The Role of Cyclic AMP in Regulation of The Slow Inward Current. In: Zipes DP, Bailey JC, Elharrar V, editors. The Slow Inward Current and Cardiac Arrhythmias. Dordrecht: Springer Netherlands; 1980. pp. 111-26. [DOI: 10.1007/978-94-009-8890-3_5]

Gap junctions in excitable cells

Gap junction channels are an integral part of the conduction or propagation of an action potential from cell to cell. Gap junctions have rather unique gating and permeability properties which permit the movement of molecules from cell to cell. These molecules may not be directly linked to action potentials but can alter nonjunctional processes within cells, which in turn can affect conduction velocity. The data described in this review reveal that, for the majority of excitable cells, there are two limiting factors, with respect to gap junctions, that affect the conduction/propagation of action potentials. These are (1) the total number of channels and (2) the selective permeability of the channels. Interestingly, voltage dependence and the time course of voltage inactivation (kinetics) are not rate limiting steps under normal physiological conditions for any of the connexins studied so far. Only specialized rectifying electrical synapses utilize strong voltage dependence and rapid kinetics to permit or deny the continued propagation of an action potential.

Effects of amrinone on the transmembrane action potential of rabbit sinus node pacemaker cells

Effects of amrinone on the membrane action potential and spontaneous activity were investigated in sinus node pacemaker cells of rabbit heart by use of microelectrode techniques. Amrinone (1 X 10(-4) M to 6 X 10(-6) M) caused a shortening of cycle length of spontaneous firing (SPCL) accompanied by an increase in the maximum upstroke velocity at phase O (Vmax) and amplitude of action potential (AAP), while it did not affect the maximum diastolic potential (MDP). All the effects of amrinone on sinus node pacemaker cells were markedly attenuated or abolished in a low calcium medium (Ca+ 0.1 mM or 0.3 mM) or in the presence of the slow channel blocking agent, verapamil (5 X 10(-7) M, 2 X 10(-6) M). The effects of amrinone were not antagonized by the beta-adrenoceptor blocking agent, pindolol (2 X 10(-7) M). These results indicate that amrinone has an intrinsic effect on sinus node pacemaker cells, increasing their spontaneous firing activity. It is also assumed that the effects of amrinone on sinus node cells are probably mediated by an augmentation of the slow calcium and/or sodium inward current through the cell membrane.