?-Adrenergic increase in the calcium conductance of cardiac myocytes studied with the patch clamp

Abstract

In bovine, cat and guinea pig myocytes the effect of bath application of adrenaline or isoprenaline and of injection of cAMP on the Ca channel was studied with the patch clamp (Hamill et al. 1981), and the following results were obtained: On β-adrenergic stimulation more activity of the single channel on repeated depolarizations and less records without activity were observed. Correspondingly, the average currents were increased. When the patch contained only one channel as judged from the lack of superpositions during all depolarizations, β-adrenergic stimulation never produced superpositions indicating that the total number of channels did not increase. Also, it was never possible to activate a channel in the mute patch. The single channel conductance was not changed by catecholamines or cAMP. Increase in probability during depolarization of the channel to be in the open state was proven by non-stationary fluctuation analysis. The kinetic analysis showed a prolongation of the open times and shortening of the shut times by catecholamines, indicating that the rate constants in a three state model C₁⇋C₂⇋O are changed in such a way that the equilibrium shifted towards the open state. In some patches clusters of channels were observed, and activity was greatly increased by adrenaline, isoprenaline and cAMP-injection. The decay of the mean current was either mono-exponential or, in most cases, double-exponential. When the decay was mono-exponential, fluctuation analysis showed an increase in open probability on β-stimulation. When the decay was double-exponential, the data could not be fitted by a single parabola. In this case it was not possible to decide whether the probability of the channels or their number, or both, had increased. From the results it is concluded that β-adrenergic stimulation increases the probability of the Ca channel to be in the open state and hence increases the slow inward Ca current. There was no indication for an increase in the total number of channels.