Ca2+ channel activation by CGP 48506, a new positive inotropic benzodiazocine derivative

Conformational changes induced in voltage-gated calcium channel Cav1.2 by BayK 8644 or FPL64176 modify the kinetics of secretion independently of Ca2+ influx

The role of the L-type calcium channel (Cav1.2) as a molecular switch that triggers secretion prior to Ca(2+) transport has previously been demonstrated in bovine chromaffin cells and rat pancreatic beta cells. Here, we examined the effect of specific Cav1.2 allosteric modulators, BayK 8644 (BayK) and FPL64176 (FPL), on the kinetics of catecholamine release, as monitored by amperometry in single bovine chromaffin cells. We show that 2 microm BayK or 0.5 microm FPL accelerates the rate of catecholamine secretion to a similar extent in the presence either of the permeable Ca(2+) and Ba(2+) or the impermeable charge carrier La(3+). These results suggest that structural rearrangements generated through the binding of BayK or FPL, by altering the channel activity, could affect depolarization-evoked secretion prior to cation transport. FPL also accelerated the rate of secretion mediated by a Ca(2+)-impermeable channel made by replacing the wild type alpha(1)1.2 subunit was replaced with the mutant alpha(1)1.2/L775P. Furthermore, BayK and FPL modified the kinetic parameters of the fusion pore formation, which represent the initial contact between the vesicle lumen and the extracellular medium. A direct link between the channel activity and evoked secretion lends additional support to the view that the voltage-gated Ca(2+) channels act as a signaling molecular switch, triggering secretion upstream to ion transport into the cell.

Calcium sensitization as new principle of inotropic therapy in end-stage heart failure?

OBJECTIVE

Due to shortage of donor hearts and increasing waiting-lists of patients with end-stage heart disease, new pharmacological principles for bridging therapies are necessary. The positive inotropic effects of cAMP-increasing drugs (e.g. catecholamines, phosphodiesterase-inhibitors) are diminished in the failing myocardium. Hence, we investigated the usefulness and mechanism of the two calcium sensitizers, levosimendan and CGP 48506 in preparations from end-stage failing human hearts since the exact mechanism of the positive inotropic effects is not yet clearly understood.

METHODS

Failing human hearts which required orthotopic heart transplantation due to idiopathic dilated cardiomyopathy were investigated. Contraction experiments were performed using muscle strips of ventricles. Calcium sensitization was investigated in skinned fibers and phosphodiesterase activity was measured in ventricular homogenate. In addition, cAMP levels were quantified in myocytes from guinea-pig hearts.

RESULTS

In muscle stripes from failing human hearts levosimendan (10 micromol/l) increased the force of contraction only to 112.8 +/- 6.7% of predrug values. In contrast, CGP 48506 increased the force of contraction to 311 +/- 59% of predrug values at 100 micromol/l. The time to peak tension and time of relaxation were increased to 175 +/- 4% and 205 +/- 15% of control levels at 100 micromol/l. Skinned fibers from failing human hearts were sensitized to calcium with an EC50 of 10 micromol/l. Other mechanisms of action were excluded since CGP 48506 affected neither the activity of phosphodiesterase isoenzymes I-IV in failing human hearts, nor cAMP levels in guinea-pig cardiomyocytes. On the other hand, levosimendan (1 micromol/l) increased cAMP content from 6.3 +/- 0.3 to 8.1 +/- 0.3 pmol/mg protein.

CONCLUSION

CGP 48506 is an inotropic agent with calcium-sensitizing properties in the human heart, that is devoid of inhibitory activity on human cardiac phosphodiesterase isoenzymes. It offers, therefore, a new form of positive inotropic therapy that can be useful for the bridging treatment of heart failure before transplantation. On the other hand, levosimendan is a calcium sensitizer showing less-effective inotropic effects accompanied by increased cAMP levels.

Relaxation effect of CGP-48506, EMD-57033, and dobutamine in ejecting and isovolumically beating rabbit hearts

Because it is not known whether ejection influences the negative effect of the Ca(2+)-sensitizing drugs on ventricular relaxation, we extended our previous analysis of stress-dependent relaxation in isovolumic beats to encompass ejecting beats and evaluated the relationships between both the time of onset of relaxation and the rate of relaxation and wall stress in a broader analysis framework. Furthermore, because the sites of action of the Ca(2+)-sensitizing drugs CGP-48506 and EMD-57033 may be different, and thus CGP-48506 may have fewer adverse effects on resting muscle length or force, we compared these two drugs to test the hypothesis that CGP-48506 would have less effect than EMD-57033 on relaxation in the isolated buffer-perfused rabbit heart. This analysis of stress-dependent relaxation in both ejecting and isovolumic beats readily differentiates between the negative lusitropic effect of 2 x 10(-6) M EMD-57033, the negligible lusitropic effect of 6 x 10(-6) M CGP-48506, and the positive lusitropic effect of 1.25 x 10(-6) M dobutamine. Furthermore, comparison of the effect of the two Ca(2+)-sensitizing drugs in ejecting versus isovolumic contractions shows that CGP-48506 affects relaxation differently in ejecting contractions than it does in isovolumic contractions, whereas EMD-57033 affects relaxation similarly in both ejecting and isovolumic contractions.