium current.

Trimetazidine improved Ca2+ handling in isoprenaline-mediated myocardial injury of rats

Dysregulation of intracellular Ca2+ homeostasis plays an important role in mediating myocardial injury. We tested the hypothesis that treatment with trimetazidine (TMZ) would improve intracellular Ca2+ handling in myocardial injury of rats. The control group received saline only (10 ml kg(-1) day(-1), i.p.) for 7 days. In a second group, isoprenaline (ISO; 5 mg kg(-1) day(-1), s.c.) was administered to rats for 2 days to induce an acute injury of the myocardium. In a third group, treatment with TMZ (10 mg kg(-1) day(-1), i.p.) was initiated 1 day before ISO administration and continued for 7 days (n = 7 rats in each group). Histopathological evaluation showed that TMZ prevented ISO-induced myocardial damage. TMZ preserved the ATP levels and decreased the maleic dialdehyde (MDA) content in the hearts compared with ISO-treated rats. The diastolic [Ca2+]i measured by loading with fura-2 AM in isolated cardiomyocytes was increased significantly in ISO-treated rats compared to the control animals. TMZ prevented the rise of diastolic [Ca2+]i and the depression of caffeine-induced Ca2+ transients caused by ISO administration. The reduction in sarcoplasmic reticulum (SR) Ca2+ content in the heart cells and in cardiac SR Ca2+-ATPase activity in ISO-treated rats was abolished by TMZ, although there were no differences in SR Ca2+-ATPase protein levels between the control, ISO and ISO + 7 mz-treated rats. In addition, TMZ prevented the reduction in sarcolemmal L-type Ca2+ current density in the heart cells induced by ISO treatment. These results demonstrate that the treatment of rats with TMZ inhibited the increase of diastolic [Ca2+]i and prevented the decrease of SR Ca2+ content, SR Ca2+-ATPase activity and L-type Ca2+ current density in cardiomyocytes in ISO-mediated myocardial injury of rats. These changes in Ca2+ handling could help to explain the favourable action of TMZ in myocardial injury.

Leukemia Inhibitory Factor Activates Cardiac L-Type Ca 2+ Channels via Phosphorylation of Serine 1829 in the Rabbit Ca v 1.2 Subunit

We have previously reported that leukemia inhibitory factor (LIF) gradually increased cardiac L-type Ca 2+ channel current ( I CaL ), which peaked at 15 minutes in both adult and neonatal rat cardiomyocytes, and this increase was blocked by the mitogen-activated protein kinase kinase inhibitor PD98059. This study investigated the molecular basis of LIF-induced augmentation of I CaL in rodent cardiomyocytes. LIF induced phosphorylation of a serine residue in the α 1c subunit (Ca v 1.2) of L-type Ca 2+ channels in cultured rat cardiomyocytes, and this phosphorylation was inhibited by PD98059. When constructs encoding either a wild-type or a carboxyl-terminal–truncated rabbit Ca v 1.2 subunit were transfected into HEK293 cells, LIF induced phosphorylation of the resultant wild-type protein but not the mutant protein. Cotransfection of constitutively active mitogen-activated protein kinase kinase also resulted in phosphorylation of the Ca v 1.2 subunit in the absence of LIF stimulation. In in-gel kinase assays, extracellular signal–regulated kinase phosphorylated a glutathione S -transferase fusion protein of the carboxyl-terminal region of Ca v 1.2 (residues 1700 through 1923), which contains the consensus sequence Pro-Leu-Ser-Pro. A point mutation within this consensus sequence, which results in a substitution of alanine for serine at residue 1829 (S1829A), was sufficient to abolish the LIF-induced phosphorylation. LIF increased I CaL in HEK cells transfected with wild-type Ca v 1.2 but not with the mutated version. These results provide direct evidence that LIF phosphorylates the serine residue at position 1829 of the Ca v 1.2 subunit via the actions of extracellular signal–regulated kinase and that this phosphorylation increases I CaL in cardiomyocytes.

Inhibition of cyclic GMP hydrolysis with zaprinast reduces basal and cyclic AMP-elevated L-type calcium current in guinea-pig ventricular myocytes

(1) Cyclic GMP (cGMP) has been shown to be an important modulator of cardiac contractile function. A major component of cGMP regulation of contractility is cGMP-mediated inhibition of the cardiac calcium current (I(Ca)). An under-appreciated aspect of cyclic nucleotide signalling is hydrolysis of the cyclic nucleotide (i.e., breakdown by phosphodiesterases (PDEs)). The role of cGMP hydrolysis in regulating I(Ca) has not been studied. Thus the purpose of this study was to investigate if inhibition of cGMP hydrolysis can modulate I(Ca) in isolated guinea-pig ventricular myocytes. (2) Zaprinast, a selective inhibitor of cGMP-specific PDE (PDE5), caused a significant increase in cGMP levels in myocytes, but was without affect on basal or beta-adrenergic stimulated cAMP levels (consistent with its actions as a specific inhibitor of PDE5). (3) Zaprinast inhibited I(Ca) that was pre-stimulated with cAMP elevating agents (isoproterenol, a beta-adrenergic agonist; or forskolin, a direct activator of adenylate cyclase). The effect of zaprinast was greatly reduced by KT5823, an inhibitor of cGMP-dependent protein kinase (PKG). (4) Zaprinast also significantly inhibited basal I(Ca) when perforated-patch or whole-cell recording with physiological pipette calcium concentration (10(-7) M) was used. However, this effect was not observed when using standard calcium-free whole-cell recording conditions. (5) These results indicate that inhibition of cGMP hydrolysis can decrease both basal and cAMP-stimulated I(Ca). Thus, cGMP hydrolysis may likely be an important step for physiological modulation of I(Ca). This regulation may also be important in disease states in which cGMP production is increased and PDE5 expression is altered, such as heart failure.

Effects of membrane potential on Na+ -dependent Mg2+ extrusion from rat ventricular myocytes

To study Mg2+ transport across the cell membrane, the cytoplasmic concentration of Mg2+ ([Mg2+](i)) in rat ventricular myocytes was measured with the fluorescent indicator furaptra (mag-fura-2) under Ca2+ -free conditions (0.1 mM EGTA) at 25 degrees C. The fluorescence ratio signal of furaptra was converted to [Mg2+](i) using calibration parameters previously estimated in myocytes (Watanabe and Konishi, Pflügers Arch 442: 35-40, 2001). After [Mg2+](i) was raised by loading the cells with Mg2+ in a solution containing 93 mM Mg(2+), the cells were voltage-clamped at a holding potential of -80 mV using the perforated patch-clamp technique with amphotericin B. At the holding potential of -80 mV, the reduction of extracellular Mg2+ to 1.0 mM caused a rapid decrease in [Mg2+](i) only in the presence of extracellular Na(+). The rate of the net Mg2+ efflux appeared to be dependent on the initial level of [Mg2+](i); the decrease in [Mg2+](i) was significantly faster in the myocytes markedly loaded with Mg2+. The rate of decrease in [Mg2+](i) was influenced little by membrane depolarization from -80 to -40 mV, but the [Mg2+](i) decrease accelerated significantly at 0 mV by, on average, approximately 40%. Hyperpolarization from -80 to -120 mV slightly but significantly slowed the decrease in [Mg2+](i) by approximately 20%. The results clearly demonstrate an extracellular Na(+)- and intracellular Mg2+ -dependent Mg2+ efflux activity, which is consistent with the Na(+)-Mg2+ exchange, in rat ventricular myocytes. We found that the apparent rate of Mg2+ transport depends slightly on the membrane potential: facilitation by depolarization and inhibition by hyperpolarization with no sign of reversal between -120 and 0 mV.

3-isobutyl-1-methylxanthine (IBMX) sensitizes cardiac myocytes to anoxia

Cardiac myocytes incubated with 3-isobutyl-1-methylxanthine (IBMX), a nonspecific cyclic nucleotide phosphodiesterase inhibitor, formed rigor complexes under anoxic conditions more readily than cells incubated with other phosphodiesterase inhibitors. Cardiac myocytes were incubated for 1 hr with either (a) no additions, (b) 150 microM zaprinast, or (c) 1 mM IBMX, and then were rendered anoxic for periods up to 60 min. Cells were >80% viable throughout the anoxic period; viability was unaffected by either drug. Rod count decreased more rapidly after the onset of anoxia in the IBMX-treated cells than in control or zaprinast-treated cells (11% rods vs. roughly 47% rods after 30 min of anoxia). IBMX-treated cell groups also formed more "contracted" myocytes (box-like rods) than their untreated or zaprinast-treated counterparts (50% contracted vs. roughly 27% contracted after 30 min of anoxia). While nucleotide degradation patterns were similar in all experimental groups, the ratio of ATP to ADP was lower in IBMX-treated cells than in control or zaprinast-treated cells. The L-type calcium channel was apparently not involved in this phenomenon; while cyclic AMP was elevated in the IBMX-incubated cells, verapamil did not protect IBMX-incubated cells from premature damage by anoxia. Incubation with 8-cyclopentyl-1,3-dipropylxanthine (CDPX), an A1 receptor antagonist, at concentrations up to 1 microM in place of 1mM IBMX did not reproduce the IBMX effect. We concluded that IBMX sensitizes cardiac myocytes to anoxia through a mechanism related to its effect on ATP/ADP, and unrelated to an elevation of intracellular calcium or preconditioning phenomena.

Mechanisms of sex differences in rat cardiac myocyte response to beta-adrenergic stimulation

The purpose of this study was to investigate sex differences in the functional response of isolated rat heart ventricular myocytes to beta-adrenergic stimulation and in isoproterenol-stimulated signal transduction. Fractional shortening was measured using a video edge-detection system in control- and isoproterenol-stimulated myocytes that had been isolated from weight-matched rats. Number and affinity of the beta-adrenergic receptors and the L-type Ca(2+) channel were measured in ventricular cardiac membranes by radioligand binding studies. Control- and isoproterenol-mediated alteration in Ca(2+) current density (I(Ca)) was determined by patch clamping and cellular cAMP content was determined by radioimmunoassay. Study results demonstrate that female myocytes have higher Ca(2+) channel density and greater I(Ca) than male myocytes. However, isoproterenol elicits a greater beta-adrenergic receptor-mediated increase cell shortening, I(Ca) and cAMP production in male myocytes. Male myocytes were also found to have a higher beta-adrenergic receptor density. These results suggest that cardiac myocytes from male rats have an enhanced response to beta-adrenergic stimulation due to augmented beta-adrenergic signaling that results in a greater transsarcolemmal Ca(2+) influx.

Increased sodium-calcium exchange current in right ventricular cell hypertrophy induced by simulated high altitude in adult rats

Ventricular hypertrophy is associated with an increase in action potential (AP) duration which is potentially arrhythmogenic. The implication of the Na-Ca exchange current (I(Na-Ca)) in the lengthening of the AP is controversial. The role of this current in the increased duration of the low plateau of the AP in hypertrophied adult rat ventricular myocytes by simulated chronic high-altitude exposure ( approximately 4500 m) was evaluated. Electrophysiological experiments were carried out on isolated right ventricular myocytes from exposed and control rats with the perforated patch or the conventional whole-cell technique in current or in voltage clamp condition. With the two techniques, a significant increase of the low plateau duration was observed in hypertrophied myocytes as compared to controls. The low plateau in hypertrophied myocytes was depressed when Na was replaced by Li and was no longer recorded when intracellular Ca was buffered with EGTA. Inward tail currents, evoked either on repolarization to -80 mV following a depolarizing pulse to +10 mV or by interrupted AP technique, were greater in hypertrophied than in control myocytes and were abolished when Na was replaced by Li or when intracellular Ca was buffered with EGTA, indicating an increased Na-Ca exchange activity. The Li-sensitive current-voltage curves, obtained by a voltage clamp ramp protocol with an intracellular calcium buffered solution, were not significantly different in both hypertrophied and control myocytes, suggesting no modification in the density of the Na-Ca exchange protein. This was corroborated by the lack of difference in NCX1 mRNA levels between right ventricles from control and exposed rats. We conclude that increased duration of the low plateau of rat ventricular AP in altitude cardiac hypertrophy may be attributed to an increase of the inward I(Na-Ca). This augmented I(Na-Ca)may result from a modification in the intracellular Ca homeostasis.

Elevated cAMP suppresses muscarinic inhibition of L-type calcium current in guinea pig ventricular myocytes

We investigated the effect of carbachol (CCh) on L-type Ca2+ current (ICa(L)) enhanced by dialyzed adenosine 3',5'-cyclic monophosphate (cAMP) and/or bath-applied 3-isobutyl-1-methylxanthine (IBMX) in guinea pig isolated ventricular myocytes. At pipette concentrations ([cAMP]pip) from 30 microM to 1 mM, cAMP increased ICa(L) to 25.8 +/- 0.9 microA/cm2 (682 +/- 24.8% increase above control). CCh (100 microM) did not inhibit ICa(L) at any [cAMP]pip. IBMX, a nonselective phosphodiesterase (PDE) inhibitor, increased ICa(L) maximally at 300 microM IBMX (17.9 +/- 0.7 microA/cm2; 449 +/- 20% increase). CCh (100 microM) inhibited ICa(L) by 92 +/- 9.5% at 30 microM IBMX and 78 +/- 4.6% at 100 microM IBMX; this effect was reduced or absent at higher IBMX concentrations (300 and 1,000 microM). Coadministration of cAMP and IBMX also progressively suppressed inhibition by CCh. CCh had a negligible effect on ICa(L) at 750 microM IBMX in the absence of pipette cAMP and at 50 microM IBMX in the presence of 100 microM [cAMP]pip. ACh-activated K+ current (IK(ACh)) was unchanged in atrial myocytes dialyzed with 100 microM cAMP; this excludes a phosphorylation-dependent desensitization of the muscarinic receptor (mAChR) or Gi by cAMP. LY83583 (100 microM), an inhibitor of cyclic guanosine monophosphate (cGMP) production, attenuated inhibition of ICa(L) by CCh in the presence of IBMX. 8-Bromo-cGMP (8-Br-cGMP), an activator of cGMP-dependent protein kinase (PKG), mimicked CCh in its actions on ICa(L) raised by both cAMP (no significant change) and IBMX (49 +/- 5.1% inhibition). Okadaic acid, an inhibitor of type 1 and 2A phosphatases, blocked inhibition of IBMX-stimulated ICa(L) by either CCh or 8-Br-cGMP. Thus the ability of CCh to inhibit ICa(L) appears caused by cGMP/PKG activation of an okadaic acid-sensitive protein phosphatase, and elevated levels of cAMP protect against this action.

Differential effects of endothelin-1 on basal and isoprenaline-enhanced Ca2+ current in guinea-pig ventricular myocytes

1. We examined the effect of endothelin-1 (ET-1) on basal and isoprenaline-enhanced L-type Ca2+ current (ICa,L) in guinea-pig ventricular myocytes under nystatin-perforated patch configuration. 2. ET-1 at concentrations of 1, 5 and 10 nM had little effect on basal ICa,L. However, ICa,L enhanced by isoprenaline (500 nM) was significantly attenuated by 5 nM ET-1 by more than 50%. This effect was reversed upon washout. ICa,L enhanced by forskolin was also decreased by ET-1. 3. The inhibitory effect of ET-1 against isoprenaline was completely blocked by the ETA receptor antagonist BQ-123 (1 microM). In myocytes incubated with pertussis toxin (PTX, 2 micrograms ml-1) for 5 h, ET-1 did not inhibit isoprenaline-enhanced ICa,L. 4. Although ET-1 has been shown to activate specific protein kinase C (PKC) isoforms, a significant inhibitory effect of ET-1 was maintained in the presence of the PKC inhibitor bisindolylmaleimide (20 nM). The nitric oxide (NO) donor SIN-1 (10 microM) attenuated but failed to prevent the ET-1 effect. 5. In summary, our results demonstrate that ET-1 is devoid of any significant effects on basal ICa,L. However, it exerts a potent inhibitory effect against isoprenaline-enhanced ICa,L. This effect is mediated through ETA receptors coupled to PTX-sensitive G-proteins and occurs in the presence of PKC inhibition and NO generation.

Changes in [Ca2+]i, [Na+]i and Ca2+ current in isolated rat ventricular myocytes following an increase in cell length

Isolated rat ventricular myocytes were stretched using carbon fibres to investigate the mechanisms underlying the increase in contraction following stretch. 2. [Ca2+]i and [Na+]i were monitored using the fluorescent indicators fura-2 and sodium-binding benzofuran isophthalate, respectively. The L-type Ca2+ current was recorded simultaneously with contraction using the perforated patch-clamp technique. 3. Mechanical stretch caused an immediate increase in contraction, followed by a slow increase. Contraction was prolonged immediately after the stretch, but did not change during the slow phase. 4. The Ca2+ transient did not change immediately after the stretch. The slow increase in contraction was accompanied by an increase in the amplitude of the Ca2+ transient. However, diastolic [Ca2+]i did not change significantly following stretch. 5. [Na+]i did not change significantly either immediately, or during the slow increase in contraction, after the stretch. 6. The L-type Ca2+ current was not significantly altered either by mechanical loading of the cell with carbon fibres or by stretching the cell. 7. These results suggest that: (1) the rapid increase in contraction following a stretch is due to an increase in myofilament Ca2+ sensitivity rather than to changes in the L-type Ca2+ current or [Na+]i; and (2) a slow increase in the Ca2+ transient underlies the slow increase in contraction in isolated myocytes, but is not caused by either an increase in diastolic [Ca2+]i or a change in [Na+]i (and hence Ca2+ influx via Na(+)-Ca2+ exchange) or a change in myofilament Ca2+ sensitivity.