Effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes.

BDF 9198 (a congener of DPI 201 - 106 and BDF 9148) was found to be a positive inotrope on guinea-pig isolated ventricular muscle strips. The effects of BDF 9198 on action potentials and ionic currents from guinea-pig isolated ventricular myocytes were studied using the whole cell patch clamp method. In normal external solution, at 37 degrees C, action potential duration at 50% repolarization (APD(50)) was 167.4+/-8.36 ms (n=37). BDF 9198 produced a concentration-dependent increase in APD(50) (no significant increase at 1x10(-10) M; and APD(50) values of 273.03+/-35.8 ms at 1x10(-9) M; n=6, P<0.01 and 694.7+/-86.3 ms at 1x10(-7) M; P<0.001, n=7). At higher concentrations in the range tested, BDF 9198 also induced early and delayed and after-depolarizations. Qualitative measurements of I(Na) with physiological [Na](o) showed prolongation of the current by BDF 9198, and the appearance of transient oscillatory inward currents at high concentrations. Quantitative recording conditions for I(Na) were established using low external [Na] and by making measurements at room temperature. The current - voltage relation, activation parameters and time-course of I(Na) were similar before and after a partial blocking dose of Tetrodotoxin (TTX, 1 microM), despite a 2 fold difference in current amplitude. This suggests that voltage-clamp during flow of I(Na) was adequately maintained under our conditions. Selective measurements of I(Na) at room temperature showed that BDF 9198 induced a concentration-dependent, sustained component of I(Na) (I(Late)) and caused a slight left-ward shift in the current - voltage relation for peak current. The drug-induced I(Late) showed a similar voltage dependence to peak current in the presence of BDF 9198. Both peak current and I(Late) were abolished by 30 microM TTX and were sensitive to external [Na]. Inactivation of control I(Na) during a 200 ms test pulse to -30 mV followed a bi-exponential time-course. In addition to inducing a sustained current component, BDF 9198 left the magnitude of the fast inactivation time-constant unchanged, but increased the magnitude of the slow inactivation time-constant. Additional experiments with a longer pulse (1 s) raised the possibility that in the presence of BDF 9198, I(Na) inactivation may be comprised of more than two phases. No significant effects of 1x10(-6) M BDF 9198 were observed on the L-type calcium current, or delayed and inward rectifying potassium currents measured at 37 degrees C. It is concluded that the prolongation of APD(50) by BDF 9198 resulted from selective modulation of I(Na). Reduced current inactivation induced a persistent I(Na), increasing the net depolarizing current during the action potential. This action of the drug indicates a potential for 'QT prolongation' of the ECG. The observation of after-depolarizations suggests a potential for proarrhythmia at some drug concentrations.

Na+-Ca2+ exchange current from rabbit isolated atrioventricular nodal and ventricular myocytes compared using action potential and ramp waveforms.

We measured and compared Na-Ca exchanger current (INa-Ca) from rabbit isolated ventricular and atrioventricular (AV) nodal myocytes, using action potential (AP) and ramp voltage commands. Whole cell patch-clamp recordings were made at 35-37 degrees C; INa-Ca was measured as 5 mM nickel (Ni)- sensitive current with major interfering voltage and calcium-activated currents blocked. In ventricular cells a 2-s descending ramp elicited INa-Ca showing outward rectification and a reversal potential (Erev) of -13.1 +/- 1. 2 mV (n = 12; mean +/- SEM). With a ventricular AP as the voltage command, the profile of INa-Ca followed the applied waveform closely. The current-voltage relation during AP repolarization was almost linear and showed an Erev of -38.3 +/- 5.3 mV (n = 6). As INa-Ca depended on the applied voltage waveform, comparisons between the two cell types utilized the same command waveform (a series of AV nodal APs). In ventricular myocytes this elicited INa-Ca that reversed near -38 mV and was inwardly directed during the pacemaker potential. This command was also applied to AV node cells; mean INa-Ca density at all voltages encompassed by the AP (-70 to +30 mV) did not differ significantly from that in ventricular myocytes (P > 0.05, ANOVA). This finding was confirmed using brief (250 ms) voltage ramp protocols (P > 0.1 ANOVA). These data represent the first direct measurements of AV nodal INa-Ca and suggest that the exchanger may be functionally expressed to similar levels in the two cell types. They may also suggest a possible role for INa-Ca during the pacemaker potential in AV node as inward INa-Ca was observed over the pacemaker potential range even with bulk internal Ca buffered to a low level.

Comparison of Na+-Ca2+ exchange current elicited from isolated rabbit ventricular myocytes by voltage ramp and step protocols.

In this study, the effects of three different voltage protocols on the Na+-Ca2+ exchange current (INa-Ca) of rabbit right ventricular myocytes were studied. Whole-cell patch-clamp recordings were made using a Cs+-based internal dialysis solution and external solutions designed to block major interfering currents. INa-Ca was measured at 35-37 degrees C as (5 mM) Ni-sensitive current elicited by: a 2 s descending ramp (DR: +80 to -120 mV); a 2 s ascending ramp (AR: -120 to +80 mV) and 500 ms voltage steps (VS) between -120 and +80 mV. DR and AR were applied from -40 mV and elicited INa-Ca with reversal potentials (Erev) of -17.6+/-2.5 mV (mean+/-SEM; n=16) and -46.2+/-4. 1 mV (n=10; P=0.0001) respectively. This difference was maintained when the holding potential was -80 mV (-44.0+/-2.1 mV, n=24 and -86. 3+/-4.8 mV, n=10; P=0.0001), when the internal Ca chelator (EGTA) was replaced with BAPTA (-19.5+/-1.8 mV and -46.3+/-1.6 mV, n=6; P=0. 0003) and when DR and AR were applied alternately to the same cell. Experiments using modified ramp waveforms suggested a possible mechanism for these differences. Increases in subsarcolemmal Ca caused by Ca entry (coupled to Na extrusion) during the initial positive potential phase of the DR might have induced INa-Ca reversal at less negative potentials than observed with AR, during the initial phase of which subsarcolemmal Ca would not have accumulated. These data suggest that INa-Ca during voltage-clamp experiments can be significantly influenced by the type of voltage protocol chosen, as the protocol appears to induce subsarcolemmal changes in Ca and Na concentration that are independent of Ca buffering in the bulk cytosol and can occur on a pulse-to-pulse basis.

Actions of myristyl-FRCRCFa, a cell-permeant blocker of the cardiac sarcolemmal Na-Ca exchanger, tested in rabbit ventricular myocytes.

In cardiac muscle, the electrogenic Na-Ca exchanger plays important roles in determining action potential shape and in the beat-to-beat homeostasis of intracellular calcium. In this study we tested the actions of a putative cell-permeant blocker of the cardiac sarcolemmal Na-Ca exchange, "Myristyl- (Myr-) FRCRCFa". Experiments were performed using isolated rabbit right ventricular myocytes and whole-cell patch-clamp at 35-37 degreesC. The Na-Ca exchange current (INa-Ca), L-type calcium current (ICa,L), inward rectifier potassium current (IK1) and delayed rectifier potassium current (IK) were compared in untreated cells and cells incubated in a solution containing N-myristylated FRCRCFa. With other major currents blocked, INa-Ca was measured as the Ni-sensitive component of current during a voltage ramp applied from the holding potential of -40 mV, between +80 and -120 mV (ramp velocity 0.1 V s-1). In untreated cells, INa-Ca at +60 mV was 7.1+/-0.6 pA/pF and at -100 mV was -2.7+/-0.3 pA/pF (n=9). After a 15-min pre-incubation with 20 microM Myr-FRCRCFa, INa-Ca was reduced to 4.2+/-0.3 pA/pF at +60 mV and -1. 5+/-0.2 pA/pF at -100 mV (P<0.02; n=7). After incubation with 20 microM Myr-FRCRCFa for 1 h, INa-Ca at both potentials was further reduced (2.3+/-0.8 pA/pF at +60 mV; -0.9+/-0.3 pA/pF at -100 mV; P<0. 008 compared with control; n=4). Under selective recording conditions for ICa,L, there was little difference in ICa,L density between untreated and cells incubated with Myr-FRCRCFa. A Boltzmann fit to the ICa,L/V relation showed no significant alteration of half-maximal activation potential or slope factor of activation. IK1 was also largely unaffected by pre-incubation of cells with Myr-FRCRCFa. IK, measured as deactivating tail current following 1-s test depolarisations to a range of test potentials, was also not significantly altered by Myr-FRCRCFa. The suppression of INa-Ca in cells incubated in Myr-FRCRCFa suggests that addition of the myristyl group to FRCRCFa peptide conveys cell permeancy to the peptide and that Myr-FRCRCFa applied externally to rabbit ventricular myocytes is moderately effective as an INa-Ca blocker. ICa,L, IK1 and IK were largely unaffected by Myr-FRCRCFa. N-Myristylation of such conformationally constrained hexapeptides may, therefore, provide a means of producing cell-permeant inhibitors of the cardiac Na-Ca exchanger.