Cardiovascular effects of E4080, a novel bradycardiac agent with coronary vasodilating properties, in anesthetized dogs

Selective inhibition by E4080, a novel bradycardic agent, of positive chronotropic responses to norepinephrine in isolated dog hearts

E4080, a novel bradycardic agent acts on various ionic currents including the hyperpolarization-activated inward current (I(f)), L-type Ca2+ current (ICa) and ATP-sensitive K+ (K+ATP) current in mammalian heart and vascular tissues. We thus investigated the chronotropic and inotropic effects of E4080 and its interaction with the positive cardiac responses to norepinephrine, 3-isobutyl-1-methyl-xanthine (IBMX) and Bay k 8644 in the isolated, blood-perfused dog right atria and left ventricles. E4080 (0.01-1 mumol) decreased the sinus rate and atrial and ventricular contractile forces in a dose-related manner. Glibenclamide (3 mumol) partly blocked the decrease in atrial force but not the decreases in sinus rate and ventricular force induced by E4080. Atropine (10 nmol) did not affect the negative cardiac responses to E4080. E4080 (0.01-1 mumol) inhibited the positive chronotropic responses to norepinephrine and IBMX dose dependently, but did not inhibit the positive inotropic ones in isolated atria. E4080 affected neither positive chronotropic nor inotropic responses to Bay k 8644. These results suggest that (1) the activation of K+ATP channels by E4080 is partly related to the decrease in atrial force but not the decreases in sinus rate and ventricular force, and (2) the selective inhibition of E4080 of the cyclic AMP-dependent positive chronotropic responses but not inotropic ones is probably due to the inhibition of I(f) rather than other properties, e.g., activation of K+ATP channels and inhibition of ICa in the dog heart.

BRL 38227 (levcromakalim)-induced hyperpolarization reduces the sensitivity to Ca2+ of contractile elements in canine coronary artery

Potassium (K+) channel openers decrease intracellular free Ca2+ concentrations ([Ca2+]i by hyperpolarizing the membrane and deactivating the Ca(2+)-channels. To examine whether the hyperpolarization produced by K(+)-channel openers has other effects on the mechanical activity of vascular smooth muscle, we investigated the effects of levcromakalim (BRL 38227) on membrane potential, [Ca2+]i, as measured with fura-2, and force of contraction induced by 30 mmol/l KCl-physiological salt solution (PSS), in canine coronary arteries. BRL 38227 hyperpolarized the membrane and reduced increases in [Ca2+]i and in contractile force induced by 30 mmol/l KCl-PSS. The [Ca2+]i-contractile force curve, determined in the presence of BRL 38227, was located to the right of the control curve determined by decreasing extracellular Ca2+ concentrations ([Ca2+]o) in 30 mmol/l KCl-PSS. The [Ca2+]i-contractile force curve, determined by decreasing extracellular K+ concentrations ([K+]o), was also located to the right of that determined by decreasing [Ca2+]o in 30 mmol/l KCl-PSS. The effect of BRL 38227, a reduction in the Ca(2+)-sensitivity of contractile elements, was antagonized by the ATP-sensitive K(+)-channel blocker, glibenclamide (10(-6) or 10(-5) mol/l). These results suggest that the membrane hyperpolarization induced by BRL 38227, or the repolarization caused by reducing [K+]o, decreases the Ca(2+)-sensitivity of contractile elements of vascular smooth muscle.

New synthetic ligands for L-type voltage-gated calcium channels

The pharmacology of the L-type Ca2+ channel has been the subject of considerable basic and clinical investigation over the past two decades primarily because of the clinical activities of nifedipine, verapamil and diltiazem. However, it is quite clear that this Ca2+ channel is, in common with other pharmacologic receptors, a multiple drug receptor. There are probably as many as six or more discrete drug binding sites associated with this Ca2+ channel. Continued investigation of these sites may yield both new therapeutic agents, structural clues to ligands active at other classes of Ca2+ channel and structures active at other classes of ion channel.

E4080 has a dual action, as a K+ channel opener and a Ca2+ channel blocker, in canine coronary artery smooth muscle

To clarify the vasodilating mechanism of action of E4080, which possesses vasodilating and bradycardic effects, we investigated its effects on intracellular Ca2+ concentrations ([Ca2+]i), as measured with fura-2, and force of contraction in canine coronary artery. E4080 reduced the increase in [Ca2+]i and force of contraction induced by 30 and 90 mM KCl physiological salt solution (PSS) in a concentration-dependent manner. The effects of E4080 in 30 mM KCl-PSS were inhibited by 10(-5) M glibenclamide. In 30 mM KCl-PSS, the slope of the [Ca2+]i-force relationship in the presence of E4080 was steeper than that of control, suggesting that E4080 decreased the sensitivity of contractile elements to Ca2+, as an effect which was also inhibited by glibenclamide. However, the [Ca2+]i-force curve was not changed by E4080 in 90 mM KCl-PSS. These results suggest that E4080 is a vasodilator in canine coronary artery, having K+ channel opening and Ca2+ channel blocking actions. The membrane hyperpolarization induced by E4080 may reduce the sensitivity of contractile elements to Ca2+.