The kinetic characteristics of L-type calcium channels in cardiac cells of hibernators. 2. Analysis of the kinetic model.

The present paper describes the experimental and theoretical investigations of the kinetic characteristics of the L-type Ca2+ channels in ground squirrels Citellus undulatus in two different physiological states (hibernation and spontaneous arousal). The perforated patch-clamp method was used in the experiments. We have shown in the previous study [1] that Ca2+ currents in hibernating and active animals may be described by the d2f1(2)f2 model. Based on that model, in this paper we studied in detail the main steps of the conductance regulation of Ca2+ channels: activation (d), slow (f1-type) and fast (f2-type) inactivations of the channel. Activation is related to the movement of the gating charge. Slow inactivation is associated with the movement of the gating charge and is current-dependent. Fast inactivation is a more complex process and cannot be represented as a single-stage conformational transition induced by the gating charge movement. It is regulated by cAMP phosphorylation. The differences in the Ca2+ current kinetics are observed virtually for all the components. In hibernating animals, the most pronounced shift (15-20 mV) towards depolarization is experienced by the normalized conductances of both inactivation components, whereas the conductance of the activation component is shifted to a lesser extent. The characteristic times of Ca2+ currents of hibernating ground squirrels are 1.5-2 times greater than those of aroused animals. The current activation gating charge of Ca2+ channels in ground squirrel cardiocytes was found to change. The gating charge was about 2 in hibernating animals and 1.5 in active squirrels. The effect of isoproterenol-induced cAMP-dependent phosphorylation on Ca2+ currents in cardiocytes from hibernating ground squirrels was studied. Isoproterenol restored the kinetic parameters of Ca2+ currents to the values close to the parameters of active animals. However, we failed to explain the suppression of the Ca2+ current in hibernating animals in terms of cAMP-dependent regulation only.