1. The protective effects of some
calcium antagonists against different forms of
calcium overload
contracture were investigated in embryonic chick cardiac myocytes. 2.
Tetrodotoxin-sensitive
sodium currents were recorded from the myocytes by the whole-cell voltage-clamp technique. Although the peak current was attenuated by
veratrine, the inactivation process was markedly inhibited, resulting in a large increase in the total inward current. Action potentials were prolonged by
veratrine, automaticity was inhibited and the membrane potential depolarized from -79 to around -45 mV. 3. Measurements of contraction were made from aggregates of myocytes using a video edge detection technique which quantified edge movement.
Veratrine caused an initial positive inotropism then inhibited automaticity of aggregates with subsequent development of a tonic
contracture to around 300% of the twitch contraction. 4.
Veratrine-induced
contractures were not significantly affected by 10 microM
diltiazem or
verapamil.
Nifedipine (5 microM),
nimodipine (5 microM) and
ryanodine (5 microM) also had little effect whilst
nicardipine and
flunarizine caused a concentration-dependent inhibition of
veratrine-induced
contractures with IC50s of 3 microM and 2 microM respectively. 5.
Veratrine-induced
contractures were found to be very sensitive to extracellular
calcium concentration with an EC50 of 32 microM. Edge movement associated with beating of the myocytes was much less sensitive to
calcium (EC50 = 1 mM). Submaximal
veratrine contractures in 20-50 microM extracellular
calcium were not potentiated by 1 microM
Bay K 8644. 6.
Tetrodotoxin also inhibited
veratrine-induced
contractures but did not affect contractions induced by
ouabain in the presence of 10 microM
diltiazem. 7.
Ouabain-induced
contractures were also inhibited by
nicardipine and
flunarizine indicating that these drugs can protect against
calcium overload in embryonic chick heart by a mechanism independent of the normal form of voltage-sensitive
sodium or
calcium channels.