Pacemaking is a typical and essential feature of the cardiac activity. In specialized pacemaker tissues, rhythmic activity is under normal conditions largely under control of two current systems: the hyperpolarizing -activated """"""""pacemaker"""""""" current i-f and the depolarizing- activated K current iK. The current i-f also mediates the chronotropic effect of adrenaline. We propose to investigate the properties of these two currects in isolated cardiac myocytes, using the suction-pipette and patch- clamp techniques. The very recent recording of single i-f-channels performed in our laboratory now allows a much more detailed investigation of the properties of this current. The problems that we wish to address include the way the current i-f is controlled by adrenaline, and in particular how intracellular cAMP and the protein kinase catalytic subunit (CS) are involved in this process at the single cell and at the single-channel level; whether the K-induced increase of i-f is mediated by a channel conductance modulation, or by a channel activating process; whether the inward rectification of iK in multicellular preparations is a channel property, and which is the role of K-accumulation/depletion processes in influencing the current kinetics; which is the cause of the apparent Ca-entry dependence of iK (that is, its dependence on agents know to modify the slow inward current, i-si). A further aim is to investigate in isolated cardiac myocytes the properties of the fast inward Na current, iNa. We are particularly interested in studying the blockade of iNa caused by Cd and other group 2b cations (Zn, Hg). This property is typical of cardiac Na channels, and is absent in nerve and skeletal muscle. Recent experimentation in our laboratory has shown that, as in Purkinje fibres, block of iNa by 2b cations also occurs in isolated ventricular myocytes.
Our aim i s to better characterize the block of iNa at the single-cell and single-channel level, and identify possible structural differences between cardiac and nerve or skeletal muscle Na channels that can explain the different sensitivity to 2b cations. We hope that these experiments will provide helpful information concerning the control of the cardiac pacemaker activity, and ultimately of the heart rate. We also feel that information on the block of iNa, and thus of the cardiac cell excitability, by 2b cations can be useful in relation to their known cardiotoxicity.
