The time course of the L-type calcium current (ICa) inactivation has been reported as prolonged in cardiac hypertrophy (Hart, 1994). These experiments, however, used rectangular voltage steps, and usually intracellular Ca2+ buffering. In this study we used the action potential clamp technique to investigate the inactivation of ICaL in left ventricular myocytes enzymatically isolated from a model of mild left ventricular hypertrophy in the guinea-pig.
Hypertrophy was induced by constriction of the abdominal aorta below the renal arteries with a silver clip (i.d. 0.5 mm) for 20 weeks; sham animals (control) underwent the same procedure without clip placement (Bryant et al. 1997). Surgery was performed under general anaesthesia (Hypnorm, 1 ml kg-1 and ketamine, 5 mg kg-1, I.M.). The guinea-pigs were humanely killed by pentobarbitone overdose (I.P., 280 mg kg-1). Current and voltage clamp experiments where conducted at 35 °C, using K+-based pipette solutions free of Ca2+ buffering, during whole-cell patch-clamp.
Action potentials, recorded from hypertrophied and control myocytes, were time-averaged and applied as the command potential during a double-pulse protocol where the action potential waveform was interrupted, at various time intervals, by a depolarization step to +10 mV (Linz et al. 1998). Cd2+-sensitive ICa, obtained during the step pulse, was plotted as a function of the maximal available ICa to obtain the inactivation variable (f ). Myocytes from clipped and sham animals were clamped with their own respective action potentials, both with a functional SR and also following inhibition of the SR with ryanodine and thapsigargin (1 mmol l-1).
There was no difference in the degree of ICa inactivation under physiological conditions between hypertrophy (n = 8) and control (n = 9) at any time point until control myocytes enter phase 3 of the action potential. With the SR inhibited ICa inactivation follows a slower time course in hypertrophy after the first 25 ms, becoming significant by 90 ms (hypertrophy 0.39 ± 0.03, control 0.30 ± 0.02, mean ± S.E.M., n = 8; P < 0.05, Student’s unpaired t test). The SR-dependent component, obtained by subtraction, was initially similar at 5 ms but thereafter was greater in hypertrophy (51 ms, hypertrophy 0.79 ± 0.03, control 0.88 ± 0.03; P < 0.05).
The time course of ICa inactivation during the action potential is unchanged in this model of hypertrophy, despite very different relative contributions to the Ca2+-dependent inactivation of ICa, the SR component being increased and the channel component being reduced in hypertrophy.
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