Effects of calcium channel antagonists and facilitators on beating of primary cultures of embryonic chick heart cells

Abstract

1 Primary aggregate cultures of embryonic chick heart have been used to investigate the effects of calcium channel antagonists and facilitators on myocardial contractility. 2 The number of aggregates showing movement was inhibited in a concentration-dependent manner by calcium antagonists from different subgroups with negative log concentrations inhibiting movement in 50% of aggregates as follows: Class 1-nisoldipine (7.20); Class 2-verapamil (6.36), diltiazem (5.83); Class 3-lidoflazine (5.68), pimozide (6.25). 3 The effects of the dihydropyridine facilitators Bay K 8644 and CGP 28392 on aggregate beating were investigated by evaluating the interaction between calcium channel facilitators and antagonists from the three subgroups of calcium antagonists. Concentrations of antagonists that inhibited beating in 85% of aggregates were used. Both Bay K 8644 and CGP 28392 reversed nisoldipine-, diltiazem-or verapamil-induced inhibition of beating. 4 Bay K 8644 was approximately 10 times more potent than CGP 28392 in reversing nisoldipine-, diltiazem-or verapamil-induced inhibition of beating. 5 For each facilitator the concentrations causing 50% reversal of inhibition of aggregate beating against the three antagonists were similar. There was little evidence for differential modulation by verapamil or diltiazem of the action of the dihydropyridine facilitators. 6 Bay K 8644 did not reverse lidoflazine-or pimozide-induced inhibition of beating, indicating that these drugs may act at a site distinct from the dihydropyridine site on the calcium channel.

Interaction of palmitoyl carnitine with calcium antagonists in myocytes

“…Bay K 8644 is a powerful tool for defining the site of action of agents at the Ca2+ channel (Su et al, 1984;Spedding & Berg, 1984;Spedding, 1985a,b). Using the present experimental protocol, Patmore & Duncan (1988) have shown that Bay K 8644 counteracts the effects of nisoldipine at nonomolar concentrations (Table 1), which is compatible with the competitive interaction between dihydropyridines observed in smooth muscle. Bay K 8644 also counteracted the effects of verapamil and diltiazem; the binding sites for these drugs are linked by an allosteric mechanism within the Ca2 + channel (Galizzi et al, 1985;Glossmann et al, 1985;.…”

“…The extent of edge movement of each aggregate, which was taken as being equivalent to an inotropic effect was dependent on the Ca2+ concentration of the medium; 1IM Ca2+ resulted in half maximal contractility with a near maximal effect occurring with 2 mm Ca2+ (Figures 1 and 2; Patmore & Duncan, 1988). Increasing the Ca2" concentration caused the beats to fuse partially so that the base-line increased; presumably relaxant processes were insufficient to account for Ca2 + entry so that cytosolic Ca2 + increased during diastole.…”

“…However, Bay K 8644 did not reverse the inhibitory effects of pimozide or lidoflazine, lipophilic class III calcium antagonists (Spedding, 1985b). Bay K 8644 does not reverse the effects of these agents in smooth muscle (Spedding & Berg, 1984) nor in heart (Boddeke et al, 1988;Patmore & Duncan, 1988) implying that these drugs either do not act at the Ca2+ channel (Spedding, 1985b;Grima et al, 1986) or act at a distinct site in the channel, perhaps trapping it in a state which has low affinity for dihydropyridines (Spedding, 1985b (Table 1) pointing to a direct interaction with the Ca2+ channel. However, the lack of stereoselectivity indicates that the interaction may be consequent to some form of selective accumulation and increased Ca2 + availability, perhaps consequent to a surface charge effect (Inoue & Papanno, 1983;Spedding, 1984).…”

“…These interactions have also been defined in chick myocytes (Patmore & Duncan, 1988). The aims of the present study were to assess whether the interactions between Bay K 8644 and the different groups of calcium-antagonists in myocardium were similar to those between palmitoyl carnitine and the calcium antagonists.…”

“…Beating was quantified with a motion detector video system which was developed to investigate the effects of Ca2+ channel activators on myocardial cells (Patmore & Whiting 1985;Patmore & Duncan 1988). Calcium-antagonists may act at various sites in VOCs to block the channels (Spedding, 1985b) and very distinct interactions have been shown to occur between the various subgroups of antagonists and dihydropyridine Ca2+ channel activators both in smooth muscle (Schramm & Towart, 1984;Su et al, 1984;Spedding & Berg, 1984;Spedding, 1985a) and in heart (Boddeke et al, 1988;Patmore & Duncan, 1988).…”