Competitive and Cooperative Effects of Bay K8644 on the L-Type Calcium Channel Current Inhibition by Calcium Channel Antagonists

Abstract

Phenylalkylamines, benzothiazepines, and dihydropyridines bind noncompetitively to the L-type calcium channel. The molecular mechanisms of this interaction were investigated in enzymatically isolated rat ventricular myocytes using the whole-cell patch-clamp technique. When applied alone, felodipine, verapamil, and diltiazem inhibited the L-type calcium current with values of inhibitory constant (K B ) of 11, 246, and 512 nM, respectively, whereas 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-[trifluoromethyl]phenyl)-3-pyridine carboxylic acid methyl ester (Bay K8644) activated I Ca with activation constant (K A ) of 33 nM. Maximal activation of I Ca by 300 nM Bay K8644 strongly reduced the inhibitory potency of felodipine (apparent K B of 165 nM), significantly reduced the inhibitory potency of verapamil (apparent K B of 737 nM), but significantly increased the inhibitory potency of diltiazem (apparent K B of 310 nM). In terms of a new pseudoequilibrium two-drug binding model, the interaction between the dihydropyridine agonist Bay K8644 and the antagonist felodipine was found purely competitive. The interaction between Bay K8644 and verapamil or diltiazem was found noncompetitive, and it could be described only by inclusion of a negative interaction factor ϭ Ϫ0.60 for verapamil and a positive interaction factor ϭ ϩ0.24 for diltiazem. These results suggest that at physiological membrane potentials, the L-type calcium channel cannot be simultaneously occupied by a dihydropyridine agonist and antagonist, whereas it can simultaneously bind a dihydropyridine agonist and a nondihydropyridine antagonist. Generally, the effects of the drugs on the L-type calcium channel support a concept of a channel domain responsible for binding of calcium channel antagonists and agonists changing dynamically with the membrane voltage and occupancy of individual binding sites.

Inhibition of the Cardiac L-Type Calcium Channel Current by Antidepressant Drugs

“…From the electrophysiological viewpoint, these drugs in every aspect resemble calcium antagonists. The shape of the concentration dependence curves and the rates of the onset and washout of inhibition at concentrations of the antidepressants close to their IC 50 values are similar to those of calcium antagonists (Uehara and Hume, 1985;Méry et al, 1996;Zahradníková et al, 2007). The inhibitory action of the antidepressants is dependent on the conformational state of the channel in a manner comparable with nondihydropyridine calcium antagonists, as demonstrated by the shift in the calcium channel availability curve.…”

“…The inability to observe competition between drugs in electrophysiological experiments is not inherent to the method used, since direct competition between drugs that act at the dihydropyridine receptor, i.e., Bay K8644 and felodipine, could be convincingly demonstrated using the same protocols (Zahradníková et al, 2007). Therefore, the antidepressant drugs most probably act at a site distinct from the benzothiazepine receptor.…”

Intercellular calcium waves are associated with the propagation of vasomotion along arterial strips

“…5C, seconds 150 -280) (n ϭ 4). The involvement of VOCCs was also tested using BAY K 8644, an L-type VOCC activator (36,37). Strips that did not show any calcium waves in response to local stimulations when superfused with 0.2 M PE (Fig.…”

A highly energetic process couples calcium influx through L-type calcium channels to insulin secretion in pancreatic β-cells

“…In contrast to stimulation of ISR by KCl, which was fully inhibited by KN-62, Bay K 8644 stimulation of OCR and ISR overcame the inhibitory effects of KN-62. This indicates that Bay K 8644 has two effects, one to increase Ca 2ϩ influx by its known interaction with the dihydropyridine binding site on the L-type Ca 2ϩ channel (50) and one that prevents KN-62 action. This could occur if Bay K 8644 affected separate proteins, each of which would carry out these roles, or, alternatively, Bay K 8644 binding to the L-type Ca 2ϩ channel has two or more actions.…”