THE EFFECT OF Ni+2 ON BLOCKING OF THE TRANSIENT Ca+2 CURRENT, (Ica, T) IN THE INTACT SINO-ATRIAL NODE OF THE HEART RABBIT

The mammalian sino-atrial node is not a uniform tissue in histological and electrophysiological terms. The differences in ionic currents underlying the regional differences in electrial activity are only just beginning to be understood. One of the ionic currents it is thought to play a role in the center of sino-atrial node for action potential upstroke should be transient Ca+2 current (ICa,T). This prediction was tested in the present study by investigating the effects of block of this current on pacemaker activity in the periphery and center of the nine intact sino-atrial node of the heart rabbit by 40 μM Ni+2 for 30 min. In all cases Ni+2 failed to change significantly action potential configuration either in the center or in the periphery of the intact sino-atrial node. It is possible that under physiological conditions ICa,T is inactive in both the center and peripheral zones of the sino-atrial node of rabbit. Alternatively it is possible that ICa,T is important in the center and pacemaker shift in the present of Ni+2 conceals the effect.