title: Ionic currents responsible for the generation of pace-maker current in the rabbit sino-atrial node.
abstract: The ionic nature of the pace-maker current (delta Ip, If, Ih) was investigated in rabbit sino-atrial node using a single sucrose-gap voltage-clamp technique. The pace-maker current was activated by hyperpolarizing clamp steps negative to -50 mV and the pace-maker potential was activated by an action potential or a depolarizing clamp step. Neither pace-maker current nor pace-maker potential were altered by addition of tetrodotoxin, but a tetrodotoxin-sensitive channel could be activated in sino-atrial nodal strips following hyperpolarizing clamp steps. Ca2+-channel blockers did not affect the voltage dependence of delta Ip or the maximum diastolic potential (m.d.p.) significantly. Removal of Ca2+ did not affect the pace-maker current at clamp potentials near the pace-maker potential range (-60 to -80 mV), but it did reduce the potential dependence of the m.d.p. Removal of Na+ suppressed completely the pace-maker current and hyperpolarized the membrane. Removal of Na+ also increased membrane conductance, most likely through an increase in resting K+ permeability. Low concentration of Cs+ (less than 5 mM), but not Ba2+ or tetraethylammonium, markedly suppressed activation delta Ip and reduced the rate of pacing slightly. Cs+ also decreased the membrane conductance and hyperpolarized the membrane. In 50% of experiments designed to determine contribution of IK to pace-maker current, a double-pulse procedure revealed a time-dependent component of delta Ip which reversed near the K+ equilibrium potential, EK. Release of depolarizing or hyperpolarizing test clamps was followed by pace-maker potentials, the magnitudes of which were dependent on the test-clamp potential. The m.d.p. approached values near EK following depolarizing clamps and near -45 mV following hyperpolarizing clamps. The results suggest that delta Ip is carried primarily by Na+ and is blocked by Cs+. It is likely, however, that Ca2+ alters the rate of pacing not only through its contribution to the Isi system, but also through activation of a K+ conductance.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002586:Cesium / Q000494:pharmacology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects; D008564:Membrane Potentials / Q000187:drug effects; D008955:Models, Cardiovascular; D011188:Potassium / Q000378:metabolism; D011817:Rabbits; D012849:Sinoatrial Node / Q000378:metabolism / Q000502:physiology*; D012964:Sodium / Q000378:metabolism / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D002121:Calcium Channel Blockers; D007473:Ion Channels; D002104:Cadmium; D002586:Cesium; D013779:Tetrodotoxin; D012964:Sodium; D011188:Potassium; D002118:Calcium